Probable Shear Strength

[releky]

Probable Shear Strength is the equivalent of Probable Moment plastic strength in seismic design of a building. How many of you use the probable shear strength for shear in beams? This would require much more stirrups than normal. I notice many colleagues don't exactly follow it but uses codes spacing of stirrups. What do you use and why?

 

[JoshPlumSE]

That's just part of concrete design for special moment frames. If you're not doing that then you've not following the code.

Though I have to admit that I have never actually designed a special concrete moment frame as I have worked mostly in Steel when in high seismic areas.

 

[releky]

Who here use the concept of probable shear strength in the design of concrete buildings? My mentor told me he didn't want to base strictly on probable shear strength because he didn't want to make the beam so strong that it would result in weak column strong beam.. he wants the plastic ductility developed at the beam and not at the column.. but he still use probable moment strength bec this is necessary. My question is how ductility is related to shear stirrups reinforcement in beams? Remember all the basic shear requirements of stirrups for deadload, liveload, sd load is strictly followed.. only the probable shear strength is what i'm describing with regards to ductility.

 

[JoshPlumSE]

Increasing the shear strength of the beam doesn't increase the moment capacity of the beam. We increase the shear capacity to make sure that we don't get a brittle shear failure when the frames starts hinging.

This is even more important for columns than it is for beams. Make sure you have adequate confinement of the column bars in the regions where plastic hinging could occur. Again, the key is to ensure that the column can continue to carry vertical load and avoid brittle failures after plastic hinging of the frame occurs.

I believe this is pretty well spelled out in ACI 318.

 

[JoshPlumSE]

Increasing the shear strength of the beam doesn't increase the moment capacity of the beam. We increase the shear capacity to make sure that we don't get a brittle shear failure when the frames starts hinging.

This is even more important for columns than it is for beams. Make sure you have adequate confinement of the column bars in the regions where plastic hinging could occur. Again, the key is to ensure that the column can continue to carry vertical load and avoid brittle failures after plastic hinging of the frame occurs.

I believe this is pretty well spelled out in ACI 318.

 

[releky]

I want to focus on the beam plastic shear strength because most only focus on beam plastic moment strength. In plastic hinging, how many percentage more important is the the moment capacity of the beam compare to the plastic shear capacity? I understand about ductility and how the bars for moment capacity needs to make plastic hinges. But what's the behavior of shear when this plastic hinges develope? How much plastic shear would develope for a given plastic moment?

 

[JoshPlumSE]

The maximum probably shear demand is usually calculated as 2*Mpr / Beam span. That assumes a constant shear which is capable of hinging the beam at both ends. If the shear capacity of your beam is greater than that max probable shear force, then it should ensure ductile hinging controls over a brittle shear failure.

I'm not as up on this for concrete as I have worked mostly in steel. However, you might also want to add in the shear due to gravity load Vg to calculate the total shear demand on your beam. At least, that's what we do in steel these days. Conceptually, it would make sense to do the same thing in concrete as well.

 

[releky]

In the US where ACI is followed. Are all concrete buildings special moment frame even in states like New York that doesn't have earthquake? Have you encountered a building that doesn't take into account plastic shear strength (meaning they only use deadload, liveload, Sload)? For these buildings which doesn't consider seismic activity... so the beams would just suffer catastrophic brittle failure... what buildings in the US have experienced this already?

 

[JoshPlumSE]

Absolutely, if you are in a low seismic area they you don't have to design your frames to ductile in this manner. I suggest you pull open the ACI, ASCE or IBC to see exactly what the cutoffs are and when you are allowed to use a frame that doesn't follow the detailing provisions of ACI chapter 21.

 

[hokie66]

releky,
Special moment frames in concrete are a rarety in most of the world. In 40 years of practice, and many concrete buildings in the US and Australia, I have never used a concrete special moment frame.

 

[releky]

In India. most buildings are special moment frames and elsewhere...so what is the seismic resisting system of your concrete buildings, Hokie? Wherever there are beams, it can take additional moment and shear during seismic activity so naturally there must be extra shear resisting system for cyclic loading...