Chapter 16.13 - Stairs cantilevering on the sides of Stringer beam

In the previous section we saw the details of transverse stairs which cantilever from the side of a stringer beam.  The following solved example illustrates the design of  such a stair.

Solved example 16.4

The reinforcement details according to the above solved example is shown in the figs.16.81, 82 and 83 given below. The cantilever waist slab has to be properly anchored into the beam. For this, the stirrups of the beam are extended into the slab as shown below in fig 16.81:

Fig.16.81
Extending the stirrups into the slab

When such an arrangement is given, we must take care of the diameter of bars and the spacing requirements. The spacing of the main bars of the slab will be the same as the spacing of the stirrups. So the stirrups of the beam should be having a uniform spacing throughout it's length. And for our case, this spacing should not be greater than 250mm c/c, and the diameter should not be less than 10mm.

If the spacing of 10mm dia. stirrups required for the beam is less than 250mm c/c, then the spacing of bars in the slab will also become less than 250mm c/c. In that case check should be done to ensure that p_t is less than p_t,lim. Other wise, the slab will become over reinforced.

The elevation and sectional views are given below:

Fig.16.82
Sectional elevation

Fig.16.83
Section XX

Now we will discuss some more features of the above figs: 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. So while bending the bars for the stirrups of the beam at the site, care should be taken to see that the extension given to the stirrup will be at the exact required level in the slab. A bar bending schedule should be prepared to give the exact measurements.

As the slab has a thickness of 110mm, a bottom layer consisting of #8 bars at 250mm c/c should be given. This is indicated by the green bars in section XX. In this layer, alternate bars should be given two 90 degree bends at the end. This is for tying them with the top layer. This bottom layer is not shown in the sectional elevation in fig.16.82.

Transverse stairs cantilevering from both sides of a stringer beam:

The following fig. 16.84 shows the part view of a stair

Fig.16.84
Part view of a stair
 

The waist slab is cantilevering on both sides of the stringer beam. The load calculation procedure is same as that of the other types of transverse stairs that we saw before. So we can use the same Eq.16.25 for the calculation of the loads. From this eq., we will get w₁ , the load per 1m length, on a 1m wide strip. If the cantilevering length on both the sides are equal, we need to do the design of only one side as shown in the line diagram below:

Fig.16.85
Line diagram for stairs cantilevering on both sides of a stringer beam

The effective span l of the cantilever (cl.22.2.c) is the length of the cantilever up to the face of the support plus half the effective depth. For initial proportioning, we can assume the thickness of waist slab to be clear length of cantilever/5 . However, this should be finalized only after the complete design and various checks.

In the next section, we will see a solved example of the above type of stairs.

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