Sizing footing "Rule of Thumb"

[kk88818]

I need to size some footings for preliminary estimate purpose? What is the best way to go about this. I have seen some engineers use the tributary area to come up with a approximate footing size. Uplift due to seismic, wind, etc. was estimated at 30 #/SF and the dead load was estimated at 10 #/SF. The resultant is, thus, 20 #/SF upwards. Say, I have a tributary area of 90 ft x 40 ft = 3600 SF. Therefor, my uplift, based on this rule of thumb, is 3600 SF * 20 #/SF = 72 Kips. For good measure, a FS of 1.5 is used, so I need to design my footing for 72 Kips * 1.5 = 108 Kips.

Thus, a footing size of 19 ft x 19 ft x 2ft would be estimated. ( Weight = 19 ft * 19 ft * 2 ft deep * 0.15 Kips/CF = 108.3 Kips)

What is the rationale for this? Did I get my numbers right? Thank you so much.

kk88818
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[fattdad]

You can also bury the footing below the ground surface and use the soil atop the footing as counterweight to uplift.

I'm not a structural engineer, but your loading rational seems reasonable to me.

f-d

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[SlideRuleEra]

You don't mention the load that the footing supports when there is no wind (typically dead load + live load times a safety factor). I would use this value, along with an allowable soil bearing pressure to determine the footing's minimum horizontal dimensions (Is this where your 19' x 19' came from?).

IMHO, a two foot thickness for a footing 19' square is thin. Of course you can use plenty of rebar to make it strong enough, but if the footing has to be 19' square to support a "downward" load, it should be rigid to minimize deflection under load. Minimal deflection helps to distribute the load evenly over all the soil, not just near the center of the footing. The easiest way to make a footing rigid is to make it thick (I would say 4' thick or so if it has to be 19' square).

After taking these steps, then you can address uplift.

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[aeoliantexan]

A 19-foot square footing is unusually large for a lightweight building. You need to estimate the downward load and make some assumption about the soil. If the column supports only the roof, the downward load might be 100 to 150 kips depending on the local live loads. Stiff clay is good for allowable bearing pressures of 2 to 4 ksf. If the soil is not good for at least 1 ksf, the site probably needs improvement. I would assume 2 ksf and state so. The footing needed for downward load is then about 7.5 to 8.5 feet square, much more reasonable.

You can use the weight of the footing and overlying soil within a 1h:2v slope to resist uplift. Assume a deep water table and a soil weight of about 120 pcf. If your frost depth is say 4 feet, the 7.5 foot square footing bearing at 4 feet can resist about 45 kips of factored uplift. That is not nearly enough, so you need to make the footing bigger or bear it deeper. I would try to keep the depth reasonable, say 8 or ten feet maximum. After sizing the footing, calculate its thickness and estimate cost. The uplift is driving the design, so you may want to consider drilled shafts instead of footings.

This would be my approach. Other practitioners will differ somewhat, especially depending on local construction practices and soil conditions.

 

[msquared48]

Seems very large, depending on the building size. Any chance of taking the uplift with a grade beam system instead to use some leverage here?

Mike McCann
MMC Engineering

 

[InDepth]

Make your life Simple. Use the CRSI Handbook - They have load tables with steel weight and concrete estimates. Of course, you'll need to add in a bit for seismic and/or wind.

 

[kk88818]

Thank you so much for all the responses, everyone. I must clarify, and I am sorry I did not, that I am doing a preliminary sizing for a footing supporting one or two columns of a pre-engineered metal building, for instance. It is not for a huge mat foundation, or anything like that.

Anyway, I was trying to size the footing, and there will be quite a number of them, based uplift from seismic and wind, minus the dead load. I am trying to understand this "rule of thumb".

Does anyone know where the 30 #/SF for uplift of seismic and wind, and the dead load of 10 #/SF come from?

BTW, the example and numbers I used are for illustrative purposes only, and not to be taken literally.

Thank you so much, and let me hear your hypotheses or theories, or if you know exactly where those loading numbers come from.

kk88818
"When I can help, I do;
When I need help, I ask;
Sharing helps everyone
Understand so much better."