Reduction of development length

[Lion06]

Let's say I have a footing that requires only the minimum steel but I am having a problem with developing these bars. If I make the footing wider, I get more development length, but the moment increases and the required area of steel increases.
If I only need say 0.25 in^2/ft, but provide 0.5 in^2/ft for the minimum, can I reduce my development length using ACI 318-05 12.5.3(d) using 0.25 in^2/ft for the required As (since that is what is needed for strength) or do I have to use the 0.5 in^2/ft in that calculation?

 

[DaveAtkins]

You can reduce development length if you don't need to develop the full 60 ksi--you would only need to develop 30 ksi in your example.

DaveAtkins

 

[civilperson]

You only need to develope the reinforcement where the moment diagram requires. Thus, the maximum moment is at the column face and goes to zero at the edge. Hooks, either 90 or 180 degree can get development near the end of the bar or cross bar welded to the reinforcement steel gives a mechanical development EVERYWHERE along the length.

 

[Lion06]

DaveAtkins-

That is the question, though. Do I really only need to develop 30ksi? If so than I am not really providing the minimum steel, correct?


civilperson-

I understand about develoopment, but I am trying not to have to hook bars or provide mechanical anchorage in a lightly loaded pavilion footing.


I can't believe I'm the first person to have encountered this. I have a 2' wide wall footing with a factored bearing pressure of 2.88 ksf (like I said, fairly lightly loaded). I'm sure plenty of others have used 2' wide footings. I even tried bumping it to three feet to get additional length, but that bumps the moment up. I then tried making it 18" deep, then 24" deep, but then the minimum jumps up.

 

[dougantholz]

There are two things here. There is minimum reinforcing for crack control and then the minimum steel you need to resist your loading.
Check the footing as unreinforced for stress and if you do need reinforcing, this will tell you exactly at what distance from the outer edge of the footing you need the first infinitesimal amount of steel.

 

[minorchord2000]

If this is a wall footing, you can rotate the bars a bit to get the required development length. Rotate them such that you can still maintain your 3" of side clearance.

 

[Lion06]

I did try it as plain concrete, but it's not even close to working. The "allowable" (I only use this term because it is really an elastic stress that causes cracking) net tensile stress is 2.75*sqrtf'c to be used against a factored loading.

Doug-
ACI says that any section that requires reinforcement must have a min of 0.0018bh in it (for footings). To me that is the min for strength calculations.

This is for a shearwall footing, so the soil stresses causing this are localized. I think I am going to try determining where I can get away with plain concrete (and stop hooking the bars in that region), and just specify hooked bars at the ends of the footing where the soil pressures are higher and I need it. Does that seem reasonable?

 

[apsix]

I can't see how a 2' wide x 2' deep footing can fail as plain concrete.

 

[Lion06]

Sorry gentleman,

I just realized an error. I didn't change my bearing pressures to k/in/ft so my moment was coming up larger than it should be by a factor of 12. I guess this is a good endorsement for including units in calcs. Boy, do I feel foolish.

 

[jike]

I have always seem to make those size footings work as plain concrete. Double check your calcs.

 

[youngstructural]

Jike: He just said it was a calculation error... Respectfully, as I have done this myself, we should all be more careful to read the whole thread when contributing.

StructuralEIT: We all have those moments, no matter how good an engineer we are. And from what I have seen, you are a credit to our Profession.

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...