Chapter 14.7 - Safe regions for providing splices in bars of beams

In the previous section we saw the basic details about 'lap splice'. In this section we will discuss some more details about splices.

Location of splices

We have seen that the splices can be used to increase the length of bars. We may be compelled to give splices wherever there is a shortage of length. But this is not allowed. Their are certain specific locations where splices should not be given. Here we will see some recommendations given by the code regarding the location of splices. These 'recommendations about location' given in cl.26.2.5, are applicable to splices in common (lapped splice, welded splice, splices by mechanical connection).

According to this clause, splices should as far as possible be away from the sections of maximum stress. This is so because, at the sections of maximum stress, the bars will be subjected to a greater amount of tension or compression. So if splices are present at these sections, the concrete will have to transfer a greater force from the stopping bar to the continuing bar. This may give rise to cracks in the concrete at these sections.

So how do we implement this recommendation? The answer is given by the code in the same clause itself:

14.12
'It is recommended that splices in flexural members shall not be at sections where the bending moment is more than 50% of the moment of resistance'. 

This means that, if the 'External factored bending moment' M_u, applied at a section is more than half of the 'Ultimate moment of resistance' M_uR offered by the section, then we cannot provide a splice at that section. So we can write:

14.13
We cannot give splices at a section XX if
M_u,xx   >  0.5M_uR,xx

The converse of this can be written as:

14.14
We can give splices at a section if
M_u,xx   ≤  0.5M_uR,xx

By considering both sides of the inequality in 14.14, we can see that when it is satisfied, even half of the ultimate moment of resistance provided at section XX is sufficient to resist the applied factored moment at that section. So there will not be much stress in the steel at that section, and we can provide splices there. The above discussion can be graphically represented as given in the fig. below:

In the above graph, M_uR,xx  will always be greater than M_u,xx  (∵ this is a primary requirement for any beam. This requirement should be satisfied at all sections of a beam when we do 'Curtailment of bars' also). In case (a), the applied factored bending moment at the section is greater than half of the 'capacity of the beam at ultimate state', M_uR at the section. So splices cannot be given there. But in (b) and in (c), it is less than half of the capacity. So splices can be given for those cases.

This can be further explained by taking a simply supported beam as an example. Fig.14.41 below shows a simply supported beam and it's factored bending moment diagram.

Fig.14.41
Simply supported beam

Suppose in the beam shown in the above fig., 'curtailment of bars' is not done. Then all the bottom bars at the midspan will continue uninterrupted to either supports. So the M_uR will be the same wherever we take a section XX along the length of the beam. But the applied M_u will vary.

• At any section, M_u will be less than M_uR. (∵ this is a primary requirement for any beam.) 
• We want the regions where M_u is less than even the 'half of M_uR'. This can be obtained as follows:

We know that the factored bending moment M_u,xx  at any section of the above beam can be obtained using the equation:

Eq.14.15
M_u,xx  =  R_A x – 0.5w_u x² (Where  R_A  is the reaction at support A, and x is the distance of the section from the support A)

We also know that, as we are not applying any curtailment, the ultimate moment of resistance M_uR  will be constant at all sections. We can find the position of a section XX at which the applied bending moment is equal to half of M_uR, simply by equating them. Thus:

Eq.14.16
M_u,xx  =  R_A x – 0.5w_u x²  = 0.5M_uR

Solving this we will get the value of x. (As the beam is symmetrical, and is symmetrically loaded, there will also be a corresponding section on the other side of the center of the span, that is near support B). In the space between this section and the support, the applied factored bending moment will be less than half of M_uR, and so we can give splices there.

This section can also be determined by superposition of the graph of 0.5M_uR over that of M_u. This is shown in the fig.14.42 below:

Fig.14.42
Method of superposition

In the above fig., the graph of 0.5M_uR is drawn above that of M_u. The points of intersection will give the positions of section XX. We do not need to draw the graph of M_uR. Here it is drawn just to show that when curtailment is not done, M_uR (which is always higher than M_u), will be having a constant value.

When curtailments are done:
When we design the beam, we design the section at the midspan, where the bending moment is the largest. But we have seen earlier that all the bars provided at the midspan region need not be given through out the entire length of the beam, because the BM progressively decreases on either side of the midspan. So, after designing the beam at the midspan, we follow the principles of 'Curtailment of bars' and do the necessary bar cut-off at various sections. When this is done, different sections of the beam will be having different values of M_uR. We will learn more details about curtailment in the next chapter. When we follow the principles of 'curtailment of bars' strictly, the M_uR at any section, (denoted as M_uR,xx) will be greater than or equal to the applied factored moment at that section (denoted as M_u,xx ). That is., M_uR,xx ≥ M_u,xx. But the steel at many sections may be just sufficient for making up the required M_uR,xx. That is., M_u,xx at many sections will be 'just below' the M_uR,xx at those sections. It will not be 'too much below' to be less than even 'half of M_uR,xx'. At such sections, the stress in steel will be maximum. So we cannot give splice at those sections. Superposition of M_uR over M_u for this case is shown below:

Fig.14.43
Superposition when curtailment is done

We can see that at any given section M_uR should be greater than M_u. But M_uR is not constant through out the length of the beam. If we want to give a splice at a particular place, we must superimpose the graph of 0.5M_uR and check whether it is safe to give splice in that region.

The graph of M_uR in the above fig. is only a schematic one. The actual graph will depend on the actual curtailments done in the beam. So a lot of calculations are involved.

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It may be noted that we have done the above discussions based on a simply supported beam. So safe regions for lapping the 'bottom bars' were considered. In the case of an intermediate support of a continuous beam, the 'top bars' will have to be considered. There, in the bending moment diagram, the hogging moment region at that support should be carefully studied. The curtailment if any, provided for the top bars should also be considered.

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In general:• For Sagging moments (bottom bars)
   ♦ M_u is greater near mid spans and lesser near supports
   ♦ So safe regions for splices may be available near supports.
   ♦ Additional calculations are required if curtailments are done

• For Hogging moments (top bars)
   ♦ M_u is greater near supports and lesser away from supports
   ♦ So  splices may be provided away from supports.
   ♦ Additional calculations are required if curtailments are done

In any case, each structure should be carefully studied to determine whether it is safe or not to give a splice at a section. And relevant clauses of various codes should be satisfied.


In the next section, we will discuss about the Staggering of splices.

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