Is it possible to specify a displacement and then have STAAD analyze a frame to give me a corresponding load (the load that would have been required to produce that displacement)?

You first need to know the pattern or arrangement of the loading which will eventually cause the

displacement you wish to see. This is because, there can be millions of loading arrangements which

cause that amount of displacement at that node, so one needs to have an idea of which of those

patterns is the one that one wants. By pattern, we are talking of details like, is the load going to consist

of concentrated forces at nodes, or distributed and trapezoidal loads on members, or pressures on

plates, etc. For example, any of these loads will cause a certain amount of displacement at a node

along a certain direction. 

So, a unit load analysis would be the best approach for solving this kind of a problem. That means, all

the components of the loading pattern would be represented by unit loads. Let us say that by applying a

member load of 100 pounds/ft, you get 0.4 inches of displacement along global X at node 43. So, if the

final desired displacement at node 43 along X is say, 1.2 inches, the applied load should be simply

(1.2/0.4)*100 = 300 pounds/ft.

I am applying a UBC seismic load on a bridge. The analysis engine reports an error message which says that:

EITHER NA OR NV FACTOR HAS NOT BEEN SPECIFIED

WHILE SEISMIC ZONE HAS BEEN SPECIFIED AS 4.

This is due to the fact that, for your model, STAAD looks at the data under the DEFINE UBC LOAD

command and concludes that you intend to analyse the structure per the UBC 1997 code. It then checks

whether all the required parameters have been specified for that code, and detects that NA and NV are

missing. You perhaps have an input similar to the one below :

DEFINE UBC LOAD

ZONE 0.4 I 1 RWX 12 RWZ 12 STYP 1.2 PX 0.2626 PZ 0.2626

For Zone 4, Na and Nv are two of the fundamental parameters necessary to calculate the base shear. If

you look at Tables 16-Q and 16-R on pages 2-34 & 2-35 of the UBC 1997 code, you will find that for

Zone 4, the coefficients Ca and Cv are dependent on Na and Nv. 

So, specify the NA and NV parameters, so that the commands look similar to the one below :

DEFINE UBC LOAD

ZONE 0.4 I 1 RWX 12 RWZ 12 STYP 1.2 NA 1.6 NV 1.6 PX 0.2626 PZ 0.2626

I would like to create a REPEAT LOAD case whose constituent load cases are themselves REPEAT LOAD cases. Is this allowed?

You can do this if you have STAAD.Pro version 2002 or later. An example of this is shown below.

LOADING 1 

SELFWEIGHT Y -1.0 

LOAD 2

REPEAT LOAD

1 1.0

JOINT LOAD

4 5 FY -15. ; 11 FY -35.

LOAD 3

REPEAT LOAD

2 1.0

MEMB LOAD 

8 TO 13 UNI Y -0.9 ; 6 UNI GY -1.2

LOAD 4

SELFWEIGHT Y -1.0 

JOINT LOAD

4 5 FY -15. ; 11 FY -35.

MEMB LOAD 

8 TO 13 UNI Y -0.9 ; 6 UNI GY -1.2

PERF ANALY

LOAD LIST 3 4

PRINT *** RES

FINISH

In the above example, load case 3 repeats load case 2, which in turn repeats load case 1.

After determining the lateral loads using Staad UBC seismic analysis in a first file, I note down the lateral loads computed at each joint. In a second separate file with the same frame model, I apply the lateral loads from the first file combining them with the gravity loads and perform the analysis. I consider this procedure of mine very tedious in case of a 3D high rise building most specifically in view of the first file. Is there any shorter procedure for this? Please take note that I am using the Command File Editor.

There is absolutely no need for you to take the lateral load data from the output of the first file, and

insert it as input into the second file. In STAAD, once the lateral loads due to UBC or IBC are generated,

they are automatically available for combining with gravity loads, or any other loads for that matter.

Consequently, there are 2 ways in which this combination can be achieved, and each is demonstrated

below :

Method 1 :

Generate the lateral load in one load case. Specify the gravity load in another load case. Then, combine

the two in a load combination case.

LOAD 1 - GENERATE LATERAL LOADS DUE TO UBC ALONG X

UBC X 1.0

LOAD 2 - SPECIFY GRAVITY LOADS

SELFWEIGHT Y -1.0

MEMBER LOAD

1 TO 25 UNI GY -1.2

JOINT LOAD

10 39 FY -10.0

LOAD COMBINATION 3 - COMBINE THE LATERAL AND GRAVITY LOADS IN ONE CASE

1 1.0 2 1.0

Method 2 :

Create a single load case in which the lateral forces are generated, and gravity loads are specified.

LOAD 1 - LATERAL LOADS + GRAVITY LOADS

UBC X 1.0

SELFWEIGHT Y -1.0

MEMBER LOAD

1 TO 25 UNI GY -1.2

JOINT LOAD

10 39 FY -10.0

I am trying to analyse a structure which consists of a large dia pipe supported at discrete points. I am unable to get STAAD to analyse this for UBC loads.

When the UBC committee came up with the recommendations for analysing structures subjected to

earthquakes, the type of structures they had in mind were conventional style buildings where the base

of the model, namely, the points where the supports are located is at the lowest elevation with respect to

the rest of the model.

If you look at the UBC procedure, it involves computation of the base shear, which then has to be

distributed over the height of the building, so that one can then calculate the inter-story shears. A certain

amount of the weight gets lumped at the highest point of the building, and the rest gets distributed along

the height. In other words, the principle is that a mass at any height of the building is subjected to an

acceleration and the force caused by the acceleration is represented by a concentrated force where the

mass is located. The summation of all such forces at a given floor cause the columns beneath that floor

to be subjected to a shear force.

When you talk of a model like a pipe which is defined as line members attached to several collinear

nodes, all of which are at the same elevation, the UBC rules become impossible to apply. The fact is, to

analyse your structure for seismic effects, you do not even need the elaborate procedure of the UBC

code. You can take the selfweight, and any imposed loads on the pipe, and apply them along a

horizontal direction like X or Z with a factor, and you will get what is normally expected in a seismic

analysis.

So, you just have to have

LOAD 2

SELF X n

where n is a number like 1.5, which represents that there is a net force of 1.5 times the weight of the

structure acting along the X direction due to an earthquake. For better handling of the distributed loads,

you might want to consider defining several nodes along the length of the pipe, between supports.

I am modelling a steel building consisting of columns and beams. The floor slab is a non-structural entity which, though capable of carrying the loads acting on itself, is not meant to be an integral part of the framing system. It merely transmits the load to the beam-column grid.  There are uniform area loads on the floor (think of the load as wooden pallets supporting boxes of paper). Since the slab is not part of the structural model, is there a way to tell the program to transmit the load to the beams without manually figuring out the beam loads on my own?

STAAD's FLOOR LOAD option is ideally suited for such cases. This is a facility where you specify the

load as a pressure, and the program converts the pressure to individual beam loads. Thus, the input

required from the user is very simple - load intensity in the form of pressure, and the region of the

structure in terms of X, Y and Z coordinates in space, of the area over which the pressure acts.

In the process of converting the pressure to beam loads, STAAD will consider the empty space between

criss-crossing beams (in plan view) to be panels, similar to the squares of a chess board. The load on

each panel is then tranferred to beams surrounding the panel, using a triangular or trapezoidal load

distribution method. 

Additional information on this facility is available in example problem 15 in the examples manual, and

section 5.32.4 in the STAAD.Pro Technical Reference manual.

When does one use FLOOR LOAD and when does one use ELEMENT LOAD?

When modelling a grid system made up of horziontal beams and the slabs which span between the

beams, we have found that there are 2 approaches that users take :

1) They model the beams only, and do not include the slabs in the model. However, they take into

account the large inplane stiffness of the slab by using the master-slave relationship to tie together the

nodes of the deck so that a rigid diaphragm effect is simulated for the horizontal plane at the slab level.

2) They model the slabs along with the beams. The slabs are modelled using plate elements.

The question that arises is, how does one account for the distributed loading (load per area of floor)

which is present on top of the slab?

If you model the structure using method (1), the load can be assumed to be transferred directly on to the

beams. The slab-beam grillage is assumed to be made up of a number of panels, similar to the squares

of a chess board. The load on each panel is then tranferred to beams surrounding the panel, using a

triangular or trapezoidal load distribution method. You can do this in STAAD by defining the load

intensity in the FLOOR LOAD command. In other words, the pressure load on the slabs (which are not

included in the model) are converted to individual beam loads by utilizing the FLOOR LOAD facility.

In method (2), the fact that the slab is part of the model makes it very easy to handle the load. The load

can be applied on individual elements using the ELEMENT LOAD facility. The connectivity between the

beams and elements ensures that the load will flow from the plates to the beams through the columns to

the supports.

