Adjust Footing Rebar Area for proper ACI 318 Development Length

[SteveGregory]

For typical 3 ft, 4 ft and 5 ft square footings (f'c = 3,000 psi and net soil bearing = 2,500 psf), it appears that I must hook the ends of the footing rebar on each end or increase the number of bars per Section 12.2.5 or increase the size of the footing. This is because the length of the bar from the critical moment location to the end of the bar is shorter than the development length [cantilever length-cover].

Most of the time, the area of steel required is governed by 0.0018bh (Temp steel). Is the "As required by analysis" in Section 12.2.5 referring to Temp steel if it is the minimum that governs or the As calculated for flexure only?

Or does 12.2.5 not apply to spread footings?

 

[hokie66]

Don't you routinely cog (90 degree hook) bars in spread footings?

 

[JAE]

Thicken your footing just a bit and the development length doesn't matter because you'd get a plain concrete footing to work.

Then put in the rebar for good measure. I think hooking (cogging per hokie) in footings like this is rarely done in my area.

Not sure if the As(req'd)/As(provided) reduction would apply to the min. 0.0018Ag steel. I wouldn't think so. The word "analysis" would imply it is the calculated As, not a code minimum.

 

[SteveGregory]

Hokie, We routinely just pull straight bars right out of a table in the CRSI handbook. I am located near your college and I have not seen any cogged or hooked bars used.

 

[hokie66]

Maybe my thinking is biased by living and working in Australia for many years, but I do seem to recall bars in spread footings in Virginia that weren't cogged. Here, the footing bars are always cogged, and the reinforcement will even be supplied that way whether specified or not. I think it is wrong not to develop the bars. Small footings are usually not flexural elements as such, and should rightly be designed by strut-tie analysis, which requires anchorage at the nodes. Larger footings that are flexural in nature are cantilevered slabs, with quite high uniform loading compared to other cantilevers. Cantilevers in suspended beams and slabs would have cogs, so why not footings?

 

[steellion]

I'm in the US, and I've never seen or used hooks in spread footings. Pilecaps: yes, spread footings: no. In my opinion, ACI does a poor job of explaining which minimum steel provisions apply to footings. I always provide at least 0.0018bH, and if flexural analysis requires more than minimum steel, then I will provide 4/3*As,req'd <= (200/fy)*bd similar to a beam. Agreed with JAE, for footings that are that small in plan, plain concrete will probably work for analysis.

 

[theonlynamenottaken]

I've noticed the same problem with developing reinforcing in smaller footings. Years ago I wrote a spreadsheet to calculate footings and hardcoded development length requirements into it. I've found that if your development length is lacking you can usually make up the difference with one additional straight (non-hooked) bar using the ratio (Provided Ldev/Yield Ldev) when your flexure demands more than the .0018bh requirement.

 

[rapt]

Agree with you on this one Hokie66.

For footings (and any cantilever) flexural design theory requires the free end of the bars to be hooked/cogged to provide at least partial development, depending on the length of the cantilever and the bar diameter. At least minimum reinforcement should be fully developed. As you suggested, for deeper footings, strut tie theory would require full development

Historical precedence should not be en excuse for continuing a bad practice!

 

[SteveGregory]

Thanks for your input everyone!

I guess I can't justify my bad habits any longer.

Back to my original post, I will need to cog my rebar (I like that word), add more bars or increase the plan dimensions of the footing. One other option is to increase the depth until it can be designed as plain concrete.

 

[Mixtli]

Hooks require more labour and are more expensive.
Have you checked the development length using fs instead of fy?


M.

 

[TXStructural]

You are required to develop the bars such that they are fully developed when required for flexure. Minimum steel put into a footing that doesn't really need them doesn't have the same development requirement.

That said, hooking bars in footings is routine in many areas of the US. In most buildings, making footings larger or using hooked bars is really irrelevant to building cost. In my opinion, don't waste engineering time saving trivial quantities of materials on a few footings, when you can just move on to the next detail. Rebar costs about $1/lb., installed, so the amounts saved will usually be very small.