Reduction of Development Length ok?

[pob11646]

I have got a question about concrete development length, and whether I can reduce the development length if the tension in my steel is less than the maximum tension that my steel reinforcement can take. Reference is ACI 318-05
Section 15.6.2 - "Calculated tension or compression in reinforcement at each section shall be developed on each
side of that section by embedment length, hook (tnesion only) or mechanical device, or a combination thereof.

Say the tension in my steel bar is only 37% of the full tensile capacity of the bar. Is it right to say that my
development length for that particular bar can only be 37%, esp. if I have no space to develop my length further?

Thanks.

 

[Lion06]

It's actually based on the ratio of req'd steel to provided steel (See 12.2.5). We don't determine stresses in steel bars (typically), we simply analyze the section to determine if it has adequate capacity.

If your analysis shows that you have twice as much steel as is necessary for your ultimate loads, then you can cut the development length in half, with certain limitations.

 

[enginerding]

The limitations StructuralEIT is talking about are those cases where the full yield strength of the bar needs to be developed. Examples would be tension ties (actually need 125% of yield), shear friction reinforcement, and shrinkage & temperature reinforcement. For a footing you would probably only have to worry about T&S steel if it governed. If this is a problem, use smaller diameter bars to get shorter lengths.

 

[Lion06]

You can use hooks or nuts to achieve development if you're really up against a wall.

 

[pob11646]

ACI 318-05 Section 12.5.3 does provide for a reduction of the development length when 90 degree hooks are used. StructuralEIT, how do we know that the ratio refers to required steel versus provided steel, and not actual tensile capacity of steel, versus allowable tensile capacity of steel?

 

[enginerding]

6 on one hand, half a dozen on the other.

The ratio is the same thing - unless the required area of steel is governed by something other than strength.

With concrete you are dealing with factored loads, not allowable loads. So technically you don't have an "allowable" capacity of the steel.

 

[NS4U]

great point by POB. adding/subtracting As is non-linear, so if you double your As you don't double your capacity.

Here's an example, using a spread sheet I have, I found that for a demand of 50 k-ft (with abritary dims)...

I need 1.4in^2 of steel. So if I provided 2.8 in^2 I could reduce my Ld by 1/2 (according to ACI 12.2.5).

However, if I calculate the capacity with 2.8 in^2 it is 87.5 kip-ft...So if you ratioed capacities by EIT's method you could only reduce Ld by by 43%.

 

[pob11646]

Thanks for the support, NS4U and for the practical example you provided. Even though a reduction of 50% (NS4U's example with area of steel) and a reduction of 43% (NS4U's example with steel capacities) are reasonably close, they are not exactly the same, and the variance may be even more if the dimensions are different.

So, it still comes back to the question of do we take the ratio to the required steel versus provided steel, or actual tensile capacity of steel versus factor capacity of steel? Or do we do both, and just go with the one that is more conservative.

Thanks for all the feedback and discussion topics, everyone.

 

[JAE]

Both 12.2.5 and 12.5.3 use the ratio:

As(required) / As(provided) as the reduction coefficient.

This is based upon, historically, the stress in the bars as the older codes used to use circumferential contact area on the bar surface as the measure of development.

NS4U, your example appears to be relating bending moment in a beam. This is not the way to look at it. The ratio of bending moments is based on moment capacity which is based on depth, beam configuration, concrete compressive stress, etc.

ACI does not recognize reductions in development length based on moment. Only based on area of steel (thus the As ratio).

 

[NS4U]

JAE, I agree, I was merely pointing out that it is not as simple as ratioing the capacities.

 

[NS4U]

sorry i meant to say... it is not as simple as ratioing the capacity to the demand.

 

[JAE]

yep

 

[bones206]

I don't think it's good practice to limit the development length. If the steel were ever overloaded, you would be promoting a non-ductile failure mode. Especially if you have seismic loads. I would avoid 12.2.5 and find a way to fully anchor the steel.

 

[Grahammachin]

In the British Standard (BS8110) there is a table for the ultimate anchorage bond lengths and lap lengths.

But there is also a clause which allows you to calculate the length of anchorage required for a specified force in the bars. A modification factor is then used to calculate the length of tension/compression lap required (there is also a minimum length beyond which the lap cannot be reduce).

Is there anything like this in the US codes.