 What is the difference between the LOAD COMB & REPEAT LOAD commands?

The difference lies in the way STAAD goes about calculating the results - joint displacements, member

forces and support reactions. For a load combination case, STAAD simply ALGEBRAICALLY

COMBINES THE RESULTS of the component cases after factoring them. In other words, for example,

in order to obtain the results of load 10, it has no need to know what exactly is it that constitutes load

cases 3, 4 and 5. It just needs to know what the results of those cases are. Thus, the structure is NOT

actually analysed for a combination load case. With a REPEAT LOAD case however, the procedure

followed is that which occurs for any other primary load case. A load vector {P} is first created, and later,

that load vector gets pre-multiplied by the inverted stiffness matrix.

I am modelling an elevated silo which will be used for storing grain. The columns which support the structure are modelled as members and the walls of the silo (containment part of the structure) are modelled using plate elements. The silo has vertical and sloping walls. The loads on the structure consist of the weight of the grain contained in the silo. What is the best method for applying the load when the silo is full of grain? As pressure loads on the inside? How should the load be applied on the sloping walls?

There are 2 segments of the tank which have to be individually considered for application of the load.

The vertical walls

------------------

The material in the tank, especially if it is a fluid, will exert a lateral pressure on the vertical walls of the

tank. This pressure load can be applied on the tank using the ELEMENT PRESSURE load facility. You

can use one of 2 options to do this.

a) A uniform pressure. If you take any individual element on the wall, if you know the pressure intensity

at the top edge, and the pressure intensity at the bottom edge, the average of these 2 intensities can be

applied as a constant pressure on the entire surface of the element, as in the following example :

45 PRESSURE -3.5

Since the load is along the local Z axis of the element, you do not have to specify the axis name in the

above command since local Z is the default for the axis. The load value must be accompanied by the

proper sign (positive or negative) which accounts for whether the load acts along or opposite to the

direction of the local Z axis.

b) A trapezoidally varying pressure.

In case (a) above, we decided to take the average of the pressures at the top and bottom edges, and

thus obtain a uniform pressure. However, this is not absolutely necessary. The load can be applied as a

trapezoidal load, in which case, the TRAP option is used and the intensities at the top and bottom edges

must be specified. An example of that is

45 PRESSURE TRAP Y -4.5 -2.5

In this example, it is assumed that the local Y axis of element 45 is along the vertical direction, and thus

the trapezoidal variation is along the local Y. The load itself acts perpendicular to the surface of the

element, and hence along local Z. If local Y is in the same sense as global Y, -4.5 indicates the intensity

at the lower edge, and -2.5 indicates the intensity at the upper edge.

If the vertical wall has many divisions along the vertical direction, there will be several "horizontal rings"

of elements. Every element contained in a ring has the same intensity at its top and bottom edge. That

means, the top & bottom intensity for each of those rings will have to be manually calculated. There is a

facility in the STAAD.Pro GUI to simplify this task. From the top of the screen, select Commands -

Loading - Load Commands - Element - Hydrostatic Trapezoidal, and provide the intensities at the top

and bottom edges of the vertical wall. The program will use the linear interpolation method to find the

intensity at each intermediate division, and then create the individual element TRAPEZOIDAL loads.

The sloping walls

-----------------

The load on the elements which make up these walls is derived from the weight of the column of

material directly above these elements, and acts along the global vertical downward direction. Since the

element TRAP load facility that is available in STAAD allows a load to be applied only along the local Z

axis, and since local Z is not parallel to any of the global directions, the TRAP load option cannot be

used here. Hence, one will have to apply these as uniform pressure loads, the value of which has to be

calculated for each sloping element as the average of the intensities at the 4 nodes of that element.

There is no generation facility currently available in the program to automate this task.

 

I modeled a curved beam using cylindrical coordinates and tried to run a moving load over the curved beam. STAAD.Pro is not allowing me to do this. Why?

Moving load on curved beams is not supported by the DEFINE MOVING LOAD command in

STAAD.Pro. The STAAD moving load generator assumes:

1)All loads are acting in the negative global vertical (Y or Z) direction. The user is advised to set up the

structure model accordingly.

2)Resultant direction of movement is determined from the X, Y and Z increments of movements as

provided by the user.

However, STAAD.beava, an automated bridge load generator, can handle moving loads for curved or

custom-defined bridge decks with beams and plates. It also generates a 3D influence surface based on

displacements, support reactions, beam forces or plate stresses for any point on the bridge. The critical

loading patterns and critical vehicle position will be identified as well. STAAD.beava is an integrated

module in the STAAD.Pro environment.

What is the significance of the Rw Value in the UBC code?

The UBC 1997 code defines Rw as a Numerical Coefficient representative of the inherent overstrength

and global ductility capacity of lateral-force resisting systems.

It is to be used in the equation for computing base shear. Its values are dependent on the type of lateral-

force resisting system in the building, such as whether the system is a Light-framed wall with shear

panels or Shear wall made of concrete or a special moment resisting frame, etc.

Values of Rw are listed in Tables 16-N and 16-P of the UBC 1994 and 1997 codes.

How is the wind load calculated/generated for a structure in STAAD.Pro ? What is the exposure factor calculated and how is it calculated? In 2002, I hear you can now define your own "panels"? What does this mean?

The DEFINE WIND LOAD command may be used to define the parameters for automatic generation of

wind loads on the structure. The user needs to define the intensity and corresponding heights along with

the exposure factors. If the exposure factor is not defined, the program takes the default value as 1.0.

A value of 1.0 means that the wind force may be applied on the full influence area associated with the

joints if they are also exposed to the wind load direction.

All loads and heights are in the current unit system. In the list of intensities, the first value of intensity

(p1) acts from the ground level up to the first height. The second intensity (p2) acts in the global vertical

direction between the first two heights (h1 and h2) and so on. The program assumes that the ground

level has the lowest global vertical coordinate of any joint entered for the structure.

The exposure factor (e) is the fraction of the influence area associated with the joint(s) on which the load

may act if it is also exposed to the wind load. Total load on a particular joint is calculated as follows.

JOINT LOAD = (Exposure Factor) x (Influence Area) x (Wind Intensity).

Exposure factor (User specified) = (Fraction of Influence Area) x (influence width for joint).

In STAAD.Pro 2002, the built-in wind load generation facility has been enhanced to allow the user to

specify the actual panels of the building which are exposed to the wind. This user-level control will now

allow the user to obtain a more accurate distribution of wind forces, especially when the exposed

surface of the building lies in several vertical zones, each reset from the one below or the one above, in

terms of the direction of wind force. Further, the basic algorithm for detecting the shape of the panels

and the amount of load which should be calculated for the panel corners too has undergone significant

improvements. The parameters for definition of the wind load types are described in Section 5.31.3 of

the STAAD.PRO Technical Reference Manual. The relevant extracts from Section 5.32.12 of the

STAAD.Pro Technical Reference Manual, where the method for applying wind loading in the form of a

data in load cases has been explained, is provided below. Note that areas bounded by beam members

(and ground), and exposed to the wind, are used to define loaded areas (plates and solids are ignored).

The loads generated are applied only at the joints at vertices of the bounded areas. For example, in the

following set of commands:

DEFINE WIND LOAD

TYPE 1

INTENSITY 0.1 0.12 HEIGHT 100 200

EXP 0.6 JOI 1 TO 25 BY 7 29 TO 37 BY 4 22 23

TYPE 2

INT 0.1 0.12 HEIGHT 100 900

EXP 0.3 YR 0 500

LOAD 1

SELF Y -1.0

LOAD 2

WIND LOAD Z 1.2 TYPE 2 ZR 10 11

LOAD 3

WIND LOAD X TYPE 1 XR 7 8

A minus sign indicates that suction occurs on the other side of the selected structure. If all of the

members are selected and X (or Z) is used and the factor is positive, then the exposed surfaces facing

in the -x (or -z) direction will be loaded in the positive x (or z) direction (normal wind in positive

direction). If X and a negative factor is used, then the exposed surfaces facing in the +x direction will be

loaded in the negative x direction (normal wind in negative direction). [If -X is entered and a negative

factor, then the exposed surfaces facing in the -x direction will be loaded in the negative x direction

(suction). If -X is entered and a positive factor, then the exposed surfaces facing in the +x direction will

be loaded in the positive x direction (suction).] A member list or a range of coordinate values (in global

system) may be used. All members which have both end coordinates within the range are assumed to

be candidates for defining a surface which may be loaded if the surface is exposed to the wind. The

loading will be in the form of joint loads (not member loads). 1, 2 or 3 ranges can be entered to form a

"layer", "tube" or "box" for selecting members in the combined ranges. Use ranges to speed up the

calculations on larger models.

