Computer-Aided Design and Detailing of Short Span Steel Bridges

Computer-Aided Design and Detailing of Short Span Steel Bridges

John C. Huang, Ph.D., PE
Principal
CHC Engineering, LLC
Herndon, VA

Introductions

steel beams The Federal Highway Administration estimates nearly one-third of America's 578,000 bridges need to be repaired or replaced. The majority of these bridges are short-span, linking vitally important state, county and municipal roads. The structural advantages of using steel include its lighter weight, which means lighter foundations and lower erection costs. Steel is ideal for short-span bridges because of its durability, ease of maintenance and ease of construction.

Today's bridge engineers face unlimited demands on limited resources. To address the needs of bridge owners and designers, the American Iron and Steel Institute (AISI) has developed design aids for cost-effective solutions for short span steel bridges. The design aids include:

Plans - A comprehensive set of pre-designed bridge superstructures covering a wide variety of roadway widths, beam/girder (stringer) types, and spacings.
Software - A computer program (AISIBEAM Version 3.0) to generate customized designs and appropriate supporting calculations.

The Plans and Software for Short Span Steel Bridges were developed using AASHTO's Load Factor Design. For a specific project, this package can be used for generating both the preliminary design alternatives and the subsequently chosen final design. The design aids were developed with input from fabricators and designers to reduce project cost and minimize design time. All final designs should be reviewed by a registered Professional Engineer in accordance with local and federal laws.

The Plans

The Plans, which are available on CD-ROM and can be purchased in a package of Plans & Software for Short Plan Steel Bridge Design by AISI, include a comprehensive set of over 1100 pre-designed superstructures, including quantities and design and details of concrete decks, cross bracings, elastomeric bearings, and integral abutments for non-skewed, single or multiple simple span bridges with individual spans ranging from 20 to 120 feet. Designs are tabulated in five foot increments. The available design options include:

1. Seven superstructure options with widths of 24, 28, 34, 40, 44 ft. The 28 ft. and 40 ft. widths have two optional stringer spacings.

2. Designs based on AASHTO M270 Grade 50 or Grade 50W steel and an HS25 live load. Each superstructure has been designed for normal weight and lightweight concrete components and for the following alternatives:

Composite plate girders without intermediate web stiffeners.
Composite plate girders with partially stiffened webs.
Non-composite or composite rolled beam.
Composite rolled beams with welded or end-bolted cover plates.

The plans also include the following design information and details:

Design tables of reactions, moments and deflections.

Details for cross frames, diaphragms and connections.

Schematic details for conventional and integral abutments, and jointless decks.

Deck slab reinforcing details.

Details of crash tested concrete parapet and steel bridge rail.

Elastomeric bearing details and design.

Quantities of concrete, reinforcing steel and structural steel for the superstructure.

The Software

The Software (AISIBEAM Computer Program), which is available for free 30-day trial and can be purchased in a package of Plans & Software for Short Plan Steel Bridge Design by AISI, can be used to design or rate single and multi-simple span bridges with a maximum skew of 20 degrees. It provides the least weight solution based on the data entered by the user. The software uses group I LFD load combinations and performs a line-girder analysis for simple-span girder and rolled beam bridges. The following items can be customized by the user:

Roadway classification and roadway width
Girder spacing
Welded plate girders with unstiffened webs
Rolled beams with optional welded cover plates
Composite and non-composite stringers
Normal weight and lightweight concrete deck material
Steel grade and concrete strength
AASHTO strength, deflection, fatigue and constructibility criteria
HS (MS) type truck load or special loading with up to 15 axle loads
Up to 25 alternate solutions for each design
Supporting design calculations
Customary U.S. units or SI (metric) units
Alternate fatigue load of HS (MS) configuration
Operating and Inventory rating
Optional automatic calculation of:
- Distribution factors for moment, shear, and deflection
- Effective width
- Impact factor

bridge splice

In the design mode, the software will iterate between a range of user-specified minimum and maximum cross section dimensions to find a minimum weight solution. In the rating mode, the user can input the exact cross section properties and the Software will then solve for both an Inventory and Operating Rating.

The Software allows the user to view and print the output file, the current design options and the design results. It also allows the user to view the moment, shear, deflection diagrams and moment, shear, deflection influence lines for the design.

Suggestions for Maximum Economy

Several steps may be taken to reduce the cost of bridges as listed below. The user is encouraged to solicit input from steel bridge fabricators to improve the cost-effectiveness of the design and details.

