INTRODUCTION - libvolume3.xyzlibvolume3.xyz/civil/btech/semester5/designofrccstructuralelements/designofstaircases/...INTRODUCTION Stairs are the conventional means of access between

STAIRS

INTRODUCTIONStairs are the conventional means of accessbetween floors in buildings. A stair is describedas a set of steps leading form one floor toanother and a staircase includes the part of thebuilding surrounding the stair.

Stairs should be constructed to provide realeasy and safe access up and downs, with stepsthat are either laborious or difficult to climbwithin a compact area so as to take up noexcessive floor area.

In the building, stairs have two main functions:firstly that of normal everyday access fromfloor to floor, and secondly they must providean easy and rapid escape, from the upper floorin case of emergency (as fire or earthquakes).

For this reason even if most buildings haveother means of access between floors aselevators (lift) or moving staircase they musthave also stairs !!!

The slope according to the use of the stair

Step - is a short horizontal surface for the footto facilitate ascent from one level to another.It commonly consists of a horizontal elementcalled tread and a vertical element called a riser.The external junction of the tread and riser iscalled a nosing and it provides as wide a surface oftread as practicable.

DEFINITION OF TERMS

Main elements of the stair: 1 - flight, 2- strings, 3- landing, 4- walking line, 5- handrail, 6- balusters, 7- tread or going, 8- riser

• Rise - is the vertical distance between the upper surface oftwo consecutive steps and the rise of a flight is the total heightbetween the floors or landings in connections. Going (or run) isthe horizontal distance between the nosing or front edges, oftwo consecutive steps.

• Flight - the word flight describes a continuous series of stepsbetween floors or landings. A flight should have no fewer thanthree and no more than sixteen steps.

• Strings or stringers - act as inclined beams, which supports thesteps. Stingers can be fixed to a wall or can be supported byother structural members.

• Strings or stringers - act as inclined beams, which supports the steps. Stingers can be fixed to a wall or can be supported by other structural members.

• Pitch or slope - the inclination of a stair that is the angle made between the line of nosing and the line of floor or landing.

• Landing - is a platform between flights. It serves as rest between flights and to turn on a stair.

Non-skidding materials must be used for the finishing of stair and landings, to prevent the sliding of the leg.

Classification according to their destination:

Classification category

Classif. Criterion Used for Notes

1 2 3 4

- monumental;

- stair flight breadth, bfl

- used as access stirs at the entrance and maxi-mum first two floors in important buildings;

- has a functional or architectural (aes-thetical) destination;

- main;- there are used in multi-storey buildings;

- provide the usual vertical circulation and escape evacuation;

- for stair breadth it is advisable tat l2.40m.

- secondary;- used for access to underground floor or roof or certain floor;

- with minimal breadth for the flight;

- stairs for fire escape;

- used just for escape risky situations;

- made usually of metal and placed outside the building;

- industrial;- access for the main-tenance or utilisation of industrial equipment.

- made usually of metal with steeply flight (60o) without risers;

classification after the dimension of their rise:

Classification category

Classif. Criterion

Used for Notes

1 2 3 4

- low steps,(hrise16.5 cm);

- rise of the step, h

- main stairs in high rise buildings (>Gfl+3F), kindergartens, schools, hospitals, etc.;

- proper for the use of children, old age or ill person;

- regular steps,(16.5hrise17.5 cm);

- main stairs in low risebuildings (<Gfl+3F) orsecondary stairs inhigh rise buildings;

- most comfortable and commonly used stairs;

- high steps,(17.5hrise22.5 cm);

- secondary stairs in low rise building, access to the underground or on the roofs;

- stairs with hrise> 19.5cm are admitted just as service stairs for one just floor.

