Converting Live Load Surcharge from PSF to K/FT 1

[CWEngineer]

I am in the process of comparing hand calculations with the USACE CTWALL Program. I am doing this to have a good understanding of what the program is really doing, just don't want to plug in numbers and get an answer. Well, right now my answers do not exactly match the results from the program.

Not sure if this is a problem or not. But in my hand calculations I have a Live Load Surchage of 325 psf extending beyond the back of the wall, which is 325 lbs for every square foot behind the wall. In the CTWALL Program, it ask for the Uniform Vertical Load in KIPS/FT. Initially, I used 0.325 KIPS/FT, but does that seem right?

Appreciate your help and advice. THANKS.

 

[RWF7437]

Yes

 

[STVU]

Not quiet. A 325 psf on the surface is multiplied by Ka or Ko, depending on if you are designing a "yielding or flexible" wall or "non yielding or rigid" wall respectively.

So let us say, you have a flexible wall, and Ka=0.3, then you will get a rectangular vertical profile, whose horizontal width is 0.3*325 = 98 psf. Then say your wall is 20 ft high, you will get an area of 20*98 = 1960 lb/ft or 1.96 k/ft This I suspect is what you are looking for. This 1.96 k/ft is a force or a thrust that acts at half the length of the wall height(including footing). So if you have 2 ft footing + 20 ft wall, you get 2 k/ft force at 11 ft below top of the wall.

 

[apriley]

I'm pretty sure the program doesn't expect you to do the conversion to an equivalent lateral load as STVU suggests. However, some programs take uniform vertical loads ALONG the wall (ie. parallel), so they essentially look like a point load in section. That might be what the program is doing...

However, it's strange to me that a wall program wouldn't take surcharge into account since it's pretty standard. If you don't have a place to put in surcharge, I would suggest putting the soil an equivalent height above the wall and "fake" surcharge. But first, take it out and make sure all other loads come out OK.

Also, if you have load factors, double check them.

 

[kslee1000]

If you can set all other loads to null (zero), only put the surcharge as an unit load, then you may be able to figure it out by inspecting the result.

 

[CWEngineer]

I made a few modifications to my Hand Calculations and now my results match exactly with those of CTWALL. In my hand calcs I use 325psf and in the CTWALL Program I input this as 0.325 K/FT. This surcharge load extends back beyond the wall and is not a point load (parrallel) to the wall.

Thanks for your help.