The Software rates and designs welded plate girders with unstiffened webs only. Unstiffened webs are generally more economical for short span bridges. The user is advised that the Software may not provide an economical design for simple spans exceeding about 120 ft (37m).
Develop designs and make preliminary cost estimates for as many alternate designs as practical and enlist the aid of fabricators and contractors who may be potential bidders on the project. Include optional details for items such as cross frame, integral/jointless abutments, and deck forms. Based on the preliminary estimates, consider including several of the least-cost alternatives in the final bid documents. Preferences for section type and details vary among fabricators and erectors depending on the equipment available, shipping considerations and experience.
It is recommended that strong consideration be given to the use of uncoated weathering steel where appropriate. The application should be in accordance with the FHWA Technical Advisory T 5140.22: Uncoated Weathering Steel in Structures. The use of uncoated weathering steel typically provides initial cost savings of 10% or more, and life-cycle cost savings of at least 30% over the life of the structure (Ref. 8). Where the bridge site conditions are questionable, contact AISI for additional help in evaluating the suitability of weathering steel before specifying a paint system (Ref. 3).
Give strong consideration to the design of integral abutments. Use continuous decks over piers of multiple-span simple-span bridges. These types of bridges eliminate the costs and certain problems associated with deck joints. Jointless bridges will also reduce significantly maintenance costs by keeping water and road salts off the girders and substructure. For more information on these types of bridges, refer to the Highway Structures Design Handbook Chapter: Integral Abutments for Steel Bridges (Ref. 5). For jointless bridges, use elastomeric bearings as shown in the Plans and refer to Steel Bridge Bearing Selection and Design Guide (Ref. 6).
Consider alternate systems for diaphragms or cross frames, including channels and bent plates for shallow sections, and cross frames consisting of angles for deeper sections. Bent plates provide adequate stiffness and may be more economical to fabricate.
When planning a bridge design, consider wider stringer spacings. Wider spacings will generate a more cost-effective design. Cost savings result from the reduced number of main elements to be fabricated, shipped and erected, and from the reduced number of secondary elements such as shear connectors, cross frames, stiffeners and bearings.
For plate girders, designs may be generated that have butt-welded flange splices to minimize material requirements. However, welded flange splices are costly. Allow the fabricator the option to continue the heavier plate to eliminate the splice. The cost saved by eliminating the splice may more than offset the cost of the heavier plate. Use a constant width flange to reduce fabrication cost.
The selection of the proper fatigue case roadway classification is an important consideration and good judgment should be employed to select the most reasonable and appropriate roadway category for a given situation. A Case I fatigue roadway classification should not be automatically applied to lower-volume roadways merely to obtain a conservative design.
Consider the use of high performance steel (HPS) (Ref. 11) to minimize the depth of the superstructure, to eliminate shoulder piers, and to minimize the cost of the bridge approach work.

The PDHonline Course

In this lesson, you need to

Study AISIBEAM User's Manual (V3.0), which provides detailed screen-by-screen instructions on how to design and rate a short span steel bridge using AISIBEAM software.

Download AISIBEAM (V3.0) (30 day free trial version) and get familiar with the software.

Use the software to solve the following three steel beam problems before taking the quiz. All three problems have the same general bridge layout, design criteria and material information except for design/rating mode, beam types/sizes, and vehicle information. Only input data in green are different from one problem to another. Therefore, the second and third input data files can be generated by modifying the first completed input data file.

Problem No. 1 - AISIBEAM Input Data for a Rolled Beam Design:

INPUTS AND OUTPUTS UNITS: U.S. CUSTOMARY UNITS
EFFECTIVE WIDTH IS CALCULATED BY: THE PROGRAM
DISTANCE BETWEEN CROSS BRACINGS ARE CALCULATED BY: THE PROGRAM
VALUE OF MODULAR RATIO (N) IS CALCULATED BY: THE PROGRAM
DISTRIBUTION FACTORS ARE CALCULATED BY: THE PROGRAM
IMPACT FACTOR IS CALCULATED BY: THE PROGRAM
NUMBER OF BEAMS: 4
CLEAR ROADWAY WIDTH (FT): 34
OVERHANG LENGTH (FT): 3.083333
BEAM SPACING (FT): 10
NUMBER OF TRAFFIC LANES: 2
CALCULATIONS ARE FOR AN: INTERIOR BEAM
SPAN LENGTH (FT): 80
NUMBER OF SEGMENTS IN SPAN: 10
DEAD LOAD (EXCLUDING SELF WEIGHT) PER GIRDER (K/FT) 1.285
SUPERIMPOSED DEAD LOAD (K/FT): 0.416
FATIGUE VEHICLE: HS - 20
LIVE LOAD VEHICLE : HS - 25
TYPE OF STEEL: M 270 GRADE 50W
BRIDGE TYPE: COMPOSITE
GIRDER TYPE: ROLLED BEAM
AASHTO DEFLECTION CRITERIA: IS CHECKED
ROAD CASE: CASE II
CALCULATION TYPE: DESIGN
AASHTO CONSTRUCTIBILITY CRITERIA: IS CHECKED
CONCRETE STRENGTH (KSI): 4
HAUNCH THICKNESS (IN.): 2
SLAB THICKNESS (IN.): 8.5
SHEAR STUDS NUMBER PER ROW: 3
SHEAR STUD DIAMETER (IN.): 0.875
UNIT WEIGHT OF CONCRETE (PCF): 150
MAXIMUM ASSUMED BEAM DEPTH (IN.): 42
MINIMUM ASSUMED BEAM DEPTH (IN.): 33
ROLLED BEAM: WITHOUT COVER PLATE

