Lateral analysis - wind and seismic load effects on shear walls

[Roukkia]

When doing a lateral analysis for a single story, rectangular shear wall building, I have typically seen it done where you first determine whether wind or seismic 'controls'. For seismic I have typically seen half the wall height of the walls perpendicular to the load direction taken into consideration. With the seismic load due to the shear wall weight being taken into consideration when designing the shear wall.

My question to you is this: If wind controls the design, do you check to see if the load effects due to:
1) seismic diaphragm load at a single shear wall
PLUS
2) the seismic load due to the wall weight

are greater than the load effects from wind?

I have provided an example that displays where my concern with this is coming from, and how this could lead to increased load effects that I have not ever heard discussed. Note that in this example I am assuming there are shear walls along the entire length of the North and South walls, but am looking at only one 30ft wall for simplicity.

 

[SJBombero]

The seismic mas is the weight of the entire diaphragm + the weight of half of all the walls (interior & exterior) connected to the diaphragm. A single story seismic model would lump all that mass at it own CG. For simplicity though we usually just assign the mass at the elevation of the diaphragm connection to the wall. Most of the time it is conservative. For sw overturning, you would not add the overturning of the sw to the seismic shear at the top. That would be double counting the effect of the mass of the wall.

 

[phamENG]

Also, be careful about the wind or seismic "controlling" stuff. If you're in seismic design category A or B, then you're generally pretty safe. But C or higher and you have to be mindful of certain prescriptive requirements and additional load factors. For instance, your total wind load on the building may be higher than the total seismic, but the overstrength requirements for certain connections or redundancy factor requirements may push certain items over the limit so the shear wall nailing for wood or reinforcing for concrete is designed for your wind load, but the anchorage force is designed for your seismic overstrength.

 

[Roukkia]

SJ - Thanks for your response. I think this is the route I will go from now on in determining my seismic load. I've seen the 'only include perpendicular walls' approach promoted in other threads on this site, so I wasn't sure if I was missing something.

pham - Definitely something I will keep in mind. We typically work in SDC B so we don't have to venture into the SDC C+ requirements too often.