Thursday, 24 May 2012

2nd year abcm - Stairs


Syllabus brief: tread & riser relation between them, Single, double (Dog legged and open well) and triple flight stairways in stone and timber, balusters and handrail in stone, timber and steel, details of joints.

Let us discuss the staircase which is one of the very important elements of the buildings. Whenever we need to climb or descend between two levels or floors in any building we have many options to use like ladders, steps, stairs and ramp  (non-mechanized) and escalators and lifts (mechanized).

The criteria to decide which one to use in our design of buildings,  depends on the following: Space available, purpose of vertical travel (for business, maintenance or leisure), age group of people, and also if private or public building, cost and number of people to be transported per unit time.

So space wise ladder will use minimum space and ramp will use maximum space whereas a stairs will use in between the two.

Also all non-mechanized means of vertical travel can be made in many different materials. For example, timber, steel, Reinforced Cement Concrete (RCC), Aluminum or other metal types, etc.

Today we will discuss Stairs and that too made of timber for our blog.

For starters you can visit this pdf file:

Before we start on actual details, let us first explore some types of stairs used design wise to negotiate two levels.

Straight flight stair: rises from floor to floor in one direction with or without an intermediate landing. As the name suggest this flight has no turns due to restriction of space.

Quarter turn stair: in this a stair rises to a landing between two floors, turns through 90 degree either to left or right and rises to floor above, hence ‘quarter turn’.

Half turn, dog leg stair: A half turn stair rises to a landing between floors, turns through 180 degree, then rises parallel to the lower flight to the floor, hence ‘half turn’. A half turn stair is often described as a ‘dog leg’ stair because it looks somewhat like the hind leg of a dog in section.

Geometrical stair: are constructed with treads that are tapered on plan, with the tapered treads around a centre support as a spiral (helical) stair, an open well circular stair or as an ellipse or part of an ellipse or plan as illustrated below.

Let us now proceed to discuss the Timber Staircase…

A staircase, which is a stair with treads and risers constructed from timber boards put together in the same way as a box or case, hence the term staircase, is the traditional stair for houses of two or more floors where the need for resistance to fire does not dictate the use if concrete. 

Each flight of a staircase is made up (cased) in a joiner's shop as a complete flight of steps, joined to strings. Landings are constructed on site and the flight or flight are fixed in position between landings and floors. The members of the staircase flight are string (or stringers), treads and risers. 

The members of the flight are usually cut from timbers of the following sizes: treads 32 or 38 mm, risers 19 or 25 mm and strings 38 or 44 mm. 

Joining risers to treads: the usual method of joining risers to treads is to cut tongues on the edges of the risers and fit them to grooves cut in the treads. 

The nosing on treads usually projects 32 mm, or the thickness of the tread, from the face of riser below. A greater projection than this would increase the likelihood of the nosing splitting away from the tread and a smaller projection would reduce the width of the tread. The nosing is rounded for appearance.

Cut or open string: A closed outer string looks somewhat lumpy and does not show the profile of the treads and risers it encloses. The appearance of a staircase is considerably improved if the outer string is cut to the profile of the treads and risers. This type of string is termed a cut or open string. Because more labor is involved, a flight with a cut string is more expensive than one with closed strings.  

A sawn softwood carriage is usually fixed below flights of a staircase to give support under fixed below flights of staircase to give support under the centre of each step. The fir (softwood) carriage is 100 x 75 mm in section and nailed to landing trimmers or joists for support with the top surface of the carriage directly under the angle of junction of treads and risers. 

Short offcuts from 175 x 25 mm boards are nailed alternate sides of the carriage, so that the top edge of these brackets bears under treads to reduce creaking of the stair.

Some more images for your reference:

Open Riser or Ladder: An open riser or ladder stair consists of strings with treads and no risers so that there is a space between the treads, with treads overlapping each other at least 16 mm. 

Recommended further readings:
1. Barry - Vol. 2 The construction of Buildings. Chapter: 3 Stairs...
2. Building Construction by Dr. J. Jha and Prof.S.K. Sinha. Khanna Publishers Chapter 14   Stairs...
3. Building Construction by Rangwala. Charotar  Publishers Chapter 19 Stairs...

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