Cantilever RC Beam / Transfer Beam / Offset Column

[RFreund]

I am wondering what the best way to design a short cantilever beam where the overhang cantilever is less than the depth of the beam. My current situation applies to a cantilevered grade beam picking up a column above.

If I design for the centerline shear and moment I need a lot of shear reinforcement which for some reason doesn't seem right or necessary. Any suggestions on how to approach this situation? It seems like there should be a Strut and Tie method that would work but I don't do much strut and tie stuff.

See attached for a quick sketch.

Thanks!

EIT
www.HowToEngineer.com

 

[MotorCity]

Design for the shear at a distance "d" from the face of the support. The shear probably drops off pretty quick from the centerline of your support. That said, depending on the column load, you still may need a few extra stirrups

 

[KootK]

I need a lot of shear reinforcement which for some reason doesn't seem right or necessary

Agreed.

It seems like there should be a Strut and Tie method that would work but I don't do much strut and tie stuff.

There is, of course, but you'll loose your shirt executing it. I'd start with the method below before resorting to fancier things. In my opinion, the most important part will be applying the rules for anchorage of positive steel at supports to the top bars.

This is one of those situations where, if you turn the sketch upside down (yours or mine), the solution jumps right off the page.

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.

 

[RFreund]

@MotorCity and Kootk - thanks for the response!

Design for the shear at a distance "d" from the face of the support

This is a situation where the columns are less than "d" away from each other. In some cases the centerlines are only a few inches different (i.e. their faces over lap).

but you'll loose your shirt executing it

Like it is difficult to calc out or it is difficult to construct in the field?

the most important part will be applying the rules for anchorage of positive steel at supports to the top bars.

Are you referring to STM rules of anchorage?

@Kootk -
Referring to your sketch - So if a goes to 0' then you would say there is no shear to design to? However you would still want your top bars to be anchored.

See attached, page 2, for quick STM sketch. Very simple so it's probably wrong. I really know nothing of STM.

Side question: I see you use cloudinary - when you post images, do you convert them from pdf, if yes, do you use cloudinary to convert? Or do you just always save a .jpg?

Thanks again!



EIT
www.HowToEngineer.com

 

[KootK]

Like it is difficult to calc out or it is difficult to construct in the field

Calculation time. Field time should be unaffected. If you only check one strut and a couple of nodes, maybe it's not too onerous.

Are you referring to STM rules of anchorage?

I was referring to the ACI clause that is typically used for anchorage of bottom steel in simple span beams at supports. STM rules will work just fine too though instead.

See attached, page 2, for quick STM sketch. Very simple so it's probably wrong. I really know nothing of STM.

Sketch some nodes in there and you've got it. The thing that you'll find is that, once you start to get into no overlap territory, the strut angle of the single strut model starts to get shallow and stops working. Of course, it's the same strut no matter what method you use, the orther methods just fail to address it. A good argument for STM I suppose.

 

[KootK]

So if a goes to 0' then you would say there is no shear to design to?

Uhhh... according to the method that I proposed, yes. Some other things to think about if there's any gap at all:

1) There's a monolithic, vertical shear friction surface between columns that can be checked.

2) If you have the latest CRSI on pile caps, they propose an equation there for Vc taken on steep angles. I've been starting to use that for some things and it would be applicable for some versions of this.

However you would still want your top bars to be anchored.

Abso-posi-lutely. I think that's the most important bit. Moreover, if you take a non-STM approach, I would base you anchorage on your original center line shear diagram. You're considering a vertical flexural crack now which isn't going to be affected by the "within d" business. Now that I think on it a bit more, I like STM much better for this part too.

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.

 

[KootK]

ide question: I see you use cloudinary - when you post images, do you convert them from pdf, if yes, do you use cloudinary to convert? Or do you just always save a .jpg

No Cloudinary for me. Here's the patented KootK system:

1) Get the image up on your screen any old way. Usually PDF for me.
2) Use the native windows snip tool to snag what you want.
3) Save the snip someplace as jpg.
4) Use the upload image tool here, three left of the quote button.

It's pretty fast and it solves that issue folks were having where pics would be three screens wide.

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.

 

[Ingenuity]

A picture worth a 1000 words...this...

 

[RFreund]

Wow, for some reason I kept trying to use the "picture" button to the left of the "upload image"... Thanks to both of you.

EIT

http://www.HowToEngineer.com

 

[RFreund]

If you have the latest CRSI on pile caps, they propose an equation there for Vc taken on steep angles. I've been starting to use that for some things and it would be applicable for some versions of this.

This was a good reference. It seems to address my situation. Basically they give an allowable shear value that increases from your typical allowable shear a 'd' from the column face to a large maximum value when the faces align.

They do mention using shear friction reinforcement similar to a corbel but say that it is not practical.

EIT

http://www.HowToEngineer.com

 

[KootK]

They do mention using shear friction reinforcement similar to a corbel but say that it is not practical

Can you reference a page or section number for that? I'm interested to check that out myself as it sounds contradictory to my own thinking.

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.

 

[RFreund]

Sure thing:

Page 5-3 See excerpt below:

EIT

http://www.HowToEngineer.com

 

[RFreund]

This is interesting:
The deep beam provisions of ACI say that your shear capacity is:
V.cn = 10*sqrt(f'c)b.w*d
Which is what CRSI uses as a maximum for the pile cap design. Meaning they increase the typical one-way slab shear strength based off how close your column and cap are up to a max of 10*sqrt(f'c). So if I am using grade beams it would seem that the allowable shear would be V.cn = 10*sqrt(f'c)b.w*d so long as your tension bars are anchored (i.e. top bars in my case are developed) and your cantilever distance is less than 2*Lc < 4*h.

Seem reasonable?

I still may just use the corbel/bracket reinforcement and be done with it...

EIT

http://www.HowToEngineer.com

 

[JAE]

I still may just use the corbel/bracket reinforcement and be done with it...

..that is what I would do.

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[DST148]

@RFreund>>>> From detailing perspective.... It is difficult to develop top tensile and horizontal shear reinforcing in short cantilevers and brackets. You may have to use U-shaped bars to satisfy development lengths..

 

[rapt]

Threaded End plates are better for development in this case, or even prestressing with live end anchorages.

If you use U bars you need to increase the bend radius to at least 10 * diameter or possibly more depending on the design code and bar size.

 

[DST148]

@rapt>>>> I agree with you. However, the OP says it's a cantilever grade beam supporting column above.... I don't see any problem so long as the length of cantilever is at least equal to
Ldh for the top reinforcing + 2". Inside diameter of bend for stirrups is 4 x db for #5 and smaller bars. Moreover, the width of the grade beam can always be made wider to suit detailing and shear requirements.
As far as possible, we like to keep things simple for below grade structural elements.....

 

[rapt]

No, it has nothing to do with the cantilever length.

This is a deep beam and must be designed by strut tie logic and the tension tie needs to develop its fill capacity at the centre of the column above.

 

[KootK]

Seem reasonable?

I interpret that as a "not to exceed" limit rather than a proposed shear capacity.

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.

 

[DST148]

@rapt>>> In reference to your post above....I have older version of the code, Section 10.7.1 of ACI 318-05: Deep beams shall be designed either taking into account nonlinear distribution of strain or by Appendix A (STM). Let us know if the code language has changed as you have posted above.
I agree with the design approach of RFreund.
The intent of my first post was to emphasize the importance of anchoring of tension reinforcement in deep beams which can be achieved either by embedment / hooks or welding to special devices.
For grade beams, I would prefer to use smaller diameter bars or wider beams to meet detailing requirements rather than the welding option.