Development length of nonstandard hooks

[cwipf]

I need to develop a #5 epoxy coated rebar from a bridge parapet into an 8" slab. The bottom clearance is 1". Therefore, the hook is only embedded 7" into the slab. Can I achieve full development by extending the tail of the #5 bar?

 

[CELinOttawa]

How far are you willing to extend? I am not sure any code will allow you to combine hook and straight lengths, even though it is clear to me this would work... Maybe I'll learn something here as well.

I cannot see how anyone could argue you're wrong to develop the bar with a full development length after the hook.

 

[CANPRO]

You may end up crushing the concrete on the inside corner of the bend before the tail of the bar becomes effective.

 

[CELinOttawa]

CANPPRO is correct, but so long as this doesn't happen until ULS, in absense of pull out it is not a failure.

 

[cwipf]

Good points. I am concerned about splitting failure in the plane of the hook inside the bend. If that occurs, the failure could progress along the hook extension (tail) and reduce any additional straight development length I have provided.

I would expect to see some research on this topic but cannot find any research mixing hooked and straight development length.

ULS?

 

[CELinOttawa]

Your "Ultimate Limite State", ie: Where the only thing that matters is that no unsuspecting member of the public is harmed...

Sorry, it is a tenant of most engineering outside the USA. I take it you are in an American ASD or LRFD jurisdiction?

 

[CANPRO]

ULS = Ultimate limit state.

There are code provisions for calculating the capacity of hooked anchor bolts in tension...I would go that route and see if you can get the capacity you need with that. I'm not sure how effective the straight tail part of the rebar would be beyond a certain distance from the bend...my gut feeling is that the concrete would crush inside the bend before the straight section became effective. If you post a cross-section of the slab and parapet you may get more responses/suggestions.

 

[JAE]

ACI has consistently held that extending the tail does you no good.

 

[cwipf]

AASHTO LRFD Bridge Design Spec.

JAE, does ACI spell that out explicitly in the code or commentary? Would you send me the reference please? I have looked through ACI 318-08 12.5 but have not seen that stated explicitly.

I am questioning the P502E bar in the Attachment.

 

[CELinOttawa]

JAE is correct, it was aq finding of 1960s research and presented in ACI's Committee 408 report in 1979. I'll try to lay my hand on a copy...

BUT, I would think that even if not developped as a hooked bar, having a full extension past this would enable you to fully develop the bar. Doesn't matter if the concrete crushes, if the result is that the bar continues to withdraw slowly without failing.

 

[rapt]

Normally, the straight length past the bend can be used if the bend radius is increased sufficiently to reduce the crushing stresses in the curve. BS8110 gives a method and I think Eurocode does also.

 

[JedClampett]

cwipf, do you need the full bar capacity?
It looks like a traffic barrier. You're looking for it to reach it's design limit once. After that, it probably needs to be repaired or replaced.

 

[cwipf]

JedClampett, yes, full capacity. It is a traffic barrier that will need to be repaired or replaced after the design collision. We use a "yield line" analysis per AASHTO LRFD BDS A13.3.1.

 

[JAE]

I can't find an ACI direct reference (Committee 408 is the development group).

However, it was taught to me in school and I found a reference in my Wang & Salmon text book that stated:
"With 90 degree hooks particularly, designers have often assumed that satisfactory anchorage is obtained by adding length to the end of the bar in excess of 12 bar diameters required as part of the hook. This is an unsatisfactory practice."

 

[cwipf]

JAE, thanks for looking.

 

[Lion06]

If it's a #5 bar, provide a perpendicular nosing bar at the bend in the hook and the bar is developed automatically.

 

[CELinOttawa]

Lion6: Code clause? The bar within the hook has some added valaue, in the range of 20% if memory serves, but I have never heard of an "automatic full development" effect in any research or code.

 

[Lion06]

It's a stirrup hook. It applies only to #3, #4, and #5 bars. Check out ACI 318-05 12.13.2.1

 

[CELinOttawa]

Ah,I see what you're doing there.... Only for a vehicular barrier this is a great principal longitudinal tension bar. I don't know if you can really apply a stirrup clause to this because of the action on the section.

 

[slickdeals]

Don't you do something similar in corbels, although it's typically a welded reinforcement of the same size.