development length for tension and conpression bars.

[chekre]

hi,
regarding the development length for a bar in tension, and in order to be able to develop fy, i should anchor it ldh + the standard hooK which is 12db according to aci318.
if my column section is not big enough to ensure the ldh and i dont want to use the reduction factors allowed in the aci, can i simply develop ld into my column and bent the tension bar inside to have a total length of ld.?

2. the development length for a compression bar is the maximum between 20cm and 18 db (plus or minus depending on the f'c of the concrete). if my section isnt big enough, am i obliged to go this 18db straight or can i go down the allowable distance and anchor the remaining length by a hook ?

 

[JAE]

can i simply develop ld into my column and bent the tension bar inside to have a total length of ld.?

I don't believe that is appropriate to bend a bar and add up the length to get ld.

Either you develop the bar with a straight length ld or you use a standard hook with the ldh.

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[chekre]

Ok. Same thing applies for the compression i assume. Any references ? Does The aci mention that u need to go straight for the ld ?

 

[MrHershey]

Compression needs to be a straight bar, period. Hooks are not effective for development of bars in compression.

 

[chekre]

Ok. Thats one thing. regarding the tension bara, any reference that u cant develop ld with a hook as stated above ?

 

[KootK]

I've been meaning to ask a similar question for a long time. Here's what I know (or think that I know):

1) I agree with Mr. Hershey that there's no hope for the compression bars. Straight development or nothing.

2) Many old school details that I've seen for slab steps etc clearly do make the assumption that bars can be tension developed by having hooks longer than standard hooks.

3) North American codes do not seem to have explicit provisions for developing bars with longer than standard hooks.

4) The eurocodes do seem to have provisions for developing tension bars with hooks longer than standard. The checks centre around calculating the bearing stresses inside the bar bend and specifying the bar bend radius accordingly. My takeaway from this is that a) small bars probably can develop around the hook with standard bend radii and b) large bars usually need to have their bend radii larger than standard to make a go of it.

5) Using the curved bar node concept of the strut and tie method, one can design themselves an extended hook bar development system that would be compliant with North American codes,.





I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Here's a link to purchase the article:
Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TXStructural]

A hooked bar may develop ahead of the bend, but one problem with developing the bar before the bend is that the concrete inside the bend can crush. Some hook length research found this to me a limit state which resulted in bar failure due to this crushing so it was considered in determining Ldh.

Another issue is whether the depth of the hook provides adequate restraint. The bar could be developed without having sufficient concrete to fix the connection.

CRSI is currently funding research on hook lengths and diameters. The first project is nearing completion and my understanding is that it has verified that the current hooks are still valid. Another set of research is underway to examine hooks in high strength reinforcement, which were not part of the older code research. The high strength research will be a longer project since higher concrete strengths will need to be examined and the need to evaluate various means of achieving high strength ductile reinforcement.