Multiple Presence Factors


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sabre
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Joined: 02/13/2015

I have a beam slab bridge configuration that requires distribution factors by lever rule. For the exterior girder, PG Super applies a multiple presence factor of 1.2 for 2 lanes loaded, and references AASHTO 4.6.2.2.2d. AASHTO 3.6.1.1.2-1 appears to designate 1.0 for 2 lanes, with some language in the last paragraph of 3.6.1.1.2 that is unclear about exterior girders using lever rule. Any comments about this program choice would be helpful. I have noted that AASHTOWare BRR does this as well.

Rick Brice
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Joined: 06/22/2010
Check the Live Load

Check the Live Load Distribution Factors chapter in the Details Report. It will detail how the software computed the distribution factor.

The multiple presence factor is selected based on the number of design lanes contributing to the distribution factor calculation. Depending on the geometry of the structure and the width of the slabs, there are cases where only one lane contributes to the LLDF when computed with the lever rule.

For a 3' wide slab, the C-C spacing of the slabs is 3'. Since a design lane is 12' wide, only one lane contributes to the LLDF. Hence, the MPF of 1.2 is used.

sabre
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Joined: 02/13/2015
Contributing Live Load Not Present

Thanks! That explains it. Since the live load of the additional lanes does not contribute to the LLDF using the lever rule, the MPF is that of a single lane regardless of lanes loaded.