Shear reinforcement for inclined member

[blake989]

I am currently working on the design of reinforced concrete stairs. The middle landing will be cast as part of the stair, with sets of stairs on each side. The stairs land at an angle to the girders. In this sense, the stair is like a folded plate. My question is how to detail the stirrups over the supports at each end. Will a shear crack develope at 45 degrees with respect to the member axis, or a global axis. Basically, should the stirrups be inclined or perpendicular to the member axis.

 

[SlideRuleEra]

If you have the proper amount of reinforcing steel, it probably does not matter. For constructability, I would suggest having the stirrups perpendicular to the axis. Otherwise you increase the chances of having the steel angled incorrectly during construction. The key issue is to put in enough rebar.

 

[blake989]

Thank you SlideRuleEra. Typically I always locate the diagnal tension steel/shear reinforcement perpendicular to the axis, and upon your advice I will with this beam as well. It is just that at each end of the beam, the ends straighten out a foot or so from the end. When I say straighten out, I mean they are no longer sloped. This occurs in an area where stirrups are req'd. I am familiar with the cornering details for the flexural reinforcement, and understand that a minimum depth usually satisfies the shear requirements for a slab, but are you aware of any standard details for my required stirrups at this change in slope?

 

[SlideRuleEra]

blake989 - I do have an old reference, "Reinforced Concrete & Masonry Structures" by Hool & Kinne, McGraw-Hill Book Company, 1944. The detail shows the stirrups in the inclined portion perpendicular to the inclined axis and stirrups in the level portion perpendicular to the level axis. No stirrups are shown at the actual "bend".

As for the reinforcing at this "bend" here is an edited quote from the reference:
"In addition to the main reinforcement ... which is the amount required to resist negative as well as positive moments, 3/8" distributing and spacing bars should be placed above the main reinforcement ... The main bars should be of such lengths as adequately to reinforce all possible regions of tension. This means that the negative reinforcement should extend about one-fourth the distance between supports from each support, and where landings are included in the span, the negative reinforcement should also be carried well into the landing slab, since the stiffness of the landing slab connected to the two inclined flights may cause negative moments in the flights for some distance out from the landings."
I have left out references to tables and diagrams in the text.

There is a lot going on at this transition point and it does need proper attention.

Hope this helps - if you can't find more modern ways to address this situation, I can e-mail you a scan of these pages, just post an address.

 

[SlideRuleEra]

blake989 - About the detail that I referenced about the sturrup locations - after adjusting my bifocals, I do see one sturrup at the "bend" between the inclined and level portions. It is postioned to bisect the angle between the two.

 

[blake989]

I have an old reference "Simplified Design of Reinforced Concrete" by Harry Parker published in 1943 with the main reinforcement details that you describe, but without the stirrup and 3/8" bar details you have provided. And no, I have not been able to locate any modern text to address this issue.

 

[JAE]

Be careful when referencing "old" (i.e. pre-1960) details and design information regarding shear as shear was not very well understood back then. Many beams and other concrete entities were relatively under-designed in that shear is an abrupt-type failure and additional safety (i.e. lower phi factors, etc.) were not utilized.

And many times the way engineers handled shear back then will not meet current codes or design requirements.

 

[SlideRuleEra]

I concur with JAE about the dangers of old methods and shear failure. Several years ago I was involved in a full scale (per ACI code) load test of a poorly constructed, all reinforced concrete building. It did not fail, but we all had white knuckles when, after the test, certain as-built drawings were found to be wrong. The building SHOULD have collapsed during the test in a sudden, catostropic shear failure.

 

[JAE]

Yikes -

Nothing like a little engineering adrenalin.

 

[reeves111]

since the definition of shear in beams is the transverse resisting force, it will always act perpendicular to the axis (centerline) of the beam. Therefore to answer your question they should be perpendicular to the member axis