Chapter 14.10 - Required increase in lap length

In the previous section we were discussing the sub clauses which gives the specifications for 'increasing lap length based on concrete cover'. In this section, we continue the discussion.

The sub clause (2), specifies another condition about ‘adjacent laps’. For discussing this, let us assume that both the bars c' and d are lap spliced at the same section. Then the section YY will be as shown below:

Fig.14.56
Section YY when laps are present in adjacent bars

In this YY section, the clear distance between the adjacent laps is shown. If this clear distance is less than the largest of 75 mm and 6Ф, then we have an example of the other condition about ‘adjacent laps’, specified in this sub clause (2). In this case also we must take the same steps as before:

• First, find L by following the 'rules for obtaining the lap length for bars in flexural tension' that we discussed above. (Figs.14.48 to 14.50). 
• Then obtain the modified value by multiplying this L by 1.4.

This modified value should be provided for the lap. Note that this condition involving the distance between adjacent laps is applicable to both top bars and bottom bars.

So in general, if a bar is such that, only the sub clause (2) is applicable (for computing required increments in lap lengths based on covers), then any one of the two points below should be satisfied by the bar:
■ It should be a corner bar
       ♦ Any of c_t /c_b OR c_s should cause concern. 
■ It should be one among two adjacent bars which are lapped at the same section, with a clear distance less than the largest of 75 mm and 6Ф.

So we now know how to determine if a bar falls into sub clauses (1) or (2). At the end of sub clause (2), one more condition is specified. That is., if a bar falls into both the sub clauses (1) and (2), then the multiplication factor to be used is '2' instead of '1.4'.

Let us now see whether this condition is applicable to any of the bars in the above continuous beam. In the section XX, consider the bars a and a'. For them, c_t causes concern. If this c_t is less than 2Ф, then they will fall into (1). For these bars c_s also causes concern (∵ the beam is not a T-beam). If this c_s is less than 2Ф, then they will fall into (2) also. If they fall into both (1) and (2), the multiplication factor to be used for them is '2' instead of '1.4'. If it is a T-beam, c_s will not cause concern, and so (2) will not be applicable. Then the multiplication factor will remain as '1.4'.

Now consider section ZZ. All the top bars e, e' and d will be in tension. So for them, c_t causes concern. If this c_t is less than 2Ф, then they will fall into (1). The bars e and e', besides being top tension bars, are corner bars also. For them c_s also cause concern. If this c_s is less than 2Ф, then they will fall into (2) also. If they fall into both (1) and (2), the multiplication factor to be used for them is '2' instead of '1.4'.

But in the above fig, the top bars are all bent, and extended into the column. So in the column portion, they are having a cover from the face of the column. If this cover is less than twice the diameter, then bar d will be in such a position that, both the conditions are applicable to it also. So in that case, the modification factor for bar d will also be '2'. For these bars, another possibility is that the top bars have the specified cover from the top portion of the beam, but do not have it from the side of the column. Then also the modification factor should be '2'.

What we saw above are only a few examples of the bars for which the cl.26.2.5.1(c) and it’s sub clauses may be applicable. The designer should carefully examine each structural member to determine the clauses which are applicable to the different bars in it. Also, any type of splicing should be avoided at regions of high stresses, as we discussed earlier.

So we have had a lengthy discussion about the sub clause (c) and the sub clauses (1) and (2) within it. It is convenient to show an abstract of the discussion in the form of a flow chart as shown below:

So we have completed the discussion about all the specifications regarding the 'length of the lap'. It is time to see the specification about 'Staggering of laps'. This is given in the cl.26.2.5.1(b). According to this clause, the lap splices can be considered to be staggered if the centre to centre distance between the lap splices is not less than 1.3 times the lap length L. This is shown in the fig.14.57 given below.

Fig.14.57
Staggering of lapped splices

Note that for this clause to be applicable, L should be calculated exactly according to the guidelines given in the cl.26.2.5.1(c), and it's sub clauses (1) and (2), which we discussed above.