Problem No. 2 - AISIBEAM Input Data for a Plate Girder Design:

INPUTS AND OUTPUTS UNITS: US CUSTOMARY UNITS
EFFECTIVE WIDTH IS CALCULATED BY: THE PROGRAM
DISTANCE BETWEEN CROSS BRACINGS ARE CALCULATED BY: THE PROGRAM
VALUE OF MODULAR RATIO (N) IS CALCULATED BY: THE PROGRAM
DISTRIBUTION FACTORS ARE CALCULATED BY: THE PROGRAM
IMPACT FACTOR IS CALCULATED BY: THE PROGRAM
NUMBER OF BEAMS: 4
CLEAR ROADWAY WIDTH (FT): 34
OVERHANG LENGTH (FT): 3.083333
BEAM SPACING (FT): 10
NUMBER OF TRAFFIC LANES: 2
CALCULATIONS ARE FOR AN: INTERIOR BEAM
SPAN LENGTH (FT): 80
NUMBER OF SEGMENTS IN SPAN: 10
DEAD LOAD (EXCLUDING SELF WEIGHT) PER GIRDER (K/FT) 1.285
SUPERIMPOSED DEAD LOAD (K/FT): 0.416
FATIGUE VEHICLE: HS - 20
LIVE LOAD VEHICLE : HS - 25
TYPE OF STEEL: M 270 GRADE 50W
BRIDGE TYPE: COMPOSITE
AASHTO DEFLECTION CRITERIA: IS CHECKED
GIRDER TYPE: PLATE GIRDER
ROAD CASE: CASE II
CALCULATION TYPE: DESIGN
AASHTO CONSTRUCTIBILITY CRITERIA: IS CHECKED
CONCRETE STRENGTH (KSI): 4
HAUNCH THICKNESS (IN.): 2
SLAB THICKNESS (IN.): 8.5
SHEAR STUDS NUMBER PER ROW: 3
SHEAR STUD DIAMETER (IN.): 0.875
UNIT WEIGHT OF CONCRETE (PCF): 150
BOTTOM FLANGE WIDTH (IN.): 16
TOP FLANGE WIDTH (IN.): 12
MINIMUM TOP FLANGE THICKNESS (IN.): 0.75
MAXIMUM TOP FLANGE THICKNESS (IN.): 1
MINIMUM BOTTOM FLANGE THICKNESS (IN.): 0.75
MAXIMUM BOTTOM FLANGE THICKNESS (IN.): 1.5
MINIMUM WEB DEPTH (IN.): 32
MAXIMUM WEB DEPTH (IN.): 42
MINIMUM WEB THICKNESS (IN.): 0.375
MAXIMUM WEB THICKNESS (IN.): 0.625
CONSTANT SECTION GIRDER: YES

Problem No. 3 - AISIBEAM Input Data for a Rolled Beam Rating for U80:

INPUTS AND OUTPUTS UNITS: US CUSTOMARY UNITS
EFFECTIVE WIDTH IS CALCULATED BY: THE PROGRAM
DISTANCE BETWEEN CROSS BRACINGS ARE CALCULATED BY: THE PROGRAM
VALUE OF MODULAR RATIO (N) IS CALCULATED BY: THE PROGRAM
DISTRIBUTION FACTORS ARE CALCULATED BY: THE PROGRAM
IMPACT FACTOR IS CALCULATED BY: THE PROGRAM
NUMBER OF BEAMS: 4
CLEAR ROADWAY WIDTH (FT): 34
OVERHANG LENGTH (FT): 3.083333
BEAM SPACING (FT): 10
NUMBER OF TRAFFIC LANES: 2
CALCULATIONS ARE FOR AN: INTERIOR BEAM
SPAN LENGTH (FT): 80
NUMBER OF SEGMENTS IN SPAN: 10
DEAD LOAD (EXCLUDING SELF WEIGHT) PER GIRDER (K/FT) 1.285
SUPERIMPOSED DEAD LOAD (K/FT): 0.416
FATIGUE VEHICLE: HS - 20

VEHICLE INFORMATION TABLE

WHEEL    WHEEL           DISTANCE BETWEEN
NUMBER   LOAD (KIPS)   SUCCESSIVE AXLES (FT)

