Standard Hooks 3

[sushilDhungana]

Dear All,

Our project is an EPC project and most of our codes in the contract is American among them for concrete we are using ACI-318M-14, ACI-350M-06 etc.

We are practising Americal code these days, it's very limitless at this stage where there is no any counsellor in our office.

One of our problems right now in "Standard Hooks" of beams/slab bars embedded in the walls/columns and other structural, contractors said that they will provide the straight bar with sufficient development length without hooks, as far as my knowledge hooks provide extra anchorage and we counsel them to provide development length with the standard hooks but they are refusing repeatedly.

I want to know what the code states regarding the configuration of development length inside the walls/columns and other structure, is the straight bar inside the wall/other structure with sufficient development length is correct or need hooks. Please give advice to me.


Regards
Sushil Dhungana

 

[rapt]

I would assume you are from Nepal, so high seismic! Therefore definitely hooks required,

Even without seismic, hooks would be required in many cases.

 

[sushilDhungana]

Please provide me clause no. of any ACI code regarding seismic hook. I have not found in ACI 318-14, ACI 350-06, ACI 315-99.

 

[JAE]

You can use either hooks or straight bars to develop reinforcement.

However, in many cases you may not have enough length to use straight reinforcement and must use hooks ( end of beams on outside columns)

You should refer to Chapter 18 (in ACI 318-14M) for seismic requirements - specifically look at 18.8.5 and also 18.8.5.3 for development of straight bars in seismic frames.

Also Section 25.4.

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

 

[sushilDhungana]

Dear JAE
Straight reinforcement is to be hooked at end of beams on outside columns i.e another case when there is not enough space we need to bend. However, I mean to say if there is enough space say 2m wall and development length is 60pi=60*25=1500mm then in seismic region 1500=1200+300 i.e 300mm should be bend (hooks) with 1200mm straight portion in the seismic region or 1500 straight bar inserted in the wall is Ok. I think 300mm bend will give extra anchorage.
My question is that in the seismic region if space is sufficient for developed bar need to bend or not.

 

[charliealphabravo]

Here is an interesting study I found on the capacity of standard hooks.

https://iri.ku.edu/sites/iri.drupal.ku.edu/files/files/pdf/SM 111.pdf

I don't have a great deal of experience in seismic design, but from a purely intuitive perspective, hooks should not necessarily be preferred when member dimensions permit the development of a straight bar. A bend in the development length causes additional stress concentrations and increases the maximum absolute stresses due to a loss of symmetry in the load transfer. Every bend increases the number of failure modes and may require additional steel to confine the hook. The same principles explain why knots, splices, and turns reduce the strength of a line or rope and why hooked anchor rods have fallen out of favor when compared to straight headed anchor rods.

 

[KootK]

The fundamental thing to recognize here is that there is much more to successful joint design than mere bar development. The bar has to be anchored into a properly designed joint. Development is just one aspect of the joint design. So:

1) You don't really need a code clause for this. The requirement stems from good old fashioned engineering.

2) Joints of this sort can be designed via strut and tie methodology, conformance to tested assemblies, and adherence to design standard recommendations in jurisdictions where that is an option. And probably some other methods I don't know about.

2) ACI has some documents covering monolithic beam column joint design. I can't spare the time to parse that out for you right now but I'm fairly certain that they allow you to bypass a rigorous joint design if you do the simple, prescribed checks and take your bars to the far side of the joint and terminate them with hooks. You only get this joint design "cheat" if you follow that the detailing recommendation of course.

In attempt to answer your question directly:

a) yeah, you can get away without hooking the bars if you can rigorously prove that doesn't compromise the joint design.
b) doing (a) is gawd awful difficult and I don't recall ever seeing it rigorously done. I'd be astonished if it has been done in your particular case.
c) a + b = hook the bars. It's definitely good practice in seismic country and it costs nothing except, perhaps, congestion. And, if that's a problem, you can just replace the hooks with headed bar anchorage.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

CAB: A real interesting study. The Conclusions are very interesting, and posted below:

