Min Area of Steel for Spread Footing 4

[ToadJones]

For minimum steel in spread footings I refer to section 10.5.4 of ACI.
That section refers to 7.12.2.1 which states that for Grade 60 bars rho = 0.0018.

Is this what others are using for footing design minimums?

I am checking a reinforcing design for a footing that is mostly sized for overturning/stability and the design moment for flxure is very small. So, I am left checking for minimum steel.

"d" for the footing is 33".
I get a min area of steel = 0.0018 x 33" x 12" = 0.71 in^2/ ft of width.
This would require a #8 bar 12" o.c.
the footing has #5 bars 12" o.c.

Is there a lesser AS min I can use for this case since flexure really is of no concern?

 

[TXStructural]

Don't lose faith in building codes. They are complicated for reasons, even if they may have strayed from the simple formula of "require only the minimum." Many complications can be avoided by simply going with the conservative solution and simple design. As with any design, if you get bogged down in the irrelevant minutia, you may never get a working design completed. Things like lap length can be simplified by using, for example, Class B, top bar Ld for the weakest concrete on the project. Alternatively, you can complicate/optimize lap splices by applying the available factors.

Minimum reinforcement provisions discussed here are not really all that complicated, it's just the layout and location of the provisions. JAE pointed out that the minimum applies to tensile reinforcement, and while it appears in the code, that's based on the principle that if you need flexural reinforcement, you probably need some minimum amount to make it effective. Rather than get into the mechanics of what happens when concrete cracks under tension, the code simply says "put in this minimum amount."

 

[dhengr]

TX:

I agree with Hokie. The ACI318-63 code is the one he and I both learned to use first, and at that time everyone pretty well understood the intent, rational, and the fundamental engineering principles and materials considerations involved in the design process, we were taught that. And, the next couple editions where pretty straight forward extensions of the previous editions, given what we had learned over the intervening years. Occasionally there was a free or inexpensive addendum because a significant issue came to light. We also read the tech. literature to keep abreast of the latest research and thinking. Without the need for a full blown code revision, every few years, these new ideas, once vetted, could be applied at our (the engineer’s) discretion. It wasn’t until code writing became a cottage industry unto itself, and that publishing frequently to support that industry and to provide full employment for some, became an end in itself, that things got out of hand for the practicing engineers. For all the confusion and complexity, extra expense of new codes and standards, we are not really producing better structures, nor are old structures falling down around us for lack of this complexity having been applied.

I also agree with the type of logic expressed in your last paragraph ‘Minimum reinforcement provisions...., so put in this minimum amount.’ We should be teaching this kind of fundamental thinking and logic (common sense) to young engineers, rather than adding another 35 pages of disjointed B.S. to the code to try to cover that; one section referring to four other sections, which refer to...., etc., and then around again, to get to the same logical solution. The problem with all the codes is that their complexity, arbitrary new factors and variables, use of other codes and standards by reference, pretty much baffles any logic, common sense or engineering judgement and experience factors. We spend all our time, when trying to use the codes trying to be sure that we have applied and understand ever new phe, phi, pho & phum, and literally end up losing sight of the real engineering principles involved in the solution of the problem. I suspect many engineers will attest to this, we aren’t doing engineering any longer, we are just following an ever more complex set of cookbooks, and then debating how much a pinch of salt really is, without knowing what the hell we are actually cooking.

While codes may be “complicated for reasons,” which are not always obvious to the experienced engineer any longer, they certainly have strayed from their intended purpose in their development and that process. The best codes are the cleanest, simplest codes possible, and that should be the code writers foremost duty and focus; so, clean up “the layout and location of the provisions,” and get rid of “the irrelevant minutia.” Their job should not be complexity to assure self employment preservation, or to complicate the practicing engineer’s life, or for no real improvement in final product. Once the intent of various code sections and concepts is taught and understood, the code should become a background document referred to for truly difficult conditions. There are plenty of times a complexity crops up in one of our designs, and we should be smart enough as engineers, to know that we must dig a little deeper to solve this one to our own satisfaction and for the public safety. Then the extra complexity is appropriate and brought on by our own design. But, 90% of what we design should not have to be dragged through this swamp of crap, just to justify a simple spread footing, as in this case. We shouldn’t be working in fear of missing some obscure factor slipped into the latest code, and causing some major problems or legal action. Rather, our work should be a continuous learning and educational process to really improve the product we design, and to more fully understand what we do and why.

I certainly hope you are on the code writing committee or have some direct input, because you seem to apply some common sense to the process, and seem to understand our difficulties. Codes are not an end in themselves, they are a means to an end, and should not overly complicate the process of getting there. They ill never be made to cover every condition under every possible situation, so let’s quite trying to have them do that. That’s what engineering judgement and experience are supposed to cover. And, maybe everyone won’t be qualified to design everything and anything, without some experienced supervision, and more complex codes won’t change this situation or dynamic.

