max. spacing between stirrup links in the beams 2

[h-badawy]

Dear experts

is there any item in ACI-code mentioned the maximum spacing between the stirrup links in concrete beams, for example if i have beam with width 600mm i can put only two links or i have to decrease the spacing between the links by add more links.

Thank you

 

[JAE]

Look at ACI 318 section 11.4.5 (for simple shear - no torsion beams). This provides the spacing limits.
11.4.6 provides for minimum amounts of shear reinforcement which sometimes controls and limits your spacing.
11.5 includes torsion design and further has limits similar to above (see 11.5.5 and 11.5.6)

The above is for 318-11 and older. 318-14 is reconfigured so you'd have different section numbers.

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[rapt]

ACI does not appear to have a specific limit on transverse spacing of the legs in links.

Most codes around the world seem to limit it to between 300 and 600mm and some relationship to Depth or Effective Depth of member (varies between codes). The depth relationship is sometimes related to the level of shear stress eg Canadian code.

 

[h-badawy]

Thank u JAE and rapt

Thank you

 

[KootK]

I don't know of a hard rule in either the ACI or CSA codes. I've taken to applying the diagram below taken from the strut and tie modelling section of Canada's concrete code. Basically, a max spacing of 12 times the flexural reinforcing bar diameter. This seems to produce reasonable results and, since we usually base shear reinforcing on a truss model anyhow, applying a strut and tie provision seems sensible to me.

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.

 

[Trenno]

In the AUS code it is 600mm or D (whichever is less).

Now adding to this discussion, how does one curtail this shear reinforcement down towards midspan?

When you go back to min. shear reo, does one still need to uphold this min. transverse spacing rule?

 

[KootK]

@Trenno: are you talking about longitudinal stirrup set spacing or transverse stirrup leg spacing? OP is interested in the latter but I suspect that you are talking about the former. Min reo still needs to satisfy max spacing. Otherwise, a shear crack could develop between stirrups and the min reo would be of no use.

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.

 

[hokie66]

KootK,
No, I think Trenno is on the same page, i.e. transverse leg spacing. I suspect he is thinking about wide flat beams which we often call band beams. A common approach is to call these one way slabs, and you often don't need shear reinforcement using the slab provisions. But I could be wrong...eh, Trenno?

 

[Trenno]

Yep, talking about transverse leg spacing here sorry.

The AUS code isn't too clear on when you can curtail this down.

For example, a 2400w x 500d beam will require 6 legs (3 loops... I hate single leg stirrups) crossing the beam's width. Having 6 legs continue, @ say max long. spacing, for the whole beam span seems overkill.

Obviously one still needs to think about constructibility issues here, beam cages etc etc...

 

[rapt]

Trenno,

As I understand it, if you need the shear reinforcement (minimum still means you need it), then the spacing limit applies. If you do not need it for shear rules but are supplying it further towards mid span than is required by the minimum shear rules, it is up to you. It then gets down to the spacing you need to support the top reinforcement mat and tendons if there. With over-weight concretors jumping up and down on it, I still like 600 - 800 maximum, so 2 ties (3 spaces) across a 2400 wide band beam.

 

[KootK]

Yeah, the same system is popular here. It's still considered a two way system, however, even though much of the design procedure echoes one way.

If one were willing to look past the letter of the code and constructability issues, I could see an argument for ditching the interior legs. If you could hypothetically narrow the beam such that spacing rules were satisfied and Vc + Vs still worked based on that hypothetical section, I'd be okay with it.

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.

 

[Trenno]

Good point KootK. Might give it a whirl next time I run into this problem.

 

[hokie66]

As rapt said, whether you need stirrups for shear or not, you need to support the top slab bars. Nothing worse than slab bars across a band beam which feel like a trampoline when you step on them.

 

[Trenno]

I agree. I highlighted those potential issues in the original post.

One work around would be maintain one full loop (full width) at say a larger spacing for midspan areas, therefore the top bars will be nicely secured and you're not being heavy with your ligs.

 

[Agent666]

I thought clause 11.4.5.1 in ACI 318-08 covered this. 'Perpendicular to the axis', i.e. legs across the width of the member? If its was spacing along the member it would say parallel to the member. But maybe I'm reading it wrong as we have a similar worded clause in the NZ concrete code that makes the distinction between 'along' and 'across'. I could have sworn in later revisions of ACI 318 that they further clarified it, but I cannot find my PDF to compare.

We have a similar clause in New Zealand concrete code as mentioned, but the spacing across the section for each leg need not be less than 250mm between vertical legs, unless the 0.33 x sqrt(f'c)b_w x d limit applies, then the lower limit is reduced to 200mm. In NZS3101 its really clear in that there are separate clauses for the spacing of stirrups along the axis of the beam and the spacing of individual legs perpendicular to the member axis. Our code is pretty much based on ACI 318 with some more stringent requirements due to the seismic side of things, so I'm surprised ACI doesn't cover it.

The reasoning for having relatively evenly spaced legs is so it prevents tension across the bottom of the section due to the strut angle for shear forces getting to the stirrups being too shallow. If you imagine a wide shallow beam with a single perimeter stirrup, there is a tension across the bottom of the section due to the shear having to strut at a shallow angle to the stirrup/corner bar.

See attached clause from NZS3101.

 

[rapt]

Trenno,

In midspan areas where it is only being provided for top reinforcement support, the logical maximum spacing is the maximum spacing for rebar chairs. The is no code shear reinforcement requirement there so shear spacing requirements do not apply.

If you do as you suggest for a 2400 wide band beam then the legs would be at about 2300 transverse and whatever spacing you select along the beam. 2300 is far too much. If there are PT tendons being supported, I would be looking at an absolute maximum of 1000 spacing, which is why I suggested 2 ties = 4 legs. For reinforcing only, with the supported reinforcement height being 350-400mm, I would think 1000 centres is still the maximum as things get pretty unstable with heavy worker loads on them.

 

[KootK]

[I thought clause 11.4.5.1 in ACI 318-08 covered this. 'Perpendicular to the axis', i.e. legs across the width of the member? If its was spacing along the member it would say parallel to the member.

Here's the ACI clause for reference. It's taken to refer to longitudinal spacing but I can see how the wording leaves room for improvement.

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.