Lateral load on vertical wall transferring to diaphragm

[tenguy23]

Ive done several of these problems, and I probably should have been keeping a closer tab on the various methods that different books used.

Say I have a tilt up wall with diaphragm at 10ft high with a 3 ft parapet (total wall ht is 13 ft), and say the wall sends its load to the horizontal diaphragm.

Case 1: the wall is subject to a constant xx psf wind load.

Case 2: the wall is subject to seismic load (cg is at 13/2= 6.5' from ground), resulting in the wall applying an inertial area load of xx psf

What is the proper "statics" to get the load to the diaphragm? (assume I'm doing a ft strip wide analysis)

Method 1: load to ground is xx psf * 5' & load to diaphragm is xx psf (5'+3') = 8 * xx psf

Method 2: load to diaphragm is based on sum moment about ground = 0 = xx psf* 1 ft strip * 13 ft * 6.5' - R,diaph * 10'; R,diaph= 8.45 * xx psf

See the difference with the methods? Which on is better? I say method 2 if the cg of the load aligns with mid height of the wall.

Thanks in advance!

 

[captaincaution]

Method 1 is a quick and dirty way to get the load in the diaphragm, but method 2 is the accurate way to get the load. Imagine the wall as a simply supported beam with a cantilever (the parapet) at one end. Method 2 addresses the load distribution in this way, while method 1 is just a tributary area calculation.

 

[McEngr]

You'll likely be quizzed on method 2.

 

[JDstruc]

Actual NCEES problems will do one of the following:

1) Give the "correct" answer option which can be arrived at by both methods (nearest answer)

2) Specifically tell you to use the "beam analysis method", or something like that

3) Specifically tell you to use the "tributary height method", or something like that

From my studies, in example problems, I usually saw Option 1 being used (you can arrive at the correct answer option by using either methods), however I did see a couple of problems using Option 2, where they specifically tell you to use the beam analogy method, etc. I don't believe I ever saw Option 3.

Anyway, point is, know how to do both options, and you'll be good to go. When it comes down to the test on multiple choice, I would typically solve it both ways and compare the (2) options to the answers. In the in-depth problems, I think I would also probably do the same, and then USE the Option 1 (Beam analogy) method, since it will lead to larger design forces, being more conservative...showing that I can do both options, but going with the more conservative, and possibly more accurate method.

 

[McEngr]

Obviously, option 2 and 3 are only the same if no parapet is given.