ACI 318-11 Shear Reinforcement 2

[precast78]

It is very hard for me to understand chapter 11.4.7.5 to 11.4.7.8. Do you guys have a sketch or photograph what these look like?

 

[cooperDBM]

11.4.7.5 - 11.4.7.7 is referring to longitudinal bars that are at the bottom of the beam in a positive moment area, which are cranked up to act as top longitudinal steel in a negative moment area. These bars are acting as flexural steel either side of the crank. The crank is the portion of the bar where it bends up from the bottom at say 45 degrees and then bends back to horizontal at the top at the same angle. These clauses say that the portion of the bar, at 45 degrees to the horizontal - the crank, acts as shear reinforcement at that point in the same way as a dedicated stirrup inclined at the same angle. I don't think these bars are used much, or at all, anymore. In slabs they've been superseded by stud rails. If you Google cranked shear reinforcement you'll see some images.

 

[cooperDBM]

Also called "bent up bars".

 

[precast78]

Cooper thanks! So basically 11.4.7.5 is a reduction even though inclining the rebars will make it more perpendicular to the shear crack? How do you do 11.4.7.6?

 

[KootK]

So basically 11.4.7.5 is a reduction even though inclining the rebars will make it more perpendicular to the shear crack?

Yes. Once the crack forms, the force to be dealt with is a vertical force. As such, vertical reinforcing is the most efficient way to go about resisting that force. If you sketch out a strut and tie model of the end of the beam treating the bent up bar(s) as a diagonal tension tie, you'll see it.

How do you do 11.4.7.6?

You use equation 11-16 with "s" as the horizontal spacing of the bent up bars.

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.

 

[TXStructural]

Also called truss bars. They are still used in bridges in some states. They are very difficult to bend properly and even more difficult to place. I watched fabrication of some 40-60 foot long truss bars. 4-5 guys along the bar, moving in unison to hold the bar as the machine slowly bent the end into shape. It takes a big, empty shop to handle fabrication like this.

The practice is now to use discrete, single-purpose bars in the form of top bars, bottom bar, and ties/stirrups. This results in more steel weight, but significant savings in cost for fabrication and placement. In buildings, the practice of using these bars has all but been abandoned.

There are even crazier configurations shown in the CRSI Vintage Steel Reinforcement book, including concentric hoops, overlapping hoops, and 3-, 4-, and 5-way reinforced slabs.