Where does the Allowable Bearing Stress Equation come from? 3

[mgator2099]

For dowels in a concrete pavement, the allowable bearing stress is calculated as:

Fb = ((4-d)/3)*Fc,

where

Fb= Allowable Bearing Stress
d= Diameter of Dowel
Fc= Ultimate Compressive Strength of Concrete.

This seems backwards to me (Allowable Stress goes down as dowel size increases. And I can't find an explanation online. Does anyone know how this formula came to be?

 

[Ron]

The allowable bearing stress is based in the remaining shear area of the concrete below the dowel. As the diameter of the diameter increases, the shear area below the dowel decreases.

 

[MiketheEngineer]

Yep

 

[Ron]

I should proofread! It should read "as the diameter of the dowel increases"

 

[BenJohnson]

The bearing strength of a steel dowel is a lot stronger than the compressive strength of concrete. So the concrete will fail before the steel. Thus compressive strength of concrete will fail long before the dowel.

 

[mgator2099]

Thanks for your help. I'm not sure I'm 100% on board just yet. If the shear area decreasing causes the bearing stress to decrease, why would the shear area not be in the equation? This equation (from ACI Committee 325, 1956) does not take into account slab thickness, thus not the area beneath the dowel.

I may be off base, and since I was not able to access the report I assume this was just a relationship found in a laboratory.