Chapter 16.4 - Bars for landings in stairs

In the previous section we designed a stair and saw it's reinforcement details. In this section we will discuss more details about the arrangement of bars.

In the fig.16.22 which shows flight AB, the bottom layer bar (bar type 'a') in the sloping portion becomes the top layer in the landing portion. For the bottom layer of landing, extra bars are given. Similar arrangement can be seen in flight CD (fig.16.23) also. We will now discuss the reason for giving such an arrangement. Consider fig. 16.24 below:

Fig.16.24
Stair bars without embedment

In the fig.16.24, the bottom bar from the sloping portion continues into the landing in such a way that it is the bottom bar in the landing also. When the loads are applied on the slab, the bar will be in tension, and it will try to straighten up. Only the concrete cover is present there to resist this tendency of the bar to straighten up. This concrete cover does not have enough thickness to adequately resist this tendency, and cracks may develop. So we must extend this bar to embed it into a 'mass of concrete'. To achieve this embedment, the bar is taken up to near the top surface of the landing, and then a bend is given to make it horizontal. The measurements required for this embedment is shown in the fig.16.25 below:

Fig.16.25
Length of embedment required by the bars

We can see that, when the type 'a' bars become the top bars of the landing, the bottom portion of the landing is left with out any bars. So some extra bars (denoted as bar type 'b') are given at the bottom layer of the landing. These bars should have the same diameter and spacing as 'a' type bars. The 'b' bars also should have sufficient embedment. So they are taken upto near the top surface of the sloping slab, and then given a bend, to make them parallel to the slope. 

The point of intersection of 'a' and 'b' is taken as the 'critical' point. The specified embedment should be measured from this point. In the fig., the length required is specified as 'Ld(min)'. Why is it specially mentioned as 'min'? The explanation is as follows: The bars we are considering are 'top bars'. Their main purpose is to resist the hogging moment (that can possibly arise even at a simple support due to partial fixity). So they must have the specified length which is more than the length over which the hogging moment can possibly act. Thus we have two lengths to consider:
• The length required for resisting the hogging moment. Which is taken as 0.25l for general cases
• The length required for the embedment to prevent straightening up. Which is Ld
The largest of the above two lengths should be used. This will satisfy both the requirements. So the mention of 'min' tells us to take both criteria into consideration. It may be noted that in Limit state design, Ld is the unique value that we saw in a previous chapter. Also, Ld  should be provided on both sides of the critical point.

Now we consider the landing portion at ‘C’ (the intermediate landing) for the flight CD in fig.16.23. Here the bar type ‘a’ will not try to straighten up. So it does not require any extra embedment. So these bars continue as bottom bars into the landing. But at the support at this intermediate landing, hogging moments can occur if a wall is constructed above the landing (causing partial fixity), as shown in the fig. below:

Fig.16.25 (a)
Hogging moment at support

We can give top bars in the landing which will continue as top bar in the sloping portion also. But when the hogging moment occurs, these bars will be in tension, and will try to straighten up. So we must give two sets of bars (types ‘c’ and ‘d’) as shown in fig.16.25(a) above.
It must be noted that in fig.16.23, the different sets of bars are shown in separate layers only for clarity. In the actual structure, they will be in same layer as shown in the fig. below:

Fig.16.25(b)
Types of bars in same layers

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Stairs with overhanging Landings
Now, we can discuss the arrangement of bars in another type of longitudinal stairs. In this type, the supports are at the ends of the sloping slab as shown in the fig.16.26 below:

Fig.16.26
Supports at the ends of sloping slab

From the fig., we can see that the supports are at the ends of the sloping slabs. The intermediate landing and the top landing are overhanging beyond the supports. In other words, the landings are cantilevers. This type of stairs are more economical. Let us see how this economy is achieved: As shown earlier, the thickness of the waist slab is taken as 1/20 of the effective span. In the fig. above, the effective span is reduced because, the supports are now nearer to each other. So the waist slab thickness can be reduced. The bending moment will also be reduced because of the reduced effective span.

A cantilever structure will produce more bending moment than a simply supported structure. So at a glance, we may feel that more steel will be required for resisting the bending moment from the cantilevers. But here, the cantilevering span is small when compared to the simply supported span between the supports. Also the load on the cantilever landings is less than the load on the sloping portion.
However, it is important to note that hogging moments will be produced at the supports because of the cantilever action. This is shown in the figs. below:

Fig.16.27
Bending moment diagram for flight AB

Fig.16.27
Bending moment diagram for flight CD

From the above figs., it is clear that hogging moments will be present at the supports in this type of stairs. The values of maximum hogging moments and sagging moments can be easily calculated from basic principles. Then we can determine the steel required to resist these moments. The steel required to resist the hogging moments should be given as top steel at the supports. The arrangement of bars for this type of stairs is shown in the figs. below:

Fig.16.28
Arrangement of bars for flight AB

Fig.16.29
Arrangement of bars for flight CD

In the above figs., a new type of bar denoted as 'd' is given as top bars at all supports where overhang is present. These are the bars which resist the hogging moment. They must have sufficient embedment as indicated by 'y' in the figs. Also note that these bars are taken to the farther face of the sloping slab, and then bent to make them parallel to the slope. This is to give maximum embedment inside concrete.

In the next section, we will discuss about another type of longitudinal stairs in which, the second flight takes a right angled turn from the first flight.

