Concrete Floor Systems - Reinforcing Terminology

[albanyYEM]

Working on a renovation project in which the building is comprised of a cast-in-place concrete joist and beam system. The original construction documents provide the beam and joist sections and reinforcing bar information. My confusion is in the way the reinforcing bars are termed. They are indicated as "Str Bars" and "Bent Bars". There is a clear bending diagram on the drawings (presumably for the bent bars) but it isn't clear to me where the "Str Bars" are placed within the beam section. I presume these reference straight bars as opposed to bent, but are they located in the bottom of the beam (M+) or straight across the top of the beam (M-)? Thank you for any insight.

 

[DaveAtkins]

In a one way joist system, the straight bars generally make up a portion of the bottom reinforcing in the joist. The bent bars make up the remainder of the bottom reinforcing (at midspan), then bend up to become the top reinforcing over the supports.

DaveAtkins

 

[albanyYEM]

Thank you for the quick response. Indeed I am referring to a 1-way system.

The bent bar reinforcing is generally much less than the straight bar reinforcing. Are these systems then typically analyzed as simple spans with little regard for resisting negative moment at the supports? Is the assumption that the concrete would crack at supports?

 

[JAE]

The bent bars from one span extend over a support and the bent bars from the opposite side of the support/column come in from the other way so you add the two sets of bent bars over the support for the total negative moment area of steel.

But even with that, I've found on many many older projects that the negative moment reinforcement is either too light or not developed out far enough per current ACI standards. This will require some judgement on your part to determine how "bad" the negative moment condition is and possibly consider some moment redistribution to the positive moment.

Also keep a look out for some conditions where the rebar schedule includes top straight bars over the support as well.

But DaveAtkins is entirely correct about the typical straight bars being bottom bars. That was the practice back then.

Check out Eng-Tips Forum's Policies here:
faq731-376