New Rooftop Equipment - SDC D

[Once20036]

We're in the early stages of looking at an existing facility that requires new rooftop equipment. Unfortunately, its heavy equipment in a heavy seismic zone.

For gravity loads - the existing structure cannot carry the weight and reinforcing isn't practical. We're planning on dropping four new legs down to the slab or new foundations to carry the weight.

For the lateral load, we discussed this in house all last week and couldn't reach a consensus. I`m posting here, hoping for more opinions.
The first option is to design the entire frame as an independent structure. I expect we'd need a concentric braced frame on all four sides. This is technically feasible, but running all these braces down to the ground isn't great for the use of that floor space. Also, the detailing is ugly trying to get the braces through the roof and waterproof. In the past we've done a braced frame below the roof and a second above the roof, but again, it's ugly.

The second option would be to utilize the existing roof diaphragm for lateral stability. The legs would extend unbraced from the floor to the bottom of the roof deck. We'd find a way to tie this in laterally (tbd), and then provide a braced from the roof deck to the new steel platform. I suspect that the increase in lateral force to the diaphragm would be minimal, given the size of the building. In SDC-D, is this a viable setup? Which R value would be most appropriate, assuming a concentric braced frame from the roof to the equipment? Are there any special provisions triggered with a setup like this?

As always, thanks.

 

[WARose]

See Table Table 15.4-1 in ASCE 7-10 (i.e. Seismic Coefficients for Nonbuilding Structures Similar to Buildings). You are talking an R value of 3.25. The height limitations (with permitted increases) are spelled out there.

To tie into the existing diaphragm for lateral stability seems like a pretty good idea. As a sanity check, you may want to do a modal/dynamic analysis of the frame (with the restraint provided by the building modeled) to back check the value from your ELF procedure.

In addition: keep in mind the new frame will have to accommodate the movements of the building during a lateral event. So I would have a good idea as to how much the movement of the two structures would be and be able to accommodate the forces that develop between them during such movements.

EDIT: If you are worried about selecting the appropriate R value for such a case (since your system won't be exactly anything in the ASCE tables).....you could take all the guess work out of it by selecting a R value for systems not detailed for seismic resistance (or "All other self-supporting structures, tanks, or vessels not covered above or by reference standards that are similar to buildings" in Table 15.4-1).

 

[KootK]

Certainly, tying into the existing diaphragm would seem to be the more elegant solution. Additionally, it relieves you of having to either isolate the seismic motions of the building and support frame from one another or somehow ensuring that they are compatible. While the impact to the diaphragm may be minimal in the macroscopic sense, I'm sure that you'll be working it pretty hard locally with the collector elements that would be required to get the equipment loads into the deck.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.