1.
The provisions of ACI 318 -14 overpredict the strength of larger hooked bars, the effect of concrete compressive strength, and the effect of transverse confining reinforcement on the anchorage capacity of hooked bars in tension.
2.
The reduction factors as applied in Section 25.4.3.2 of ACI 318-14 for concrete cover and confining transverse reinforcement are unconservative.
3.
Increasing concrete side cover from 2.5 to 3.5 in. does not increase the anchorage capacity of hooked bars.
4.
Hooked bars with 90° and 180° bend angles produce similar anchorage capacities and can
be used interchangeably. This includes hooked bars with a 180° bend angle confined by
transverse reinforcement parallel to the straight portion of the bar spaced over the region required in Section 25.4.3.2 of ACI 318-14 to allow use of the 0.8 development length reduction factor for 90° hooks.
5.
For hooked bars with a 90° bend angle, confining transverse reinforcement placed perpendicular to the straight portion of the bars results in lower anchorage capacity than confining transverse reinforcement with a similar spacing placed parallel to the straight portion of the bars.
6.
The effect of concrete compressive strength on the anchorage capacity of hooked can be
represented by the compressive strength to the 0.29 power for hooked bars not confined
by transverse reinforcement and the compressive strength to the 0.24 power for hooked bars confined by transverse reinforcement.
7.
Transverse reinforcement results in an incremental rather than percentage increase in the anchorage capacity of hooked bars.
8.
For a given embedment length, the anchorage capacity of hooked bars increases wi
th bar diameter; this effect is greater with the addition of confining transverse reinforcement.
9.
If closely spaced, three or more hooked bars exhibit a decrease in force per bar compared to joints with just two hooked bars.
10.
Hooked bars cast outside the column core exhibit lower anchorage capacity than hooked
bars cast inside the core.
11.
Hooked bars anchored in the middle of the column core exhibit lower anchorage capacity
than hooked bars anchored on the far side of the column.
12.
High longitudinal reinforcement ratios (> 0.04) in a column provide additional confinement to hooked bars, increasing their anchorage capacity.
13.
The high side cover typically present for hooked bars in walls provides confinement similar to that provided to hooked bars inside a column core.
14.
The proposed Code provisions in Section 5.6 provide a conservative basis for the development length of hooked bars.

The text was badly formatted; I don't know how it will show up.

Dik

 

[charliealphabravo]

KootK, I think I am not understanding what you are saying or we are not understanding what OP is asking. Why does anchorage imply a hook? I don't see language in 318 or 352 indicating that development of a straight bar is inferior to a hook, all other things being equal. On the contrary, it describes the use of a hook when member dimensions do not permit development of a straight bar, implying that the hook is not default.

I was envisioning OP asking about something like this where there is adequate space for development of a straight bar past the critical section. Why would one insist on hooking the top bar for the cantilever into the beam when straight bar development is superior?

The same configuration would apply anywhere you had negative moment steel being developed for a beam or slab through a column. The top steel passes through and is not hooked.

 

[KootK]

Why does anchorage imply a hook?

I didn't really say that "anchorage" implies "hook" Charlie. What I said is that joints require rigorous design or adherence to code "cheats" that obviate the need for rigorous design. The latter path is what designers do 90% of the time and, for beam/column joints designed in conformance with the ACI doc that I mentioned, that requires a hook. If one wants to ignore the cheat and design joints using straight development and STM procedures etc, then that is there prerogative. It's slow going and they'll loose their shirts doing it but that's another discussion.

I don't see language in 318 or 352 indicating that development of a straight bar is inferior to a hook, all other things being equal. On the contrary, it describes the use of a hook when member dimensions do not permit development of a straight bar, implying that the hook is not default.

Again, the key here is understanding that "development" is not the end of joint design. If you draw a strut ant tie model of a beam to column perimeter or corner joint, you'll quickly see that a hooked bar is a vastly superior choice regardless of which bar end style is better for development.

Why would one insist on hooking the top bar for the cantilever into the beam when straight bar development is superior?

One wouldn't insist upon that. One can easily design such a joint with straight development because there are no geometric constraints at play on the embedment length. This is not the case with the classic beam to column or beam to wall joint which is what I assumed OP was getting at.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.