1              6.0               0.0
2             18.5             14.0
3             18.5             18.5
4             18.5             32.5
5             18.5             37.0

TYPE OF STEEL: M 270 GRADE 36
BRIDGE TYPE: COMPOSITE
GIRDER TYPE: ROLLED BEAM
AASHTO DEFLECTION CRITERIA: IS CHECKED
ROAD CASE: CASE II
CALCULATION TYPE: RATING
AASHTO CONSTRUCTIBILITY CRITERIA: IS NOT CHECKED
CONCRETE STRENGTH (KSI): 4
HAUNCH THICKNESS (IN.): 2
SLAB THICKNESS (IN.): 8.5
SHEAR STUDS NUMBER PER ROW: 3
SHEAR STUD DIAMETER (IN.): 0.875
UNIT WEIGHT OF CONCRETE (PCF): 150
ROLLED BEAM SIZE: W36X300
LENGTH OF COVER PLATE (FOOT): 0
WIDTH OF COVER PLATE (IN.): 0
COVER PLATE THICKNESS (IN.): 0

The following are basic steps required for running the software:

Step 1 -	To develop a new input file for a design or rating select File from the Main Menu screen, then select menu item New Project.
Step 2 -	Beam Editor Project Information screen appears. Select the appropriate options and enter the input data in the input text boxes. If you wish to continue, click on the Next button. If you do not wish to continue, click on the Cancel button. This takes you to the Main Menu screen.
Step 3 -	After clicking on the Next button a new screen appears, enter the input data in the appropriate text boxes and answer all of the questions in this new screen. At any time you can go back to the previous screen by clicking on the Back button.
Step 4 -	After answering all the questions on each screen and clicking on the Next button, the Save Input File (Save Menu) screen appears. At this time you can save the input file or select Restart to change or edit the input file.
Step 5 -	After saving the input file, the Main Menu screen will appear. Now you can click on the Run menu to run the program. A new screen will appear. Enter the name of your data input file and output file.
Step 6 -	Click on the OK button. A new screen appears indicating that the program is running. After the run is complete, the Main Menu screen will reappear.
Step 7 -	You can now view or print the output or options files. If you click on the Graphs menu you can look at the moment, shear and deflection diagrams and influence lines for the last run.
Step 8 -	To print or view the design or rating output file, click on the File menu. Then click on the Print menu item or the View File menu item and enter the file name.

The AISIBEAM User's Manual can also be downloaded and be printed free of charge. Choose any of the formats below to start downloading:

Download AISIBEAM User's Manual (a Word file, 92 KB)

Download AISIBEAM User's Manual (a PDF file, 1.4 MB)
You may need to download Acrobat Reader to view the document in PDF format.

Once you finish studying the User's Manual of AISIBEAM and completing three problems, you need to take a quiz to obtain the PDH credits.

***

Additional Technical Resource and Further Readings:

1. American Association of State Highway and Transportation Officials, Manual for Condition Evaluation of Bridges, Washington, DC, 1994.

2. American Association of State Highway and Transportation Officials, Standard Specification for Highway Bridges, 1996, Edition, 1997,1998 and 1999 Interim Specifications - Bridges, Washington, DC.

3. American Association of State Highway and Transportation Officials, Guide for Painting Steel Structures, Washington, DC, 1997.

4. American Institute of Steel Construction, AISC Database for IBM Compatible PC, Chicago, IL, 1997.

5. American Iron and Steel Institute, Integral Abutments for Steel Bridges, by Ed Wasserman, PE, and J. Houston Walker, PE, 1996. (Available from the National Steel Bridge Alliance, Chicago, IL).

6. American Iron and Steel Institute, Steel Bridge Bearing Selection and Design Guide, by Profs. Charles Roeder, PE, and John Stanton, PE, 1996. (Available from the National Steel Bridge Alliance, Chicago, IL).

7. Federal Highway Administration, Uncoated Weathering Steel in Structures, FHWA Technical Advisory T5140.22, Washington, DC, 1989.

8. Missouri Highway and Transportation Department. Task Force Report on Weathering Steel, January 1996.

9. Rubeiz, C.G., and Gorman, C.D., Pre-Engineered Short Span Steel Bridges, ASCE Structures Congress, Atlanta, GA, 1994.

10. Taavoni, S., Comprehensive Package for the Design of Short Span Steel Bridges, National Symposium on Steel Bridge Construction, Atlanta, GA, 1993.

11. AASHTO, Guide Specifications for Highway Bridge Fabrication with HPS70W Steel, Washington DC 2000.

For more information on AISIBEAM software, please click on the following links: Short Span Bridge/American Iron and Steel Institute (AISI).

***

Acknowledgment:

PDHonline.com acknowledges the sponsorship of American Iron and Steel Institute (AISI) for this online course.

***

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDHonline.com or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered professional engineer. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.