It is advisable not to use the SET Z UP command in a model with wind load. A closed surface is

generated by the program based on the members in the ranges above and their end joints. The area

within this closed surface is determined and the share of this area (influence area) for each node in the

list is then calculated. The individual bounded areas must be planar surfaces, to a close tolerance, or

they will not be loaded. Hence, one should make sure that the members/joints that are exposed to the

wind make up a closed surface (ground may form an edge of the closed surface). Without a proper

closed surface, the area calculated for the region may be indeterminate and the joint force values may

be erroneous. Consequently, the number of exposed joints should be at least 3.

I am using the moving load generation. The truck that I am specifying is so wide (dimension perpendicular to direction of traffic) that within the width of one lane of traffic, there are 3 or more parallel beams along the direction of traffic. How does STAAD determine how the truck load should be converted to beam loads?

Based on the data you provide under the DEFINE MOVING LOAD command, each truck is treated as a

set of axles. If the WIDTH option is NOT specified, each axle is assumed to be comprised of 1 tire. If the

WIDTH option is specified, each axle is assumed to be comprised of 2 tires.

The program looks at each tire independently. For any given tire, it looks for one longitudinal beam to

the left of the tire, and another longitudinal beam to the right of the tire. Then it distributes the tire weight

on those 2 beams as though the tire is located on a simply supported cross beam that spans the two

longitudinal members on either side.

Thus, even if a lane spans across 3 longitudinal beams or for that matter several beams, the above

approach ensures that the tire weights get properly applied on the correct set of beams as concentrated

member loads.

You can get a listing of these concentrated member loads by using the command:

PERFORM ANALYSIS PRINT LOAD DATA

For moving load generation, does STAAD provide the location of all the moving point loads in terms of member number and distance from the start of the member?

Yes. Please use the PRINT LOAD DATA option with your PERFORM ANALYSIS command and you will

get the information in your output file.

How does STAAD consider the moving load over the beams if the load is not applied over a beam exactly?

If a wheel falls inside a panel composed of beams on either side of the wheel running parallel to the

direction of movement of the vehicle, the load is distributed on the 2 beams as simply supported

reactions. Hence, if the wheel load is 10 kips, and if the distance from the wheel to the beam on the left

is 7 ft, and the distance to the beam on the right is 3 ft, the beam on the left gets a 3 kip load, and the

beam on the right gets a 7 kip load.

If we have a wind load on a bracing system (perpendicular to the bracing plane), can we apply the wind loading directly to the brace as a uniform load instead of resolving the force into point loads? How does Staad handle this type of loading on members that are declared trusses?

If a transverse load such as a uniform distributed load or a concentrated force is applied on a truss

member, STAAD converts it to the equivalent concentrated shears at the 2 ends of the member. The

member end force output will show them as shears on the member under the output terms SHEAR-Y or

SHEAR-Z depending on the local axis direction the load is applied in.

However, if you determine the equivalent end shears and apply them as joint loads instead, and not as

a member load, the truss members at that node will not experience any shear force due to that load.

I am using the moving load generation facility to generate a set of load cases for a truck moving on a bridge. Can STAAD provide the support reactions for the critical position that produces the maximum effects on the system flooring?

This would require that the support reactions for all generated load cases be produced in a report form

sorted in a descending order based upon the specific support reaction criteria we are interested in, such

as the FY force, or the MZ moment.

To get this report, first run the analysis. Go to the Post processing mode. Select the support node(s) at

which you want the information you are seeking. From the top of the screen, select Report | Support

Reactions. In the dialog box that comes up, select the degree of freedom (FY, MZ, etc.) which should be

used as the criteria for sorting. Set the sorting order (high to low or low to high). From the loading tab,

select the load cases that you want considered. Click on OK. A report of the results will be displayed in

tabular form.

I have some distributed loads on some members of the model. I would like to consider the weights due to these loads in the base shear calculation for UBC load generation. Can you explain the process for doing this?

When analysing a structure for UBC loads, there 2 stages in the input. The first stage is the one where

one defines data such as the various parameters (zone factor, importance factor, soil structure

interaction factor, etc.) as well as the weights. In terms of the STAAD command language, it is initiated

using the DEFINE UBC LOAD command, and an example for this may be found in Example 14 of the

STAAD.Pro Examples manual.

Graphically, one may assign the data in the following manner.

Select the beam or beams you want to assign the distributed weights to. Next, from the top of the

screen, select Commands | Loading | Define Load | Seismic Load. In the Parameters tab, select the

type, and enter the relevant

values for the parameters. Press the "Save" button. A new tab called "Weights" should come up. Press

the "Member Weight" button. For the loading type, choose UNI, enter the distributed weight value,

distances to where the load starts and the load ends, and press "OK". Press the "Assign" button to

actually assign them to the selected members. Finally, press the "Close" button.

What is JOINT WEIGHT? I'm trying to learn how to use the seismic load generator and I don't see anything explaining what JOINT WEIGHT is or what it is used for.

In the block of commands which fall under the DEFINE UBC LOAD heading or any of the other ones like

AIJ, 1893, etc., the weight data which goes into the calculation of the total weight consists of

SELFWEIGHT

MEMBER WEIGHT

JOINT WEIGHT

If at any of the joints of the structure, there are any weights which you want included in the total weight

calculation, you specify them using the JOINT WEIGHT option.

How do I get STAAD to automatically combine static load cases with load cases generated using the MOVING LOAD generation facility?

You should use the option called ADD LOAD along with the LOAD GENERATION command.

Shown below is an example:

DEFINE MOVING LOAD

TYPE 1 LOAD 20. 20. 10. DISTANCE 10. 5. WIDTH 10.

LOAD 1 STATIC LOAD

SELF Y -1.0

* GENERATE MOVING LOADS AND ADD THE SELFWEIGHT

* LOAD TO EACH GENERATED LOAD CASE

LOAD GENERATION 10 ADD LOAD 1

TYPE 1 7.5 0. 0. ZI 10.

PERFORM ANALYSIS PRINT LOAD DATA

How is the wind typically applied to the model?

The slab edge determines the exposure (Ram Frame – Loads – Exposure) and we assume some kind

of vertically spanning cladding transfers the loads to the diaphragms. The program calculates a total

force based on total exposure, considering one windward surface and one leeward surface per

diaphragm. Where the wind may hit more area, user defined loads should be used.

How are the wind pressures viewed?

You can see the total force using Process – Results – Applied Story Forces, or Report the “Loads and

Applied Forces” to see the intermediate wind pressure calculations. The Report – “Exposure

Boundaries” is useful for visualizing the exposed surfaces of the model.

Is wind uplift considered?

No, the program only applies the total horizontal force for each story or diaphragm. Uplift loads are not

yet considered.

While there is no good work around for this limitation some users apply negative magnitude Live Loads

in order to check the beams (or foundations) for uplift. It's important to note that the program still sees

this as a Live load and factors it accordingly, so some adjustments in the magnitude of the applied load

or customization of the load combinations might be required.

How does sloped framing affect the wind loads?

The program currently applies the wind loads based on the simple rectangular area of the structure

defined by the story data. Adjustments in the column and wall elevations do not affect the total applied

wind loads at this time.

Use the Report - "Exposure Boundaries" to see a visualization of the exposed wind surfaces whenever

there is some concern over the height or tributary width being used to calculate total story forces.

Can wind loads on open structures be generated?

No, when the program generates wind loads it always assumes the structure is fully clad all the way

down to the ground level with one complete windward and leeward surface.

For any other condition where rigid diaphragms exist, "User defined story forces" should be used (these

are defined in RAM Frame under loads - load cases). When there is no rigid diaphragm use "Nodal

loads" (defined in the Modeler in the Elevation view). The program generated wind loads are not

completely accurate for any of the following situations:

open structures,

partially exposed structures,

structures with multiple windward surfaces (e.g. a "U" shaped plan),

structures with a stepping foundation.

How are torsional irregularities considered?

In general, the program automatically accounts for any eccentricity in the stiffness of the structure during

the finite element analysis. For each structure, there is a center of rigidity (which you can report if you

create a special center of rigidity load case). If the load is applied to the diaphragm eccentric to this

center of rigidity location, then torsion in the structure develops.

Accidental torsion is also considered based on the percentage set under loads - masses (default is 5%

of the diaphragm dimension). Currently, the application of accidental torsion is limited to rigid diaphragm

analysis. A method for incorporating accidental torsion in semi-rigid diaphragm analysis is in

development now.

What the program does NOT do, is amplify these torsion effects according to any specific

code provisions (e.g. "Ax" from ASCE 7-02 12.8-14) . It is up to the user to account for additional torsion

resulting from plan or vertical irregularities. Most people increase the mass eccentricity under loads -

masses from 5% to some larger value to account for the extra torsion required by code, though user

defined story forces with a modified location also work well.

Are the seismic results ultimate?