- steep stairs,(22.5 hrise 30cm);

- vertical

Classification according the resistance to fire action:

1 23 4

- incombustible;

- fire resis-tance of materials used for construction

- in the buildings with fire resistance class I, II, and III;

- reinforced concrete, stone and ceramic materials stairs;

- hardly combustible;

- in the buildings with fire resistance class I, II, and III;

- metallic stairs;

- semi-combustible; - fireproofed timber

stairs;

- combustible;- timber stairs.

Classification after the materials used to construct:

12

3 4

- timber stairs

-materials used for construct

ion

- for temporary buildings, internal stairs in duplex apartments;

- are lightweight, warm,aesthetical and easy toconstruct but combustible;-of oak wood for main stairsand fir for secondary stairs;

- metallic stairs;

- aces for ramps or platforms, fire escape, service or maintenance stairs for tall special buildings (reservoir, tanks, towers, chimney shafts);

- are lightweight, easy toconstruct but determine highsteel consumption;- metallic stair elementsmust be protected againstcorrosion,

- stone stairs;

- for external access in the buildings;

- the stones( as granite, gritstone, limestone) must resistto friction, aggressive waterand atmospheric agent action;

- ceramic stairs;

- internal stairs in chalets or rest houses

- of pressed bricks withminimal class M100;

- reinforced concrete stairs;

- main and secondary stairs social - cultural or dwelling buildings;

- there are incombustible,with high rigidity and bearingcapacity and can be made indifferent shapes;

- most common stairs.

Based on the plan form there are:

•The simplest type of stair is a straight flightbetween floors, without landings. Such astaircase occupies a long narrow area and it is auseful form of stair when the total rise is notoo great.

•If the floor height is greater, the straightflight is tiring to ascend, and the flight can bedivided by a landing placed in the length of thestair.

If the floor height isgreater, the straightflight is tiring toascend, and the flightcan be divided by alanding placed in thelength of the stair.

Most common stairsused in timberconstruction arecalled dogleg. Thisstair presents twoflights, which returnon each other about asingle newel so thatthe handrail of thelower flight stopsslight against theunderside of theupper flight.

The open well is a form of stair that has twoflights returning each other but with a spacecalled well between the two flights.

When the flights are at right angles quarterspace landing is formed, and when they returna half space landing is formed.

With landings, the flights of steps are not solong and there is grater flexibility in planning,but the area occupied by staircase isincreased.

Considering the construction or structural form of the stair :

- Newel stair when vertical posts,called newel or newel posts, are used intimber or metallic stairs at the end ofthe flight to support the strings andconnect them to the floors.

- Another form is the geometrical stair. Inthis type there are no newels, the stringsand handrails being continuous from floor tofloor

Dimensional design of stairs

Historical trends

•Rules of thumb for determining satisfactory rise/run ratios have existed at

least since Classical times. In De Architectura, Vitruvius suggests a unit rise of

between 9 and 10 inches, and a unit run between 18 and 24 inches. To modern

tastes, this proportion would create a very stately stair. Sir Henry Wooten

(1568–1639) suggested that the unit rise not exceed 6 inches and that the unit

run be between 1 and 1½ feet. Both authors, however, are describing public

buildings or grand houses.

•Jacques-Francois Blondel (1705–1774) argued in his Cours d'Architecture that

the rise/run ratio should be based on the length of the human pace, which he took

to be 25.5 inches. Since in one step on a staircase a foot travels by two risers and

a tread, Blondel arrived at the formula two times the unit rise, plus the unit

run = 25.5", or unit rise = (25.5" - unit run) over 2. This formula works well only

for moderate values for unit rise (or unit run). It was, nonetheless, enshrined in

the National Fire Code.

Width of the flight – In most cases it will be necessary to allowsufficient width for two persons to pass side by side, which requires aminimum of 100 cm to 170 cm. For domestic stairs, a space of 87...90cm from wall face to the handrail is accepted as reasonable. Forstaircases in offices, public buildings and institutions, the width shouldbe enough for the people to pass comfortably in concurdance uith thenumber of fluxes. The number of fluxes is calculated with the relation:

F=N/C

Landing breadth near lifts: A -lift with hinged doors opening in landing direction, B - lift with sliding door.

