AASHTO LL Surcharge

[bridge_engineer1]

Hi,

For abutment & retaining wall design AASHTO requires a live load surcharge.

My question is, is this load applied to the back face of the wall horizontally (ie no vertical component) or is the soil pressure applied at at angle due to angle of the wall and the soil-wall friction angle (like the soil pressure EH load is when using the Coulomb method)?

The equation in AASHTO for LL surcharge pressure is k * density * heq, where h equivalent is an equivalent height of soil for vehicular load, density is based on soil density and k is either at-rest or active depending on the wall flexibility. Since ka is based on coulomb earth pressure, which takes into account wall friction angle and wall angle, I usually assume the live load surcharge acts at an angle from horizontal equal to the wall friction angle and wall angle. I'm second guessing that because I found an example online that just applies it horizontally.


Thanks!

 

[LOTE]

For retaining walls, I apply it at an angle per Coulomb for external stability. For internal stability (MSE walls) I apply as straight horizontal load.

 

[DoubleStud]

It is basically just adding 2 ft of soil of whatever soil condition you have. To figure out surcharge load of a truck, it is pretty complicated. So AASHTO just made it easy and basically just add more soil height on your design of horizontal surcharge.

 

[BridgeSmith]

It's not always 2 ft of soil. For shorter walls, if the wheel load can be close to the wall, it can be up to 4 ft.

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