For program generated seismic load cases from any modern code (e.g. ASCE 7-05), the force

magnitudes are at an ultimate level.

It's important to note, however, that the drift associated with any static seismic load is the elastic

deformation (δxe from ASCE 7-02 Eq 12.8-15). The user should amplify the program drift results to

determine design deflection for comparison against the allowable drift (δx  from ASCE 7-05 Eq. 12.8-15)

Also note, the vertical component of the earthquake (Ev) is handled though the generation of load

combinations by increasing or decreasing the Dead load factor, it is not part of the individual seismic

load cases themselves. Furthermore, increases in the seismic force required by a lack of redundancy

(Rho) are only accounted for in the load factors applied to the seismic loads in generated combinations.

What is the difference between seismic loads that use provisons for member forces and provisions for drift?

When creating a seismic load case suing the IBC/ASCE7 equivalent lateral force procedure, there is an

option to use provision for member forces or provision for drift (see screenshot below).

The difference between these options is the upper limit of the calculated period used to calculate the

seismic loads.  When provisions for member forces are used, an upper limit of T = CuTa is used for the

calculated period per ASCE 7-05 12.8.2. When provisions for drift are used, the upper limit on the period

is not used per ASCE 7-05 12.8.6.2

See Also

While I can look at the model with Stress Ratio values annotated next to the steel members that I have asked to be checked, when I do the member query (double

clicking on the members) I don't see the Design Property and Steel Design boxes anymore. Why is that?

 Design Property and Steel Design tabs are not displayed for members which have not been designed.

Are you sure you are clicking a member for which the design has been done? Sometimes, when ratios

are annotated on the screen, the picture may become quite cluttered with data and in an effort to double

click on a designed member, one may end up clicking on a member for which design has not been

performed. So, first check that the member you are double-clicking has indeed been designed. If you

are certain that STAAD has done the design and evidence of that exists in the analysis output file and in

the postprocessing Unity Check tables, but still you are not able to see these tabs in the dialog box

which comes up when you double click on the member, please send us your .std model and our support

representatives will look into that. Our email address is support@reiusa.com

STAAD is checking deflection for beams or girders for all the load combinations in my model. Is there a way to tell STAAD which load combination to check?

You have to use the LOAD LIST command to achieve this. Supposing you want to check deflection for

combination cases 81 and 82. And assume that L/Deflection has a limit of 240. The command sequence

required to achieve this is

LOAD LIST 81 82

PARAMETER

CODE AISC

DFF 240 ALL

CHECK CODE ALL

However, after these commands, you have to reset DFF to a very small number so that deflection does

not become a criteria for any further design operations. That is because, once a parameter is specified

in STAAD, it stays that way till it is changed again. So, after the above, you need to specify

PARAMETER

CODE AISC

DFF 1 ALL

The steel design output indicates a slenderness failure (KL/r exceeds allowable). Why? The axial force on the member is very small.

The code has requirements which say that the KL/r ratios for a member should not exceed certain

allowable limits. For members subjected to tensile forces, the code suggests one limit, and for members

subjected to compressive forces, there is another limit.

This check does not consider the amount of the axial force. It only looks at the sign of the force to

determine if it is a tensile force or compressive force.

In most codes, this is the first check STAAD does on a member. If the member fails the check, no

further calculations are done for that member.

So, STAAD performs these checks by default. However, the code does not offer any guidelines on what

must be the minimum magnitude of the axial force for the member to become a candidate for this check.

So, in STAAD, two parameters are available - one called MAIN and another called TMAIN if you wish to

bypass this check (TMAIN is available for some codes only). MAIN=1 is for bypassing the slenderness

check in compression, and TMAIN=1 is for bypassing the slenderness check in tension.

I set my deflection limit to L/360, but the maximum deflection indicated in the summary of node displacements in PostProcessing shows a deflection of 1.5 inches. Isn't this above the limit that I set?

During steel design per the AISC ASD code, there are two types of deflection checks you can perform

with STAAD. They are

1. Check for local deflection. This is usually applicable to members which are connected at both

their ends to other members.

2. Check for the relative displacements between the nodes such as for a cantilever beam.

LOCAL DEFLECTION is defined as the maximum deflection between the 2 ends of the beam relative to

a straight line connecting the 2 ends of that member in its deflected position.

If you go to

Help - Contents - Technical Reference - Commands and Input Instructions - Printing Section

Displacements for Members

you will find a diagram indicating this is in figure 5.41.

To obtain more information on the difference between the 2 methods of deflection checking, please go

to

Help - Contents - Technical Reference - American Steel Design - Design Parameters (which comes

after Allowables per AISC code)

It will bring up section "2.4 Design Parameters"

At the end of the parameters table, you will see several notes. Please read Notes items 1 through 4 for

the description of the two methods.

As you can see there, the default condition, which is also represented by a value of zero for the CAN

parameter, is to perform the LOCAL DEFLECTION check.

Your question indicates that what you are looking for is a check of the nodal deflections. The cantilever

style check STAAD offers is probably the solution for your problem. If so, specify the CAN parameter

with a value of 1.

THE VALUE OF E FOR MEMBER NNN DOES NOT SEEM RIGHT. What does this mean?

The steel design output for several members is accompanied by the following warning message :

WARNING : THE VALUE OF E FOR MEMBER 21 DOES NOT SEEM RIGHT.

WARNING : THE VALUE OF E FOR MEMBER 22 DOES NOT SEEM RIGHT.

WARNING : THE VALUE OF E FOR MEMBER 23 DOES NOT SEEM RIGHT.

During steel design, there is a check for ensuring that the Modulus of Elasticity (E) specified for the

member is within the range that is normal for steel. This is because, E is a crucial term that appears in

many equations for calculating section capacities and the program wants you to know if the value

appears to be abnormal.

In STAAD, you specify E either explicitly under the CONSTANTS command block or through the

DEFINE MATERIAL block, as in the examples below.

Example 1 :

UNIT KIP INCH

CONSTANTS

E 29000 ALL

DENSITY 0.283E-3 ALL

Example 2 :

UNIT METER KNS

DEFINE MATERIAL START

ISOTROPIC STEEL

E 2.05e+008

POISSON 0.3

DENSITY 76.8195

ALPHA 1.2e-005

DAMP 0.03

END DEFINE MATERIAL

CONSTANTS

MATERIAL STEEL MEMBER 101 TO 121

So, if you are specifying an E value which is significantly different from that for steel, such as say,

Aluminum, and then later asking the member to be designed according to a steel code, as in the

following example, the above-mentioned warning message will appear.

 

UNIT FEET POUND

DEFINE MATERIAL START

ISOTROPIC ALUMINUM

E 1.44e+009

POISSON 0.33

DENSITY 169.344

ALPHA 1.28e-005

DAMP 0.03

END DEFINE MATERIAL

CONSTANTS

MATERIAL ALUMINUM MEMBER 21 TO 30

..

..

PARAMETER

CODE AISC

CHECK CODE MEMBER 21 TO 30

The KL/r value that STAAD reports for the Y axis for a single angle does not match what I get from my hand calculation. Can you explain why? 

For single angles, the local Y and Z axes are the principal axes as shown below:

 

The KL/r value is computed using ry and rz which are based on the principal axis system. Chances are

that your handculation uses the geometric axes.

I have a large model with several hundred members which have been assigned steel sections. I am doing a code check and I want to find out which of those members have failed. Can I get a list of just those members without having to scroll through hundreds of pages of steel design output?

There are 2 methods for finding just those members which have failed the steel design checks.

1. From the Select menu, choose By Specification - All Failed beams. The members which fail

the check will be highlighted. You can then isolate them into a New View to examine them in

greater detail. Double click on those members or use Tools - Query - Member to access a

dialog box with tabs called Steel Design and Design Property to see the cause of the failure

along with allowable and actual stresses and critical conditions.

2. In the Post processing mode, go to the Beam page along the left side of the screen. One of the

sub-pages will beUnity Check. A table will appear along the right side of the screen. One of

the tabs of that table is Failed Members. Select this tab, and click on each row of the table to

look at each such member individually.

I am running STAAD.Pro 2003. In the TRACK 2 output for the American LRFD code, I find some terms that I am not familiar with. Can you tell me what those are?

The terms reported in the TRACK 2 output for American LRFD are :

AX = Cross section Area.

AY : Area used in computing shear stresses along local Y axis.

AZ : Area used in computing shear stresses along local Z axis.

PY : Plastic Section modulus about local Y axis.

PZ : Plastic Section modulus about local Z axis.

RY : Radius of gyration about local Y axis.

RZ : Radius of gyration about local Z axis.

PNC : Axial compression capacity.

pnc : Axial compressive force used in critical condition.

PNT : Axial tensile capacity.

pnt : Axial tensile force used in critical condition.

