Reinforcement Details and Splices

1. Excess Reinforcement

Ref: ACI 12.2.5

In many cases we will have more reinforcement than is (exactly) required. I mean, first of all, we pick standard sizes of rebar and whole numbers of pieces, not fractions. Or maybe in our particular design we want to use only a particular size, and are even farther off of what is exactly needed. In our number crunching, for example, if we calculate that we need 3.8 # 4 pieces of rebar, we'll install 4 # 4, and have a little bit excess. And the Code will only require that we `develop' exactly what we need. (Though there are some exceptions to this, but beyond what we need to know for now.) Hence, in this example we could say that our development length is ...

... that big long equation from last time ... multiplied by 3.8 / 4 ...

2. Extension of Reinforcement

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.

The Commentary to the Code says that this is for `shifting moments' and recognizes that our calculated moments may not be exactly those realized in the field.

So, for example, let's say we have a simple beam supporting a uniformly distributed load. And let's say we calculate that we need 4 # 4 bars at the point of maximum moment (at mid-span). We could say that at the quarter points of the beam (half way from the mid-span to the supports) that the moment has dropped to three-quarters of the maximum, and thus there is a point pretty close nearby where (we could argue) we only need three of the four pieces of rebar. And we would be right, technically. But the Code will require that we keep going a distance d or 12 d_bpast where we calculate we no longer need that piece (or pieces).

3. Extension into Supports

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.

My intent here is not to keep you from buying the Code ... anything but! I can't imagine anyone designing or constructing reinforcement concrete without their own (well used) copy of the Code. But, I quote because if I were to paraphrase ... it would probably take three or four times as many words, and not be as clear.

The Commentary to the Code again mentions shifting moments due to changes in the loading, settlement of supports, etc.. In my opinion this is (also?) a huge safety issue. If we end the reinforcement `exactly' at the support, all it takes is a little mis-construction, or mis-something-else, and we potentially have a `plain' section of concrete beam (a place where there is flexural tension stress but no rebar). That is scary! Bang! Beam breaks and is on the floor (below).

4. Points of Zero Moment

Ref: ACI 12.11.3

This is what I call the `pullout calc'. And, as I said earlier, it is based on this thing called the development length. As I understand it, the calculation basically ensures that we can develop the reinforcement as fast or faster than we need it. Consider, for example, the end of a simple beam. Theoretically at the end we need zero reinforcement since we have zero moment. But as we move away from the support we start picking up moment, and the need for reinforcement. But not only does it need to be there, ... there needs to be enough length of it to develop what we need there. This is accomplished by requiring the bar size be small enough so that it `develops bond' fast enough. And we do this by requiring that the development length (be small enough to) satisfy ...

... ʆ_d ≤ (1.0 or 1.3) M_n / V_u + l _a ... (ACI Equation 12-3, slightly re-written)

where,

M_n = nominal moment strength assuming all steel developed to f_y,

V_u = factored shear at the section,

ʆ _a = the embedment of the rebar past the centerline of the support, and,

where the 1.3 represents an increase of 30% allowed if the reinforcement being developed is confined by a compressive reaction; otherwise use 1.0.

We'll hammer out several example calculations as the course drags on ... but for now let me just say `watch the units' in your calculations. Development length is expressed in inches, and if you divide nominal moment in lb-in. ... by factored shear in lb, you'll get a number with units of inches. Generally if we are using the smaller size reinforcements this equation will work out.

5. Splices

Ref: ACI 12.15

Especially in foundations we have structural elements that are much longer than our pieces of rebar. Hence, we need to `splice' the rebar. In this course we will only deal with lap splices of tension reinforcement. Our lap splices may be contact or `non-contact' splices. Contact and non-contact are as the terms imply.

For the non-contact splice the maximum distance apart of the two pieces being spliced is one-fifth the required splice length but not greater than 6 in.

Splice lengths (as I warned) are in terms of the development length ...

Class B lap splice ... 1.3 ʆ_d

Class A lap splice ... 1.0 ʆ_d .

Generally we use Class B splices.

We may specify Class A splices ONLY where:

1. we have excess reinforcement ... to the extent that the area of reinforcement required is not more than half the area provided, AND,

2. not more than half of the reinforcement is being spliced (within the required lap length).

Item 1 above will generally require that the lap locations be specified clearly, since the Contractor will in general not necessarily know where there is enough excess reinforcement to do the splice.

By specifying Class B splices we generally give the Contractor liberty to use (place) them where needed (convenient).

6. Conclusion

So, whereas Construction Documents will generally not specify development lengths, the will specify lap lengths, and perhaps where reinforcement is terminated, and all that (stuff) depends on development lengths, and so that's why I have included it all in the same lesson(s).

7. References

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