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Design Live Load Moment per Girder

Mediumlive loadload combinationdistribution factor
At one girder section, concurrent per-lane moments are Mtruck = 900 kN·m and Mlane = 180 kN·m. Dynamic allowance IM = 0.33 applies only to the truck. The girder distribution factor g = 0.55 lanes/girder already includes multiple presence. Use Mgirder = g[(1 + IM)Mtruck + Mlane]. What is the live-load moment on one girder?
  1. A790.02 kN·m
  2. B757.35 kN·m
  3. C416.54 kN·m
  4. D1,377 kN·m
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Answer: B. 757.35 kN·m

Truck with allowance: 1.33(900) = 1,197 kN·m. Per-lane total: 1,197 + 180 = 1,377 kN·m. Distribute: 0.55(1,377) = 757.35 kN·m. 790.02 kN·m applies the 1.33 impact to the lane load too (0.55·1.33·1,080). 416.54 kN·m applies the distribution factor twice, as if multiple presence were added again (0.55²·1,377). 1,377 kN·m is the per-lane value without distributing to the girder.
kN·m
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