Wind and Seismic Loads on a Gas Flare 2

[Anna611]

It has been quite a few years since I've done foundation design, I usually quit right after I come up with the soil's bearing capacity. I need to determine what size foundation I will need to overcome wind and seismic loads on a gas flare.

Any help on which equations I need would be appreciated as my foundation's book seems to have lost itself in the last move.

Assumptions
wind load 110 mph
seismic zone 4
shear at base 17.0 kip
moment @ base 480 kip-ft
deadload 43.0 kip
diameter of flare 11 feet
height of flare 50 feet
soil's bearing capacity 2000psf

These are the variables I have. I appreciate any help I can get on this one.

Thanks!

 

[civilperson]

For a spread footing, (probably not the most cost effective): q = (P/A + or - Mc/I). Which load case gives the biggest moment at the base, wind or seismic? Using LC 7, 0.6D + W + H or LC 8, 0.6D + 0.7E + H, calculate P and then iterate with different sizes of foundation to find maximum and minimum soil loads. Use of drilled shaft or piles will allow some tension capacity and be more cost effective.

 

[tchiggins]

You question is a little vague- without knowing exactly which area you're unfamiliar with, this is a basic list of design functions to step through after you've calculated the loads to the foundation:
1. Stability - (service load combinations)
a. Overturning
b. Sliding
c. Bearing (as civilperson touched on)
2. Concrete design - (factored load combinations)
a. 1-way shear (.75x2xbxdxsqrt(fc'))
b. 2-way shear -
c. bending

Judging by the broadness of your question, this is probably bearly getting you started or maybe the general procedure you are already aware of. If that's the case or not, bring on more questions.

Civilperson makes a good point about drilled shaft foundations. These are usually the norm for tall vessels and stacks. However with 11'dia x 50'ht., you straddle the fence (in my opinion). Something relavant to your design is frost depth, and if you're including a pedestal on top of a spread footing (or pier cap if drilled shaft). Pedestal shapes can be square or octagon if desired.

Commonly on tall vessels and stacks, the shear from seismic is applied at 2/3xht. to arrive at the overturning moment. This can be due to top heaviness from items such as nozzles and assoc. piping/valves, or flare tips and platforms.

 

[Anna611]

Good points. I don't need any concrete or rebar design help since I have an example set of equations for a gas flare for that portion. I specifically need help with the overturning portion (I didn't do so hot in statics with moments).

A deep foundation is out due to equipment and materials readily available. I agree that it if it was much taller, piles would be the way to go.

My current plan (please correct me if it is a bad idea) is to put a ringwall type footer in to 6" below frost depth then place a mat footing over it. General practice seems to be that a 16'x16' or larger concrete mat is used.

Thank you for your help so far.

 

[Anna611]

What FS is typically used for overturning when it is a non critical structure?

 

[Anna611]

I just wanted to let you all know that an old college friend called and got me straightened out. Thank you for your help!

 

[swearingen]

It was my understanding that any part of the U.S. that used to be in a seismic Zone 4 doesn't use zones any more. Is this in the U.S.? If so, I believe you're working with an outdated code.



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