Reinforcing Bar Development Length

[lumbee]

I am designing a spread footing for a lightly loaded metal building. I am using a 24" x 24" x 18" deep footing. The punching shear and beam shear check okay and there is enough reinforcing to help with the bending moment. The baseplate is 12" x 8". I am using 3000psi concrete, 60,000 psi yield strength #4 bars. With this combination I end up needing approximately 13.5" of development. As you can see with the 24" x 24" footing and the baseplate above I don't have it. Is there anything I can do to achieve my development length without going to a larger footing.

 

[SlideRuleEra]

Basic development length for a Grade 60, #4 rebar, 3 ksi concrete, with a standard hook is 11 inches.
If builiding codes, type loading, etc. allow, perhaps you could consider using an unreinforced footing.

 

[dirtsqueezer]

Guys,

Sorry to get in before the question is resolved, but I wonder where you're getting these different values for Db? I've tried to chase down development length in the 97 UBC (I assume still the same in the IBC), but wasn't able to nail it down to a specific formula. This should make for a simple answer. Thanks.

 

[SlideRuleEra]

dirtsqueezer - The 11 in length for a hooked bar came from a reference book quoting an older edition of the ACI (American Concrete Institute) Code. It may not be the latest, but these type things usually don't change often. Most U.S. building codes take the calculations and values established by the ACI.

lumbee - Other ways to look at your situation:

1. If you could increase the compression strength of the concrete, this would shorten the development length required (a minimum value may apply however).

2. If your design could be based on Grade 40 reinforcing steel stress the development length would also tend to shorten. You would have a few more bars, but it might solve your problem. You could still use Grade 60 (it is probably more readily available than Grade 40) as long as the placement was in accordance with your Grade 40 calculations.

 

[DaveAtkins]

The ACI Code also allows one to reduce development length based on providing more reinforcing than is necessary. The multiplier is As required/As provided.

DaveAtkins

 

[dicksewerrat]

Can the bar extend into the floor of the bldg.?

 

[zamrube]

You should use the old 1963 code which requires bond stress checks only. There's no way you can have a smaller footing using the latest codes which limits the ld in tension to 12in.

 

[UcfSE]

If you look at the reinforcing design as a cantilever beam as usual, a 24" wide footing yields a 12" long beam that is 18" deep. You don't need to develop a bar, just provide minimum. You probably don't need any rebar anyway since you can't get that much bending out a such a short deep beam. Check that the bending stress is under the modulus of rupture and leave it at the minimum reinforcing.

If you really want to develop the bar why don't you specify a "U" shaped bar with a standard hook on each end and turn the hook up into the depth of the footing?

 

[jike]

With those proportions, I would suspect that an unreinforced footing would work. Then you wouldn't have to be concerned about development length. Check it out.

 

[DRC1]

The last version of ACI 318 changed required development lenths. It is now based on a formula and is a real dog's breakfast. However, development length is the length to fully develop the bar. If you are not fully utilizin te bar which I will bet you are not even close,the requred development is proportinal to the load in the bar vs. design capacity. Thus if your load is 50% capacity, you need 1/2 the development length.