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Re: Rebar Spacers in PC beams

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Jim & Gerard,

Thank you for the responses. The dimensions I listed for the beam did not include the 5" slab (3" fill over 2" mtl deck) - which gives an overall depth of 18". Also, there is compression steel in the slab above the beam (4 - #4's) and 2 - #4's in the top of the PC portion of the beam. Reinforcing and depths from top of slab are: 0.80 @ 1.50 in, 0.40 @ 7.00 in, 2.40 @ 14", and 2.40 @ 16". Effective flange width is 60" including the 12" web, so over-reinforcement is not a problem. Mu = 238 k-ft. Span = 22.25'.

Glen A. Underwood, SE
Clark Pacific


Glen,

I concur with Gerard: your beam should be deeper (depending on the thickness of your slab). I have seen cracks at tie spacings in a tensile zone, so I think all your concerns are valid and further cracking is likely. I have also seen bars added in lots of places for lots of reasons. They are what the cage tier (sp?) has, so he uses them - just like I use a small Phillips head screwdriver to start a screw hole if I do not have my gimlet with me.

Jim Getaz Winchester, Virginia



Doesn't answer your questions, but that a lot of steel for such a small beam... is it Overreinforced? Is it ductile? -gm


I've designed a conventionally reinforced precast beam with two layers of longitudinal tension reinforcement - specifically - 8 - #7's in two layers of 4 each. The design clear spacing between the two layers is 1". The beam is 12" x 13" and will be a composite tee-beam when the slab is cast on top.

The production crew has achieved the 1" clear spacing by placing a short stub of #8 bar between the two layers of longitudinal steel. This happens every 5 feet or so along the beam length.

Knowing that the concrete will crack (and crack control requirements are met), is there any problem with this method? I have never seen this done before in the tension zone of a beam. My experience has been that longitudinal bar elevs are set by tie wire hangers, and it looks odd to me to introduce a weakened plane in the tension zone of a beam. Would the solid bar/weakened plane cause normal flexural cracks to propagate further up the depth of the beam?

Thanks in advance.

Sincerely,

Glen A. Underwood, SE
Clark Pacific

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