Development Lenghts 3

[marcploks]

Sirs,

Good Morning!

Regarding development lengths, is it okay to ignore ldh for hooked bars but still hook it and let it equal to Ld for straight bars?

For more details please see attachment

Thank You in advance

 

[slickdeals]

I have seen this detail in many drawings in India and but it seems research done shows that the extension of the tail is not beneficial. If you follow ACI provisions, then what you have said is not allowed.

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN

 

[marcploks]

may i ask if which reasearch studied this? thanks slick

 

[dcarr82775]

I would say no. There is a certain minimum length on the hook, after that you are not gaining anything, just wasting bar.

 

[cooperDBM]

No. The stress profile along the bar and the mode of failure between straight and bent bars is different. That's why they have different requirements.

 

[dik]

That seems counterintuitive.

If it requires an Ld to develop a straight bar, then if you have the same length of straight bar and put a bend in it, one would think it would be at least as strong.

The only reason that I can think of is that the bar is 'pulling away' from the concrete at the outside of the radius... but, then it's pushing against the inside of the radius.

Can anyone explain why it would not be as strong?

Dik

 

[slickdeals]

The failure is partly due to the compressive stress failure (crushing) at the bend. I think the MacGregor text book has a fairly detailed explanation.

 

[slickdeals]

http://www.dot.state.fl.us/research-center/Completed_Proj/Summary_STR/FDOT_BD545_40_rpt.pdf

http://www.dspace.rice.edu/bitstream/handle/1911/14759/7126324.PDF?sequence=1

 

[slickdeals]

http://repositories.lib.utexas.edu/...009-08-385/Younghye_Kim_thesis.pdf?sequence=2

 

[dik]

Thanks... some light reading for the weekend coming up...

 

[rapt]

If you want to develop extra bar development over that which a standard cog allows, you have to design the cog for it. The radius of the bend must be increased to reduce the compression/bearing stresses in the bend and then the bonus you get from the cog is lost and the bar is treated as a normal bar for development. BS8110 and I thing Eurocode 2 give design methods for this. The increase in capacity is dependant on the radius of the curve in the cog.

 

[dik]

Do you have information on the increased radii?

Dik

 

[charliealphabravo]

The dimensions in the hook and lap tables are intended to provide full capacities in all cases where coverage requirements are met, subject to stated limitations of course. There is not enough information in the posted detail to ensure that.

Take the extreme case in the detail. If you ignore ldh entirely you have the potential for the hook to be turned down immediately after the bar passes the face of the support. That can't work and would result in minimum coverage and the hook pulling through the face of the support and possibly from the face of the beam before reaching full capacity. This would also happen with thin walls and columns where ldh could not be observed.

I could see the detail working in some scenarios. Perhaps where there was supplemental reinforcement under the hook or where a reasonable ldh was "unofficially" observed.

 

[charliealphabravo]

The detail could also work if you assigned some reduced resistance to the hook based on the actual provided ldh.

 

[slickdeals]

The papers I posted talks in detail about this "lead in" length from the face of the column and it's effect on development length.

 

[charliealphabravo]

yes, "lead in" that's a better term since ldh refers to a constant number.

 

[dik]

CAB... generally have a horizontal bar between the hook and the concrete face that precludes 'breakout'... does that have an impact on the Ldh value?

Dik

 

[rapt]

Dik,

I assume that question was for me.

BS8110 has a formula for bearing stress in the curve which is dependant on the radius and the force in the bar in clause 3.12.8.25.2. You can rearrange the formula to give a minimum radius for the amount of development of the bar required.

AS3600 says that the bar can be considered to develop stress around a curve (ignore cog benefit and treat it as a normal length of bar) if the radius of the bend is greater than 10 times the bar diameter. But really this should be related to concrete strength etc as BS8110 does.

 

[IDS]

AS3600 says that the bar can be considered to develop stress around a curve (ignore cog benefit and treat it as a normal length of bar) if the radius of the bend is greater than 10 times the bar diameter. But really this should be related to concrete strength etc as BS8110 does.

On the list for the next revision?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[marcploks]

Saw this at ACI 315-99 though its a little vague.

My understanding is that the question i raised is OK as per ACI code.

Please correct me if I am wrong..