Civil Engineering Reference
In-Depth Information
will be able to develop at least 125% of the specified yield strength of the bars. It is consid-
ered desirable to butt the bars against each other, particularly for #7 and larger bars. Splices
not meeting this strength requirement can be used at points where the bars are not stressed to
their maximum tensile stresses. It should be realized that welded splices are usually the most
expensive due to the high labor costs and due to the costs of proper inspection.
Mechanical connectors usually consist of some type of sleeve splice, which fits over
the ends of the bars to be joined and into which a metallic grout filler is placed to interlock
the grooves inside the sleeve with the bar deformations. From the standpoint of stress
transfer, good mechanical connectors are next best to welded splices. They do have the
disadvantage that some slippage may occur in the connections; as a result, there may be
some concrete cracks in the area of the splices.
Before the specific provisions of the ACI Code are introduced, the background for
these provisions should be explained briefly. The following remarks are taken from a
paper by George F. Leyh of the CRSI.
8
1.
Splicing of reinforcement can never reproduce exactly the same effect as continu-
ous reinforcing.
2.
The goal of the splice provisions is to require a ductile situation where the rein-
forcing will yield before the splices fail. Splice failures occur suddenly without
warning and with dangerous results.
3.
Lap splices fail by splitting of the concrete along the bars. If some type of closed
reinforcing is wrapped around the main reinforcing (such as ties and spirals de-
scribed for columns in Chapter 9), the changes of splitting are reduced and smaller
splice lengths are needed.
4.
When stresses in reinforcement are reduced at splice locations, the chances of
splice failure are correspondingly reduced. For this reason, the Code requirements
are less restrictive where stresses are low.
Splices should be located away from points of maximum tensile stress. Furthermore,
not all of the bars should be spliced at the same locations—that is, the splices should be
staggered. Should two bars of different diameters be lap spliced, the lap length used
should equal the splice length required for the smaller bar or the development length re-
quired for the larger bar, whichever is greater (ACI Code 12.16.2).
The length of lap splices for bundled bars must be equal to the required lap lengths
for individual bars of the same size, but increased by 20% for three-bar bundles and 33%
for four-bar bundles (ACI Code 12.4) because there is a smaller area of contact between
the bars and the concrete, and thus less bond. Furthermore, individual splices within the
bundles are not permitted to overlap each other.
7.13
TENSION SPLICES
The Code (12.15) divides tension lap splices into two classes, A and B. The class of splice
used is dependent on the level of stress in the reinforcing and on the percentage of steel
that is spliced at a particular location.
8
Portland Cement Association, 1972,
Proceedings of the PCA-ACI Teleconference on ACI 318-71 Building
Code Requirements
(Skokie, IL: Portland Cement Association), p. 14-1.