Cl.26.2.5.1(d)
This clause is about the lap length required in compression. According to this clause, the lap length in compression should not be less than the largest of the following: (1) Development length L_d of the bar in compression, and (2) 24Ф. We can make a fig similar to the one we used to show the lap length for bars in tension. Such a fig is shown below:

Fig.14.58
Lap length in compression

We have to take special care about the item (1), the L_d in compression. For a bar of a particular diameter, L_d in compression will be different from L_d in tension. We have already discussed how to calculate L_d in compression. See the notes given below Table 14.1 and the example of a doubly reinforced cantilever beam shown in fig.14.5

When bends or hooks are provided at the lapped ends, only their 'projected length' can be considered for calculating the L_d. We discussed about this based on fig.14.16 in the section: 'Bends and hooks for compression reinforcement'. If bends and hooks are provided at the lapped ends of bars in compression, it is better to ignore their projected length, and consider the straight portion only. This is shown in the figs. below.

Fig.14.59
Lap length in compression when hooks are provided

Fig.14.60
Lap length in compression when bends are provided

It may be noted that for determining the lap length for bars in tension, the code gives different methods for ‘flexural tension’ and ‘direct tension’. But there is no such differentiation for the laps of bars in compression.

Cl.26.2.5.1(e)
This clause is about the lapping of two bars having different diameters. We have seen that Ф, the diameter of the bar comes into the calculation of development length L_d, Lap length L, etc., This clause tells us to use the smaller Ф in the calculations, when bars of two diameters have to be lap spliced.

Cl.26.2.5.1(f)
This clause is about the splicing of welded wire fabric.

Cl.26.2.5.1(g)
This clause is about the splicing of bars within a bundle. Bundles as a whole should not be spliced. If the bars of a bundle fall short of length, they can be lap spliced one bar at a time. Care should be taken to see that these splices are staggered, and are not at sections of maximum stress. The modification factors that we saw earlier in sub clause (c) should be applied where ever necessary. Also, the increase in development lengths to be applied particularly for bars in a bundle, that we discussed based on fig.14.9 should also be taken into account.

This completes the discussion about lapped splices. So we will now see some basic details about welded splices and mechanical connections.
For large diameter bars, welded splices and mechanical connections are more suitable. When these methods are adopted, extra length of steel bars for the lapping is not required. So it will result in a lower consumption of steel. Cl.12.4 of the code gives the recommendations regarding welded joints or mechanical connections. According to this clause, tests must be conducted on the bars spliced by these methods, to ensure that they have the full strength as the bars which are connected. Cl.26.2.5.2 gives the value of the strength of the weld. According to this clause, if the tests prove that the weld in a bar used as tensile reinforcement, has the same strength as the parent bar, then the design should be in such a way that, only 80 per cent of the full design strength of the bar will be applied on that welded bar.

More details about welded splices and mechanical connections, and the details of tests that have to be performed, can be obtained from the relevant codes.

So we have completed the discussion on splices. The next chapter discusses the details about 'Curtailment of bars'.

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Chapter 14.8 - Conditions for providing splices in bars of beams

In the previous section we saw the safe regions for providing splices. In this section we will discuss about staggering of splices.

Staggering of splices

We have seen how to determine the safe sections where splices can be given according to cl.26.2.5 of the code. This same clause mentions about one more requirement. That is., the splices should be staggered. Staggering of splices is essential because of the following reason: We have seen that at a splice, the force in the stopping bar is transferred to the continuing bar by means of the bond stress in concrete. If more splices are provided at a single section, there will be a cumulative effect from the bond stress in all the spliced bars at that section. This may lead to cracks. 

The method for achieving proper staggering is also mentioned by the clause: 'not more than half the bars shall be spliced at a section'. This is a simple rule to follow. If there are two bars at a section, only one of them can be spliced. If there are 3 bars at a section, then also only one of them can be spliced at that section because if we take more than one, it will become greater than half the number of bars at the section. In this way we can limit the number of splices at a section. Consider the fig.14.42 in the previous section. Suppose in that beam, there are 3 bottom bars of the same diameter, continuing uninterrupted from support to support, and two of them fall short of length. In that case, one can be spliced at the safe zone near support A, and the other can be spliced at the safe zone near support B.

Precautions to be taken when conditions are not satisfied
So we have seen two requirements from this clause. One is about avoiding splices at sections of maximum stress, and the other is about staggering of the splices. In addition to these, the clause also tells us about the precautions to be taken when the above requirements cannot be satisfied. That is., these precautions are to be taken when
• Splice is given at a section of maximum stress. OR
• More than half the bars is spliced at a section.

