Brace Footings

[SteelPE]

I have a building that is concentrically braced with some large uplift loads. The building is not very large only 90’x40’x56’ (LxWxH). The large loads are due to narrow bracing limits (13’-6” wide).

To resist the overturning forces of the brace, I will place large footing under the brace that will satisfy the code overturning requirements (essentially mass concrete). Does anyone else have a better way of handling this problem? My way seems really inefficient.

 

[msquared48]

A concrete grade beam(s) extending to other column footings to pick up their dead loads might be the better solution.

Mike McCann
MMC Engineering

 

[kslee1000]

Mike's idea shall work.
Would continuous footing helps (along the 40' edges)?

 

[SteelPE]

Thank you for your responses.

I am leaning towards msquared48’s grade beam idea. I have to play with the column locations to make it work. I had previously thought I could play around with the steel detailing to reduce the seismic loads on the building (use a SCBF R=6 instead of R=3) but once I did this wind quickly became the controlling factor. Actually the wind overturning is as much as the seismic. For most I my buildings seismic is the controlling lateral load.

My existing braces are 10’ in from the end walls of the building (buried in the stair openings). So the end wall grade beams/frost walls are of no help. In one location I am near the elevator so I have to drop the footing another 4’ (to the top) to get the footing in.

I just can’t believe that I need a 40’ long grade beam/footing to stabilize the footing against overturning. Seemed inefficient to me.

 

[civilperson]

The uplift is not your problem, horizontal loads are the controlling load case.

 

[msquared48]

It's a trade off of the cost of the materials and labor for the footing verses the grade beam.

Mike McCann
MMC Engineering

 

[kslee1000]

Looks like the brace is jammed in a busy area (with so many geometry changes/constraints). Well, if you don't mind, a plan view might attract more (to-the-point) better/easier solutions.

 

[SteelPE]

Civilperson, overturning is my problem, which is produced from the horizontal loads. I was figuring if I could reduce the lateral loads by increasing the R then I would be all set, then wind became the controlling factor. There goes that idea.

Attached is a quick sketch of the plan.

 

[kslee1000]

You may be able to resist the uplift by providing grade beams in between 6 columns in the NE corner & SE corner of the building (I assume there are two N-S column rows aligned with the columns at the end of the horiz. braces). Depending on how the slab is to be attached, you might be able to utilize it too.

The elevator shaft possess some geometric challenges, can you shift it a few feet north?

 

[prsconsultant]

An easier & economical way to deal with uplift is dropping the spread footing. This way you can count on the weight of soil (go at 45 degree angle from the edge of footing) and the slab on grade. This along with a grade beam (if needed to tranfer horizontal shear) may solve your problem.

Another alternate would be to look at adding some mini piles, tension anchors etc. It would not be a bad idea to check with the geotech engineer for local practices and suggestions.