Lever Rule on narrow bridge

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mdmetzger21
Lever Rule on narrow bridge
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Hello, I'm designing a bridge with roadway width of 20'. I'm having trouble matching the LLDF's calculated by PGSuper for the interior girder.

In PGSuper's calculation of the 2+ lane LLDF per lever rule, it seems to be placing one wheel load at the center of the bridge, one 4' away, and one 6' away. I think this would actually place the design lanes off of the bridge (see attached sketch).

Per AASHTO 3.6.1.1.1, for bridges with roadway width between 20' and 24', design lane width = 1/2 roadway width, 10' in this case.

Per AASHTO 3.6.1.3.1, position wheel load 2' from edge of design lane.

I have attached the PGS file, along with my lever-rule calculation/sketches.

Thanks for your time!

Rick Brice
Lever Rule on narrow bridge

Your assumption about how PGSuper is positioning the trucks is incorrect. Please look at the Live Load Distribution Factor Details chapter in the Details Report.

For the interior girder with two loaded lanes, the first wheel line is placed as far to the left side of the bridge as possible (d = 7.5ft). The next wheel line is 6 feet to the right of the first (d=1.5ft). A spacing of 4 ft is used to place the next wheel line (left wheel line for second loaded lane). This places the load at d=3.5ft. The fourth wheel line is off the bridge so it is neglected.

g = (7.5*P/2 + 1.5*P/2 + 3.5*P/2)/(7.5*P) = 0.833. MPF = 1.0.

Per LRFD 4.6.2.2.1-1, for Nb = 3 use lessor of equation and lever rule. Equation gives 0.630. The LLDF per the equation governs.

Your 2 loaded lane case with the two outside wheel lines off the bridge is no different conceptually then PGSuper having one wheel line off the bridge. The difference is the configuration used by PGSuper is a higher load distribution to the interior girder and is probably a more realistic truck placement configuration.

Also note that you have the Project Criteria set to use the WSDOT distribution factor method. This method is different then AASHTO LRFD. Once key difference is the MPF for 1 loaded load, when LLDF are computed by lever rule, is taken to be 1.0 not 1.2.

If you think the LLDF should be different values, you can compute them outside the program and then enter them as user defined values.

mdmetzger21
Re: Lever Rule on narrow bridge

Thanks Rick, I think that answers my question, and thanks for pointing out about the Project Criteria, I will switch to the AASHTO LRFD method to use the MP = 1.2.

If I understand correctly, the difference in lever-rule computation is that PGS interprets AASHTO's placement of live-load procedure to allow part of a design lane to be outside of the face of curb or even off of the bridge, as long as the centroid of the wheel load is inside or coincident with the face of curb. I also think the sketch I attached does reflect what PGS is doing, maybe just mirrored to the other side. So the program is following the AASHTO guideline of not placing a wheel closer than 2' from edge of lane, but it's allowing the design lane to be off the bridge, as long as the wheel load is still on the roadway. I guess AASHTO doesn't specifically say the design lane has to be on the bridge, but I would argue it's implied and a more realistic design approach. In my attached sketch for the 2 loaded lane case, the two outside wheel lines are not off the bridge, only outside the exterior girder reactions, but still within the roadway width. While the PGS approach is conservative, I will follow your suggestion and input my own LLDF's, calculated by saying the design lane must be between the bridge rails/on the roadway surface.

- Matt Metzger, P.E.

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