AASHTO LRFD retaining wall footing bearing pressure distribution

[OSUCivlEng]

I've been writing a spreadsheet for retaining wall design and just realized that in section 11.6.3.2 of the LRFD Bridge Specifications, AASHTO specifies a uniform bearing pressure for spread footings on soil. They do specify the typical trapezoidal pressure diagram for spread footings on rock. This seems to go against everything I was taught and have read about retaining wall design. Is there any particular reason for this?

 

[bridgebuster]

Could it be due to the use of reduced bearing area? I looked at a spreadsheet a former coworker developed; he didn't account for this. I have some FHWA publications on substructures; maybe they can shed some light on the matter. I'll look for them later today.

 

[fattdad]

It's acceptable to make the bearing stresses uniformly distributed on the footing, by reducing the bearing area.

So, I get it!

f-d

ípapß gordo ainÆt no madre flaca!

 

[bridgebuster]

Here's an FHWA Manual that may help. There's also an NHI substructure course available. It's a very large PDF and I'm unable to find a link to download it.

 

[Guest090822]

I'm pretty sure those diagrams in Chapter 11 are for determining the bearing resistance, not the actual exerted heel/toe pressures. Then again, the way college professors are wreaking havoc on fundamentals of engineering, they've probably tried to reinvent the wheel - again.

 

[Doctormo]

The equivalent uniform pressure distribution was originally proposed for MSE Walls many years ago since there was not a rigid foundation interface (soil on soil, I think it was attributed to Myerhoff). The resultant location is determined by the eccentricity of the loading on the structure and distributed over the Base - 2*ecc thus a higher uniform pressure.

At some point in recent years, AASHTO adopted this model for all gravity walls except those with a rigid foundation (ie: on rock as you noted). Even worse than that, overturning is not directly checked anymore and only an eccentricity check is required, ecc < B/3 or something like that. Just about the time you have figured that out, add load factors to the calculation and you get an eccentricity value that is pretty close to meaningless and just an empirical value to be measured against another calculated value.

In the older AASHTO ASD criteria, eccentricity criteria was < B/6 and made some sense (eccentricty in middle third of footing thus no negative heel pressure.) Now it is just numbers. Most people still check overturning but you don't have to.

Just a few thought..

 

[OSUCivlEng]

Rick it is true that the figure comes from section 11.6.3.2 - Bearing Resistance. However it states: Bearing resistance shall be investigated at the strength limit state using factored loads and resistances, assuming the following soil pressure distributions:
Where the wall is supported by a soil foundation: the vertical stress shall be calculated assuming a uniformly distributed pressure over an effective base area as shown in Figure 11.6.3.2-1.

bridgebuster I found the answer I was looking for in that FHWA Manual. In section 8.3.2 of that manual it states: As the load on a footing is rarely concentric, the bearing pressure varies across the base of the
footing. The variation in bearing pressure is generally assumed to be linear. For foundations on soil, compression of soil beneath the footing results in a redistribution of bearing stress to a more
uniform value. Therefore, for foundations on soil, the factored bearing resistance is compared to the factored uniform unit bearing stress acting on the base of a concentrically-loaded footing area

That is definitely not what my soil mechanics prof. taught me in school or what is in any of my textbooks. I have usually used the PennDOT software ABLRFD for any substantial retaining wall design, and I never really paid that much attention to how AASHTO specified to calculate the bearing pressures for footing on soil. I guess I learned something.