Seismic Load Distribution in Buildings with Flexible Diaphragms and Asymmetrical Wall Loads

[AYPE]

Good Day!

I am designing a building with a brick veneer (50 psf) on the front sidewall and metal panels (4 psf) on the back sidewall (see attached sketch). Since the roof’s metal panels are classified as a flexible diaphragm, should the seismic load be distributed based on tributary area, with each sidewall bracing system responsible for half of the total seismic load? Or should each sidewall resist the full load applied to it, meaning the bracing system on the front sidewall would carry more load due to the 50 psf brick veneer?

 

[Harbringer80]

For longitudinal seismic the diaphragm will not see the load from the long walls, therefore, you would add the diaphragm reaction to the seismic from the respective long wall in your brace design for each wall line.

For transverse seismic diaphragm will distribute the seismic load due to the long walls to the short walls by tributary area.

 

[Celt83]

In my opinion distribute based on tributary area but also ensure that your applied loading is distributed according to the masses.

If the seismic story shear is 10 kips but 2/3 of the mass is all on the front wall then 10 kips (2/3) = 6.67 kips would be directly applied at that lumped mass at the front wall. The story load application should mimic the story mass distribution, text books do a very poor job teaching this and almost always show the load applied at the center of the diaphragm which only holds true for uniform mass distribution on the floor plate.

 

[Once20036]

I think of these diaphragms as being a simple span beam.
Distributed loads are based on tributary area, but any point loads will be distributed to the lateral supports based on their specific locations.

 

[KootK]

I assume that we're talking about the following:

1) Seismic load originating in the walls themselves.

2) Seismic load parallel to the walls being considered.

3) How [1] gets distributed to the walls.

For this set of assumptions I would say that:

a) For flexible diaphragms, the seismic load originating from each wall stays in that wall.

b) For rigid diaphragms, I would most often still say that the load originating from each wall stays in that wall. I can envision some cases where this would not be strictly true but those situations would be pretty rare. And even rarer in the context of a very simple low rise structure like a PEMB.