Development Length - Concrete vs Grout

[slickdeals]

A standard hook can be developed for tension per Section 3.3.3.2 of ACI 530-05 in 13 d_b. This is 6.5" for a #4 standard hook.

However, if I used 2500 psi concrete, then I would need 0.02*60000*db /sqrt(2500) = 12" for a #4 bar.

Can someone please explain why you need only half the development length in grout than concrete?

We are Virginia Tech
Go HOKIES

 

[ToadJones]

I believe some of the development is based on conc./grout bearing on the bar deformations.

 

[Lion06]

I asked this question of some engineers in my office about three years ago. I never did get an explanation and I didn't follow up. Maybe it has something to do with grout having a built-in sort of confinement (with the CMU providing the conefinement.

 

[concretemasonry]

Sorry, but I can only offer information on ACI 530-02 based on concepts and information when it was written. Next week, I will be back and able to refernce the 05 version.

In general, there is a difference between grout in concrete when it comes down to the intended use, physical properties and/or testing. Masonry grout must meet the specifications of ASTM C476.

Poured/placed concrete has a much lower slump, while masonry grout has a much higher required strength to do the job of filling and bonding. This can be a factor on smaller bars.

The testing methods for masonry grout and concrete are dramatically different, so comparisons on a psi level are very difficult if not impossible. - Two different materials. Good engineers often have set limit on the maximum strength (f'g) of grout to insure a wall acts in unison as a structural element as designed.

Because the NCMA does significant amounts of testing in their own lab or jointly there may be new information, especially because much of the masonry deals with the smaller bar sizes. Newer results could have had an effect on this subject that is "hot" in some applications.

If the formula is in ACI 530, it should refer to masonry grout and its associated testing.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[TXStructural]

Using your numbers, ACI 318 gives a basic hook development length (ldh) of 24db. Allowing the masonry unit to confine the bar, use the 0.7 factor in 12.5.3(a) to get 16.8 db.

Under 530, the equivalent embedment length (le) is measured from the point of tangency (ACI 530 1.5 for "le"), which differs from 318. ACI 318 Fig R12.5 shows to add 4db to the 13db in ACI 530 since they are not measured from the same place.

This yields about the same 17 db.

 

[ron9876]

My understanding of ACI 530-05 is that a hook will provide a tension capacity equivalent to 11.25db embedment. So a straight bar embedment length less 11.25db when a hook is provided for full tension capacity of a hooked dowel.

 

[slickdeals]

I think a hook provides a stress in rebar of 7500 psi and then you need to provide development length to transfer the remainder stress (24000psi - 7500psi).

I think TX's response makes sense, but what is throwing me in a loop is the statement in the commentary which states:

"The allowable stress developed by a standard hook, 7,500 psi (51.7 MPa), is the accepted permissible value in masonry design. Substituting this value into Eq. (2-8) yields the
equivalent embedment length given. This value is less
than half that given in Reference 1.14 (ACI 318)"

We are Virginia Tech
Go HOKIES

 

[slickdeals]

@Ron9876,
Going by your concept, for an Fs = 24000, the l_d for a straight bar is 36 d_b. Subtracting 11.25 d_b from it gives 24.75 d_b. Is the 24.75 d_b = 12.375" the amount of straight bar needed beyond the point of tangency to develop this bar?

We are Virginia Tech
Go HOKIES

 

[ToadJones]

does it have anything to do with Fs=24,000 psi vs. 60,000 psi. ?

 

[slickdeals]

@Ron9876,
Re-reading the code very slowly and carefully, I think what you said makes sense. I might had a brain fart when I was reading it.

We are Virginia Tech
Go HOKIES

 

[ron9876]

I was doing something with this a few days ago.

I think that is what it says but it doesn't address the needed embedment to get that force. Say you are hooking a #6 vertical bar 3" into an 8" wide bond beam. It would seem that it couldn't resist (11.25/48)x 0.44 x 24,000 x 1.33=3,292 lbs.

 

[slickdeals]

If you embed a #6 bar 3" into a bond beam, then it can develop only 2,666 psi (or 0.44 * 2,666 = 1,170 lb). You will need to embed it 27" in order to get 24,000 psi

We are Virginia Tech
Go HOKIES

 

[slickdeals]

See attached.

We are Virginia Tech
Go HOKIES

 

[jenofstructures]

is it the eq 3-15 of ACI 530 is for the development length and not for the ldh or standard hook?

Poems are made by fools like me, but only God can make a tree. engineers creates wonderful buildings, but only God can creates wonderful minds

 

[slickdeals]

Eq 3-15 is for the development length of straight bars. You get a reduction in this length of 13 db if you have a standard hook.

We are Virginia Tech
Go HOKIES