guide for using risa3d to calc freq and mode shapes

8/19/2019 Guide for Using RISA3D to Calc Freq and Mode Shapes

http://slidepdf.com/reader/full/guide-for-using-risa3d-to-calc-freq-and-mode-shapes 1/9

CE 533, Fall 2014 Guide for Using RISA3D 1 / 9

to Calculate Natural Frequencies and Mode Shapes

Example Structure. The procedure for calculating frequencies and modes shapes of a multi‐

degree of freedom (MDOF) system will be demonstrated using the following example.

3‐Story, 1‐bay x 1‐bay structure.

Plan dimensions are 30’ x 30’.

All floors

have

a 7"

‐thick

reinforced

concrete slab (f’c = 4 ksi, unit

weight = 150 pcf).

All four columns are W14x30 steel

columns, Izz = 291 in4

1. Define the material properties.

Activate the “Data Entry” menu if it’s not already visible, and click on “Materials”.

1.1. Select the “Hot Rolled” tab and “A992”. Make sure that the modulus is set to 29,000

ksi.

1.2. Select the “Concrete” tab and “Conc4000NW” (for 4000 psi normal weight concrete)

and make

sure

“Density”

= 0.150

kcf.

10'

10'

10'

v1

v2

v3

30'





2. Define the Sections.

Select “Section Sets” from the “Data Entry” menu.

2.1. Select the “Hot Rolled” tab, type in a label (e.g. “Columns”), select the W14x30 shape,

select the A992 Material, and check that the moment of inertia about the strong axis

(Izz) = 291 in4.

2.2. Select the “Concrete” tab, type in a label (e.g. “Slab”), specify a rectangular element 7

inches deep by 180” wide (half the building width).

3. Set up your drawing grid

3.1. If the Graphic Editing Toolbar is not visible, right‐click anywhere in the white part of

the screen and select it.





3.2. Select the “Drawing Grid” icon, type in “1@30” under “X Axis” and “3@10” under “Y

Axis”.

4. Layout the structure. Select “Draw Members”, and

4.1.

Draw the

columns

after

selecting:

“Hot

Rolled”,

“Assign

a Section

Set”,

“Columns”

(make sure to select the nodes at each level to provide an intersection for the slab

floors).





4.2. Draw the slab floors after selecting: “Concrete”, “Assign a Section Set”, “Slabs”.

4.3. Check that your model is input correctly by selecting “Plot Options”, “Members”,

“Wireframe”, and “Shape”.

5. Specify the Boundary Conditions. Since we are building a 2‐dimensional model and RISA is

3D program, the first task is to constrain the model to a single plane (the X‐Y plane). Then

we specify the boundary conditions at the support (assume fixed‐base).

5.1. To constrain the model to the X‐Y plane, select the “Modify Boundary Conditions” icon,

then select

“Fixed”

and

check

the

“Use?”

box

for

“Z

Translation”,

“X

Rotation”,

and

“Y

Rotation”. Then select the “Apply Entries to All Selected Joints”, and select “Apply”.





5.2. To specify the support conditions, select “Reaction” and check the “Use?” box for “X

Translation”, “Y Translation”, and “Z Rotation”. The only difference between “Fixed”

and “Reaction” boundary conditions is reactions are not calculated for “Fixed”.

5.3. Check the boundary conditions by selecting ”Boundary Conditions” from the “Data

Entry” menu.

Your

boundary

conditions

should

appear

as

below.

6.

Specify

the

Loads.

RISA

is

set

up

for

structural

design

in

which

the

engineer

checks

the

structural response to multiple combinations of loads. For this example, since we are only

calculating the natural frequencies and mode shapes, we will only have one basic load case





(self weight, others could be super‐imposed dead load, live load, seismic loads, . . .) and one

load combination (self weight times a factor of 1.0, others could be 1.2 D + 1.6 L, . . .).

6.1. Select the “Basic Load Cases” icon and the “Load Combinations” icon and arrange the

spreadsheets as shown below.

On the “Basic Load Case” spreadsheet, type a label under “BLC Description” (e.g. “self

weight”) and

type

a “‐1”

under

“Y

Gravity”

to

specify

that

the

member

self

weights

will

be applied in the negative Y direction (downward).

On the “Load Combinations” spreadsheet, type another label under “Description” (e.g.

“self weight only”) and type a “1” under “BLC” (refers to Line 1 of the BLC spreadsheet)

and type a “1” under “Factor”

6.2. Check your model by calculating the structural response to self weight only. With

Load Combination 1 selected, select the “Solve Current” icon.

Display the

deflected

shape

of

the

structure

by

selecting

the

“Plot

Options”

icon,

the

“Deflection Diagrams” tab, “Load Combination”, “Include Undeflected Shadow” and





“Apply”. Check the indicated deformations for reasonableness (e.g. no rotation at

fixed‐base supports, small rotations at column‐to‐floor connection, etc.).

7. Calculate the Natural Frequencies and Mode Shapes.

7.1. To calculate the natural frequencies, select the “Solution” icon, “Dynamics”, and

“Solve”.

Select

“Start

Solution”

on

the

“Dynamics”

spreadsheet

that

pops

up.

The resulting frequencies are displayed to the right:





7.2. To display the mode shape for Mode 1, select “Plot Options”, “Deflection Diagrams”,

“Mode Shape”, “Mode 1 Period .9371 Sec”, “Include Undeflected Shadow”, and

“Apply”. Display the mode shapes for the other modes using a similar procedure.

Mode 2 Mode 3

8. Adjust the RISA model to match the hand‐calculated (spreadsheet) frequencies and mode

shapes.

We made several simplifying assumptions when calculating the frequencies and mode

shapes by hand.

8.1 Make the following changes to your RISA model so that it’s frequencies match the

hand‐calculated frequencies.

On the “Boundary Conditions” menu, “Fix” the rotation about the Z‐axis at all

column‐slab joints.





On the “Global Parameters” menu, “Solution” tab: uncheck “Shear Deformation”

On the “Materials” menu, specify a density of “0” for A992 steel.

8.2 Now compare the RISA vs. hand‐calculated frequencies and mode shapes. You can

copy the RISA mode shapes into your Excel sheet and normalize them so that the maximum

modal displacement

equals

1.0

for

each

mode.

guide for using risa3d to calc freq and mode shapes

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