 

[ToadJones]

Well said dhengr....

I sorry to say that AISC has been traveling this same troubled path.

For shyts and giggles sometime...take a look at Chapter F in the 2005 AISC Code and try to apply it to a built up, unsymmetrical girder section with non-compact elements as often seen in mill building and turbine building crane runways.
It is a complete joke & endlessly frustrating.

Its like code writers went straight from university textbook example problems to writing codes...."look, works perfect for a W30x90!"
I believe it is section F12 that might as well say "go figure it out yourself" ....you could also replace "figure it out" with another word starting with "f".

I will not apologize for hijacking my own thread.

 

[ToadJones]

oh, and your right....my code digging was a CYA.

 

[TXStructural]

Yes, I am on several ACI committees. Not on 318, but am well-connected to that group. ACI code development committees are entirely volunteer. My employer supports the effort because of its critical interest to our industry. Many of the code committee members are academics who frequently know a great deal about a portion of the code, but possibly do not really understand how a provision will work in practice. Very few are practicing engineers, primarily because working engineers cannot take the time needed, nor have sufficient employer support for travel required.

ACI committees meet twice a year at the spring and fall conventions, possibly over a few conference calls, and infrequently, additional face-to-face meetings. Codes are generally on 6-year cycle, with 3 year minor cycle. This correlates to the International Building Code cycle. There are calls for less frequent updates, but every time they try, there are problems identified mid-cycle which require "urgent" fixing. Even now, there are advances in alternative reinforcing materials and concrete mix design which need to be addressed, at least until a performance standard is implemented.

Even if you do not join ACI and cannot participate in meetings regularly, you can get involved in the process of code development. All ANSI-accredited standards-making organizations must accept and address public comment for changes.

 

[DST148]

Hi List,
Although I have been a member for quite some time, I recently started participating.
I am referring to ACI 318-05.
The minimum reinforcement in flexural members depends on the type of member. For two-dimensional elements like slabs the provisions of Chapter 7 would apply and for one-dimensional elements like beams, Chapter 10:
Typically this is what we follow in our office:
Structural slabs, footing pads, foundation mats - section 7.12.
Pile caps - Guidelines given in CRSI Handbook
Combined footings - Longitudinal Direction Sections 10.5.1, 10.5.3
Transverse Direction - 7.12 (Distribution varies)
Strap beams in footings - Sections 10.5.1, 10.5.3
Cantilever slabs like balconies which are statically determinate and where there is no possibility of redistribution of moments, we
use sections 10.5.1 and 10.5.3.

 

[SAIL3]

I agree completely with Dhengr but do not see any hope that the practicing engineers would ever be able to bring about any meaningful change in this money-making code industry that has taken root.
We as engineers just don't have the capability/mentality to bring about this needed change.
So we are left to whine or whimper and feel on some primitive level that we have accomplished something.

 

[dhengr]

Sail3:

One small way that we, as practicing engineers, can have some effect on this code mania is to talk assertively and often with our legislators, or local officials who pass the laws and regs. which force/allow the adoption of the latest codes every cycle. And, in particular, for us, that means the IBC and its various brothers, which then adopt all the other, almost latest eds. of, codes by reference. We have to explain to them that buildings won’t start falling down around them if we skip the next couple, or every other, edition of the IBC. They don’t know any better, they assume the newest must be the bestist, and that is the code writers pitch. And, we have to educate them and explain that there really isn’t much improvement or advancement in the latest edition as relates to a better structure for them. As part of our standard practice, we keep up with changes in the industry, and the current code allows us to apply these, without a full blown code revision, and all the other complex changes that entails. Maybe we should also explain to them that they should give some of this authority and responsibility back to the EOR, rather than relying on a bunch of new minutia in the form of a new code. We have to explain to them that this current rate of code change is just adding confusion and commotion in the entire construction industry, actually costing more money, and not providing better infrastructure for their dollar. Most of the state and local building depts. will actually work with us on this, since they are as confused and put-upon, as we are, by not actually learning to use and interpret the codes, before they change again. The fact is, that this rate of change and commotion likely adds to the potential of error, since nobody really learns to use, interpret, or inspect to a version of the code, before it changes, and starts all over again.

The outcome of this would be that the code writers could change and print to their hearts content, we would follow the research in tech. literature, and apply it as we saw fit and were willing to assume responsibility for it; BUT they wouldn’t sell many of their newly printed editions, every other cycle. This would get their attention, because if we don’t have to buy it, they don’t have anything to sell.