MNZ : Nominal bending capacity about local Z axis.

mnz : Bending moment about local Z axis, used in critical condition.

MNY : Nominal bending capacity about local Y axis.

mny : Bending moment about local Y axis, used in critical condition.

VN : Shear capacity.

vn : Shear force associated with critical load case and section location.

DFF : Permissible limit for checking length to deflection ratio.

dff : Actual length to deflection ratio.

I am running STAAD.Pro 2002. In the TRACK 2 output for the AISC ASD code, I find some terms that I am not familiar with. Can you tell me what those are?

The terms reported in the TRACK 2 output for AISC ASD are :

AX = Cross section Area

AY : Area used in computing shear stresses along local Y axis

AZ : Area used in computing shear stresses along local Z axis

SY : Elastic Section modulus about local Y axis

SZ : Elastic Section modulus about local Z axis

RY : Radius of gyration about local Y axis

RZ : Radius of gyration about local Z axis

FA : Allowable axial stress. If failure condition involves axial tension, this is the allowable axial tensile

stress. If failure condition involves axial compression, this is the allowable axial compressive stress.

fa : Actual axial stress.

FCZ : Allowable bending compressive stress about local Z axis.

FTZ : Allowable bending tensile stress about local Z axis.

FCY : Allowable bending compressive stress about local Y axis

FTY : Allowable bending tensile stress about local Y axis.

fbz : Actual bending stress about local Z axis, used in the design condition

fby : Actual bending stress about local Y axis, used in the design condition.

FV : Allowable shear stress.

Fey : Euler stress for buckling about local Y axis.

Fez : Euler stress for buckling about local Z axis.

DFF : Permissible limit for checking length to deflection ratio.

dff : Actual length to deflection ratio.

I am using STAAD.Pro 2003 and I want to use physical members to do a steel design. I know how to manually create physical members by selecting the individual members, right-clicking the mouse and choosing Form

Member. But if I have hundreds of these members, can I do it faster?

In STAAD.Pro 2003, you can use the Auto-Form member option to let the program automatically create

physical members for you. From the Member Design page in the Steel Design Mode, go to Member

Design | Physical Members | Auto Form Members. The rules it uses to create physical members are as

follows:

1. All elements must form a single continuous line. But they do not have to form a straight line.

Thus curved members may be formed.

2. There must be a free end. Whilst curved members are allowed, they cannot form a closed loop.

3. All elements should have the same beta angle.

4. All elements must point in the same direction. Check with the orientation labels if necessary.

Use the reverse element command on elements that point the wrong way.

5. None of the elements can be part of another member.

6. The section properties must be consistent at each element end. Elements can taper along their

length, but where one element ends and the next starts, they must have the same section

reference.

7. All elements must be made from the same material.

8. Vertical segments are converted into columns first.

I want STAAD.Pro to perform a steel design based on the LRFD 3rd Ed rather than the 2nd Edition. The output always says "LRFD 1994". How do I tell it what code to use?

If you wish to use LRFD 3rd Edition Code, you can write CODE LRFD3 when providing the design

parameters.

The 3rd edition of the American LRFD steel code has been implemented along with the 2nd edition. In

general, the principles outlined in the code for design for axial tension, compression, flexure, shear etc.,

are quite similar to those in earlier versions of the code. The major differences are in the form of

incorporation of the Young’s modulus of steel in the various equations for determining various limits like

slenderness and capacities. 

Consequently, the general procedure used in STAAD for design of steel members per the AISC-LRFD

code has not changed significantly. Users may refer to Section 2 of the STAAD.Pro Technical

Reference manual for these procedures.

Those who wish to use the 1994 edition of the code can still do so by specifying the code name as:

CODE LRFD2

An example of commands used for performing design based on the new and old codes are as shown.

Example for the LRFD-2001 code (3rd Ed)

UNIT KIP INCH

PARAMETER

CODE LRFD

or

CODE LRFD3

FYLD 50 ALL

UNT 72 MEMBER 1 TO 10

UNB 72 MEMB 1 TO 10

MAIN 1.0 MEMB 17 20

SELECT MEMB 30 TO 40

CHECK CODE MEMB 1 TO 30

Example for the LRFD-1994 code (2nd Ed)

UNIT KIP INCH

PARAMETER

CODE LRFD2

FYLD 50 ALL

UNT 72 MEMBER 1 TO 10

UNB 72 MEMB 1 TO 10

MAIN 1.0 MEMB 17 20

SELECT MEMB 30 TO 40

CHECK CODE MEMB 1 TO 30

I am not sure how STAAD deals with the specifications of the unsupported length for top flange compression.

For example, if I have a truss whose top chord is laterally supported at every other node (i.e. two

member lengths being unsupported), then should I highlight every two members (of the top chord)

seperately and then tell the program to take their combined length as being unsupported, or should I

highlight the entire top chord and then specify the correct unsupported length.

The value you specify for UNL is what STAAD uses for the expression Lb which you will find in Chapter

F of the AISC ASD & LRFD codes. Starting from Version 2001, UNL has been replaced with UNT and

UNB for these codes. If the Lb value for the top flange is different from that for the bottom flange, you

have to specify the corresponding values for UNT & UNB.

So if the bracing points are at every alternate node, first determine the distance between the alternate

nodes. Then assign that value for both beams which exist between those nodes.

For example, if you have

Member 5 connected between nodes 10 and 11, and is 6.5 ft long

Member 6 connected between nodes 11 and 12, and is 7.3 ft long

and both the top and bottom flanges are braced at nodes 10 & 12, you can assign

UNIT FEET

PARAMETER

CODE AISC

UNT 13.8 MEMB 5 6

UNB 13.8 MEMB 5 6

To assign these parameters using the GUI, while in the Modelling mode, select the Design page from

the left side of the screen. Make sure the focus is on the Steel sub-page. On the right side, select the

proper code name from the list box on the top. Click on the Define Parameters button along the bottom

right side. In the dialog box which comes up, select the tab for UNT and UNB, specify the value, and

assign it to the appropriate members.

I would like to perform code checking on a 8" x 2 1/2" x 10 Gage channel per the AISI Coldformed steel code. But this channel is not listed in the sections available in your database. Can I assign it using a user provided table?

At present, sections whose data is specified using a "User Provided Table" (see section 5.19 of the

Technical reference manual for details) cannot be designed or checked per the AISI code. However, the

following approach may be used to get around this limitation.

You may add your section to the STAAD AISI section database, so that your section becomes a

permanent part of the database. This can be done using the following method.

From the Tools menu, select Modify Section database. The various steel databases available in the

program will be listed in a dialog box. You will find ColdFormed (US) at the end of this list. Expand this

list, and choose Channel with Lips or Channel without Lips as the case may be. On the right half of the

dialog box, the Add option will become activated. Select that, and you will now be provided with an

interface through which you can add your channel to the list. Save and Close it.

You can now go to the Commands menu, and choose Member property - Steel Table - AISI Table to

obtain visual confirmation that this new section is permanently included among the list of channel

sections. You should now be able to assign this new section to the members through the usual property

pages and menus.

Increasing the NSF value in Steel Design does not change the Failure Ratio for a member, Why?

In the design input parameters, I set NSF to .85 for my steel design. The design output result showed a

failure ratio of 1.063 on Member 1. I then proceeded to change the NSF parameter to 1.0. This time, the

design output result showed the same failure ratio of 1.063. It seems that nothing has changed. I

increased the net section factor by 0.15, but the stress ratio hasn't changed? 

The NSF value has an effect only on allowable axial tensile capacity, and the actual tensile stress.

If axial tension, or axial tension plus bending, are not what determine the critical condition, changing the

value of NSF will not have any impact on the failure ratio. For example, if the critical failure condition for

a member is compression, changing NSF will have no impact.

Check to see what the critical condition is. It will show up in the form of expressions such as:

AISC H1-1 or Slenderness, etc.

I ran my STAAD model and got an error message which stated that "This version does not design prismatic sections". What does this mean?

In the earlier versions of STAAD (STAAD-III), the code check for prismatic sections was done using

allowable stresses which are arbitrarily chosen as 0.6 x Fy. However, this assumption of 0.6Fy was not

based on any code specific requirements. The word PRISMATIC is meant to indicate a section of any

arbitrary shape. But neither the AISC nor LRFD codes provide guidelines for design of arbitrary shapes.

Section capacities are dependent upon aspects such as the width to thickness ratio of flanges and

webs, lateral torsional buckling etc. From that standpoint, using an allowable stress of 0.6Fy for

PRISMATIC sections was not always conservative.

