Interpretation of 2 Design Trucks Load Case in AASHTO

[Quade999]

Hello,

AASHTO has a clause where you consider 2 design trucks at 90% weight with a spacing of 50 feet between them for negative moment and reaction at interior piers.
When they say reaction at interior piers do they mean to include the bearing in this case as well? It just mentions the piers themselves and says nothing about the bearings so I'm not sure how this is to be interpreted. i.e. is it a single design truck for the forces on the bearings but 2 design trucks for the forces on the piers? or is it 2 design trucks for the forces on the bearings and the piers?

Thanks

 

[Lomarandil]

The wording is slightly ambiguous, but I wiuld include the bearing as part of the interior pier.

----
just call me Lo.

 

[Settingsun]

AASHTO are repeat offenders.

https://www.eng-tips.com/viewthread.cfm?qid=492585

I'd say the reaction means the bearing. It's pretty common in bridges to talk about the superstructure in isolation.

I like how the two trucks in the third dot point materialise to produce hogging moment but then at least one disappears before they both load a span in sagging.

 

[bridgebuster]

I agree with steve49. The force on the pier is applied through the bearings.

 

[BridgeSmith]

We use the 90% Truck Train + Lane load for designing the bearings, and for calculating the seismic loading.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Quade999]

One thing is that when looking at the shear in the girders, you only look at the single design truck instead of 2.

Another question is it also mentions putting the trucks in adjacent spans to produce maximum force effect. What about when looking at any uplift forces? For a single bearing this would place the two trucks in non-adjacent spans.

 

[BridgeSmith]

One thing is that when looking at the shear in the girders, you only look at the single design truck instead of 2.

The Truck + Lane load effect for shear will be larger than the 90% Truck Train + Lane because shear in the girder is only affected by the load on one side of the pier.

Another question is it also mentions putting the trucks in adjacent spans to produce maximum force effect. What about when looking at any uplift forces? For a single bearing this would place the two trucks in non-adjacent spans.

The Truck Train loading combination is not used for uplift; only for negative moments and reactions at interior piers.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Quade999]

Would uplift forces not count as a reaction on the bearings or piers though?

 

[BridgeSmith]

Net uplift (+DL + -LL) on an interior pier would be a very odd case for a highway bridge, if it's even possible. However, the Truck Train loading is specified "For negative moment between points of contraflexure...and reaction at interior piers only". The Truck Train is to be applied with the 2 trucks "in adjacent spans". At an interior pier, uplift would occur with the opposite exterior span loaded. Any loading in the span adjacent to the pier under consideration would decrease the uplift at that pier.

Rod Smith, P.E., The artist formerly known as HotRod10