Concrete Cantilevered Retaining Wall - LRFD Moment at Heel

[SKIAK]

I'm working on a basic concrete cantilevered retaining wall, nothing special.

I have done all of the stability calculations (overturning, sliding, soil pressures) with "allowable" loads and everything works fine and has appropiate factor of safeties.

I'm try to find the "ultimate" moment in the heel of the footing. My first hunch was to apply the LRFD factors to the loads and find my new soil pressures to find the moment, only to find out that it creates negative soil pressures ("uplift") in the heel of the footing (because of the higher emphasis on the horizontal earth pressure).

I've seen some calculations simply multiply the soil pressures I obtained with the "allowable" loads by 1.4 (in what I assume is a compromise between 1.2D and 1.6H), in combination with the gravity loads, to find an ultimate design moment. I think I see their reasoning, but it seems to me that you could end up with an low design moment. The soil pressures effectively work to reduce the design moment, it seems to me that arbitrarly increasing these could lead to undersized footings.

Is there a better way to deal with this?

 

[SKIAK]

I have found one resouce (Bowles, Third Edition) that suggestes applying a load factor to the shears and moments. It suggests (page 460 under step #2):

"Estimate the load factor. Use at lease 1.7 for cohesionless backfill in the full Rankine zone. Use more than 1.7 for cohesive backfill, or limited backfill zone. Limit the load factor to an equivalent Ka = 1.10 regardless of the backfill."

So far as I can tell this load factor, it states later at the end of step 9, is applied to the shears and moments to get a design value. Although, I'm not sure I fully understand what the author is doing there.

 

[FixedEarth]

See attached design example.

http://www.FoundEng.com

 

[SKIAK]

Thanks for the reference FixedEarth, but I think that design procedure could end up with highly conservative results.

The design procedure you posted (on page 6 under section 7. Heel Design) only considers the weight of the heel + fill + surcharge to calculate the moment and neglects the soil pressure. The soil pressure is acting in the opposite direction and will reduce the design moment.

Although that procedure would certainly be the worst case scenario, I think it could be unnecessarily conservative.

 

[FixedEarth]

For few years now, I have been using commercial software for proportioning retaining walls. Just last month, I started working on my own Retwall spreadsheet. When I get to heel design, I will be able to comment better on your post. So far, I find the approach by John Cernica (Foundation Design 1995 +/-) to be the most rigorous. Keep me posted.

http://www.FoundEng.com

 

[MIBridgeEng]

In the transportation industry, we are governed by the AASHTO LRFD Bridge Design Specifications. It requires factored loads for the overturning, bearing pressures and sliding. You would then check the heel bending using the factored loads of the soil above the heel and the resultant factored pressures on the bottom of the footing to determine the factored moment and shear at the back face of the wall. So your initial suggestion is correct. Unfortunatly we have found the LRFD approach to be conservative due to the crazy high load factor placed on the horizontal earth pressures. I think the theory is rational, but the load factors themselves don't seem correct.

You can find a design example from MnDOT (page 11-39). It is for a pile foundation, but I think you'd get good information from it.

http://www.dot.state.mn.us/bridge/manuals/LRFD/pdf/section11-6302010.pdf

 

[dcarr82775]

Remember any resisting forces get a load factor of 0.9, while the driving forces should get a load factor of 1.6.

The load factor of 1.4 could be related to the load case of 1.4D, which in my opinion doesn't apply. I would not consider soil even remotely well enough defined to use anything less than a load factor of 1.6.

 

[SKIAK]

I've found that the 2008 CRSI Design Handbook has a pretty good procedure. They basically use a 1.6 factor on the service loads, and reduce the soil pressures by 25% (a 0.75 factor). If you have their book, it's on page 14-10. That's probably what I'll be using.

 

[FixedEarth]

I have found Ferguson/Breen/Jirsa's Reinforced Concrete Fundamentals, 5th ed, '88 to cover the structural portion the best and Cernica's (Geotechnical Engineering:Foundation Design, '95) to cover the geotechnical portion the best.