Double Shear Stirrup Vertical Leg Transverse Spacing Requirements 1

[bridgeengineer2007]

Does anyone know of any requirements or specifications in AASHTO or ACI regarding the minimum spacing between legs of double vertical stirrups in a cap or beam for shear design? I'm referring to the spacing across the width of the member, not the longitudinal spacing. For wide beams, it seems that you'd want to maintain a minimum spacing of where the double stirrups overlap, but I can't find any code reference indicating this...

 

[BridgeSmith]

So you're concerned about the minimum spacing of the vertical legs of the stirrups? I don't think there is one.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Greenalleycat]

I don't think we have an explicit requirement on that, but it's implicitly captured by requirements such as providing buckling restraint to compression reinforcement. So it will be determined by how many longitudinal bars you have and their spacing across the member cross-section. Also the seismicity of your area/ductility demands on the beam. An example clause is below:

 

[KootK]

ACI does have something for this that is meant for the tension bars and not the compression bars. The name of the game is preventing the struts that represent the shear truss compression field from having to travel too far laterally.

 

[rapt]

If you want some non-ACI guidance to give an idea of possible limits, as well as the restraint requirements for compression bars, other codes would suggest a maximum of about D or 600mm.

 

[BridgeSmith]

The OP said "minimum spacing between legs". Maximum spacing is a whole different conversation.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[bugbus]

I would have though there's no particular minimum spacing, unless it's causing a congestion issue.

E.g. The example below, I don't think this would raise many eyebrows

 

[rapt]

Oops.

Probably 1.5 times max concrete aggregate size.

 

[BridgeSmith]

Probably 1.5 times max concrete aggregate size.

In the strictest reading of the spec, I suppose that spacing requirement could apply. It's really meant to apply where there's more than just one 2 bars, and the concrete needs to flow between the bars to get to the other side, such as in a column, where if all the spaces were that close, the concrete may not flow well into the annular space between the reinforcing cage and the outside face of the column. In this case, I don't see how 2 vertical legs of the stirrups in the middle of a beam would restrict the concrete flow in any way, no matter how close they were.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[KootK]

In this case, I don't see how 2 vertical legs of the stirrups in the middle of a beam would restrict the concrete flow in any way, no matter how close they were.

I think that a designer would have two paths to choose from in this regard:

1) Provide enough space between the bars (1.5 Agg or whatever) such that both could achieve full bond independently OR;

2) Consider the two bars as a bundled group of bars for development and anchorage.

This may be moot if one chooses to assume that small diameter bars are anchored instantaneously when hooked around transvers rebar, which is often the case. I've never been clear as to whether or not this "out" can be applied to bundled stirrups.

The OP said "minimum spacing between legs". Maximum spacing is a whole different conversation.

I feel that they are related conversations. I envision a situations where designers are needing more than two stirrup legs and asking themselves what the optimal disposition of those would be. In wide beams, consideration of the maximum stirrup spacing across the beam would play into this. Why put your second hoop 3" inboard, or whatever, if you'll just wind up needing another hoop to satisfy the maximums?

This is the stuff that ACI has to say about this.

 

[BridgeSmith]

KootK, I hadn't thought about anchorage, since the only 'stirrups' we would overlap would be in a concrete cap, where we would used closed ties. I agree that anchorage would have to be considered in the spacing of stirrups. If they're too close, the anchorage zones could definitely overlap.

I think you're stretching a little to relate minimum spacing and maximum spacing, though. I get what you're saying about if you put the legs close together in one area, they may be too far apart in others, but I see that as strictly a maximum spacing issue (and generally the result of careless detailing).

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Agent666]

I would not be worried about the spacing between the two legs in that arrangement (as gusmurr posted), having said that I'd never personally detail a stirrup set like that with two overlapping stirrups over one bar.

For one it is bloody hard to build in that arrangement and secondly, it's just a poorer detail compared to a single stirrup with a central link, especially for a capping beam as per OP's post which I assume is on top of some piles in a retaining wall or something like that where you'd usually have some horizontal shear as well that you'd need to resist?

For example, we often have stirrups like those shown below in a beam and do not worry about clear spacing along the top and bottom surfaces at the green cloud between the individual stirrups where stirrups are pushed hard against one another..... does anyone worry about this... ?

For spacing of individual legs across a section follow the table from ACI Kootk posted. The NZ code which I mainly deal with has had similar provisions long before ACI added them. To some degree, you can exercise some judgement as long as an average spacing is satisfied. These spacings are required to prevent the vertical struts within a beam from becoming too shallow and causing perpendicular tension across the top or bottom face of the member causing concrete tension cracks (which your proposed arrangement would also be rubbish for resisting by the way....)



Greenalleycat, have a look at clause 9.3.9.4.12(b) in NZS3101, there are specific provisions for NZ code to cover this .... it's not worded that well, but it is there. A lot of people seem to miss the 'at right angles' thing...

https://engineervsheep.com

 

[KootK]

KootK, I hadn't thought about anchorage, since the only 'stirrups' we would overlap would be in a concrete cap, where we would used closed ties.