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Chapter 15.13 - Curtailment of Bent-up bars

In the previous section we saw the anchorage requirements of bent-up bars at simply supported ends and at end supports in frames. In this section we will see the case (iv) and (v) of fig.15.59.

Case (iv): Intermediate supports in continuous beams and slabs

The following fig.15.73 shows such a support.

Fig.15.73
Bent up bar at continuous support

In the fig.15.67, the bar provided as bottom bar in span BC is bent-up at Q. It then becomes top steel at support C and continues as top steel into the span CD. Now, when the span BC bends downwards due to the applied loads, the bar will experience tension. So it will try to pull out from support C. To prevent this, we know that adequate development length should be provided, and from the discussion that we had about the 'development length of bent-up bars', we can say that the length RST should be greater than or equal to L_d. This will prevent the bar in span BC from pulling out from the support C.

This same bar will experience another tension also: At the support, the beam is bending upwards, so the bar is being pulled from span BC as well as from span CD. So the bar will try to pull into the support C from both sides. That is., from span BC and from span CD.

We will first consider span BC. The portion inside BC has a bent shape. So a bar with a bent shape is preventing the 'pulling into support C'. This situation requires a different method of measurement for L_d. It is shown in the fig. below:

Fig.15.74
Method of measuring L_d when the bent up bar is under hogging moment

From the fig.15.74, we can see that the procedure is simpler. We do not need to mark the point R. The measurement is taken from the bottom bend point Q towards the left side. The available length towards the left of Q should be greater than or equal to L_d. If this is satisfied, the bar will not 'contract' into support C from the span BC.

In a similar way, the bar will try to 'contract' into the support C from span DC also. To prevent this we must provide L_d from the face of the support towards the right side.

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Now we will discuss about the performance of the bent-up bar in resisting the hogging moment at the support C. We have seen figs.15.51 and 52 which give the requirements of top bars at an intermediate support in a continuous beam. Fig.15.51 is shown here again for easy comparison.

Fig.15.51
Curtailment of 'straight' top bars

We can see that a bar is curtailed at a distance of L_a from the theoretical cut-off point. The other bars continue beyond the point of inflection, (two of them will continue even beyond the next support to act as stirrup hangers and also to take part in resisting the hogging moment at the next support. In a general simple case, there will be three bars at the top, and the middle one will be curtailed. If the hogging moment is of a larger magnitude, more than three bars will have to be provided in layers). Here we concentrate our attention on the bar which is cut-off at a distance L_a from the theoretical cut-off point. This actual cut-off point is close to the support. We can think of using a bent-up bar in the place of this bar. This is shown in the fig.15.75 below:

Fig.15.75
Bent up bar in place of curtailed bar at top of continuous support

From the fig., we can see that the horizontal portion ST can be extended to the desired point in the right side of support C. But it is not possible to do so in the left side, and thus it may not have the required length in the left side. So the bar will perform well on the right side of the support C. And any contribution that it makes to resist the hogging moment on the left side of C should be ignored. But this arrangement will create a sort of asymmetry because, on the right of support C, three bars (two stirrup hangers and one bent-up bar) are available, while on the left side, only the two stirrup hangers are available to take up the hogging moment. This is solved by bringing in the bent-up bar of span CD as top bar into span BC as shown in the fig. below:

Fig.15.76
Bent up bar at a support from adjacent spans

In the above fig., the two bars are shown at different levels only for clarity. In the actual beam, they are provided at the same level as indicated by the ‘0 mm’ clearance between the bars.

Now we will see the curtailment requirements when bent-up bars are provided. Earlier we have seen fig.15.51 (shown again above) which showed the curtailment details when straight bars are used. This fig. is based on the cl.26.2.3.4 of the code. The same requirements apply here also. So we can draw a fig. for bent-up bars by making suitable modifications to fig.15.47. This is given in fig.15.77 below:

Fig.15.77
Curtailment of top bars at an intermediate support when bent up bars are used

We can see that the bent-up bar from span CD is not included in the calculation of A_st1. This is because, it's top horizontal portion does not have the required length inside span CD. So it's contribution cannot be taken into account in span CD. But it's contribution can be taken into account in the span BC.

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So we have completed case (iv) of fig.15.59. The next case (v) is similar to case (iv). To show a final diagram showing the curtailment details, all we need to do is to change the support condition in the above fig.15.77. This is shown below:

Fig.15.78
Bent up bars at the intermediate supports of a frame

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So we have seen the final case (v) also of fig.15.59. It may be noted that for the cases (iii), (iv) and(v), Moment envelope should be used instead of Bending moment diagram where ever applicable.

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Shape of Bent-up bars

We have seen the usage of bent-up bars in various situations. Now we will see an important aspect that we have to consider in the 'actual making' of a bent-up bar. In the figs. that we saw above, the bends are all shown as ‘sharp’, as in the fig. below:

Fig.15.79
Sharp bends shown in illustrations

But this is used only for illustration purpose. In an actual beam, such sharp bends should be avoided. This can be achieved by introducing curved portions in between straight segments as shown in the fig. below:

Fig.15.80
Actual method for forming a bent-up bar

Each of the curved portions at the top and bottom bend points is a part of a perfect ring having inner radius r and outer radius r + Φ. More details about the formation of such bent bars are given in IS 2502-1963: Code of practice for bending and fixing of bars for concrete reinforcement.

So we have completed the discussion about bent-up bars. In the next section, we will see the method of curtailment when the continuous members are designed using moment coefficients.

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