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Stirrups placement in cantilvered beams
4

Stirrups placement in cantilvered beams

Stirrups placement in cantilvered beams

(OP)
As per attached sketch, please suggest the best arrangement and orientation of rebars and stirrups in cantilver beam. Beam is also supporting a secondary beam at the tip of cantilver (Not shown in sketch).
Appreciate for guidance.
 

RE: Stirrups placement in cantilvered beams

I swould still use "B" as the diagonal tension crack does not know whether the "beam" is horizontal or sloped.  It should still form roughly 45 degrees relative to the main axis of the cantilever.   

Mike McCann
MMC Engineering

RE: Stirrups placement in cantilvered beams

I would have said C because the direction of the shear load is still straight down

RE: Stirrups placement in cantilvered beams

I would use 'B' for the stirrups as the gravity load will be seen as axial and perpendicular components by the beam.

Top bars; obviously anchorage into the column is crucial and use laps as far away from the column as possible.

RE: Stirrups placement in cantilvered beams

Mike is correct.  The stirrups must be oriented so that a diagonal tension crack will intersect the reinforcement.  A diagonal tension crack in the inclined member would tend to be roughly vertical for downward loading, and roughly horizontal for upward loading.  So orienting the stirrups perpendicular to the member suits either case, but orienting the vertically could result in a crack parallel to the stirrups.

RE: Stirrups placement in cantilvered beams

What is the mechanism that causes the crack to be oriented to the beam orientation?

is that the same for a flat cantilevered beam with a tapered soffit?

RE: Stirrups placement in cantilvered beams

The mechanism of shear failure is actually tension in the concrete, and this tends to occur diagonally across a member.  Nothing is precise in concrete, but we assume that diagonal tension causes cracking to occur at 45 degrees to the member axis.  Yes, this applies to any member, regardless of the orientation.  I suggest you consult a concrete design text.  Any good text will have an illustration of what is assumed for shear crack orientation.

RE: Stirrups placement in cantilvered beams

Option B & C are ok options. The stirrups don't necessarily have to be in line with the loading. i.e. they can be diagonal. Draw out your strut& ties on each model. Graphically represent the force flow like a truss. Make sure the compression strut is within 30-45 degrees.  

RE: Stirrups placement in cantilvered beams

B is better. As noted by others, C stirrups are too much aligned with potential diagonal tension cracks.

Your column/beam connection is critical here. I would make the cog a large hook, and have well anchored bars running perpendicular to the crack that will form at the internal angle.

RE: Stirrups placement in cantilvered beams

(OP)
Thanks for all.

Is there any reference material for beam reinforcement in tapered sloped cantilver beams.

I have to use arrangement "A", type "Y" for varying stirrup sizes, otherwise each stirrup have to be made different. Two stirrup legs should have development length to act as one.   

RE: Stirrups placement in cantilvered beams

Does your code allow lapped splices in beam shear reinforcing?  The Australian code AS3600 does not accept that lapped splices provide adequate anchorage unless they are welded splices.

RE: Stirrups placement in cantilvered beams

It is always better not to splice anything if you can. But it is also worth to remember that one of the flexibilities of concrete comes form its ability to pass tension from some bar to another through splices; otherwise many concrete spans would be limited to just the length of the ordinary rebar being sold (9 to 12 m) and such is not the case. So with proper splices and anchor lengths, we are working its technology.

 

RE: Stirrups placement in cantilvered beams

(OP)
Normally stirrup hooks are on top for simply supported beams, is it not good that hook for cantilever should be at bottom face on compression side.

RE: Stirrups placement in cantilvered beams

It's ok to have hooks in the tension zone, you just have to discount the stirrup capacity (by 20% in aust. code)

RE: Stirrups placement in cantilvered beams

demayeng,

And that rule is disappearing in AS3600-2009 which should be available in the next month or so.

Yes AS3600 says no lap splices for stirrups, but I cannot understand why as long as proper development calculations are done allowing for lapping 100% of bars in a full tension zone.

RE: Stirrups placement in cantilvered beams

I would look at using closed stirrups with a range of set depths.
The varying beam depth can be accommodated by inclining the stirrups as the beam depth decreases, but inclining in the opposite direction to Option 'C'.
45 deg maximum inclination is typically allowed.

RE: Stirrups placement in cantilvered beams

I like apsix's solution best.

RE: Stirrups placement in cantilvered beams

Just be sure to account for the inclination of the stirrups when you are checking the capacity.  


 

RE: Stirrups placement in cantilvered beams

I would say option C with the 135deg cog at the bottom of the beam face and inclined so it is perpendicular to the plane of bending.

The Australian code had incorporated a 20% additional reduction factor when the shear reinforcement cog is in a tensile zone because it does not achieve the same development as it would in a compression zone (micro-cracking I assume). The need for the reduction factor was discussed in the AS/NZS codes forum not so long ago and it is the only code that has this requirement. As Rapt has just informed us the new issue of AS3600 will not have this provision, after I just had all spreadsheets in our office updated to include this 0.8 factor.  

RE: Stirrups placement in cantilvered beams

(OP)
Hi Apsix,

Appreciate if you can give a sketch for your idea.  

RE: Stirrups placement in cantilvered beams

Sorry ali07, it can't be till next week, I'm home for the weekend.

RE: Stirrups placement in cantilvered beams

(OP)
Thanks BAretired,

Just wondering, is it easy to control that spacing pattern  on site. Any limitations expected for placement, which would be dangerous, i mean any arrangement which cannot be monitored on site easily.  

RE: Stirrups placement in cantilvered beams

Not too easy.  If you can talk the architect into using a prismatic beam, it would be easier.  

Otherwise, determine the spacing you need at mid-height, then calculate what that means the spacing should be at the top of beam.  You can check that with a tape measure before pouring.

Personally, I think I would make life easier for the guys on site by varying the height of each stirrup.  It is not too difficult for the fabrication shop to do that.
  

BA

RE: Stirrups placement in cantilvered beams

Quote:

Personally, I think I would make life easier for the guys on site by varying the height of each stirrup.  It is not too difficult for the fabrication shop to do that.

I agree. This is the simplest way.

RE: Stirrups placement in cantilvered beams

How do you attach a sketch to these threads

RE: Stirrups placement in cantilvered beams

If your sketch is on your computer (either PDF or JPG), click on..."or upload your file to ENGINEERING.com" just below the area you are typing in.  A dialog box will appear.  Click on "Browse" in the dialog box, then find your file with the sketch.  Then click on "Upload File".

BA

RE: Stirrups placement in cantilvered beams

(OP)
I agree, if the architect does not want a simple shape, how it can be built simple. I am also thinking to specify varying stiruups sizes.    

RE: Stirrups placement in cantilvered beams

BAretired has drawn what I meant.

I also agree that varying each stirrup length is probably preferable.

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