Transverse Tension in Bars

[Lutfi]

I am being told by another fellow engineer that one can count on a transverse continuous horizontal bar that a hooked bar goes over will carry tensile load hence the development length of the hook does not have to develop! I never heard of such a thing before.

Does anyone know if ACI has any provisions allowing continuous transverse bars to carry load in tension transversely?
I searched ACI 318 and did not find any.


Regards,
Lutfi

 

[frv]

Are you referring about a bar placed in the center of the radius of the bend of the hooked bars?

I've seen this detailed only as an aid to ease of construction, but never as something to count on to develop.

 

[Lutfi]

frv, yes as to the location of the bar. I agree in that helps in the construction and holding bars in position. I was taught the development length depends on bond between bars and concrete. That is why the development length calculations has variable to cover NWC vs. LWC, steel strength, compressive strength and the coating of bars.

Regards,
Lutfi

 

[JAE]

Lutfi, the only place I've ever seen anything like that documented would be at the ends of corbels where the main top As steel is indicated in 318 chapter 11 as welded to a transverse bar at the top tip of the corbel. This is to tie in that transverse bar an avoid any bearing or diagonal shear failure at the tip.

There is nothing in Chapter 12 of 318 that indicates a transverse bar helps reduce or eliminate the need for l_dh.

A question could be sent to ACI via their website to see what they say.

 

[Lutfi]

JAE, you are correct about the corbel but that is not where the condition I am speaking of.

Regards,
Lutfi

 

[Ron]

Lutfi...the only provision I know of in ACI 318 is the one that allows a modification factor for the hook development length if closely spaced ties are perpendicular to the length of the hooked bar. This amounts to reducing the development length by 20 to 25%.

In theory, I believe the other engineer is correct; however, it is not necessarily recognized as a code provision (the perpendicular rebar interrupts the "splitting tensile" stress distribution caused by compression of the concrete from the hook).

 

[Lutfi]

Ron,

I agree with the modification factors.

Interesting point that you bring up about the horizontal bar theory; but as you said, it is not a code recognized provision.

I should be on the beach enjoying the wonderful Florida weather today!


Regards,
Lutfi

 

[Lion06]

I don't believe that the transverse bar actually carries any tensile force, but I think that the principle is similar to the anchorage requirements for stirriup. A stirrup is considered fully anchored by virtue of the hook around the top bars.

There are other requirements for larger stirrups, and these provisions apply specifically to stirrups per ACI 318, but my best guess is that he's referencing that type of behavior. I hope he's not really suggesting that the transverse bar somehow carries tension transverse to the longitudinal axis of the bar.

 

[Ron]

Lutfi...same here. The weather is great!

 

[JAE]

Ron - we are at 84 degrees today. Low humidity. Just sayin'.

 

[Ron]

JAE...84 degrees is great! But low humidity? Who can take that? My gills would dry out!

 

[grantstructure]

To argue that a bar placed on the inside radius of a hook bar will result in mechanical anchorage that develops yield in the hooked bar is scary. What, exactly, is the load path being proposed? As someone noted above, if the bars were welded together, then slip between the hooked bar and the transverse bar is prevented, and the anchoring effect of the transverse bar would be engaged. Simply having bars in contact doesn't have anything like the same effect--the hooked bar could still pull out and "slide" right past the transverse bar.

Regarding Lion06's analogy to stirrups--I've never heard that the hook of a stirrup is considered to develop the stirrup at, say, the start of the radius of the hook. Stirrup anchorage details are prescriptive, not predicated on developing the bar from some critical section--that would be at the location of the shear crack, which is of course random, as is stirrup placement, to a degree. I think it's more a concern of having stirrups spaced closely enough that developing yield isn't necessarily critical for any individual stirrup.

 

[Ron]

grantstructure...

The initial stress distribution is that the hook puts the concrete above the hook in compression. That holds as long as the paste-to-bar bond holds or the concrete breaks in splitting tension from the compression concentration along the line of the hook. If you reinforce the area of concrete perpendicular to the expected tension crack, you will then throw the stress distribution back to a bond failure. Studies have shown that with higher strength concrete, the hooked bars can actually straighten and pull out without failing the concrete (except locally at the bond interface).

 

[Lion06]

Grantstructure-

I respectfully disagree. Because an inclined shear crack can develop anywhere and it is assumed that any stirrup crossing the plane is effective it has to be assumed that any stirrup crossing the plane is fully anchored on both sides of the crack, otherwise it wouldn't be effective. Additionally, ACI 318 11.5.4 says that both ends of the shear reinforcing shall be developed according to 12.13. 12.13.2.1 says that for #5 bars and smaller, a standard hook around a longitudinal bar shall be provided. For #6 bars and larger an additional development length is provided. The point I was trying to make is that for typical stirrup sizes, they are considered completely anchored (100% developed) just by virtue of the standard hook around the longitudinal bar regardless of where the crack occurs.

 

[msquared48]

D^** the 84 degree braggarts! It snowed here this AM as they were saying on the radio in Seattle that was raining!

As for the tension steel, I use the detail all the time to theoretically increase the size of any pullout cone - as shear reinforcing per se. If the embediment is too short, I put a plate on the end and run a couple of transverse bars over it. Never had a problem with it.

But, perhaps this is not the concern here...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com