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Section 14: Straight Plate Girders

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Materials

Use A 709 Grade 50W or HPS 70W steel for unpainted bridges. Use A 709 Grade 50 steel for painted bridges. Use A 709 Grade HPS 70W steel for painted bridges if it is economical or otherwise beneficial to do so.

Use 0.875-in. or 1-in. diameter bolts for bolted connections.

For bridges in the Amarillo District only, specify tension components to meet Zone 2 tension component impact test requirements.

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Geometric Constraints

Minimum flange width is 0.20D, 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.

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Structural Analysis

Girder 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 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.
  • Do not take the live load distribution factor for moment or shear as less than the number of lanes divided by the number of girders, including the multiple presence factor per Article 3.6.1.1.2.
  • 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.
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Design Criteria

Standard girder designs must meet the following requirements:

  • Regarding Article 6.7.2, do not specify girders to be out-of-plumb in the steel-dead-load-only or theoretical-no-load condition if field-welded diaphragm/cross-frame connections are used. Diaphragms and cross-frames have traditionally been field-welded to plumb girders without problems to date. If diaphragms/cross-frames are to be field-bolted to the girder, consult with a steel bridge girder fabricator on an appropriate load condition for detailing cross-frame/diaphragm fit. If 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 interior diaphragms/cross-frames parallel to skew up to 20 degrees. Set perpendicular to girders beyond 20 degrees.
    • Check the limiting slenderness ratio of cross-frame members using bracing compression member criteria provided in Article 6.9.3.
    • Lean-on bracing design, as described in Cross-Frame and Diaphragm Behavior for Steel Bridges with Skewed Supports, Helwig and Wang, Research Report 1772-1, 2003, is permissible.

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 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.
  • 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.
  • Include an assumed stay-in-place formwork weight of 15 psf in design.
  • Specify continuous placement of bridge deck where possible, and staged placement only if required. If staged placement is specified, base girder design on the worst case effect of staged and continuous placement. Base dead load deflection and camber on an analysis for continuous placement, even if staged placement is specified in the plans.
  • 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 ASTM F3125 Grade A325 bolts. Use galvanized Grade A325 bolts for painted structures. Use Grade A490 bolts only if the connection cannot be designed with A325 bolts. Do not specify galvanized Grade A490 bolts for any structure.
  • 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 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 Equation 6.13.6.1.5-1.

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