Point load distribution in retaining wall

[Enginerdad]

Hi everybody,
I am in the process of designing a concrete cantilever retaining wall which will have a vehicle barrier attached to the top of it. I was wondering about the method of distributing that point load (AASHTO 10 kip) over the height of the wall to find the moment-per-foot at the base of the stem. General knowledge says that the stress will distribute at a 45 degree angle in each direction, thus making the distribution width = 2*h. However, I seem to remember seeing something in a code about the distribution width being =h instead of =2h. It may have been in ACI of AASHTO, but I can't see to find it. Does anybody have any insight on this?

 

[BridgeEI]

One thing I've never been able to find is where the 10k is at in the code. I know it's not in the LRFD, at least from what I can find. That being said, if you're using the LRFD, then I'd look at chapter 13 since it gives a table of impact load, distribution width, etc and design off that depending on what test level criteria you want to use. In a deck design, the force is distributed out at 30 degrees for impact so that's probably how I would do it. It's all engineering judgement, in my opinion.

Either way, if your wall seems excessive in size then I'd consider using a moment distribution slab and have the rail tied into it. Sometimes I think this is the easiest alternative.

 

[Teguci]

Mappleby - Figure out how the wall is going to fail under an impact loading. If we're talking about overturning, the wall is going to have significant bending and torsional restraint over a significant distance. If deflection is not a concern (its not) then you could conceivably consider even a 60 ft width of wall being activated by an impact load. Verify that local failures will not occur and have it. As always, take a step back and try to imagine ways to fail your system and then run the numbers to isolate that possibility.

As for the 10 kip load, its 6 kips service which is 9.6 kips ultimate. This load occurs at 18" above the road surface and acts on a 12"x12" square (18" barrier is unacceptable, 24" min).

 

[tbone73]

Teguci, the IBC 2009 has added a new increased height of impact load check of 27 inches now. This will control most designs for moments now.

I have designed many retaining walls with considering impact loads, and I have used a 45 degree distribution with acceptable results. As Teguci has mentioned consider what you're designing for. For localized wall bending and shear I would use more conservative distribution widths. However, if you are considering overall wall overturning you could use a larger distribution, because the whole wall will act to resist the overturning, provided you don't have a localized failure first.