crossties ACI-318 - correct placement ??

[boffintech]

Considering that ACI-318 reads:
Crosstie — A continuous reinforcing bar having a
seismic hook at one end and a hook not less than 90
degrees with at least a six-diameter extension at the
other end. The hooks shall engage peripheral longitudinal
bars. The 90 degree hooks of two successive
crossties engaging the same longitudinal bars shall be
alternated end for end.

I take this to mean that ALL crossties in structural elements (with the one exception of spandrel beams) are alternated end for end. I see lots of crossties in columns and shearwalls, and a few in pile caps for shearwalls, but not so often in beams.

Am inspecting on a job now where the EOR shows a beam detail with a vertical crosstie.

This 36x36 beam has 6#11T w/2nd layer of 4#11T and 7#11B w/ 2nd layer of 3#11 and one closed tie (135 hks at corner) and one center vertical crosstie.

Structural engineers would expect to see this vertical crosstie alternated end for end, right?

 

[ishvaaag]

Right. The shear reinforcement must always wholly engage the main longitudinal bars. If less than this is provided, it is logical to alternate for the better behaviour.

As an example of that these things are old practice, a village home of my aunts 200 year old has over 6 m main wood members with vaulted gypsum in between, points of the trunks of the wood members alternated, so the vaults' supports run parallel.

 

[hokie66]

Beam shear stirrups are not necessarily required to be alternated. It depends on where you are located. Seismic detailing requirements should be clearly shown on the structural drawings.

 

[BAretired]

Beam shear stirrups should be hooked in the compression zone and continuous in the tension zone.

BA

 

[msquared48]

BA:

Ideally, yes, but really it is impossible where alternating loads are seen, to include continuous beams.

Mike McCann
MMC Engineering

 

[BAretired]

Hi Mike,

I agree that the point of inflection may move back and forth a bit, but it is usually possible to place the stirrups correctly in the support region, where shear is maximum.

BA