Bent Bar Reinforcement 1

[RFreund]

Where can I find information on bent bar reinforcement.

I'm curious if the bar should be fully effective immediately after the bend (along the straight section). Also what, if any, contribution of the sloped portion has.

Thanks

EIT

http://www.HowToEngineer.com

 

[CELinOttawa]

CRSI

http://www.crsi.org/index.cfm

Also read the appropriate ACI report (408 I believe) or a good text on reinforced concrete design....

I believe you'll find that the hook reduces Ld, but that the length after the hook continues to develop the anchorage, so you cannot call the bar developped immediately after the radius.

 

[TXStructural]

For code-design purposes, a bar can be considered as developed in a length described by the Ldh equation in ACI 318 (12.5), which defines the distance from the outside of the hook to the spot on the tension end of the bar where it is fully developed.
The old thought was that half of development occurs in the hook, and half in the associated straight part of the Ldh. This may or may not be true, but was part of the reasoning behind the lengths. The problem with considering a hook to be fully developed at the hook (or on the tail end of a hook) is that relying upon this means that the forces in the concrete inside the bend are much higher - too high for some bar-concrete combinations.

CRSI is currently funding research on hook development, to answer a few fundamental questions, like what bend diameter is needed for various combinations of bar yield strength and concrete compressive strength.

 

[grogannc]

There are some modifications but much of the basis for ACI code regarding hooks can be found in this paper.

Link

 

[RFreund]

Sorry for some reason my second paragraph is not on the OP.

I'm curious about bent bars found in old joists and older beams. Usually (at least in the Midwest, USA) they would have straight bottom bars but also bent bottom bars, which would act as bottom reinforcement at throughout midspan and then be bent up to act as top (negative moment) reinforcement over supports. I'm wondering can you use the full bar as effective in negative/positive reinforcement as soon as it straightens from this bend?

https://dl.dropboxusercontent.com/u/65240332/Eng-Tips/Concrete/Bent Bar.JPG

EIT

http://www.HowToEngineer.com

 

[CELinOttawa]

Ah, well that is a much more clear question...

The designers often considered them as being immediately effective, I do not. You need a reliable load path, and in this case that means reliable anchoring. The stresses must transfer through the bond with the concrete; Personally I consider the bar as being effective after a hooked bar down velopment length in from the bend.

 

[RFreund]

Do you know if there is any literature out there on this?

Thanks again!

EIT

http://www.HowToEngineer.com

 

[dhengr]

RFreund :
I’d look at older CRSI Handbooks, ACI Codes and Reinforced Concrete Design textbooks from the 50's, 60's & 70's. That was a pretty common means on setting up the rebar. It was a bit of a prescriptive method of locating rebars and bends in some cases, but was thought to save some rebar because rather than extending a straight bar for development, the bend and extension into the adjacent region (from -M to +M or visa-versa) added the development length and the steel for that region too. The bent bars could also be considered as some web reinforcement when this was needed, particularly in beams and joists. Note that the bends took place in lower moment regions of the beam, joist or slab; where most of the development had already taken place or where full development was not required.

 

[wannabeSE]

RFreud,
Are you looking for some clarification or commentary for ACI 318 section 12.10.1?

 

[RFreund]

@DHengr - Thanks I'll see if I can find something

@wannabeSE - Thanks for pointing out that section. I was really just looking for some old paper that showed how these bars would typically be designed.

The issue I'm having is that most of these joists have plenty of bottom reinforcement (at midspan) but not much top reinforcement over the supports (more than not they even work as simple span beams). So I was just trying to clarify at what point you can start using these bent bars as top reinforcement for moment at supports (typically considered negative moment).

EIT

http://www.HowToEngineer.com

 

[TXStructural]

These are called truss bars and are fully developed along their length, between the end anchorage lengths. In modern design vernacular, they act as the tension ties in a strut-and-tie system. The compression struts act at the bends.

As far as their use these days, you will find some DOT applications that still require them, but they are very difficult to bend and place - DO NOT USE THEM IN NEW CONSTRUCTION DESIGN.