A way around this limitation (lack of specific guidelines) would have been to use the rules of a known

shape, such as a Wide Flange, for designing prismatic shapes. That would require knowledge of

equivalent flange and web dimensions. When the properties are defined using the PRISMATIC option,

there is no means to convey information such as dimensions of flanges or webs to the STAAD design

facility. Hence, the design of PRISMATIC shapes is not supported in STAAD/Pro. You may get around

this problem by defining the properties using the GENERAL section in a User Provided Table. For a

GENERAL section, STAAD provides the means for providing dimensions of the components that are

critical from the standpoint of computing allowable stresses. The allowable stresses for a GENERAL

section are computed using the rules of a wide flange shape (I shape). As a result, the allowable stress

value will be dependent on attributes such as dimensions of the cross section, length of the member,

etc.

The KL/ry reported for a double angle does not match my hand calculations. I am designing the section per the AISC ASD 9th edition code.

For singly symmetric shapes such as Tees and Double Angles, the KL/r value for the Y axis is

calculated by STAAD using the rules for flexural torsional buckling as explained in page 3-53 of the

AISC ASD manual. It is not calculated as Ky multiplied by Ly divided by ry. 

I am using the composite beam design capabilities. But the output does not show any evidence of this design. Why?

There are 2 sets of data associated with analysing and designing a composite beam.

Step 1 : Define the member properties as a composite beam. To do this, one has to use the "TA CM"

option as explained in Section 5.20.1 of the STAAD.Pro Technical reference Manual. For example, if

member 1 is a composite beam made up of a 3.0 inch thick slab on top of a W18X35, and the grade of

concrete is 4.0ksi, one would have to specify

UNIT INCH KIP

MEMBER PROPERTIES

1 TA CM W18X35 CT 3.0 FC 4.0

Step 2 : Parameters for steel design. This is what you find in Section 2.9 of the STAAD.Pro Technical

reference Manual. These are the attributes which are to be used in the actual design equations, using

the expression PARAMETER, as in,

PARAMETER

CODE AISC

BEAM 1 ALL

TRACK 2 ALL

FYLD 50 ALL

CMP 1 ALL

DR1 0.3 ALL

WID 60 ALL

FPC 4 ALL

THK 4 ALL

SHR 0 ALL

DIA 0.75 ALL

HGT 4 ALL

RBH 2 ALL

CHECK CODE ALL

The most important thing to note here is the usage of the parameter CMP. Unless it is set to 1.0,

STAAD does not design the beam as a composite section. The beam will be designed as a pure steel

beam section in the absence of the "CMP 1" parameter.

How does one change the value of the yield strength of steel?

FYLD is one of the items specified as parameters for steel design. The STAAD Technical Reference

manual and International Design Codes manual contain information on specifying parameters for steel

design.

There are example problems in the STAAD Example manual demonstrating how parameters are

specified for design. The example below shows some typical post-analysis commands.

PERFORM ANALYSIS PRINT STATICS CHECK

PRINT MEMBER FORCES LIST 5 7

PRINT ELEMENT STRESSES LIST 10 TO 16

UNIT KIP INCH

PARAMETERS

CODE AISC

UNT 1.0 ALL

UNB 20.0 ALL

LY 60 MEMBER 36 40

LZ 60 MEMBER 36 40

FYLD 46.0 MEMBER 47 50

CHECK CODE ALL

FINISH

If you prefer to use the graphical method, this is how you can specify it. From the left side of the screen,

select the Design page. Make sure the sub-page says Steel. On the right hand side of the screen, go to

the top, and choose the appropriate code.

Select the members on the structure for which you wish to assign the FYLD parameter.

Then, on the bottom right hand side of the screen, you will find a button called Define Parameters. Click

on that button. Select the FYLD tab. Specify the value, and click on Assign.

In STAAD/Pro 2000 and STAAD.Pro, I no longer see the UNL parameter for the AISC ASD and LRFD codes. Instead, I see the parameters UNT and UNB. Why?

In versions of STAAD prior to STAAD/Pro 2000, the mechanism for specifying the unsupported length of

the compression flange was through the means of the UNL parameter. However, the drawback of this

command is that if the value for the top flange is different from that of the bottom flange, there wasn't

any means to communicate that information to STAAD.

Consequently, 2 new commands were introduced, namely, UNT and UNB.

UNT stands for the unsupported length of the TOP flange of the member for calculating the capacity in

bending compression and bending tension.

UNB stands for the unsupported length of the BOTTOM flange for calculating the capacity in bending

compression and bending tension.

To avoid the confusion that may arise from having 3 separate parameters to specify 2 items of input, we

no longer mention the UNL parameter. However, to enable the current versions of STAAD to analyze

input files created using the older versions of STAAD, the UNL parameter continues to work the way it

did.

These 2 new parameters are to be used in place of UNL. If UNT/UNB is specified in addition to UNL,

UNL will be ignored. If neither UNT nor UNB are specified, but UNL is specified, the value of UNL will be

used for both top and bottom flange.

The steel design output for a tube section checked per the AISC ASD code indicates an SY and SZ substantially different from the values which are reported in the AISC publication. Why?

In steel design per the AISC ASD code, the elements of the cross section (flange, web etc.) have to be

put through some tests per Chapter B of the code. These tests are required to classify the cross section

into one of 3 types - Compact, Non-compact, Slender.

If a section is classified as slender, the allowable stresses on the section have to be determined per the

rules of Appendix B of the code. For slender "stiffened elements", which is the type a tube falls into, the

effective section properties have to be calculated and those effective properties must then be used in

computing the actual stresses.

The extent of the cross section deemed effective depends on the bending moment on that section. It is

very likely that for the critical load case, the effective properties are less than the gross section

properties, which is why you see the reduced Sz and Sy in the output.

How can I check whether the story drift of the floors are within allowable limits?

If you have STAAD.Pro 2001 Build 1005 or Build 1006, you can specify a command called

PRINT STORY DRIFT

in your input file. Run the analysis. Then check your output file, The drift for each story will be reported.

You will have to manually verify that this is within your allowable limits.

Utilizing DFF in STAAD only helps one check the local deflection. What if I want to check the drift of a column / beam frame?

If my joint displacement printout says that joint of a column/beam joint has moved 1.42 inch in the global

X, then my drift ratio is 18x12/1.42 = 152.11, but the "dff" says 1072 for the same column, then where is

the dff being measured?

When the DFF parameter is specified, the deflection checks during steel design are performed on the

basis of so called "local axis deflection", not the nodal displacements in the global axis. For this reason,

it is not possible to include storey drift checks into the steel design calculations at present.

If you want additional information on local axis deflection, please refer to example # 13, and Section

5.42 of the STAAD Technical Reference Manual.

Can I get STAAD to check deflection in both axes?

 Yes. However, rather than check the deflection for each axis independently, STAAD finds the resultant

deflection "d" and compares the "L/d" (length to deflection ratio) against the allowable limit specified by

you through the DFF parameter.

Will STAAD explicitly state that the beam has passed the deflection criteria?

When STAAD performs steel design (code checking as well as member selection), it checks several

conditions required by the code. The one which gives rise to the highest unity check is the one

determined as critical. If the deflection criteria ends up being the worst condition, you will see it being

reported as the critical condition.

You can verify whether a member has passed the deflection check by looking at the terms "DFF" and

"dff" in the steel design output. "DFF" is the value you input. "dff" is the value the program calculates as

the actual "L/d" ratio. If "dff" is larger than "DFF", the member is deemed safe for deflection.

What are the design parameters which control deflection check?

1) DFF : This is the value which indicates the allowable limit for L/d ratio. For example, if a user wishes

to instruct the program that L/d cannot be smaller than 900, the DFF value should be specified as 900.

The default value for DFF is 0. In other words, if this parameter is not specified as an input, a deflection

check will not be performed.

2) DJ1 and DJ2 : These 2 quantities affect the "L" as well as the "d" in the calculated L/d ratio. They

represent node numbers that form the basis for determining L and d.

By default, DJ1 and DJ2 are the start and end nodes of the member for which the design is being

performed, and "L" is the length of the member, namely, the distance between DJ1 and DJ2. However, if

that member is a component segment of a larger beam, and the user wishes to instruct STAAD that the

end nodes of the larger beam are to be used in the evaluation of L/d, then he/she may input DJ1 and

DJ2 as the end nodes of the larger beam. Also, the "d" in L/d is calculated as the maximum local

displacement of the member between the points DJ1 and DJ2. The definition of local displacement is

available in Section 5.42 of the STAADPro Technical Reference Manual, as well as in Example problem

# 13 in the STAADPro Examples Manual.

A pictorial representation of DJ1 and DJ2, as well additional information on these topics is available

under the "Notes" section following Table 2.1 in Section 2.8 of the STAADPro Technical Reference

Manual.

If you use the design parameter TRACK 2.0, you will see a term called "dff" in the STAAD output file.

This terms stands for the actual length to deflection ratio computed by STAAD. If "dff" is smaller than

"DFF", it means the member has violated the safety requirement for deflection, and will be treated as

having failed.

