max. spacing between stirrup links in the beams 1

[SBRC]

Greetings,

AS per this thread link

https://www.eng-tips.com/viewthread.cfm?qid=397909As

Majority of engineers refer to ACI 318, 11.4.5 and state that transverse stirrup spacing shall be as prescribed in 11.4.5.1.

In my opinion, ACI speaks about spacing of stirrups perpendicular to longitudinal axis of member which is vertical axis and it specify vertical stirrups spacing(S) not any horizontal spacing of stirrup.

Lateral spacing is of no concern since we calculate "Av" which itself depends on width of beam"bw" More wider beam will need more Av.

Would any engineer shed some light over it?Is my understanding Correct?

 

[hokie66]

The longitudinal axis of a beam is normally horizontal, not vertical. The d/2 requirement is to ensure that shear reinforcement crosses any potential diagonal tension crack.

The link you gave above does not seem to exist. Does it relate to the number of vertical legs required at a given beam cross section? Wide beams do require more vertical legs.

 

[JAE]

In my opinion, ACI speaks about spacing of stirrups perpendicular to longitudinal axis of member which is vertical axis and it specify vertical stirrups spacing(S) not any horizontal spacing of stirrup.

ACI is speaking about the spacing of “vertical stirrups”. It is not speaking of the “vertical spacing” of stirrups.

Lateral spacing is of no concern since we calculate "Av" which itself depends on width of beam"bw" More wider beam will need more Av.

Lateral (or horizontal) spacing is the concern here. For beams, the code provides for three conditions:

Vu > phiVc
Calculate and provide Av based on the magnitude of Vu but no less than the minimums (d/2 etc)

phiVc/2 < Vu < phiVc
Provide the minimum Av required

Vu < phiVc/2
No stirrups are required
(I usually still provide min stirrups in this condition anyway, or something near to minimum.

 

[r13]

Spacing is the distance between two stirrups, measured along the axial axial axis of the element.

- The spacing of two vertical stirrups in a beam, is the "horizontal" distance in between.
- The spacing of two horizontal stirrups in a column, is the "vertical" distance in between.
- The spacing of two stirrups in a sloped girder, is the distance in between the two, measured "laterally" along the axial axis.

 

[JAE]

Stirrups are placed in beams.
Ties are placed in columns, not stirrups.

 

[r13]

Exchangeable terminology - same type/shape (enclosed hoop), and same function (shear/torsion). I prefer to call J-hooks (one end 180°, the other 90°), and straight hooks with 135° legs at ends as ties.

As in my example, how you call the enclosed hoop shear reinforcement in a girder tilted 45°? Or you might argue the girder essentially a column?

 

[JAE]

Not exchangeable per ACI.
Different purposes.
Stirrups - takes shear forces in beams.
Ties - Takes shear forces and primarily confines compression reinforcement.

Beam stirrups can be open.
Column ties have critical detailing requirements pertaining to closure, cross ties, and spacing that is totally different than in beams.

For 45 degree "beams" the issue is whether the member has significant axial compression to warrant the use of ties, which have different spacing requirements than stirrups as well.

 

[r13]

Argument accepted, except the stirrups in beams are meant to confine the longitudinal bars in compression too, just not so pronounced as in columns. The U stirrups require hooked ends, and most often require the ties mentioned above.

 

[Agent666]

This particular clause is talking about the spacing of vertical stirrup legs across the cross sections width. I don't know if that was 100% clear in reading others comments to be honest.

The reason for this requirement is to ensure a relatively well distributed reinforcement shear capacity across the width of a member. If you say have a single leg at the perimeter of a wide beam and no legs in the middle then the tension via strut and tie across the base of the beam can exceed the tension in the vertical legs leading primarily to the possibility of longitudinal cracks along the axis of the beam. This clause is meant to alleviate this concern by ensuring the legs are never more than a certain distance apart across the width so the horizontal parts of the stirrups are not working too hard.

 

[hokie66]

Agent666, I don't think that is what Clause 11.4.5.1 is about. The d/2 is not typical for the vertical leg spacing across the cross section. Your points are well taken, and I am sure there is an ACI provision which addresses the issue, but don't think it is that one.

