Chapter 16 (cont..16)

The following solved example illustrates the design of a typical structurally independent tread slab of a stair

Solved example 16.6 

The reinforcement details according to the above solved example is shown in the fig.16.97 below:

Fig.16.97
Reinforcement details

We will now discuss about the above fig.
In cantilever members, the main tensile steel is given at the top. So in our case, the main bars are given as the top most layer. The distributor bars are given as the second layer from the top. This arrangement will give maximum possible effective depth ‘d’ for the section.

The 10 mm dia. main bars at the top should be given sufficient anchorage. This is obtained by giving a standard 90^o bend, and extending each of them into the wall. The length of this extension from the face of the wall should be determined:

Details about development length can be seen here: unique value of Ld

(a) The 10mm dia. bar that we are considering is mild steel and so they are not deformed
(b) The bar is in tension
(c) The grade of concrete is M20

From (c) above, we get  τ _bd  = 1.2 N/mm². From (a), the bar is not deformed. So we cannot increase this τ _bd  by 60%. Substituting the values in Eq.14.6 we get L_d  = 453 mm. So each of the 10 mm dia. main bars at the top should be given an extension which is not less than 453 mm.

As the slab has a thickness of 100 mm, a bottom layer consisting of 3 Nos. of 8 mm mild steel bars should be given. This is indicated by the green bars at the bottom layer in section XX. These 3 bars should be given two 90 degree bends at the free end. This is for tying them with the top layer. Distributor bars for this layer consists of 5 Nos. of 6mm mild steel bars. These 3 bars at the bottom most layer also take up loads when stress reversal occurs when the slab is subjected to seismic loads. So these bars should also be given extension into the wall.

In this type of stairs with independent tread slabs, a vertical gap will be present between adjacent slabs. It is recommended that this gap should be filled up with materials of low density such as Light weight concrete, before applying cement plaster or laying tiles. This filling is done in the overlapping 10cm. So the ‘Tread’ of the slabs will not be reduced. This is shown in the fig.16.98 below. As this is done before the finishing works, the use of a different material for the filling will not be visible afterwards.

Fig.16.98
Filling of the vertical gap
 

Structurally independent tread slabs simply supported at ends

Fig.16.99
Part view of a stair

The above fig. shows the part view of a stair. It consists of independent tread slabs. These slabs are simply supported on masonry walls on either sides of the stair. This type of stair is used only for climbing small heights of about 45 cm to 100 cm. This is because, for climbing higher, the walls on either side will have to be built higher. When climbing higher, the length of stair and the walls will also increase, and this will lead to loss of space under the stair.

The Rise, Tread, and other details are same as that we saw earlier in fig.16.90 in a previous section

The design of a typical tread slab of this type of stair is easier than that of a cantilevering tread slab because alternate Live loads need not be considered. But all other load calculation procedure are the same.

So the three items that constitute the load on 1m length of the tread slab are given by Eq.16.26, 27 and 28. Thus we can write:

Eq.16.34
The load per meter length of a tread slab simply supported on walls = w₁ = Eq.16.26 +27 +28

When w₁ is determined, we can draw the line diagram as shown below:

Fig.16.100
Line diagram for analysis

From this, we will get the maximum Bending moment at the midspan, and the design of reinforcement can be done. For preliminary proportioning, a thickness of 80mm can be assumed for span upto 1m.

The following Solved example illustrates the design of a typical structurally independent tread slab supported on masonry walls on either side of the stair.

Solved example 16.7

The reinforcement details according to the above solved example is shown in the fig.16.97 below:

Fig.16.101
Reinforcement details

In the above fig., we can see that the main bars are given in the bottom most layer. They are given a 90^o bend at both ends. This bend give better anchorage for the bars.

It may be noted that in this type of stairs also, it is better to fill up the vertical gap between adjacent tread slabs, as we saw earlier in fig.16.98 above.

We have seen the commonly used types of stairs in the above discussions. Before moving to the next chapter, which is about Two way slabs, we will discuss about the design of a ‘Open well staircase’ in the next two sections.

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