Development lengths of non-standard hooked rebar

[TehMightyEngineer]

We've got a need for hooks of a #5 bar at the edge of a precast bridge plank with a tighter bend radius than a standard hook (to maintain clear cover requirements).

I've been unable to locate any research or recommendation on how this will effect the development length of the bar. Obviously a tighter bend means more crushing of the concrete in the hook and likely a reduction in the development length contribution of the hook; but the question is by how much.

We're not changing the hook by much, going from a 3.75 inch inside diameter bend to a 3.25 inch diameter bend.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[dlawrence529]

Are you able to tilt the bars to maintain clear cover? Check out CRSI RN-2009-2 Evaluation of the Orientation of 90° and 180° Reinforcing Bar Hooks in Wide Members. It's available for free on the CRSI website bookstore. I've only skimmed through it, but I think it showed very little to no reduction in capacity for hook bars that are tilted to maintain clear cover.

 

[TehMightyEngineer]

Yep, I have a copy of that and we've used tiled bars before in this situation but the congestion is higher on this project. We can tilt some bars but not all of the hooks. Headed bars are also an option but not ideal as we're not sure yet if we can fit them all. We're looking into a combination of tilted hooks and headed bars as an alternative but tighter hook bends would definitely be the most favorable.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[JAE]

Doesn't a tighter bend radius risk cracking the bar?

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[TehMightyEngineer]

JAE: Yes, but the standard hook bends are much larger than the stirrup and tie hook bends. We've found through experience that going much tighter than the stirrup and tie hook bend diameters is where you start overstressing the bar.

I believe I've found my solution but would still be interested if anyone can dig up research on development length of non-standard hooks.

Our solution we're going to use for this specific project is the project specs reference the CRSI Manual of Standard Practice which allows a ±0.5 inch tolerance on 180° hook diameter, so we're just going to deliberately use up all the bend tolerance for these hooks.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[TLHS]

How about threading a head onto the rebar to develop it? Something like this:

https://www.erico.com/catalog/literature/CP7E-WWEN.pdf

 

[TehMightyEngineer]

Yeah, we also considered headed bars but unfortunately many bars are bundled or otherwise spaced so close together that the heads would start conflicting.

To give you an idea, here's a previous project with a less congested cage but similar to what we're trying to work on. For this one we could turn the hooks into the thicker curb area and not have them tilted. For the new panels we have no such curb to fit the hooks into.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/