Scott,
Sorry for the late response. I haven't been here much lately. EMT is not sufficient under any circumstance for seismic support. The calculations are pretty complicated too. We are always responsible for the design by spec and use either ISAT through Tommarco, Mason Industries or power strut. They do the seismic design based on our rack layout and conduit quantity and sizes. Then they supply kits for each required seismic location. I can give you contact names for future reference if you would like.
We try and stay with cable bracing as it is cheaper than strut and can be used when the vertical rack length is more than 10ft. We are in a spot right now on the Orange County Water District job I am doing where cable bracing is not allowed and we have massive amounts of cable tray with vertical rod lengths of up to 15ft. Strut is only good to about 14ft on the angled bracing so we are looking at having to install tube steel bracing. It gets really expensive in situations like this. This is also a NEMA 4 area so the tube steel will probably have to be 316 stainless.
I have a couple pictures of an impressive three tiered seismically braced cable tray installation that seems to be on my work computer. I send it in Monday to be added in. It had to be hung with 3/4" rod (they originally wanted 7/8" rod)and each tier is braced with double strut.
When designing seismic bracing you have to take into account things like rack size IE: conduit count, conduit size cable tray weight per foot rack width and max load distance from support rods. Conduit weights are always calculated at max fill. You take into account rack spacing, rod length, weather you are using cable or strut. In the calcs the rack hight above grade is a factor. If you are running on the sixth floor of a building it is going to react differently than on the ground floor. I have even seen a factor for the soil condition the building is built on and most engineers want a safety factor built in on top as well.
I understand the basics of seismic bracing but am in now way qualified to officially do the calculations. The always have to be stamped by either a civil or structural engineer anyway so it always gets sent out for design. Now, if you are dealing with any hospital work where OSHPOD is involved, well that is a whole new thread!
I am going to send you an excel spread sheet one of my project managers made that I use to help in rack design and layout. It is intended to help get us going while the seismic is being designed. It helps us make sure our rod size is sufficient and that we are not overloading the strut. It also lets us know if the seismic locations are going to require vertical rod bracing among other things. I can't help designing a bracing system per California regulations but I can connect you with people that can!
