In the previous section we completed the discussion on curtailment of bars. In this section we will discuss the design of staircases.
Fig.16.1
View of a stair
In the above fig., the stair portion is shown at a condition before finishing works like plastering, painting, side railings etc., are done on it. This is to show the structural details. We can see that:
• The stair begins with an inclined slab from the ground surface. This inclined slab becomes horizontal towards the end, and finds support on a masonry wall.
• Then the stair changes direction. In the new direction, first there is a horizontal portion. Then the inclination begins. Towards the end, it becomes horizontal again and finds support on a beam, which is projecting out from the main building.
• At this end point, the stair has reached the terrace of the building.
The view from the rear side of the stair is shown below in fig.16.2.
Fig.16.2
View from rear side
In the two views, we can see that in the inclined portions, steps are made in brick masonry. These steps makes the upward and downward movement easier. No such masonry work is required over the horizontal portions of the slabs. To show more technical details of a stair, we need to draw elevation and plan views. These drawings related to the above stair, are given in the figs. 16.3 and 16.4 below:
Fig.16.3
Side elevation of Stair
Fig.16.4
Plan of Stair
In the above elevation and plan views, the reinforced concrete slabs are shown in yellow colour, and the steps made in masonry work are shown in blue colour.
• The measurement of the horizontal top portion of a step is called Tread, denoted by the letter 'T'.
• The vertical distance between two adjacent treads is called Rise denoted by the letter 'R'.
Generally, a value of 30 mm is given for T and 15 mm is given for R, in public buildings. But for residential buildings, lower values can be used.
It may be noted that T can be given a small increase when the finishing works are applied to the steps. This increase is obtained by a projection (called Nosing) beyond the vertical face of the step. An example is shown below:
Fig.16.5
Nosing given to steps
In the above fig., the vertical piece of tile will not help in increasing T because, it will be coming in all steps. The horizontal piece of tile is given a projection beyond the vertical piece.
The width of stairs vary from 1.1 m to 1.6 m. Larger widths are given for stairs of public buildings. Stairs in residential buildings can be of lower widths. But they should not be less than 85 cm. In the above plan view, the width is shown as 90 cm. The horizontal portion of the stair is called the Landing. The horizontal distance between the first and last risers is called the Going. In the above elevation and plan views, we can see that, the length of the intermediate landing is 180 cm. It's width is 90 cm.
• On the side of the first flight, the last masonry step of this flight is contributing 25 cm for making up the required width of 90 cm for the intermediate landing. This is because, the horizontal portion of the concrete slab has a width of only 65 cm on the side of the first flight.
• But on the side of the second flight, the horizontal portion of the concrete slab readily have a width of 90 cm. So no contribution is required from the masonry steps.
• At the top most landing also, the contribution is made by the last masonry step.
Stairs can be considered to consist of Flights. In the above plan view, there are two flights. Flight AB and Flight CD. AB has one Going and one Landing. CD has one Going and two landings. The number of steps in a flight should not exceed 12. After 12 steps, there must be a Landing.
Supports of Stairs
Now we will discuss about the supports that are given to the above stair. First of all, we must consider the two flights separately. This can be explained as follows: At the intermediate landing, the two flights are connected together. But as we will soon see in the ‘design of reinforcements for the stairs’, the main bars of the two flights are not connected together. Only the distributor bars in the intermediate landing will be common to the two flights. So there is no transfer of force between the two flights. In other words, the loads and forces in one flight will not have any effect on the other flight. So we can draw the line diagram of the two flights separately as shown in the fig.16.6 below:
Fig.16.6
Line diagram of Stairs
We are able to separate the individual flights of the above stairs easily. For stairs of other shapes, care should be taken while separating the flights. Now we will see the support at A. In the elevation shown in fig.16.3, we can see that the inclined slab rests directly on the ground. But at the point of contact with the ground, the slab is given an increased thickness as shown in the fig. below:
Fig.16.7
Increased thickness at support
The increase in thickness is given in such a way that the resulting enlarged portion will become the first step. The extra portions are shown in red color. We can see that there is one large red triangle above and a small red triangle below the slab. The larger triangle has the same size as the steps. The increase is achieved not only by providing more concrete. The reinforcement of the slab is also bent in such a way as to get this final shape. We will discuss about it in the section on reinforcements of stairs. This method gives a good seating for the slab. Appropriate foundation is provided below this enlarged portion. Extra bars are provided in the foundation, which will be bent into the inclined slab of the stair. This will prevent sliding of the inclined slab.
This type of arrangement gives a 'Simple support' at A. In the next section we will discuss about the other supports at B, C and D.
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