bend rebars - tension + shear reinforcement?

[n3jc]

Hi, im looking at some old projects.
When it comes to RC beams I see that almost always bottom (tension) reinforcement is bent to 45[sup]o[/sup][sup][/sup] before it reaches supports. Does that mean that reinf. bars that are bend like this actually have 2 functions - as bending reinforcement and a shear reinforcement? im asking this because there are as always classic stirrups too.

Do they bend bars like that for that purpose (extra shear resistance) or is that just a way to anchor bars at the top of a beam?

In my country they rarely use bend bars for shear reinforcement and if they do, they do not add extra stirrups. So whats the deal?
I hardly believe that they design shear reinforcement in a way that some are stirrups and some are bend bars - that is in my opinion unnecessary complication.

 

[dik]

You are correct... truss bars acted as shear reinforcing, too... once did modifications to an old building and when the concrete was cut out, some of the rebar was trussed... and still bundled and wired together as it was delivered to site with the metal ID tag still in place... square twisted bars...

 

[TXStructural]

These bar serve as negative moment reinforcement over supports, shear reinforcement, and as midspan moment reinforcement. If you have discrete shear reinforcement, the bent bars are probably designed only to resist positive and negative beam moments. They are commonly called trussed bars but the methods of trussed bars in the 1900-1920 period were a bit more complicated than single bent bars. (See CRSI Vintage Steel Reinforcement in Concrete Structures, 2014) While some DOTs still require them, they are seldom used in the US these days. Bending such bars is a pain in the fabrication plant unless the fabricator has dedicated equipment that can make multiple bends on both ends without the long ends of the bar having to sweep across the shop floor. Schnell makes this kind of bender but they are not common in the US so I cannot speak to the economy of the practice. Where minimizing steel weight is important and seismic exposure is a minimum, this might still be a viable method.

I am currently working with this kind of reinforcement in a building in the US dating from 1911/1916/1924 (depending on which part of the building). I have also see it in joist construction elsewhere from the 1905-1930 period, and later.