Plain concrete wall footings-4 story building 1

[WWTEng]

4 story precast building w/ foot print of about 200 ftx70 ft
Seismic loads govern with SDC = B (project site is just outside Detroit so low seismic activity area). Allowable soil capacity = 4ksf.

The client has asked for an option of plain concrete footings. The typical D+L load on foundations range from 16 klf to 23 klf. Lateral loads do not result in any net uplift and the effect of downward force (distributed over a larger area based on the footing thickness) also do no result in forces larger then D+L.

The typ exterior footing size I am using is 3'Dx6'-6" wide. I am checking factored moment and shear at the center line of the footing (conservative) using chap 22 of ACI-318. Vc = 4/3xsqrt f'c and Mn = 5 x sqrt f'c x S. Phi = 0.55. I am neglecting 6" of footing thickness for my S calc.

The numbers seem to work for the given loads and even though the footing is designed as plain, I am going to place 3 #5s T&B and also #5 vert dowels @ 48" oc.

I haven't designed a plain footing for a structure this magnitude so I wanted to get a second opinion if I was missing anything. From what I can understand from IBC-06 & ACI 318-05, I can use a plain wall footing for a structure in SDC-B.

 

[hokie66]

I have nothing against plain concrete footings under concrete walls, but for a footing that wide, I would want some transverse reinforcement. With a reinforced footing, you should be able to reduce the depth of the footing.

 

[concretemasonry]

Hokie is right about transverse steel for a footing that wide. Under some codes (at least for residential) longitudinal steel is not required or is minimal.

For a masonry wall or a poured in place wall system the wall becomes a part of the structural system because the masonry wall or concrete wall is bonded and creates a deep beam that is far stiffer than the footing.

Since you are dealing with precast walls, you have the traditional and recurring problem of connection and load distribution to the footings.

The important factor is if your connection between the wall and footing will allow the interaction that you get with site connections.

Dick




Engineer and international traveler interested in construction techniques, problems and proper design.

 

[WWTEng]

hokie, CM

Thanks for that reminder. I actually was going to put #5 @ 16" oc horz only along the bottom of the footing.

 

[WWTEng]

Also, the reason I want a thicker footing is not because of shear or flexure, I want to be able to distribute the downward point load from the preacast panel under lateral load over a larger area. So for 3' thick footing I use 3'+1'+3' of length of distribution.

 

[spats]

I don't understand. If the owner wants plain concrete footings, why are you adding reinforcing? What is wrong with designing per Chapter 22, and not adding steel?

 

[hokie66]

What I don't understand is why the owner should be interfering with the design of the footings. Don't they usually want the most economical solution?

 

[WWTEng]

hokie,

Its a project funded by the state. Our understanding is that someone at the state is trying to justify their salary.

 

[WWTEng]

In any case, we intend to give them both options: reinforced footings and not-so-reinforced footings.

 

[JAE]

I think you should go either PLAIN or REINFORCED and not mix the two as this simply makes no sense. The owner apparently thinks there is savings in a plain footing - and there might be depending on the relative costs of raw concrete vs. rebar and labor to set rebar.

If you have a 36" deep footing, and the numbers work, transverse rebar isn't required - so why negate the advantage of a plain footing by adding rebar?

If you are going to add rebar to "feel good" about the design - design the thing as a reinforced footing and reduce the thickness per hokie66's suggestion above.

 

[Teguci]

Well if its a state project, I suggest providing a study to examine the two options. We wouldn't want the taxpayers to pay for a more expensive option. Have they considered timber piles? You might consider a precast footing in the study as well. Geopiers might also reduce the size of the footing. If you need help with the study let me know!

Detroit? Hey maybe the Ambassador Bridge owners can help you out now that they are out of jail. They seem to have some political ties.

PS - please read this sarcastically.

 

[WWTEng]

JAE,

Thanks. The client has asked for Plain concrete and thats what we am going to provide. And indeed the rebar I mentioned is a feel-good hunch but the footing with this reinf is no where near a reinf design. Also, the only bar they'll have to set before the pour is the bottom rebar. I have seen them "muck in" the top rebar and dowels. Plus I think the cost of rebar that I am adding is minimal compared to the overall structure.

But now I have another question. ACI 318 section 1909.3 talks about contraction/isolation joints. Does this apply to trench footings too?

 

[WWTEng]

Teguci,

They were give a preliminary design with plain and reif options. The opted for plain.

 

[Teguci]

WWTEng,

One thing worse than someone spending their money foolishly is someone spending other people's money foolishly. I just had to spout off.

For a smaller footing, plain might make sense, but a 6'-6" wide strip with excavation costs for the added depth, I can't help but feel sorry for people in Michigan (even with their voting record).

 

[dhengr]

Teguci:
They are trying to save the steel for future cars they’ll never build, with the same thinking.

I think you meant ‘please read this as a bit of my sarcasm.’ I tried to ‘read it sarcastically’ and all I could find was honesty and truth and sometimes that hurts those deserving of it.