Uplift design for x braced frame

[kushal2944]

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.

 

[hokie66]

Additional mass (big concrete footing), deep foundation (possibly bored pile), or some type tension anchor would be options.

 

[EngineeringEric]

I would think having two would be better than one giant one. But you are going to need a massive foundation or some deep piles. Using a grouping of helicals or some other pile seems needed. Cannot say i have designed a foundation for 70kips, sounds fun.

 

[jayrod12]

I have a hard time believing that you have 70 Kips on the brace when you've got a total of 31 Kips at the eave.

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.

 

[jayrod12]

Disregard my previous post. I missed the fact you're talking about splitting the 287 foot dimension in two.

However my second point still stands. Your cmu walls will take quite a bit more than a straight up flexible distribution.

 

[KootK]

Three more possibilities, depending on how far you're willing to drift from the original concept:

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.

 

[KootK]

4) Use the shear corresponding to OT as your upper limit of capacity in the brace and assume that, once you reach that limit, the remainder of the load will split between the two end walLs.

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.

 

[kushal2944]

@hokie66 : yes additional mass is a good way.

@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.

 

[KootK]

Definitely reasonable. In retrospect, I'm surprised that we neglected to mention soil engagement. Our bad.

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.