Large quantities of rebar in isolated footing 1

[Thoughtclaw]

I'm fairly new to footing design, and I'm designing the column footings for a pre-engineered steel building. I have just finished calculating what would be needed for an interior column, using the procedure outlined in the "SE Structural Engineering Reference Manual" by Alan Williams. It takes an axial load of 83.47 kips, and I have determined it requires a 7.5' by 7' footing. The rebar required is much higher than I expected: I've got 11 #14 bars in the 7.5' direction and 10 #14s in the 7' direction. I've checked and re-checked my numbers, but that's what I'm coming up with. Does that seem reasonable to you folks?

As always, appreciate your time and effort.

 

[Rabbit12]

That does not seem right.

 

[BAretired]

Sounds high to me. That is a very low bearing pressure.

Looking at an old Reinforced Concrete Design Handbook, the lowest soil pressure considered was 2000 psf. For a column load of 100 kips, 12"x12" column, 7'-5"x7'-5"x14" footing the reinforcement recommended was 14-#5 each way.

BA

 

[JAE]

Agree with Rabbit12.
How thick is the footing?


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[WinelandV]

How thick is your 7.5' x 7' mat? Is there any uplift on this interior column?

Using 2000psf allowable bearing, I'm getting a quick and dirty bottom steel amount of 8's @ 12", for a d = 8" (running parallel to the 7.5' direction).

 

[Thoughtclaw]

It's 2' thick. I'm using 1500 psf, which is what the Ohio Building Code specifies unless you do soil borings. Uplift is 13.79 kips.

 

[Celt83]

I get about (7)#5 which lines up with CRSI.

What are the column dimensions and foundation thickness?
Is the 83.47 kips a factored or service load?
Assuming service that load and foundation dimensions yields a bearing pressure of about 1.6 ksf is the allowable bearing 2.0 ksf?

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[WARose]

Are you using the 0.0033*(Ag) min or the 4/3*(As_req'd) clause?

For footings, it typically makes a difference. (Temperature steel [0.0018*Ag] governs a lot of the time.)

 

[Celt83]

2' thick for that load you're probably going to be governed by minimum steel 0.0018*b*h = 0.0018*7.5ft * 12 in/ft * 2ft * 12 in/ft = 3.888 in2 or about (7)#7's edit: had ft/in instead of in/ft

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[WinelandV]

Thoughtclaw,

Using your numbers, it appears you footing is a touch small - 83.47 kips/ 1500psf = 55.65 sq.ft minimum. 7.5' x 7' = 52.5 sq.ft.

That being said, how are you calculating the moment in your slab? The way I learned was to apply the soil load to the footing, and treat the column point as fixed and design as a cantilever. Assuming 2000psf, and a LF = 1.6, you get (3.2ksf)*((7.5'/2)^2)/2 = 22.5 kip-ft / foot

Since the 83.47 is factored, moment becomes 11.2 kip-ft/ft - (4) #5's in the 7.5' direction.

And as Celt83 and WARose mentioned, minimum steel will likely govern.

 

[Thoughtclaw]

That's a factored load. Can I ask what procedure you guys are using? I'm obviously doing something wrong here.

 

[Thoughtclaw]

The moment equation I'm using is M_u=q_uL(B/2-c₁/2)²/2

 

[Celt83]

that's the right formula, maybe a missed decimal place or unit conversion?

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[Ingenuity]

Or you are calculating your moment over the full footing width, and not per foot width. Divide by the orthogonal footing width and you should be ok.

 

[WinelandV]

The moment equation I'm using is M_u=q_uL(B/2-c1/2)²/2

Yep, that's the right formula. I just zero'd out the column/pier dimension for fast calculation, and did mine on a per foot basis.

qu = 83.47k/7.5'/7.0' = 1.59 ksf

L= 7'

B/2 = 7.5'/2 = 3.75'

Using the above formula nets me 78.3 kip*ft (with c = 0) = 939.1 kip*in

What are you getting for a moment?

 

[Thoughtclaw]

Oh, I see what I did. I failed to do it on a per-foot basis. Well, I'm embarrassed! … But I'd rather be embarrassed and have the answer I need.

Thanks so much, everybody!

 

[BAretired]

If there are many of these footings, a soil report might save the owner a few bucks.

BA

 

[Thoughtclaw]

The architect told the owner that, but I guess he figured he'd take his chances.

 

[JAE]

The architect told the owner that, but I guess he figured he'd pay extra money for nothing

Fixed it for you.

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[Ron247]

In my area, uplift tends to control footing weight more than gravity loads. I generally design interior columns for uplift first and then check gravity. I use working loads and my actual soil bearing under gravity loads tends to be 800 to 900 psf for interior columns. I do not know whether all loads you listed are factored or not but double check your FOS for uplift. I tend to use 1.5 for uplift.

If uplift is controlling footing size, soil bearing will be less of any issue on interior columns but may be more needed on exterior columns since they have overturning.

Isn't the 1500 allowed in your code an allowed and not factored value?