THIS VERSION DOES NOT DESIGN TAPERED POLE SECTIONS (MEMBER 1). What does this error message mean?

 I am using tapered tubular section properties in my model. When I try to design those members using

the AISC code.

The AISC code currently does not have the rules for designing tubular sections which are 6 sided, 8

sided, 12 sided, etc. That is why you cannot currently design them per the AISC code.

There is a code from ASCE called the ASCE publication # 72. That document contains the rules for

designing these shapes. Those rules are implemented in STAAD's transmission tower code, and if you

have purchased that code, you should be able to design them.

I am using STAAD to do steel design per the AISC code. For 2 members with similar cross sections, one passes, the other fails. Fact is, the one which fails has almost no load on it. The other is significantly more stressed but still passes. Is something wrong in the steel design calculations that STAAD is doing?

You will notice that, for the member which failed, the cause of the failure is reported using the phrase

"L/R-EXCEEDS". This means that the member has failed the slenderness check.

When STAAD performs steel design on a member per the AISC code, it adopts the following sequence :

It first sets the allowable KL/r in compression to 200 and the allowable KL/r in tension to 300.

For the member being designed, it goes through all the active load cases to see if the member is

subjected to axial compression and/or axial tension.

Next, it compares the actual KL/r against the allowable KL/r. If this check results in a FAILure, the

member is declared as FAILed, and design for that member is immediately terminated. The requirement

to check this condition is in Section B of the AISC specifications.

If the member passes the KL/r check, only then does the program go on to do the remainder of the

checks such as axial compression + bending, shear, etc.

It must be noted that failure to satisfy the KL/r check is a reflection of the slenderness of the member,

not the capacity of the section to carry the loads which act on it. Even if the axial load or bending

moment acting on the member is a negligible quantity, the fact is, failure to satisfy KL/r will result in the

member being declared as unsafe as per the code requirement.

If you do not want the KL/r condition to be checked, you can switch off that check using a parameter

called MAIN. Set MAIN to 1.0 for a specific member and it won't be checked for slenderness. See Table

2.1 of the STAAD.Pro Technical Reference Manual for details.

What do the following parameters mean?

NSF 0.85 ALL

BEAM 1.0 ALL

KY 1.2 ALL

RATIO 0.9 ALL

LY 18 ALL

LZ 18 ALL

CHECK CODE ALL

NSF 0.85: This parameter is called Net Section Factor. One of the criteria used in determining the

capacity of a section in Axial Tension is fracture of the net section. The capacity is calculated as NSF X

Gross Area X Ultimate Tensile Strength of steel in tension

BEAM 1.0: This means the design or code checking of the member will be done by determining the

safety of the member at a total of 13 points along the length of the member. Those 13 points are the

start, the end, and 11 intermediate points along the length. If this parameter is not set, design will be

performed by checking the safety at only those locations governed by the SECTION command.

KY 1.2: The KY value is used to determine the KL/r for the Y axis -

Ky multipled by Ly divided by Ry.

RATIO 0.9: The code requires one to check the safety of a member by verifying several interaction

equations for compression, bending, tension, etc. The right hand side of these equations is usually 1.0.

The RATIO parameter allows one to set the right hand side to the value of the RATIO parameter, in this

case 0.9.

LY 18: The LY value is used in calculating the KL/r for the Y axis -

Ky multipled by Ly divided by Ry.

LZ 18: The LZ value is used in calculating the KL/r for the Z axis -

Kz multipled by Lz divided by Rz.

CHECK CODE ALL : For ALL members, the safety of the section is determined by evaluating the ratio of

applied loading to section capacity as per the code requirements.

 

When one does the AISC code check or member selection, what are the calculations the program is performing?

The checks done as per the AISC ASD 9th edition code are :

1. Slenderness - Checks for KL/r limits per Chapter B

2. Local Buckling per Chapter B

3. Axial Compression + Bending per Section H

4. Axial Tension + Bending per Section H

5. Shear per Section F

 

When I run code checking [as per BS5950] of the steel prismatic members which were defined in the User Provided Table, I get the following message in my output file:

CHECK CODE ALL

DESIGN NOT PERFORMED WITH PRISMATIC PROPERTIES

USER-TABLE MAY BE USED TO DESIGN PRISMATIC SECTIONS

The program is not designing the steel members defined as "Prismatic" in the UP Table, whereas

all other members defined otherwise as Tee, Channel etc are being designed. Also I couldn't

understand the meaning of the last line "User-Table may be used to design prismatic sections".

Since PRISMATIC sections by definition are those whose section shape is not one of the standard

shapes like a W, C, Angle, etc., there are no readily available rules in the code to follow. Due to this

reason, prismatic shapes are presently not designed per the BS code nor the ACI code.

You may get around this problem by defining the properties using the GENERAL section in a User

Provided Table. For a GENERAL section, STAAD provides the means for providing dimensions of the

components that are critical from the standpoint of computing allowable stresses, such as flange, web,

etc. The allowable stresses for a GENERAL section are computed using the rules of a wide flange

shape (I shape). As a result, the allowable stress value will be dependent on attributes such as

dimensions of the cross section, length of the member, etc.

I am using STAAD to perform steel design on a member per the AISC ASD code. I want the column to be designed based on an unbraced length of 20 ft. I have set the UNT and UNB values to 20 ft, but STAAD appears to consider only a 10 feet length in its KL/r calculations. How do I correct this problem?

The parameters UNT and UNB are for specifying the unsupported length of the compression flange for

the purpose of computing allowable stresses in bending compression.

If you want to specify the unbraced length for the purpose of computing allowable stresses in axial

compression, use the parameters LY and LZ. See Table 2.1 of the STAAD.Pro Technical Reference

Manual for details.

How do I get a design parameter, say the RATIO parameter, to be applied only to certain load cases?

You would need to use the "LOAD LIST" command. For example, if you only were interested in the 1st,

3rd and 5th load cases for the RATIO parameter you would need to write:

LOAD LIST 1 3 5

RATIO 0.5

In your input file.

I run the analysis of a 3-D bridge truss model and requested a CODE CHECK of the members. The results

of this code check do not correspond to my hand calculation results.

The results of this code check show some very strange numbers in as far as code ratio using AISC- H1-

1 formulation is concerned. Reference result output for members number 62 to 74 for example. Other

ratios do not seem right either.

If you look at the AISC equation H1-1, you will find that there are 2 terms in the denominator, called

(1-fa/Fey)

and

(1-fa/Fez)

If the value of fa equals or exceeds Fey or Fez (Euler stresses), the respective terms become zero or

negative, which is not a desirable event. In such a situation, STAAD replaces that negative number with

the value 0.0001. The consequence of this is that, that part of the interaction equation becomes

magnified by 10000, which will cause the overall value of the left hand side of equation H1-1 to increase

significantly.

The above scenario is what occurs in the case of several of the members in the list 62 to 74. If you want

to obtain proof of this, you may do the following. Change the value of the TRACK parameter from 1 to 2,

and you will get a more detailed design output. That output will include the values of fa, Fey, Fez, etc.

To remedy the problem, you need to use a larger cross section so that "fa" becomes smaller, or use one

with a smaller KL/r value so that Fey and/or Fez become larger.

What is the LX parameter used for?

The LX is the parameter used in calculating the axial compression capacity for flexural torsional

buckling 

The KL/r value that STAAD reports for a single angle member does not match my hand calculation. Design is per the AISC ASD 9th edition code.

A single angle is subjected to 2 buckling modes :

1. Column buckling. This is determined using the simple expressions (Ky.Ly/ry) and (Kz.Lz/rz),

where ry and rz are the radii of gyration about the principal axes.

2. Flexural torsional buckling : This mode of buckling uses an equivalent KL/r, which is computed

on the basis of equation (4-4) on page 5-311 of the AISC ASD 9th edition code. Generally, this

mode of failure produces a higher KL/r than the ones from the column buckling mode.

You should check whether the flexural torsional buckling mode governs in your case. The KL/r

calculated for the flexural torsional mode, if it happens to the largest of the 3 values, is reported only

with a TRACK 1.0 detail of output. It does not get reported for TRACK 0 or TRACK 2 level of detail of

output. In other words, if you want to see the KL/r in the flexural torsional buckling mode, use the

parameter TRACK 1.0.

What are the SSY and SSZ parameters for AISC ASD based steel design?

SSY and SSZ are terms which dictate how sidesway criteria should be used in computing the Cm

coefficients. For both of them, a value of 0.0 means sidesway is present for the corresponding axis, and,

a value of 1.0 means sidesway is not present for the corresponding axis.

When SSY is set to 0.0, Cmy is set to 0.85 as per page 5-55 of AISC ASD.

When SSZ is set to 0.0, Cmz is set to 0.85 as per page 5-55 of AISC ASD.

