LRFD Footing Shear Design

[jparke]

How do we properly design for one way shear in an abutment or retaining wall footing using the latest AASHTO LRFD specifications? We have found so far that the AASHTO LRFD specification requires a much greater thickness of footing to meet the shear requirement than the the latest standard AASHTO specification. It doesn't make a lot of sense to have a deep footing for a 30 foot tall retaining wall when normally a 3 to 4 foot deep footing would work using the standard specification.

Thanks.

 

[curvbridger]

I think we're going to have to get used to strut and tie modeling. Most footings will not qualify as slender flexural members (it would require a toe length of more than 6-8 feet for a wall such as you describe - see Section 5.6.3.1).

In the strut and tie model, just as in a truss, there is no shear per se, just axial loads in the assumed members.

curvbridger

 

[jheidt2543]

jparke,

You raise a really good point. Because of the code change, do we revisit the structures designed under the old code and "upgrade" them? No! (Assuming there is no major flaw in the structure.)

When significant code changes come along and result in a new design that doesn't seem to make sense based on older codes and COMMON sense, shouldn't the new code be revised? How do all these older structures stand up? Every provision of a new code doesn't always point us in the direction of progress, there has to be some common sense too.

 

[PM]

jparke;

Just as a matter of interest; @ 30' high, why aren't you considering a counterforted wall rather than a cantilevered wall that I assume you are designing? I presume the economical minimum height for counterforted or buttressed walls is somewhere in the vicinity of 28'???
Regards,

 

[jparke]

The 30' wall is actually a wing wall for a bridge with multiple steps in height with the 30' being the maximum height. This shear problem not only occurs with cantilevered retaining walls, but also with any type of deep concrete beam that also has an applied moment. Pile cap footings, spread footings for abutments...etc. I havn't realized any help in reducing the footing thickness of pile cap footings using the strut and tie method. How would you model the compression struts for a spread footing?