chapter 3: designing steel bridges using dcalc · designing steel bridges using dcalc p.3-1 ... a...

Designing Steel Bridges Using DCALC

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Chapter 3: Designing Steel Bridges Using DCALC By Karl Hanson, S.E., P.E.

July 2006

3.1 Introduction:

This chapter is an overview showing how to a design typical highway bridges using

DCALC. We will look at the overall process, and direct the reader to the various

programs.

DCALC is installed with several examples that are located in the “C:\DCALC\DEMO”

directory. Two bridge design examples are included: a steel wide flange bridge and a

precast prestressed concrete beam bridge. There is essentially little different in the

approaches used in the design of both types of bridges, except for the design of the

beams.

3.2 Steel Beam Bridge Design Example:

We can explain the process of using DCALC by walking through an example showing

how to design a steel beam bridge. This example will be taken from an actual bridge that

was constructed in Chicago. There are ramps on each side of the main bridge, which we

will ignore for the purposes of this exercise. The main bridge consists of ten straight wide

flange W36 beams having steel with a yield stress of 50 ksi.

Actual structure consists of two ramps and “Unit 1” of a viaduct


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3.2 Steel Beam Bridge Design Example (Cont’d):

Profile Grade

Cross-Section Thru Superstructure

Deck Plan


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3.3. Starting The Design

As general housekeeping practice, with any bridge design the first thing that you will

need to do is collect geometrical information. This following information is required to

run DCALC, after which the design will develop quickly. You should prepare sketches

showing:

• Bridge cross-section

• Deck plan

• Vertical Curve

You will also need to collect other important to be included with your calculations:

• Geotechnical Borings and Recommendations

• Transmittals

• Survey Data

• Design References

Typically, engineers draw hand sketches showing this information in their calculations.

However, if CAD drawings are available, why not go directly to CAD? This decision will

depend on the individual. We each approach things in our own way, and some may argue

that jumping directly into CAD is juggling too many tasks at once. DCALC has a CAD

interface, via DesignCAD. If you do not have a copy of DesignCAD, DCALC allows you

to skip method. As the saying goes, “There are many ways to skin a cat”.

So, here is a way to start: Draw a preliminary framing plan in CAD or a pencil sketch,

such as the one shown below.

Preliminary framing plan

Typically, the horizontal alignment is developed by roadway engineers and is provided in

AutoCAD or MicroStation format, and the structural engineer develops a framing plan,

“X-Referencing” the roadway alignment files. AutoCAD and MicroStation drawings can

easily be converted to DesignCAD, to be used by DCALC. However, it is not absolutely

necessary to use DesignCAD. We will explain how to use DCALC with DesignCAD and

also how to use it without DesignCAD.


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3.4. The Bridge Design FlowChart:

After you start DCALC, you will see the main menu selection:

Select “Make a Calculation”. Then select “View Bridge Design Flowchart”. You will

then see the following screen:

This flowchart shows the connectivity of DCALC’s calculations used for designing

bridges. The best way to design a bridge is to use a “top down” approach. (The same can


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be said for designing a building, in which case you begin by designing the roof, then

work your way down to the foundation). You start with the deck, then the beams, then the

bearings, then, finally the substructure. DCALC helps you follow this logic by having a

bridge design flowchart, showing the sequence of the programs to be used.

3.5. Collecting Bridge Geometry Using DCBRIDGE:

There is an example calculation located in “C:\DCALC\DEMO”, “Example DCBRIDGE

calculation – Steel Bridge”. The user should run this file to become familiar with the

operations. DCBRIDGE is a little bit more involved than other DCALC calculations, and

a step-by-step tutorial is included in the DesignCalcs website.

There are two methods that can be used for entering geometry:

Method 1 – Manual Entry:

If you do not have DesignCAD, you will be able to use DCBRIDGE. In fact, for simpler

bridges, there is an “alternative entry” method that is about as fast as than using

DesignCAD for entering points.

“Altenative entry” involves the following steps:

• Defining the geometry of the station line

• Defining the parallel offset of each beam from the station line

• Defining the location of each pier/bearing using the station and angle with respect

to the station line.

Method 2 – Using DesignCAD:

If you have DesignCAD, you can use it to collect bridge geometrical information. There

is elegance to this approach, since it avoids a duplication of effort. Essentially the idea is,

if you’ve already drawn the beam and lines accurately in CAD, why not use that

information for design purposes?

Interestingly, you can design an entire bridge from a CAD drawing, without computing

any geometry (or loads)! This is a distinctively different approach than how engineers

have worked in the past, when engineers did very difficult geometrical calculations.

There can be an increase in efficiency and accuracy using the newer approach utilizing

CAD – in the right hands. It is important that engineers be able to check geometrical data

independent of CAD. As we progress into the future, we need to recognize how previous

engineers worked and what was good about the way they worked, and what was tedious

and can be improved upon.

3.6 Defining Vertical Geometry Using ELEVS:

Roadways and bridges alike have vertical geometry defined by a “profile grade line”,

which is related to the horizontal geometry which is defined by the “station line”. We’ve

already defined the bridge horizontal geometry using DCBRIDGE, so now we need to

define the bridge vertical geometry. Program “ELEVS” is used for this purpose.


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There is an example calculation in the “C:\DCALC\DEMO” directory, “Example ELEVS

calculation” that was developed for this example. The reader should also run this

calculation to try out the features.

