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

 

[rrmiv]

One thing to make sure of is your safety factor. If your DL has the 0.6 factor applied for allowable stress design, in my opinion, this includes the safety factor for uplift. So, you wouldn't need to multiply by that again. Also, check your uplift number - 30 psf is pretty high, depending on where it is.

You can also count on the weight of the soil and slab on grade, etc. above the footing that would be picked up by it. And make sure your base plate and anchor rods can pick up the footing. Otherwise your numbers seem accurate to me.

 

[DavidStructural08]

I do not know if these rule of thumb numbers are correct, but I am wondering what is the definition of "tributary area".

1, To be specific, I think this "tributary area" is different for dead load and lateral induced uplift load. You should have a much smaller one for dead load and a much larger one for lateral load. In other words, you may under-estiamte your ftg size for uplift since you over-count dead load while under-estmate lateral one;

2, you also need to estimate the ftg size for downwrad load which includeds D + L + E (W). Here the lateral ones are compression of the same value as uplift. Of course you can always use the combination factors as in IBC.

Only my 2 cents

 

[rrmiv]

DavidStructural08, I'm not sure what you mean. The tributary area getting to a column footing would be the same for DL or uplift.

One thing that must be considered though is vertical, downward DL vs. "real DL" used to resist uplift. You may choose to use a heavier DL for the gravity design of roof members, but not use all of that DL in the resistance of uplift forces. However the area that contributes to this design is the same.

One thing I don't understand in ASCE7 is the limit of 100 sf for the Effective Wind Area in finding the value of GCp (Figure 6-11B). Seems like there should be a reduction for the uplift of a column footing as compared to the uplift for one steel joist. An analogy would be the floor LL reduction for a column vs. the LL reduction for one filler beam. Does anyone know the rationale behind this limit?

 

[DavidStructural08]

rrmiv,

Well, I do not think the tributary area is the same for DL and lateral induced uplift for a braced frame ftg.

Assume only braced frames are used, then one ftg in one frame will take an uplift load from a tributary area the same frame will be exposed to. However, the tributary area for dead load on this ftg is whatever the column will take, which is much smaller than the first one.

Say for example, you have 6 bays @ 30' with two braced frames at center (30') and the transverse dimension in the braced frame direction is all 60' with 15' each edge.

You have a tributary area for dead load on one footing is 30' x 22.5'.

Well, one braced frame would have to take a lateral load from a tributary area of half the building 90' x 60' and uplift load of 90' x 60' as well.

 

[rrmiv]

DavidStructural08, perhaps I misunderstood your initial response, but I think the original question from kk88818 was about direct uplift, not uplift due to a lateral element.

 

[DavidStructural08]

rrmiv:

"Uplift due to seismic, wind, etc. " in original post.

Just curisou. What is the direct uplift due to seismic?

 

[rrmiv]

I noticed that also, assuming it was poor wording. What say you, kk88818? Please clarify.

 

[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."

 

[JLNJ]

It seems simple enough that you don't need a "rule of thumb". A few actual calcs should do the trick.

You might consider a deeper footing rather than one 20' square!

 

[mrpid]

kk88818,

To review you first post wit regard to a "rule of thumb" I would use as rrmiv suggested 0.6DL + 1.0WL for uplift and 1.0DL+0.75LL+.75WL for bearing calculations (although a very quick inspection will probably show that uplift will govern, unless you have extremely pour allowable bearing capacties). When you consider an uplift FOS of 1.5 the generic numbers you calculated actually would fail with regard to load combinations req'd. i.e. First the 72 kips is low in the it would be 3600*(30psf - 0.6(10psf))=86.4kips Then the footing size based on 2' thick footing would be (86.4kips/(0.6*0.15*2))^(1/2)= 21.9ft each direction. (assuming you have not "burried" the foundation to utilize the soil above as additional DL)

Now considering your second post and a PEMB, you have probably been asked to design and/or do some take-offs for a building that has not been purchased otherwise you would have the reactions. One method to determine the reactions preliminarily is to actually calculate the wind loads yourself per the code. This is not difficult and can be done quickly by hand or use a simple 3-D model program. Also the 10 psf seems a little heavy for a PEMB as they typically are reducing the weight of the building as much as possible (I would not use anything higher than 4psf...2 1/2 for building steel + 1 1/2 for roofing materials). And since uplift will be probably govern a smaller Dead Load would be the conservation approach.

Another way to guestimate the reactions, which I use regularly for preliminary work, is to use a catlog from a building supplier. You can get them free from suppliers (or at least used to be able to, and maybe can be found on teh internet). I have a catalog from CECO Buildings (2003) that give reactions for standard configurations, bay spacing, and spans with factor to use for atypical loading scenerios. These loads are good for preliminary design/take-offs, but would need to be verified once the actual reactions are available. The catalog also give some simple formulas for calculating reactions at end wall columns as well.

Another thing to consider is the horizontal forces at the base as they will have to be resolved as well.

Regards,
Mr Pid!