Uplift design for x braced frame
Uplift design for x braced frame
(OP)
I am designing a commercial building that is 287 ft x 62 ft with CMU on three (2 short, 1 long) sides. Its one story building with metal roof deck. Due to the nature of the job I can only use an additional lateral force resisting system at one location (162 ft from the left end). I have about 500 plf wind load at the diaphragm level. Assuming flexible diaphragm, the load on the central lateral force resisting system (x-braced frame) would be around 70 kips. Neglecting the compression brace, the uplift therefore would be 70 kips too. I plan to use a giant base plate and 8 bolts to resist the uplift. But apparently, this is very high force and this could actually lift the pier cap or the pier itself.
How do I solve this problem? Please suggest your views.
One way would be to have two x-braced frames. Still the uplift would be 35 kips and the pier will not be able to handle this level of uplift.
How do I solve this problem? Please suggest your views.
One way would be to have two x-braced frames. Still the uplift would be 35 kips and the pier will not be able to handle this level of uplift.






RE: Uplift design for x braced frame
RE: Uplift design for x braced frame
RE: Uplift design for x braced frame
500×62=31000. How does that become 70,000?
I also feel that the steel deck will perform somewhere between flexible and rigid And therefore your 287 foot long cmu wall will take the majority of the load.
By just my thoughts. I might be in the minority.
RE: Uplift design for x braced frame
However my second point still stands. Your cmu walls will take quite a bit more than a straight up flexible distribution.
RE: Uplift design for x braced frame
1) Run a deep grade beam across the short dimension of the building, beneath the brace. This should increase the lever arm and engage additional dead load.
2) Scrap the central brace altogether. Your diaphragm aspect ratio would still only be 4.63:1. If your code allows a diaphragm aspect ratio this high, just design the diaphragm with extra care, paying close attention to diaphragm deflection and P-delta effects. If your code doesn't allow this, forget about the steel deck and design yourself a horizontal truss for your roof diaphragm using steel strapping etc.
3) Ditch the brace and turn each transverse framing line into a moment frame using joist girders.
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.
RE: Uplift design for x braced frame
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.
RE: Uplift design for x braced frame
@engineeringeric: yes, i have space to provide 2, but we want to give client the flexibility if he wants to make openings later.
@jayrod12: i agree, but analysis start getting complicated when you want to design somewhere between flexible and rigid.
@kootk : all your suggestions are agreeable. and your signature is impressive too.
Thanks everybody.
I had meeting with my supervisor today and he said I can count on the weight of the soil to act against the uplift.
I will be using a 24/72 pier. If the pier has to lift, it should take the soil with it. We are assuming a 60 degree cone failure of the soil.
The weight of the concrete and the soil would be sufficient to counteract the uplift.
Meanwhile, i plan to reduce uplift by increasing my bay length.
Please let me know if this assumption is unreasonable.
RE: Uplift design for x braced frame
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.