 

[Agent666]

We have a similar provision in NZS3101 (which is predominantly based on ACI318), and that exactly what that clause is intended for. It uses similar wording but with more stringent spacing requirements. I'll post tomorrow for comparison (and actually review ACI318) to show the wording and people can make up their own mind up.

 

[hokie66]

Thanks. Will be waiting on your post. I am at a disadvantage, as I don't have access to the ACI code at the moment, but to me that clause looks like what we have always used for spacing of stirrups, not the number of stirrup legs required at a given section.

 

[r13]

ACI terminology definition - shear reinforcement

stirrup — bar or wire reinforcement oriented normal to or at an acute angle to the longitudinal reinforcement in a flexural member and extending as close as practical to the extreme tension and compression fibers of the cross section. (See also transverse reinforcement and tie.)

tie — (1) loop of reinforcing bars encircling the longitudinal steel in columns; (2) a tensile unit adapted to holding concrete forms secure against the lateral pressure of unhardened concrete; (3) a tension member in a strutand-tie model.

transverse reinforcement — reinforcement at right angles to the longitudinal reinforcement.

 

[JAE]

I would side with hokie66 on 11.4.5.1. That section is about the longitudinal spacing of stirrups down the length of the member.

For wider beams, ACI 318 doesn't say anything directly about multiple stirrup leg transverse spacing.

This has been a subject of discussion here before:
thread167-397909
thread592-153245

 

[Settingsun]

The NZ code has two sub-clauses with clear wording that one applies to longitudinal spacing and the other to transverse. In ACI 318-14, the corresponding clause to the one shown in the first post in this topic is clause 9.7.6.2.2. The commentary to clause 22.5.10.5 mentions transverse spacing (no values given though), which suggests to me that 9.7.6.2.2 isn't intended to apply to transverse spacing. I have one US textbook (Wight) which refers to the FIB model code for transverse spacing rather than to the ACI code.

Clause 9.7.6.2.3 applies to inclined stirrups and it appears clear that longitudinal spacing is the intent of that clause, so it would follow that 9.7.6.2.2 is also intended to apply to longitudinal spacing.

 

[JAE]

Yes, I'd agree that in ACI 318-14, section 9.7.6.2.3 applies to spacing along the axis of the beam - not transverse spacing.
You have multiple occasions when a 45 degree line (crack) can occur down the length of the member and so you need multiple stirrups crossing multiple 45-degree cracks. Again - nothing about transverse spacing of multiple stirrup legs.

Your reference to 22.5.10.5 does speak directly to this and suggests that an engineer should consider multiple (i.e. more than 2) legs across a wider beam - but for whatever reason, 318 doesn't limit leg spacing transverse. The links I posted above have a few comments (mine included) that suggest using some sort of guide. In my case I used my mentors recommendations. In NZ it appears the code demands it.

 

[rapt]

My understanding is that is the maximum longitudinal spacing of beam stirrups in ACI. I have never found a transverse spacing limit in ACI.

Different codes use the minimum of 450 to 600 and .75 to 1 times D as the transverse spacing limit.

 

[r13]

ACI does not have rigid limit on transverse spacing of transverse shear reinforcement in cross section, but I recall that in seismic detailing, it requires intermediate cross ties in addition to stirrups. I don't know if it is still required on these days.

 

[Agent666]

Here you go, the requirements from NZS3101, 'at right angles to the longitudinal axis of the beam' is the same essentially as 'perpendicular to the member axis' which was my actual train of thought. This is clearly differentiated from item (a) which is the stirrup spacing along the member.

In ACI318 there are spacing requirements across the section, take a look at CL 9.7.6.2.2 in ACI318-19 (cut and paste below). Maybe this was only a recent addition in 2019 version, following 15 years after we first adopted it. OP didn't say what version of ACI they are looking at though....

Here in NZ stirrups are called stirrups for the most part, whether they are in columns or beams really has no practical difference, it's just a term most people feel comfortable with describing essentially the same thing. In our design standards however beam 'stirrups' are referred to as 'stirrups' and column 'stirrups' are referred to as 'stirrup-ties' sometimes. You are trying to make a contractor learn too many words in my opinion ....... KISS.

 

[Agent666]

FYI, looking through ACI318-14, the above across width spacing isn't present. Therefore added in 2019 version.