Foundation Reinforcing Details 4

[macan1]

I have a concrete foundation supporting concrete column.
Size of foundation is 3000 mm x 3000 mm x 600 mm thick.
There are bottom and top reinforcing for the foundation.
At the vertical face (perimeter) of the foundation, I would put vertical rebars 10M @300 mm spacing with horizontal hook at each end. Also I would put one horizontal 10M bars all around at mid-height.
Reason for this is to prevent concrete cracking/splitting.
Question: Are these extra reinforcing necessary? Is there any code related requirements for this?

 

[slickdeals]

No, and I don't think it is code mandated. You would not need it for a footing 600mm thick.

Why is there top reinforcing in the foundation?

 

[ToadJones]

I think the vertical bars around perimeter would be unnecessary. I have never done that or seen it.

Slick, I've often wondered why so many put top reinforcing in where it seems unnecessary.
I understand that some times it is necessary if the footing has shears and moments that can cause uplift on one side of the footing and thus the soil/overburden wt. causing some tension on the top side of the footing, but I have seen in a lot in footings that I wouldnt think require it.

I admit, I have had a couple very large footings over the years where I put top reinforcing in where it may not have been necessary. I was no where near over-reinforcing it so I just put it in.

I'd love to hear some thoughts from others.

 

[macan1]

Top reinforcing is for uplift forces on the foundation.
The question is about the perimeter reinforcing for crack control, is there any code related requirements?

 

[rowingengineer]

doing a lot of wind farm footings, I put top steel in and shear reo to boot.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[Teguci]

OK, if the footing were to fail in a horizontal split, we'd be talking about uplift combined with significant shear loads (can't think of any other load combo that would excite me). Since you have uplift, the column anchors extend to the bottom of the footing. At this point, I can't think of any justification for vertical bars around the exterior of the footing. If you needed shear reinforcement, it would be placed where the shear forces are highest. If you need additional flexural reinforcement, you'd place it where it would be the most effective (the bottom or top extreme). I've had vertical bars placed in my footings for shear friction, but that was because the contractor couldn't prevent a horizontal cold joint in the footing.

If I saw a footing detailed the way you described, I'd get my red pen out.

 

[hokie66]

Sometimes, hooks are required on the footing bars for adequate development. If both bottom and top bars have hooks, it also assists in supporting the top bars.

 

[DST148]

@macan1: The question you raised in your OP has already been answered by the above responses - there is no codal requirement for reinforcing on the vertical sides of the footing. However,.....
As per ACI 318, for relatively deep flexural members(depth, h > 900mm) longitudinal skin reinforcement shall be uniformly distributed along both side faces for a distance of h/2 from the tension face to control cracking in the web. Without skin reinforcing, the width of the cracks may exceed the crack widths at the level of flexural tension reinforcement.
For discussion purposes, let us assume an usual case of a narrow footing of 1200(W) x 3500(L) x 1200mm(D) depth. Unlike pad footings, this footing would behave as a flexural member in one direction only. Now as per the codal requirement, skin reinforcing is required on the longitudinal sides of the footing. A similar condition exists in flexural members like grade beams, strap beams etc., which are more than 900 mm thick. However, skin reinforcing in such situations is generally not provided since the members are buried in the ground. One may exercise caution, though, if the members are below the water table.

 

[shobroco]

I can't say that I have ever designed a 3000X3000 footing that had to be 600 thick, because if the bearing capacity of the soil only called for 3X3m the bending in the footing from the soil resistance sure wouldn't require 600mm to handle it. I can't imagine what kind of load calculations you are doing to come up with loads that your soil will support but that need skin reinforcing in your footing. I'd say ask someone to check your numbers.

 

[slickdeals]

Since he is talking about top reinforcement, I would assume he is needing the weight to hold the footing down. This is where the 0.6 *killer* factor hurts. I have designed enough of these large footings for one story warehouses where wind uplift controlled.

Would be good to get some more information from the OP.

 

[ToadJones]

A 10'x10' footing 2' thick is pretty common in the places I work.

 

[shobroco]

It might be common, but is that just to reduce the bending in the footing & substitute concrete cost for rebar cost? It might be more reliable to be able to see the thick footing afterwards & not be sure if the reinforcing was installed than to hope the reinforcing you specified is there, but it won't be cheaper. I have trouble imagining a soil with compressibility and bearing capacity that produces bending stresses this high in this size of footing. Footing thickness has to be one of the most commonly oversized items in any building.

 

[ToadJones]

Factor in large footing moments, uplift and vertical load etc...

I have done may jobs where a 10'x10'x2'(~ 7 yards of concrete) footing was the smallest footing on the job. I don't know what industry you work in, but the buildings I design I have had mud mats that took 7 yards.

Call me what you will, but concrete is cheap and foundation upgrades are expensive if not impossible....see my thread on existing footings...

http://www.eng-tips.com/viewthread.cfm?qid=315201

 

[concretemasonry]

Toad -

You are right about the cheap cost of "dumped" concrete in pours of 10 to 100 yards because there is minimal forming and little finishing. It does not always fit into the text book estimating costs unless it is a large project where ther are other factors.

Dick

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