Section 12: Straight Plate Girders
Anchor: #i1359516Materials
Use A 709 Grade 50W steel for unpainted bridges. Use A 709 Grade 50 steel for painted bridges. You can use A 709 Grade HPS 70W steel for both unpainted and painted bridges if it is economical or otherwise beneficial to do so.
Use 0.875-in. or 1-in. diameter bolts for bolted connections.
Anchor: #i1359577Geometric Constraints
Minimum flange width is 0.25D, where D=web depth, but not less than 15 in.
Minimum flange thickness is 0.75 in.
Minimum web thickness is 0.50 in.
Minimum stiffener thickness used to connect cross frames or diaphragms to girder is 0.50 in.
Anchor: #i1359649Structural Analysis
Beam designs must meet the following requirements:
- Distribute the weight of one railing to no more than three girders, applied to the composite cross section.
- Assume no slab haunch when determining composite section properties.
- Live load distribution factors must conform to AASHTO
LRFD Bridge Design Specifications, Article 4.6.2.2.2
for flexural moment and Article 4.6.2.2.3 for shear, except as follows:
- For exterior girder design with a slab cantilever equal to or less than half the adjacent girder interior spacing, use the live load distribution factor for the interior girder. The slab cantilever is the distance from the centerline of the exterior girder to the edge of the slab.
- For exterior girder design with a slab cantilever length greater than half the adjacent interior girder spacing, use the lever rule with the multiple presence factor of 1.0 for single lane to determine the live load distribution. The live load used to design the exterior girder must never be less than the live load used to design an interior girder.
- For interior as well as exterior girders, do
not take the live load distribution factor, for moment or shear,
as less than
Where,
- m = multiple presence factor per AASHTO LRFD 3.6.1.1.2
-
= number of lanes -
= number of beams
or girders
- When checking the Fatigue and Fracture Limit State, remove the 1.2 multiple presence factor from the one-design-lane-loaded empirical live load distribution factors.
- Use only one lane of live load in the structure model when checking the Fatigue and Fracture Limit State.
Design Criteria
Standard beam designs must meet the following requirements:
- Regarding AASHTO LRFD Bridge Design Specifications, Article 6.7.2, do not specify girders to be out-of-plumb in the steel-dead-load-only or theoretical-no-load condition. Diaphragms and cross frames have traditionally been installed with girders plumb and no significant problems have been reported to date. If the designer believes that analysis indicates that girders will be significantly beyond plumb after slab concrete is placed, contact the Director of the Bridge Division for guidance.
- Diaphragm and cross-frame designs must meet the following
requirements:
- The maximum spacing is 30 ft. if all limit states requirements are met.
- Provide diaphragms/cross frames at all end bearings. At least two interior bearings at a bent must have a diaphragm/cross frame intersecting them.
- Set diaphragms/cross frames parallel to skew up to 20 degrees. Set radial to girders beyond 20 degrees.
- Check the limiting slenderness ratio of cross-frame members using bracing compression member criteria provided in AASHTO LRFD Bridge Design Specifications, Article 6.9.3.
Girder designs must meet the following requirements:
- Use composite design and place shear connectors the full girder length.
- Do not use longitudinal stiffeners unless web depth exceeds 120 in.
- Use short-term modular ratio equal to 8 and long-term modular ratio equal to 24.
- Provide longitudinal slab reinforcement in accordance with AASHTO LRFD Bridge Design Specifications, Article 6.10.1.7.
- Assume the composite slab is effective in negative bending regions for Deflection check, Fatigue and Fracture Limit State, and Service Limit State.
- Do not use longitudinal slab reinforcement as part of the negative bending section for Strength Limit State.
- Unbraced flange length must satisfy either, Equation 6.10.1.6-2 or Equation 6.10.1.6-3.
- At flange splices, extend thicker flanges beyond the theoretical flange splice location by a length equal to the flange width but not more than 2 ft.
For stud connector designs, minimum longitudinal stud connector spacing is limited to 4d, where d is the stud connector diameter.
Bolted field splices must meet the following requirements:
- Use of A 325 bolts is preferred over A 490 bolts.
- Assume Class A surface conditions. Class B surface conditions may be used only when slip controls the number of required bolts. Always note the surface condition assumed for design in the plans.
- Add at least 0.125 in., and preferably 0.25 in., to minimum edge distances shown in AASHTO LRFD Bridge Design Specifications, Table 6.13.2.6.6-1.
- Do not extend and develop fill plates equal to or thicker than 0.25 in. Instead, reduce bolt shear strength with AASHTO LRFD Bridge Design Specifications, Equation 6.13.6.1.5-1.