Concrete Cantilevered Retaining Wall - LRFD Moment at Heel
Concrete Cantilevered Retaining Wall - LRFD Moment at Heel
(OP)
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?
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?





RE: Concrete Cantilevered Retaining Wall - LRFD Moment at Heel
"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.
RE: Concrete Cantilevered Retaining Wall - LRFD Moment at Heel
www.FoundEng.com
RE: Concrete Cantilevered Retaining Wall - LRFD Moment at Heel
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.
RE: Concrete Cantilevered Retaining Wall - LRFD Moment at Heel
www.FoundEng.com
RE: Concrete Cantilevered Retaining Wall - LRFD Moment at Heel
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.
ht
RE: Concrete Cantilevered Retaining Wall - LRFD Moment at Heel
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.
RE: Concrete Cantilevered Retaining Wall - LRFD Moment at Heel
RE: Concrete Cantilevered Retaining Wall - LRFD Moment at Heel