Shear Reinforcement On Wide Concrete Beams 4

Think of the shear-truss analogy, you don't want the shear going from the middle of the section (concrete strut) to the perimeter (tension ties). You'd rather it an even distribution across the cross section.

I know the Aus code has a limit of around 600mm max distance between an adjacent leg/stirrup/link/fitment.

Also, the additional legs help tie up the beam cage.

Image your wide beam as a two end cantilever with support in the middle. Then provide shear reinforcement accordingly (zero at tips, maximum at support, critical face can be d, or d/2, depends on the depth).

Once the beam becomes wider than its depth you start losing stirrup efficiency. Have a look at these two articles: Link #1, Link #2.

abusementpark said:

...but haven't seen any specific requirements for when you need to do this.

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I believe that ACI covers this in the latest edition.

The more critical issue, in my opinion, is detailing shear transfer where wide beams pass over narrower columns and all of the shear is forced out of the beam width and into that connection.

And speaking to Trenno's comments about STM thinking, I've applied the following from Canada's concrete code in the past using similar logic. [12 x db] for high [Vs] situations which seems fairly reasonable to me in practical situations.

Kootk,

For wide beams on narrow columns we would always check punching shear as well. Where the beam is wide enough and shallow enough for a punching perimeter to form within the beam dimensions and that perimeter is shorter than the 2 * beam width, I would always check punching shear as well as one way shear.

KootK said:

I believe that ACI covers this in the latest edition.

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Thanks for that screenshot! That is way more stringent than I would have imagined. Basically, if Vs is high enough, you could have a situation where, for example, an 18" wide x 30" deep beam needs an interior tie. That seems a little ridiculous.

KootK said:

The more critical issue, in my opinion, is detailing shear transfer where wide beams pass over narrower columns and all of the shear is forced out of the beam width and into that connection.

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At what point would you start worrying about this?

abusementpark,

I think the logic is that concrete shear capacity Vuc is dependent on aggregate interlock. As the shear crack width increases aggregate interlock reduces so shear capacity reduces. The spacing of the legs needs to be restricted to reduce the shear crack width and that is more critical as the shear stress carried by the stirrups increases.

That seems a little ridiculous.

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Thats very relative though to your perspective, for parts of the rest of the world (you know outside of America), certain countries have had this in their codes for literally decades. Why? Because it is fundamentally required to address how shear works in the real world vs simplified code equations for the reason(s) Rapt notes.

Suddenly because ACI have finally had it codified in a similar manner and you chaps have been effectively getting away with not doing it previously, what is probably considered "standard international best practice" gets called out as being ridiculous.

To add to what Rapt notes, it is also I believe about keeping the horizontal forces developed in the cross section in check as well by limiting the strut angles to the vertical if you consider limiting cracking longitudinally along the beam if you get larger horizontal components to the vertical shear as it resolves itself to the vertical stirrup locations across the width of the cross section. The wider spaced your verticals are across the width of the section, then the higher these horizontal stresses are.

There was another recent thread where this was discussed in some detail, but for the life of me cannot locate it.

At what point would you start worrying about this?

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Most codes cover this, but it may be discussed in the scenario of effective joint widths or effective beam widths, like you can only utilise longitudinal reinforcement some distance either side of the column or something similar for beam/column joint design. The latest incarnation of ACI318-19 covers this in several clauses, one obvious one being the commentary to 18.6.2. I'm sure a search will turn up other restrictions or considerations in the scenario of wider beams supported on narrower columns. Consideration of some strut and tie model across the width of the beam may be required to ensure the shear out in the extremities of the beam can transfer into axial load in the columns.

My first thought with that shot was the Florida pedestrian bridge collapse.

Dik