Flexible Diaphragm Design 1

[RM87]

Hello all,

I was hoping someone could give me some pointers - I'm concerned about the flexible diaphragm shear on the lower level of a three story home. Specifically, I have a proposed 26'-0" span between shear lines and I want to know if these existing conditions comply

Please see attached:

http://files.engineering.com/getfil...eb69f721&file=860_S._El_Monte_Shear_Calcs.pdf

I intend to calculate the worst case scenario situation at the lower level and I was wondering if:

1) I wanted to calculate the actual force being transferred into my lower diaphragm, do I have to sum the diaphragm shear of the upper floors with that of the lower floors to see how much shear is actually being transferred into the diaphragm, or is the load isolated? I'm drawing a blank - can someone set me straight?

2) Can someone also confirm that the maximum collector forces I expect to find along any given shear line does not equate to the diaphragm shear I need to design for? Again, I'm drawing a blank but I'm leaning towards the concept that they're separate - one defines the shear in the diaphragm, and the other is the axial load expected to be carried by the collecting member.

Thanks a lot folks,
R

 

[msquared48]

In general, floor diaphragms only see the lateral load from 1/2 story heights above and below the diaphragm UNLESS...

There is a shear wall above that ends at the diaphragm and the force has to be transferred into the diaphragm and re-distributed to the shear walls below through the diaphragm. Basically a beam action...

Mike McCann
MMC Engineering

 

[wannabeSE]

1. Provided there is a flexible diaphragm with no offset shear walls, the 2nd floor diaphragm governs with 21.1 kips. The lateral force from the story above is already into the shear walls, it does not transfer through the other diaphragm.

2. Assuming the building is fairly regular with even distribution of forces to shear walls on each side, then 1/2 diaphragm shear goes to each collector. ASCE 7-10 clarified (or codified) collector forces. For light frame shear walls, it is easy and the diaphragm forces from 12.10.1.1 are used with the load combinations in 12.4.2.3. If it weren't light frame shear walls there are other cases to consider and some require the overstrength factor.

 

[RM87]

Thanks a lot. I appreciate the help.