Bridge Abutment Footing

[KOTOR1]

I have point load maximum of 21.0 k at each wheel (42 k axle load) Do I have to design the footing assuming all the load acts directly on the footing or I can assume the load spreads along more of the footing area through the medium of the abutment. I was concerned about punching shear at the footing.

In other words, should I design the footing assuming all the 21.0 k acts at a single point on the footing?

 

[Qshake]

No, you shouldn't design the footing assuming a single point load.

But the answer of how to distribute the load to the foundation gets a little murky from there. Some points to consider are:

1. Does the endbent/abutment have a concrete diaphragm encasing the longitudinal girders? If yes, the distribution may follow more along the lines of a 2:1 through the diaphragm, though AASHTO does have distribution formulas.

2. If there is no diaphragm encasing the girders, then the wheel load(s) is positioned on or between the girders where it is transferred to the supporting girders by simple beam distribution. The reactions of the girders now become point loads acting on the footing or abutment beam.

I hope this helps.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[KOTOR1]

I have 4' wide voided slabs basically forming the bridge. So I can basically develop the point loads in a 4' wide area? So instead of using 21.0 k directly, I can assume it be distributed along a 4'-0" wide length. ie 21k/4 = 5.25 k on the footing.

Am I right in that assumption?

Thanks

 

[Qshake]

That's 21k/4' or 5.25k/ft on the footing, not including the length of the bearing.

If there is a shearing mechanism between the girders, a shear key or post-tensioning bar (assuming you have adjacent box beams or deck beams for your bridge) you could potentially get distribution across the joint. You have to check AASHTO.



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[KOTOR1]

Qshake, thanks for the response.

I do have shearing mechanisms between the adjacent box beams.

However, to be conservative, I will assume the load is 5.25k/ft across the whole footing.

 

[crossframe]

How tall is the abutment stem? Unless very short, you'll get a fair amount of distribution along the length of the abutment, likely approaching uniformity.