In this section we will see a situation which is very similar to a lap splice. The fig. below shows a portion of two pieces of a same material, which are joined by applying glue between them.



The two pieces, when joined together, act as one single piece. This single piece is in tension. The tensile force in one piece is first transferred to the glue. This transfer takes place with the help of the bond between the material of the piece and the glue. From the glue, the force is transferred to the second piece again by the bond

Now we can look at the portion which is common to both the pieces. This is the lap length. The piece is trying to pull away from the glue. The glue must exert sufficient grip on the piece to stop it from moving away.The glue can exert a definite quantity of force per unit area. So there must be sufficient area for the glue to exert the required force so that the piece can be kept from moving away. There is no way to increase the width for the purpose of increasing the area. The only option is to increase the length. If this required length is available, then the grip exerted by the glue will be equal to the tensile force in the piece, and so the piece will not move. This is similar to a situation that we discussed: the grip exerted by the concrete, to stop the top bar of the cantilever from pulling out, and based on that, we saw the concept of development length Ld. So if length of the lap is increased, the splice will be stronger. The fig. below shows an increased lap length.



Based on the above discussion, we can think of concrete as the glue, and the pieces shown in the fig as the steel bars being lap spliced. Obviously, concrete is a rigid material, and will be covering the entire bars, just as if the lapped portion of the pieces in the above figs is completely embedded in glue as shown in the fig. below.



In this case, the contact area between the pieces and the glue is greater, and so the joint will be stronger. So we can see that for the splice to work perfectly, the concrete or glue should be in tact. There should not be any formation of cracks, displacements, spalling or such similar defects. For this, we provide, extra stirrups, compacts stirrups, spirals, etc.,

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