When SSY is set to 1.0, Cmy is calculated as per the equations on page 5-55 of AISC ASD.

When SSZ is set to 1.0, Cmz is calculated as per the equations on page 5-55 of AISC ASD.

If the CMY parameter is specified (and the value is a valid one), that value is used, regardless of what

the value of SSY is.

If the CMZ parameter specified (and the value is a valid one), that value is used, regardless of what the

value of SSZ is.

The KL/ry reported for a T shape does not match my hand calculations. I am designing the section per the AISC ASD 9th edition code. 

For singly symmetric shapes such as Tees and Double Angles, the KL/r value for the Y axis is

calculated by STAAD using the rules for flexural torsional buckling as explained in page 3-53 of the

AISC ASD manual. It is not calculated as Ky multiplied by Ly divided by ry.

Can you provide me with some help on how I can include deflection check as one of the criteria in steel design?

Deflection of a beam or a column can be included as one of the criteria during code checking or

member selection with most steel design codes in

STAAD. The ratio of length to maximum deflection of a beam (L/d ratio) will be calculated by STAAD.

STAAD will then check that quantity against the allowable limit which the user specifies under the

PARAMETERS option.

What are the design parameters which control deflection check ?

1. DFF : This is the value which indicates the allowable limit for L/d ratio. For example, if a user wishes

to instruct the program that L/d

cannot be smaller than 900, the DFF value should be specified as 900. The default value for DFF is 0.

In other words, if this parameter is not

specified as an input, a deflection check will not be performed.

2. DJ1 and DJ2 : These 2 quantities affect the "L" as well as the "d" in the calculated L/d ratio. They

represent node numbers that form the basis for determining L and d.

By default, DJ1 and DJ2 are the start and end nodes of the member for which the design is being

performed, and "L" is the length of the member, namely, the distance between DJ1 and DJ2. However, if

that member is a component segment of a larger beam, and the user wishes to instruct STAAD that the

end nodes of the larger beam are to be used in the evaluation of L/d, then

he/she may input DJ1 and DJ2 as the end nodes of the larger beam. Also, the "d" in L/d is calculated as

the maximum local displacement of the member between the points DJ1 and DJ2. The definition of local

displacement is available in Section 5.42 of the STAADPro Technical Reference Manual, as well as in

Example problem # 13 in the STAADPro Examples Manual.

A pictorial representation of DJ1 and DJ2, as well additional information on these topics is available

under the "Notes" section following Table 2.1 in Section 2.8 of the STAADPro Technical Reference

Manual.

What are the results one gets from STAAD for the deflection check?

If the steel design parameter called TRACK is set to 2.0, the L/d ratio calculated for the member can be

obtained in the STAAD output file. The value is reported against the term "dff". Notice that the

expression is in lower-case letters as opposed to the upper-case "DFF" which stands for the allowable

L/d.

If "dff" is smaller than "DFF", that means that the displacements exceeds the allowable limit, and that

leads to the unity check exceeding 1.0. This is usually a cause for failure, unless the RATIO parameter

is set to a value higher than 1.0. If "DFF" divided by "dff" exceeds the value of the parameter RATIO, the

member is assumed to have failed the deflection check.

What are the limitations of this check?

Since the "d" in L/d is the local deflection, this approach is not applicable in the case of a member which

deflects like a cantilever beam.

That is because, the maximum deflection in a cantilever beam is the absolute quantity at the free end,

rather than the local deflection. Check whether STAAD offers a parameter called CAN for the code that

you are designing to. If it is available, set CAN to 1 for a cantilever style deflection check.

Since the deflection which is checked is a span deflection and not a node displacement, the check is

also not useful if the user wishes to limit story drift on a structure.

In the output for steel design, what does the term "dff" represent?

"dff" is the value of actual length divided by local deflection. The actual length value is the distance

between the nodes DJ1 and DJ2 which default to the actual end nodes of the member. The deflection

used is the maximum local deflection between the points DJ1 and DJ2. You can get the Max. Local

Displacement value by looking at the output of the PRINT SECTION DISPLACEMENT command. The

definition of DFF, DJ1 and DJ2 may be found in Table 2.1 of the Technical Reference Manual for

STAAD/Pro.The word PRISMATIC is meant to indicate a section of any arbitrary shape. But the AISC

code does not provide guidelines for design of arbitrary shapes.

Section capacities are dependent upon aspects such as the width to thickness ratio of flanges and

webs, lateral torsional buckling etc. From that standpoint, using an allowable stress of 0.6Fy for

PRISMATIC sections was not always conservative. 

In STAAD-III, I was able to get a steel design for members defined using the PRISMATIC property attribute per the AISC ASD code. I cannot do this in STAAD/Pro. Why?

In the earlier versions of STAAD, the code check for prismatic sections was done using allowable

stresses which are arbitrarily chosen as 0.6 Fy. However, this assumption of 0.6Fy was not based on

any code specific requirements.

The word PRISMATIC is meant to indicate a section of any arbitrary shape. But the AISC code does not

provide guidelines for design of arbitrary shapes. Section capacities are dependent upon aspects such

as the width to thickness ratio of flanges and webs, lateral torsional buckling etc. From that standpoint,

using an allowable stress of 0.6Fy for PRISMATIC sections was not always conservative.

A way around this limitation (lack of specific guidelines) would have been to use the rules of a known

shape, such as a Wide Flange, for designing prismatic shapes. That would require knowledge of

equivalent flange and web dimensions. When the properties are defined using the PRISMATIC option,

there is no means to convey information such as dimensions of flanges or webs to the STAAD design

facility. Hence, the design of PRISMATIC shapes is not supported in STAAD/Pro.

You may get around this problem by defining the properties using the GENERAL section in a User

Provided Table. For a GENERAL section, STAAD provides the means for providing dimensions of the

components that are critical from the standpoint of computing allowable stresses. The allowable

stresses for a GENERAL section are computed using the rules of a wide flange shape (I shape). As a

result, the allowable stress value will be dependent on attributes such as dimensions of the cross

section, length of the member, etc.

In the context of design, what is meant by the term Ratio?

In steel design, the Pass/Fail status of a member is determined according to various conditions.

According to most design codes, the member has to be checked for failure against axial compression

and axial tension, slenderness, compressive & tensile stresses caused by axial compressive force +

bending moments, failure caused by shear stresses, etc. For each of these conditions, determination of

whether the member is safe or unsafe is done by checking whether the actual values due to the loading

exceed or are less than the allowable values. The amount by which the member is stressed for each of

these conditions is quantified in the form of the Ratio. For example, take the case of equation H1-1 of

Section H of the AISC-89 specifications. The number obtained by computing the left hand side of that

equation is the Ratio corresponding to that equation.

I have multiple sets of design in the same STAAD file and I am only able to see the results for the final set in the Postprocessing mode (GUI). How can I view the results for all design sets in the GUI ?

The postprocessing Beam >Unity Check page can report the design results only for the final set of

design. This is a limitation in STAAD.Pro as the program architecture does not allow that results of

multiple design sets to be made available at the same time graphically. The analysis output file is the

only place where you can view results for all design sets. The only way to view the results of a previous

design cycle graphically is 

1. to go to the editor and comment out the subsequent design sets and rerun the analysis

2. reverse the order for the design data blocks so that the set, for which the GUI data is needed,

becomes the last set.     

 

How is the shear stress calculated in STAAD.Pro for AISC design code ? 

The shear stress calculated by STAAD is the maximum shear stress by default which is based on the

standard formula VQ/Ib, where

V = Shear force

Q = Moment of area of the part of the cross section that

is above ( or below ) the plane where shear stress is being calculated, about

the neutral axis

I = Moment of Inertia

b= Width of the section at the plane where the stress is

being calculated

So the term Ib/Q is reported as the shear area that corresponds to this shear stress calculation.

If required one can get STAAD to calculate the average shear stress instead of the maximum. There is

a SHE design parameter that can be used to influence how STAAD calculates the shear stress. When

the parameter is set to 0 ( default ), stress is calculated as mentioned above. However when this

parameter is set to 1, average shear stress will be calculated based on the formula V/Ay (or Az ) where

Ay or Az are the shear area for the cross section.  

 

You got Cf (force co-efficient) from charts / formulae which is dependent on Solidity ratio, number of parellal frames, frame spacing & exposed width of frames. You should divide this Cf by number of parellal frames to arrive at wind load per frame if you plan to apply wind loads to each frame. 

However, it would still be OK to apply full wind load (calculated Cf) on windward frame & estimate effect of this total wind load on brace system / foundations. 

Summarizing, Cf you get from "ASCE Guidelines for wind load on petrochemical facilities & anchor bolt design" formulae is total Cf considering all frames. For design, you should distribute this Cf to all parellal frames and normal practice is to distribute it equally. 

Regards, Rushikesh