Stirrups placement in cantilvered beams 4

[ali07]

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.

 

[msquared48]

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

 

[demayeng]

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

 

[apsix]

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.

 

[hokie66]

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.

 

[demayeng]

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?

 

[hokie66]

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.

 

[InDepth]

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.

 

[Tomfh]

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.

 

[ali07]

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.

 

[hokie66]

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.

 

[ali07]

As per Canadian Code, it is allowed, only concern is placement problem. I don't like either lapping the stirrups too.
See Canadian code recommendations regarding stirrup lapping.

 

[ishvaaag]

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.

 

[ali07]

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.

 

[demayeng]

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

 

[rapt]

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.

 

[apsix]

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.

 

[hokie66]

I like apsix's solution best.

 

[Lion06]

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

 

[asixth]

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.