The precautions to be taken are:
• Increasing the length of the lap AND
• Using spirals or closely spaced stirrups around the length of the splice.
The code does not specify the amount of increase to be given to the lap. We can use the details given in 25.2.5 of SP 24, the explanatory hand book to the code. There, the increase in length to be provided is given in the form of a graph. This is shown in fig.14.44 below:

Fig.14.44
Graph for determining the increase in length

Let us now see the details of this graph. '% of bars spliced at the section' is plotted along the X axis. So if 3 out of 4 bars having the same diameter is spliced at the section that we have under consideration, we will mark a point at 75% on the X axis, and draw a vertical line through it.

Along the Y axis, '% of design stress in steel' is plotted. So we have to first find the stress in the steel at our section. Let us denote it as f_s. It can be calculated using the basic formula: 
Applied factored bending moment = Stress x Area of steel x Lever arm
That is., M_u = f_s x A_st x z . From this, f_s can be calculated.

Now, the maximum stress possible is 0.87f_y. So the percentage of design stress is: (f_s/0.87f_y)x 100.

---

(It may be noted that calculating the percentage of design stress in this way is equivalent to calculating the percentage of applied moment. Which is equal to (M_u/M_uR)x 100.  That is., (f_s/0.87f_y)x 100 = (M_u/M_uR)x 100. The proof for this can be seen here).

---

If we get this as say 60% , we mark this value of 60% on the Y axis and draw a horizontal line through it. The point of intersection of the horizontal and vertical lines is our point of interest. It will fall within one of the four rectangular areas. The lap length to be given will depend upon the rectangle into which the point of intersection falls. For example, the intersection of horizontal line through 60% and vertical line through 75% will fall within the red rectangle as shown in the fig.14.45 below. So the lap length to be provided will be 1.7L_d.

Fig.14.45
Sample calculation

In the above two graphs, we can see that the blue rectangle is specially marked as 'Code recommended area'. This is so because, it falls within the 50% limit of 'both the requirements' of the cl.26.2.5 of the code. We have already seen these two requirements: 
(1) Splice should not be provided at sections where applied factored bending moment is greater than 50% of the moment of resistance, (Y axis) and 
(2) More than 50% of bars should not be spliced at a section, (X axis) 

These two requirements are satisfied in that area. Because it falls within '50' on X axis and also '50' on Y axis. 

The other precautionary measure is to use spirals and/or extra stirrups. We have seen the details of spirals to be provided in the case of bars larger than 36 mm in diameter. It was explained with the help of figs.14.39 and 14.40. The same procedure can be adopted here also.

If spirals cannot be provided, we must use closely spaced stirrups around the length of the splice. The procedure for this is given in SP34, the handbook on concrete reinforcement and detailing. This can be explained based on the fig.14.46 given below.

Fig.14.46
Extra stirrups around the splice

First step of the procedure is to calculate the number of these extra stirrups. This can be done as follows: 
• The total area of cross section of all the stirrups (A_sv) multiplied by the ultimate stress in the steel of the stirrups (0.87f_y) will give the ultimate force which these extra stirrups will be able to resist. 
• This must be equal to the total tension in all the bars that are spliced at the section. 
• So we can write: T = 0.87f_y A_sv. (Where T is the total tension in all the bars that are spliced at the section) From this we will get A_sv. 

[It may be noted that A_sv here is the total area of all the stirrups. So A_sv = No. of stirrups (N) x No. of legs of a single stirrup x Area of cross section of the bar of the stirrup.]

• If we assume the number of legs and the diameter of the bar of a single stirrup, we can calculate the area of one single stirrup. So, A_sv divided by this area of a single stirrup will give us 'N', the number of extra stirrups to be provided.
• Next, one third of the lap length is marked off at both the ends of the lap.
• In the final step, 'N' number of stirrups is provided at each of these marked regions at uniform spacing, starting from the inner side.

It should be noted that
• a minimum of three extra stirrups should be provided at each of the marked off region, and 
• the spacing should not be greater than 150 mm.

If the diameter of the spliced bars in the above fig. is greater than 28 mm, then it is compulsory to provide spirals or compact stirrups even if the extra stirrups are provided. A model showing the shape of compact stirrups is given in the fig.14.47 below. In the fig., the compact stirrups are shown in yellow color.

Fig.14.47
Shape of compact stirrups

So we have had a lengthy discussion about the precautionary measures alone. It will be convenient to have a flowchart like representation of the above discussion, as shown below:



In the next section we will see the provisions in the code, which are related specifically to lap splices.

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