3.7 Defining Bridge Loads Using LOADER:

After you’ve defined horizontal and vertical geometry, next is the process is the

computation of loads. Program “LOADER” computes the beam loads that will be used by

the beam design programs, for both steel and precast prestressed concrete beams.

LOADER computes the “dead load per unit length” and the “live load wheel distribution

factor” for each beam – the traditional approach used by engineers to design bridge

beams. It is an observation that this approach is in itself an approximation, since the true

loads experienced by beams is dependent on the three dimensional characteristics of a

bridge system, rather than line elements that we assume. LOADER computes live load

distribution factors for both the AASHTO Standard Specifications and the LRFD

Specifications.

There is an example calculation in the “C:\DCALC\DEMO” directory, “Example

LOADER calculation – Steel Bridge” developed for this example. The reader should run

this calculation to become familiar with the operation of this program.

3.8 Preliminary Sizing of Beams Using BRIJBEAM:

Program BRIJBEAM is used design typical straight wide flange beams or plate girders.

This is an iterative process:

• After defining horizontal geometry (DCBRIDGE) and vertical geometry

(ELEVS), and loads (LOADER), define beam sizes using BRIJBEAM. You will

need to make some assumptions here, so this is a guessing process.

o Guess at a required wide flange size or plate girder size

o Guess at splice locations

• After defining preliminary sizes, you will need to save this information in a

“beam file”.

• Prepare input for the next program to be used, “CBRIDGE”. From the main

menu, select “Save CBRIDGE input”.

• Exit BRIJBEAM.

There is a example calculation located in the “C:\DCALC\DEMO” directory, “Example

BRIJBEAM calculation”. The user should also run this calculation to become familiar

with the operation of this program.


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3.9 Computing Beam Moments and Shears Using CBRIDGE:

Program CBRIDGE is used to compute moments and shears for typical straight beam

type bridges.

• After starting CBRIDGE, when asked “Is this a new or existing file”, answer

“existing”.

• Retrieve the CBRIDGE input file that you created using BRIJBEAM.

• Run it


CBRIDGE calculation – Steel Bridge”. The user should also run this calculation to

become familiar with the operation of this program.

3.10 Returning to BRIJBEAM to Design The Beams:

Now you’ve computed moments and shears using CBRIDGE, we can check if the beams

that you sized are adequate.

• Start BRIJBEAM

• “Retrieve Beam file” (the beam data that you had previously guessed at)

• “Retrieve CBRIDGE output” (the moments and shears)

• Select “Design A Span”.

• For each span, you will see in a table the capacity of the member, and whether it

is adequate (“OK”) or not (“NG”).

• Change the size of the beam as required to make the capacity adequate.

• After changing sizes as required, once again “Save Beam File”.

• Again, “Save CBRIDGE input”.

• Exit BRIJBEAM

• Go back to CBRIDGE and recomputed moments

After one or two iterations, you will get a feel for the beam sizes that are required.

3.11 Design Splices using SPLICE:

Continuous steel beam bridge have splices, usually located close to the points of dead

load contraflexure. The usual practice is to locate the splices about 5 feet away from the

dead load contraflexure point, to avoid placing the studs on the splice plates.

SPLICE is used to design the splices. This program will use the output file from

BRIJBEAM. Alternatively, the user can manually input design moments and shears.

Again, this is an interative design process, where the engineer tries various bolts patterns

and plate dimensions until a satisfactory splice capacity results.


SPLICE calculation”. The user should also run this calculation to become familiar with

the operation of this program.


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3.12 Design Bearings using BEARINGS:

DCALC has an automated bearing design program, based on the methods and practices

used by the Illinois Department of Transportation. IDOT has standardized many of its

bearing designs, based on typical bridge loadings and expansion lengths. For locations

with high seismic forces, additional requirements may be required, beyond the

capabilities of this program.

The operation of this program is easy and self-explanatory.

3.13 Compute Deck Elevations using DECKELEV:

The final shape of a bridge deck must fit the vertical profile specified for the roadway.

Since all bridges deflect under the weight of the wet concrete, the concrete forms must be

adjusted to compensate for the calculated deflection. For this purpose, bridge plans

typically include “deck elevation tables” that show “top of deck elevations adjusted for

dead load deflection”.

Knowing the vertical profile and the beam deflections due to the weight of the wet

concrete, the bridge engineer computes deck elevations using DECKELEV. This program

automatically reads the vertical profile created by the ELEVS program and deflections

computed from either BRIJBEAM (steel bridges) or PCBRIDGE (concrete bridges).

Alternatively, the user can enter deflections computed by other methods.


DECKELEV calculation”. Also, there is a further write-up on deck elevations included

in the Tutorials on the DesignCalcs website.

3.14 Compute Bearing Seat Elevations using BEAMELEV:

The elevation of each bearing must be indicated in bridge plans. In order to calculate

bearing seat elevations, the bridge engineer requires the following information:

• Vertical profile

• Fillet height (distance above beam/girder to bottom of deck)

• Beam depth

• Is the beam straight or cambered?

• Beam deflections

• Bearing heights

This calculation can be especially complicated by hand, even using a spreadsheet. The

BEAMELEV program computes bearing seat elevations, top of beam elevations and

camber diagrams. If you have followed the flowchart, BEAMELEV will look for data

previously computed by the other programs required for this calculation.


BEAMELEV calculation – Steel Bridge”.

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