Reinforcement Details for an Experimental Beam

1. Additional Embedment

From our lesson on Reinforcement Details ... (here) ...

Ref: ACI 12.10.3 ... quoting ...

Reinforcement shall extend beyond the point at which it is no longer required to resist flexure for a distance equal to the effective depth of the member, or 12 d_b, whichever is greater, except at supports of simple spans and at free ends of cantilevers.

Since the bars in all of our beams are essentially the same length (none terminate early) ... we don't have to deal with this Code issue.

The possibility arises, however, where we might terminate rebar where no longer needed. For example, for our beam loaded primarily with a concentrated load at midspan, the moment drops off essentially linearly toward the supports. Half way to each support the moment has dropped to half the maximum. We should theoretically need only half as much reinforcement. If we had two bars, for example, one bar could terminated at each of these `quarter points'. And so they can, but the Code requires that we extend them just a bit, just in case. If, say, we needed 2 - # 4 at midspan, and decided to drop to 1 - # 4 at the quarter points, we would need to extend these terminated bars a bit past the quarter points ... the greater of d ... 7.5 inches, or 12 (4/8 in.) = 6 in. ... or 7.5 in.

I should probably sketch this, but not for now.

2. Extension of Reinforcement

Ref: ACI 12.11.1 ... again quoting ...

At least one-third of the positive moment reinforcement in simple members and one-fourth the positive moment reinforcement in continuous members shall extend along the same face of member into the support. In beams, such reinforcement shall extend into the support at least 6 in.

Now we have a problem. In our example the beam extends past (into) the supports only 6 in. on each side, and the rebar only 2 in. of this. This is dangerous! ... If the rebar is somehow short, or if the location of the support changes, we could end up with a portion of beam subject to flexure but without reinforcement. Even if the amount of flexure is small, the result can be catastrophic as unreinforced concrete is both weak, and failure sudden and catastrophic. Also, we could have the rebar go the entire 7 ft, but then we wouldn't have any end cover. So, either way, we are in violation.

Possible remedies ...

1) Provide a `manufactured' bracket or something that controls the location of the support and rebar, and necessitates some minimum amount of extension into the support. We would probably have to have this tested, and provide tests to the Building Official.

2) Make the gross length of the beam longer and thus allow for a full 6 in. of extension.

References

Reinforcement Details and Splices, Jeff Filler, Associated Content.

Building Code Requirements for Structural Concrete, ACI 318, American Concrete Institute, P.O. Box 9094, Farmington hills, Michigan, 48333.