While I realize that it's conventional dogma to think other wise, rationally, I don't see why closed stirrups should be all that much better than 90 hook. Frankly, when it comes to shear stirrup anchorage, I'd prefer open stirrups with 135's to either 90's or closed ties. The truthy-ness of this is, in part, a function of the beam width of course.

I think you're stretching a little to relate minimum spacing and maximum spacing, though.

So be it. Hopefully OP will resurface and give us some more context for here particular problem.

 

[KootK]

For example, we often have stirrups like those shown below in a beam and do not worry about clear spacing along the top and bottom surfaces at the green cloud between the individual stirrups where stirrups are pushed hard against one another..... does anyone worry about this... ?

I feel that it's selve solving to a degree. The two situations in which one might attempt to exploit the inner hoop top and bottom legs would be:

1) Shear in the lateral direction and;

2) Torsion.

I would not expect most seasoned engineers to rely on anything but the outer hoops for either of those purposes.

In my mind, inner hoops are mostly only useful for additional shear resistance in the direction in which they cross the entire depth of the member.

 

[Agent666]

Yeah, that and plain round bars typically utilised for stirrups are not so reliant on bond along the straight length as a deformed longitudinal bar I guess, so stacking a few bars next to each other without maintaining minimum spacing probably isn't so much of a concern (as anchorage achieved via the bends around main longitudinal reinforcement at ends/corners typically).

I'd definitely place some reliance on the smaller stirrup in the cross direction in a beam-column joint for example, if code allows it no reason not to. It's not as effective depending on the geometry but helps with reducing congestion (provided your code provides some rules for this I guess though, unsure of ACI's position on this). But for torsion most codes only let you use the outermost closed stirrup anyway, seasoned engineer or not.

https://engineervsheep.com

 

[KootK]

...plain round bars typically utilised for stirrups

Must be a regional difference. I've yet to see a plain round bar used as a stirrup in a newbuild outside of, perhaps, welded wire fabric setups.

I'd definitely place some reliance on the smaller stirrup in the cross direction in a beam-column joint for example

I'm having trouble visualizing the purpose for which you'd be using those legs in a beam-column connection. Can you elaborate. I could probably get behind using the smaller stirrup, partial width legs for shear friction given the right shear plan and proportions.

 

[HTURKAK]

I did not look to the previous posts in detail. The minimum spacing in AASHTO LRFD Bridge spec.is defined for CIP concrete;
Copy and paste of relevant clause;

5.10.3.1 Minimum Spacing of Reinforcing Bars
5.10.3.1.1—Cast-in-Place Concrete
For cast-in-place concrete, the clear distance between parallel bars in a layer shall not be less than:
• 1.5 times the nominal diameter of the bars,
• 1.5 times the maximum size of the coarse aggregate,
or
• 1.5 in.

Regarding max . spacing ( across the width )

5.10.7—Transverse Reinforcement for Flexural Members addresses 5.10.6

Compression reinforcement in flexural members, except deck slabs, shall be enclosed by ties or stirrups
satisfying the size and spacing requirements of Article 5.10.6

..and Article 5.10.6 limits the max. spacing 48 in.

IMO, the minimum spacing should be decided acc. to concreting , and vibration method..










Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[rapt]

Kootk,

Plain round bars for shear reinforcement were prevalent when I started in Australia in the 1970's.

In PT we had problems with them as tendons in beams tended to be supported by short horizontal bars tied to the vertical legs. They tended to slip during the concrete pour so your profile was stuffed!

So for PT we changed to deformed bars for stirrups purely to solve this slip problem.

The remainder of the industry in Australia then changed to deformed bars as a result of this.

Not sure about the rest of the world.

 

[BridgeSmith]

A note on the AASHTO minimum bar spacing quoted by HTURKAK: A layer would, by necessity, consist of more that 2 bars, otherwise lap splices, including most non-contact lap splices could not be used. Furthermore, if you read on a little, you find this:

5.10.3.1.3—Multilayers
Except in decks where parallel reinforcement is placed in two or more layers, with clear distance between layers not exceeding 6.0 in., the bars in the upper layers shall be placed directly above those in the bottom layer, and the clear distance between layers shall not be less than 1.0 in. or the nominal diameter of the bars.

which clearly indicates a "layer" is an arrangement where the bars are oriented horizontally and spaced horizontally.

There's also the next subsection:

5.10.3.1.4—Splices
The clear distance limitations between bars that are specified in Articles 5.10.3.1.1 and 5.10.3.1.2 shall also
apply to the clear distance between a contact lap splice and adjacent splices or bars.

Clearly indicating that 2 bars can be touching one another, so long as the spacing to other adjacent bars meets the minimum spacing requirements

The entire reason for the minimum spacing of bars is to allow concrete to flow around and through the bars. In the middle of a concrete beam, 2 vertical stirrup legs, no matter how close together they are, are not a restriction to concrete being able to flow around them.


Rod Smith, P.E., The artist formerly known as HotRod10

 

[KootK]

@rapt: thanks for the historical context... I love that kind of thing.