CONSTRUCTION OF STAIRS

1. Timber stairs

2. Metal stairs

3. Stone stairs

4. Concrete stairs

1. Timber Stairs

Joints between risers and treads

2. Metal stairsCast-iron escape stair The oldest type of metal stair is probably theexternal cast-iron and steel fire escape stair. Metal stairs are also used astemporary stairs in the process of constructing the building.

•strings made of mild

steel, 10 mm thick and

175 mm high in

section. The metal

treads are bolted to

the strings;

•treads are made of

perforated cast-iron

or mild steel chequer-

plates. Perforated

cast-iron risers can

also be fitted if

require;

•landings are formed

of 13 mm cast-iron or

mild steel chequers

plates.

Spiral stairs -Using the same materials standard newel-type spiral stairs can be constructed.

Fixed metallic ladders - used as industrial stairs (to reach some high equipment) or as means of

escape from fire.

3. Stone stairsStone staircases are little used today, mainly because of their cost and the weight of construct.

Bricks are used for external steps and occasionally for internal use. The steps must be formed of good hard, square bricks to withstand wear and are bedded in cement mortar on concrete.

4. Concrete stairs

Reinforced concrete stairs are widely used inalmost all types of buildings. They are strong,

hard-wearing, fire-resisting and make possible a

large variety of forms.

The reinforced concrete offers the possibility to

build better stairs in different constructive

solutions and for this it is used for access and as

means of fire escape stairs in most buildings of

more than two storeys.

String stairs -Where there are load-bearing walls around the staircase it is generally economic to build the landings into the side walls as one-way spanning slab or supported by landingtrimmers.

The strings may span between landings to support the flights.

The presence of the strings allows using a thinnerflight than in the case of slab type therefore this stairhas somewhat lighter weight.

Inclined slab stair - Excepting the cases when the span of theflight is very long, the stair can be designed without strings.

The flight is designed to act as a slab spanning between the trimmers ona distance that is measured on a horizontal line between the centres ofthe trimmers. The effective depth is the waist thickness of the slab.

Cranked slab stair - This type of stair doesn't has trimmers.In this case, the top and bottom landings, together with the flight, aredesigned as an individual structural slab spanning between enclosingwalls or frame.

Stairs with stepped soffit flights

Monolithic cantilever stair - In this case the flightsand landings are cast in situ and cantilevered out from a wall and castin situ together with this. The wall that supports the stair elements canbe the enclosing wall of the entire staircase or a central spine one.

Continuous slab stair – It consists structurally of a continuous slab,monolithic with the floors, which runs from one floor to the landing level, turnson itself and continues without any support to the next floor. The stair issupported at the floors above and below and therefore it has to resist notonly to the normal stresses of bending and shear, but also to torsion. In theseconditions it is more expensive

THE STRUCTURAL FORM OF THE STAIRS AND DESIGN ELEMENTS

The choice of the structural form for the stairs is determined by functional and aesthetical requirements in accordance with the architectural plane and the position of other structural elements of the building in the proximity of the stair.

There are many structural forms for the stairs

The estimation of stresses and deformations, for the design of thestairs, is made taking in consideration the structural type, thebearing condition for the stair elements, and the supported loads(permanent or dead loads and live loads from the traffic on thestairs). For the flights the dead loads is uniformly distributed onthe sloping surface and the live load is uniformly distributed on ahorizontal surface .

For the stair with cantilevered steps or slab, the fixing and stability conditions in the supporting wall, must be provided. For this the loadbearing wall, that support the stair, must be loaded with enough dead load, from the upper floors, to fix strongly the stair elements.The degree of fixingdepends on the relative values of vertical reactive forces Ns and Ni and of the stability Ms and fixing Mimoments. If Ns and Msincrease comparative to Ni and Mi than the degree of fixing will increase also.