Bridges & Structures

Design Step 3.1 - Obtain Design Criteria
Design Step 3.2 - Select Trial Girder Section
Design Step 3.3 - Compute Section Properties
Design Step 3.4 - Compute Dead Load Effects
Design Step 3.5 - Compute Live Load Effects
Design Step 3.6 - Combine Load Effects
Positive Moment Region: Design Step 3.7 - Check Section Proportion Limits
Design Step 3.8 - Compute Plastic Moment Capacity
Design Step 3.9 - Determine if Section is Compact or Noncompact
Design Step 3.10 - Design for Flexure - Strength Limit State
Design Step 3.11 - Design for Shear
Design Step 3.12 - Design Transverse Intermediate Stiffeners
Design Step 3.14 - Design for Flexure - Fatigue and Fracture
Design Step 3.15 - Design for Flexure - Service Limit State
Design Step 3.16 - Design for Flexure - Constructibility Check
Design Step 3.17 - Check Wind Effects on Girder Flanges

Design Step 3.7 - Check Section Proportion Limits
Design Step 3.8 - Compute Plastic Moment Capacity
Design Step 3.9 - Determine if Section is Compact or Noncompact
Design Step 3.10 - Design for Flexure - Strength Limit State
Design Step 3.11 - Design for Shear
Design Step 3.12 - Design Transverse Intermediate Stiffeners
Design Step 3.14 - Design for Flexure - Fatigue and Fracture
Design Step 3.15 - Design for Flexure - Service Limit State
Design Step 3.16 - Design for Flexure - Constructibility Check
Design Step 3.17 - Check Wind Effects on Girder Flanges
Design Step 3.18 - Draw Schematic of Final Steel Girder Design

Design Step 3.1 - Obtain Design Criteria

The first design step for a steel girder is to choose the correct design criteria.

The steel girder design criteria are obtained from Figures 3-1 through 3-3 (shown below), from the concrete deck design example, and from the referenced articles and tables in the AASHTO LRFD Bridge Design Specifications (through 2002 interims). For this steel girder design example, a plate girder will be designed for an HL-93 live load. The girder is assumed to be composite throughout.

Refer to Design Step 1 for introductory information about this design example. Additional information is presented about the design assumptions, methodology, and criteria for the entire bridge, including the steel girder.

Figure 3-1 Span Configuration

Figure 3-2 Superstructure Cross Section

Girder Spacing

Overhang Width

Figure 3-3 Framing Plan

Cross-frame Spacing

A common rule of thumb, based on previous editions of the AASHTO Specifications, is to use a maximum cross-frame spacing of 25 feet.

For this design example, a cross-frame spacing of 20 feet is used because it facilitates a reduction in the required flange thicknesses in the girder section at the pier.

This spacing also affects constructibility checks for stability before the deck is cured. Currently, stay-in-place forms should not be considered to provide adequate bracing to the top flange.

The following units are defined for use in this design example:

Design criteria:

For this design example, transverse stiffeners will be designed in Step 3.12. In addition, a bolted field splice will be designed in Step 4, shear connectors will be designed in Step 5.1, bearing stiffeners will be designed in Step 5.2, welded connections will be designed in Step 5.3, cross-frames are described in Step 5.4, and an elastomeric bearing will be designed in Step 6. Longitudinal stiffeners will not be used, and a deck pouring sequence will not be considered in this design example.

Design factors from AASHTO LRFD Bridge Design Specifications:

STable 3.4.1-1 & STable 3.4.1-2

Table 3-1 Load Combinations and Load Factors

The abbreviations used in Table 3-1 are as defined in S3.3.2.

The extreme event limit state (including earthquake load) is generally not considered for a steel girder design.

Table 3-2 Resistance Factors

Multiple Presence Factors

Multiple presence factors are described in S3.6.1.1.2. They are already included in the computation of live load distribution factors, as presented in S4.6.2.2. An exception, however, is that they must be included when the live load distribution factor for an exterior girder is computed assuming that the cross section deflects and rotates as a rigid cross section, as presented in S4.6.2.2.2d.

Since S3.6.1.1.2 states that the effects of the multiple presence factor are not to be applied to the fatigue limit state, all emperically determined distribution factors for one-lane loaded that are applied to the single fatigue truck must be divided by 1.20 (that is, the multiple presence factor for one lane loaded). In addition, for distribution factors computed using the lever rule or based on S4.6.2.2.2d, the 1.20 factor should not be included when computing the distribution factor for one-lane loaded for the fatigue limit state. It should also be noted that the multiple presence factor still applies to the distribution factors for one-lane loaded for strength limit states.

Dynamic load allowance:

Table 3-3 Dynamic Load Allowance

Dynamic load allowance is the same as impact. The term "impact" was used in previous editions of the AASHTO Specifications. However, the term "dynamic load allowance" is used in the AASHTO LRFD Bridge Design Specifications.

Design Step 3.2 - Select Trial Girder Section

Before the dead load effects can be computed, a trial girder section must be selected. This trial girder section is selected based on previous experience and based on preliminary design. For this design example, the trial girder section presented in Figure 3-4 will be used. Based on this trial girder section, section properties and dead load effects will be computed. Then specification checks will be performed to determine if the trial girder section successfully resists the applied loads. If the trial girder section does not pass all specification checks or if the girder optimization is not acceptable, then a new trial girder section must be selected and the design process must be repeated.

Figure 3-4 Plate Girder Elevation

For this design example, the 5/8" top flange thickness in the positive moment region was used to optimize the plate girder. It also satisfies the requirements of S6.7.3. However, it should be noted that some state requirements and some fabricator concerns may call for a 3/4" minimum flange thickness. In addition, the AASHTO/NSBA Steel Bridge Collaboration Document "Guidelines for Design for Constructibility" recommends a 3/4" minimum flange thickness.

Girder Depth

Web Thickness

Plate Transitions