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AASHTO LRFD (LL+IM on piles) bridge design

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evie

evie

Structural
Jul 22, 2003
2
Using the AASHTO LRFD code (3.6.2.1), I understand that the load used to design piles at end bents, which are entirely below ground, would not have the dynamic load allowance (IM) included with the truck live load calculations. As for an interior bent, where piles are partially above ground, I understand that IM would be included with the truck live load calculations in determining the loads used to design the piles.

My question then is...when stating the tonnage needed for piles (i.e. based on bearing--some distance below ground on stable soil), would I still include IM with the live load? I plan on designing the piles for LL + IM, but I'm not sure if I need to include IM on the truck load when stating the Factored Design Load in tons, specifically because the tonnage needed is resisted by bearing (or skin friction) of the pile below ground, where IM is not usually included. Hopefully, you see why I'm torn whether or not I should include IM in the tonnage; if I do, it increases the Required Ultimate Bearing (which is the Factored Design Load divided by the Geotechnical Resistance Factor) by about 60 tons!

I appreciate your suggestions...
 
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eve,
There should be someone in your office with experience. Bridge design isn't something you want to go off and do on your own. But, it reads like you can speak English--so here's a reply.

LRFD is great for sizing structural elements. However, for retaining walls LRFD is a mess. It would be nice if ACI increased the allowable stress beyond 24 ksi for the ADM. I am not familiar with AASHTO LRFD.

Foundations and soils are not designed via load factor. There are too many unknowns in geotechnical engineering, it is highly subjective. Soils engineers use service loads and usually include a safety factor of 3.

There is no impact loading included in the substructure. If it is above ground it does not qualify as part of the substructure. Impact takes into account the bounce of the truck. By the time the bounce makes it to the piles it will have dissipated throughout the structure. The bounce is a transfer of momentum to the bridge. Thus, the loading is temporary, and is a momentum transfer between a small mass and a big mass. It makes since that impact should not be applied to the substructure.
F=(dP/dt) Newton was possessed.
 
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