In the previous section we completed the analysis and design of a stair in which flights are supported on the landings. In this section we will discuss about a different kind of 'load distribution', that occurs in special type of longitudinal stair. Fig.16.43 below shows the external view of a building.
Fig.16.43
External stair of a building
A room is constructed as an 'extension' to the Main building. The stair climbs upto the roof of this extension room. This roof is at a lower level than the roof of the main building. From the top of this lower roof, another smaller stair is provided, which can be used to climb up to the roof of the main building. This smaller stair is built at the rear of the building, separately from the main stair, and is not seen in the view. We are concerned about the main stair in our present discussion. The section and plan of the stair are shown below:
Fig.16.44
Section of stair
Fig.16.45
Plan of Stair
We can see that the stair is supported on the ground at the bottom end, and on a beam at the top end. This is a longitudinal stair. The lower roof acts as the landing. The loads on the Going and the Landing are calculated below:
Four items constitute the load on the Going, and is given by Eq.16.9.These four items are:
1) Self wt. of waist slab on a horizontal 1m2 area (in kN/m2) =
= 5.991 kN/m2 .
2) Self wt of steps per 1m2 of horizontal area (in kN/m2) = (R γs) ⁄2 = 0.5Rγs
The value of γs for brick masonry is 20kN/m3
= 1.65 kN/m2
3) Self wt. of finishes per 1m2 of horizontal area (in kN/m2)
= 0.8 kN/m2 (obtained from code)
4) Live Load per 1m2 of horizontal area (in kN/m2)
= 5 kN/m2 (obtained from loading code)
Sum of above four = 13.441 kN/m2
So factored load = w1 = 13.441 x 1.5 = 20.16 kN/m2
Similarly, Three items constitute the load on the Landing, and is given by Eq.16.10. These three items are:
1) Self wt of steps per 1m2 of horizontal area (in kN/m2) = 25t (where ‘t’ is the thickness of the landing)
= 25 x 0.12 = 3.0 kN/m2
2) Self wt. of finishes per 1m2 of horizontal area (in kN/m2)
= 0.8 kN/m2 (same as for Going)
3) Live Load per 1m2 of horizontal area (in kN/m2)
= 5 kN/m2 (same as for Going)
Sum of above three = 8.8 kN/m2
So factored load = w2 = 8.8 x 1.5 = 13.2 kN/m2
Half of this load (0.5 x 13.2 = 6.6 kN/m2) can be assigned in the design of stairs because the full load will be used for the design of the landing slab. The line diagram of the stair is shown in the fig.16.46 below. Note that it is a continuous structure. This because of the intermediate support at B.
Fig.16.46
Line diagram for the stair
It can be seen that the above line diagram is incomplete. The length l2 is missing. We can easily obtain l2 from the architectural drawings. Once l2 is known, we can analyse the stair as a strip of a continuous slab, and the design can be done. But as mentioned earlier, the aim of our present discussion is to learn the details about a special kind of load distribution in longitudinal stairs. We are not doing the analysis and design of this stair. We are not concerned about the portion BC in our present discussion. In fact, we are going to focus our attention only on the portion from A to B, the ‘Going’ portion. We will discuss about it in the next section.
Fig.16.43
External stair of a building
A room is constructed as an 'extension' to the Main building. The stair climbs upto the roof of this extension room. This roof is at a lower level than the roof of the main building. From the top of this lower roof, another smaller stair is provided, which can be used to climb up to the roof of the main building. This smaller stair is built at the rear of the building, separately from the main stair, and is not seen in the view. We are concerned about the main stair in our present discussion. The section and plan of the stair are shown below:
Fig.16.44
Section of stair
Fig.16.45
Plan of Stair
We can see that the stair is supported on the ground at the bottom end, and on a beam at the top end. This is a longitudinal stair. The lower roof acts as the landing. The loads on the Going and the Landing are calculated below:
Four items constitute the load on the Going, and is given by Eq.16.9.These four items are:
1) Self wt. of waist slab on a horizontal 1m2 area (in kN/m2) =
2) Self wt of steps per 1m2 of horizontal area (in kN/m2) = (R γs) ⁄2 = 0.5Rγs
The value of γs for brick masonry is 20kN/m3
= 1.65 kN/m2
3) Self wt. of finishes per 1m2 of horizontal area (in kN/m2)
= 0.8 kN/m2 (obtained from code)
4) Live Load per 1m2 of horizontal area (in kN/m2)
= 5 kN/m2 (obtained from loading code)
Sum of above four = 13.441 kN/m2
So factored load = w1 = 13.441 x 1.5 = 20.16 kN/m2
Similarly, Three items constitute the load on the Landing, and is given by Eq.16.10. These three items are:
1) Self wt of steps per 1m2 of horizontal area (in kN/m2) = 25t (where ‘t’ is the thickness of the landing)
= 25 x 0.12 = 3.0 kN/m2
2) Self wt. of finishes per 1m2 of horizontal area (in kN/m2)
= 0.8 kN/m2 (same as for Going)
3) Live Load per 1m2 of horizontal area (in kN/m2)
= 5 kN/m2 (same as for Going)
Sum of above three = 8.8 kN/m2
So factored load = w2 = 8.8 x 1.5 = 13.2 kN/m2
Half of this load (0.5 x 13.2 = 6.6 kN/m2) can be assigned in the design of stairs because the full load will be used for the design of the landing slab. The line diagram of the stair is shown in the fig.16.46 below. Note that it is a continuous structure. This because of the intermediate support at B.
Fig.16.46
Line diagram for the stair
It can be seen that the above line diagram is incomplete. The length l2 is missing. We can easily obtain l2 from the architectural drawings. Once l2 is known, we can analyse the stair as a strip of a continuous slab, and the design can be done. But as mentioned earlier, the aim of our present discussion is to learn the details about a special kind of load distribution in longitudinal stairs. We are not doing the analysis and design of this stair. We are not concerned about the portion BC in our present discussion. In fact, we are going to focus our attention only on the portion from A to B, the ‘Going’ portion. We will discuss about it in the next section.
hats of you sir ,,, u reached an extreame imagination rcc topics through your diagrams and demonstrations
ReplyDeleteVery glad to know that these lessons are helpful for Students and Professionals.
Deletethank you so much sir,
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