Beam Construction Joints

[slickdeals]

I have seen typical details from a few firms that show "X" shaped rebar passing through a construction joint. Usually this consists of 2 "S" shaped bars crossing each other at mid-height.

What is the engineering reason for doing this?

 

[hokie66]

I have never seen that detail, but my guess is that it is an attempt to provide reinforcement to handle diagonal shear in both directions. Perhaps the engineer doesn't know whether the shear is up or down. Or maybe it is in both directions to prevent the contractor from picking the wrong way. Locating construction joints where shear is minimal prevents having to worry too much about this type of thing.

 

[slickdeals]

Example of that detail here

http://12.139.234.2/project_docs/do...T CD 5-4-11/Plans/Volume2/Structure/S5.21.pdf

 

[hokie66]

Looks like overkill to me. Maybe it is some specific seismic detail, but the beam elevation at the column joint doesn't show ties in the joint as would be required in seismic detailing.

 

[CELinOttawa]

Hokie's first instinct as to the use is spot on: This is a seismic detail. I have specified something similar many times...

Think of this this way: What better way to ensure you know EXACTLY where a hinge will form than to place a bond-broken tension section and extra "focussing" bars where your modelling tells you hinges are likely to form.

 

[CELinOttawa]

Oh, and just by the way: This job (ie: with the attachement linked above) is a shear wall job... So plastic hinging areas in the beams MAYBE, beam-column joist seismic detailing *not* required so long as the engineer ensures sufficient strength and stiffness from other systems to preclude hinge formation in the columns. They are actually non-seismic perimeter columns here.

 

[hokie66]

I can't see how that detail would aid in forming a hinge. All the bars are continuous through the joint. You may be right about the lack of seismic detailing of the beam/column joints. I just don't know.

 

[CELinOttawa]

It depends on what the designer is looking for... Having a closer looking at the detail I find that it is missing some of the hallmarks I would expect for a seismic hinge (the length of the zone being completely unrealistically short), and it is entirely possible that this is meant to simply be a construction joint. In that case I agree it is overkill.

In the case of a seismic hinge, it is the fact that there are fewer bars in the zone which can be used to increase the likelihood of hinge formation where you want.... The X-bars should extend into the primary steel in such a case, and the locations of the hinges should be clearly spelled out on the plans. Neigther being the case here.

I think I'm wrong, and misjudged the detail due to past experience. *sigh* I'll try to save what face I can by pointing out that a very similar detail can be used seismically, but would not find a place in a shear wall building in any case.