When does a cantilever concrete beam become classified as a deep beam? 2

[failuremode]

ACI 318 defines a deep beam as:

9.9.1.1 Deep beams are members that are loaded on one face and supported on the opposite face such that strut-like compression elements can develop between the loads and supports and that satisfy (a) or (b):
(a) Clear span does not exceed four times the overall member depth h
(b) Concentrated loads exist within a distance 2h from the face of the support

A cantilever beam does not have a "clear span" in my mind since it is only supported on one side. At what point does a cantilever beam need to be considered a deep beam and designed using Strut and Tie method? Is it bound to the 4h criteria? RISA flags a cantilever as a deep beam within the 4h criteria.

 

[KootK]

bound to the 4h criteria?

Yup. I would say that it applies when your cantilever span does not exceed 2h. A cantilever greatly resembling half of a simple span beam twice the length.

 

[bugbus]

1.5h according to the Australian standards, but 2h and maybe even up to 2.5h would probably warrant considering S&T modelling

 

[failuremode]

Thanks for the responses. After I posted this, I kind of came to a similar conclusion. 2h seems like the right approach because that fits within the 9.9.1.1 b criteria of having a load within 2h of the support.

For my project this has come up on, I have the ability to move my cantilever support further from the end of the member to avoid STM and wanted this discussion to decide how far to move it since RISA 3D only flags 4h criteria for every member including cantilevers.

Side note:
One thing I find interesting is that it the code section it only mentions: "Deep beams are members that are loaded on one face and supported on the opposite face..."

Only in the commentary does it talk about loads applied to the member through the sides or bottom of such a member. It still says we should use STM to distribute the loads to the top first before to the support.

 

[JoshPlumSE]

For my project this has come up on, I have the ability to move my cantilever support further from the end of the member to avoid STM and wanted this discussion to decide how far to move it since RISA 3D only flags 4h criteria for every member including cantilevers.

I think the key is that RISA-3D has no way of knowing that a beam is a cantilever. Right?

RISAFloor should be smart enough to know. Maybe there are some Revit flags that RISA-3D might import that could tell it. But, I'm not sure if those are acknowledged for models that didn't come from RISAFloor.

 

[KootK]

For my project this has come up on, I have the ability to move my cantilever support further from the end of the member to avoid STM and wanted this discussion to decide how far to move it since RISA 3D only flags 4h criteria for every member including cantilevers.

I don't love the sound of that. In my book, short cantilevers are usually structurally preferable to long ones. So I'd not be lengthening my cantilevers just to sidestep RISA's (or the code's) concern for STM design.

I apply the STM recommendations pretty judiciously and there are very few situations where I would go that route in practice, regardless of the proportions. I might go STM if the cantilever is supporting a major column out at the free end. I wouldn't bother if it's just a typical exterior wall support situation. The most important thing about most end loaded cantilevers is quick anchorage of the top steel at the free end. This is what STM will likely tell you and the answer is usually to hook your top steel there which you're probably doing already as matter of standard detailing convention.

STM is fun to talk about and useful to think about but it sucks as a production tool unless you have it automated. I mostly let that particular tool collect dust in my tool chest.

 

[failuremode]

I think the key is that RISA-3D has no way of knowing that a beam is a cantilever. Right?

RISA does seem to know its cantilevered. In the "Additional Properties" when clicking on the member, it says "Start Support Cantilever".

I don't love the sound of that. In my book, short cantilevers are usually structurally preferable to long ones. So I'd not be lengthening my cantilevers just to sidestep RISA's (or the code's) concern for STM design.

I am only talking inches, not feet. I agree with your thought process though.

 

[wiktor]

The beams formulas become invalid when span to depth ratio exceed 7:1, so for a cantilever it's 3.5:1

 

[rapt]

Unless you have a heavy point load on the end of the cantilever, why are you worried about S/T modelling anyway? Whatever reinforcement you need at the end column for the internal span would be adequate for the short cantilever.

 

[failuremode]

I agree. The decouple moment in the STM model would pretty much tell you that you need tension/tie reinforcement equal to the load multiplied by the cantilever length and that would continue over the supporting column into the backspan.

I think it is more of looking at the end anchorage, like KootK said, to have the reinforcement immediately developed at the end of the cantilever. Also, applying the other skin reinforcement/stirrup ACI requirements for deep beams.