Stirrups as punching shear reinforcement over columns

[canadiancastor]

I've used stud rails before and it was a good experience, though maybe a bit expensive. Does anyone have any experience specifying stirrups as punching shear reinforcement above columns? I know the both ACI and CSA standards allow it, but I'm wondering if it the additional bars make the column areas too congested?

 

[EngDM]

It does make it very conjested. Typically we use stirrups only if we need a drop pannel. You get additional shear capacity from the drop of course, but also from the stirrups. It makes it easier to fit all of the steel, considering you also have integrity steel to add and it moves the hooks for the stirrups out of the way of your slab/drop panel top mat. Without drop panels I would lean towards stud rails.

 

[Luceid]

Congestion really is the primary difference. There is a really minor calculation difference in a shear capacity - see ACI for this, but it's really not enough of a difference to worry about. Stud rails were originally invented to combat congestion from what I have been told

 

[cliff234]

Installation of loose stirrups in slabs around columns in flat plates is problematic. Studrails are faster to install (versus stirrups), improve constructability, and are usually less expensive (when considering installation cost). When shear reinforcing is required in flat plates, the stirrups/studrails are required at very close spacings (d/2). It’s time consuming (and expensive) to accurately install lots of loose stirrups. Strips of studrails on the other hand are quick to install (prior to placing the slab flexural steel), with the primary requirement being that the first studrail stud (closest to the column) must be accurately installed. (If the first studrail stud is too far from the column, the studrails are useless).

Another disadvantage of using loose single leg stirrups is that they require time-consuming installation transverse bars inside the top and bottom hooks. When those transverse bars are the top and bottom flexural steel, then that steel must be lowered (for the top bars) or raised (for the bottom bars) by ½” to maintain ¾” clear cover of the hooks. Studrails on the other hand don’t have hooks, so there’s no need to raise or lower the slab reinforcing. If additional bars are installed inside the stirrup hooks (versus raising and lowering the slab flexural steel) then there’s no need to raise or lower the slab flexural steel, however the additional bars add to the congestion. Bottom line: Use studrails (not loose stirrups).

It is important to first accurately nail the studrails to the formwork before any slab rebar is installed. This is easy to do because there is nothing else in the slab to prevent precisely installing them. It is impossible to accurately install studrails after the rebar is installed. Some studrail manufacturers say you can install studrail strips upside down if the slab rebar is already installed, however this is always a bad idea, because in my experience it’s impossible to accurately install upside down studrail strips due to interference with all of the each way T&B rebar crisscrossing over the column. Plus, if the slab bars were accurately installed with 3/4” top cover, having the studrail strips laying on top of the top bars and will result in less than ¾” cover.

When we need drop caps/panels, we always make the drops deep enough so that we don’t also need studrails.

 

[Tomfh]

It can be done, but as noted above there can be issues installing them.

I’ve inspected many a flat plate where the stirrups have taken a back seat to everything else going on around the columns. The stirrups often end up pushed aside, or leant over so they’re not effective.

Stud rails on the other hand are installed early, fixed in place, and tend to assert themselves in a way the stirrups don’t. They tend to remain in place, ready to do their job.