Engineering

TYPES OF UPPER FLOORS

Depending upon the arrangement of beams and girders of the frame-work for supporting the flooring and the materials used in the construction of the entire system.

Types of Upper Floors

The upper floors may be broadly classified into the following different types:

  • Timber Floors

There are three main types of timber floors:

1. Single floors

They are the cheapest form of wooden floors. They are easy in construction and the frequently used in domestic buildings. These floors are constructed by fixing 25 to 30 mm. thick planking or boarding to wooden joists known as “bridging joists” spaced at 30 to 40 cm. apart. The bridging joists are placed along the short span of the room and are nailed to timber wall plates fixed in the walls on either end. The span for single floor is limited to 3.66 m., as for greater span the joists become very deep and consequently this system proves uneconomical. The depth of joists depends upon the loading condition of the flooring. The joists are designed both for strength and rigidity. In other words, the joists provided should be such that they may safely carry the floor loads without deflection. The joists should not be allowed to come in connect with green mortar or masonry as this would cause the wood work to rot and decay. Thus a gap is left between the end of the joists and the wall, so that three main circulation of air around the ends of the joists. If by any chance, there is a slight sag in the joists, there is every possibility of the ceiling (if any) fixed to joists to crack.

2.Double Floors

In this case the shortest span of the room is more than 3.66 m., and it is desired to provide a timber floor, double type of floor may be constructed. The span of double floors is restricted to 7.32 m., beyond which framed floors have to be adopted. In double floors the bridging joists do not span the whole room, but rest on other joists placed at right angles to them. The joists are called binding joists or binders and are placed across the short span. Thus, in double floor, the bridging joists support the boarding and the binders support the floor load through bridging joists. The center to center spacing of the binders varies from 1.8 to 2.4 m. At their ends the binders rest or stone on concrete templates embedded in the walls. As far as possible, the binders should not be placed immediately over door or window openings. In case this can not be avoided, they should rest on lintel or beams of R.C.C. which may span over piers between the openings.

3. Framed Floors

When the shortest span of the room or hall exceeds 7.6 m., framed floors are provided. In this system of wooden flooring, boarding is supported by bridging joists laid parallel to the long span or at right angles to bridging joists. The binders are further supported or fixed into large and heavy beams called girders which are placed parallel to the short span. The center to center spacing of the girders depends upon the loading conditions and the size of the binders. At their ends, the girders are supported on stone or concrete templates embedded in walls. To avoid the loss of headway or to reduce the thickness of floor, the binders are normally framed into the girders by means of tusk the tenon joint. A gap is left between the walls and end of girder so as to ensure free circulation of air at the ends to avoid the defect of rot or decay of timber. It is important to note that the ends of two binders from opposite sides should never be framed into the girder at one place, as it would weaken the girder considerably. The binders should be framed in staggered fashion.

  • Timber Floors Supported on Rolled Steel Joists

This system of flooring is similar to ordinary timber floor except that wooden joists are replaced by rolled steel sections which are available in longer lengths. This enables the use of lighter sections of wooden member which ultimately makes the floor economical. In order to reduce the depth of floor, the joists supporting the flooring are notched over the top flanges of the girder and are made to rest upon angle iron cleats bolted on either side of each rolled steel joist.

  • Flagstone Floors Resting on Steel Joists

It is a cheap and easy form of floor construction. If the width of the room is not more than 3.66 m., rolled steel joists (R.S.J’s) of small sections are spaced at 30 to 45 cm., center to center along the width of the room, 38 mm., thick flagstone of width equal to the center to center spacing of the joists are placed on the lower flanges of the rolled steel joists (R.S.J’s). Portion above the flagstone is then filled with light weight concrete, lime concrete, moorum or mud etc., so that the consolidated thickness of filling lies about 13 mm., above the flanges of joists. Finally, tiles, cement concrete, flagstone or any other desired type of flooring can be laid upon the filled material. If lime concrete is used as filling material, the steel joists are protected from the rusting or corrosive action of lime by providing a screed of cement concrete around them or by applying a coat of bitumen or paint on the surface of the joists.

If the shortest span of the room exceeds 3.66 in its dimensions, the smaller steel joist are supported by bigger rolled steel joists or steel beams of the designed section. The steel beams span the width of room and are spaced at 2,4 to 3 m, apart.

  • Jack Arch Flooring

This type of flooring may be constructed by using brick or concrete. The arches span between the lower flanges of rolled steel joists spaced at center to center distance not exceeding 1.2 to 1.5 m. The arch given a rise of 1/12th of the span. As the arch action of floor develops tension in the supporting walls, mild steel ties (not less than three in a bay) are provided in end spans at suitable distance apart. The diameter of the tie rod varies from 18 to 25 mm., and they are usually spaced 1.8 to 2.4 m. apart. The tie rod are anchored into each wall with a stout washer. The floor may be finished with tiles cement concrete mosaic or any other desired finishing after filling the space above the arch with lime concrete or other filling material. The ceiling in this type of construction is not plain and this is considered a drawback of this flooring.

  •  Ribbed Floors

It is a form of reinforced concrete floor slab construction in which the portion of concrete below the neutral axis of slab is removed so as to form ribs at short distances apart. The ribs act monolithic-ally with the concrete topping and behave like T-beams and slab construction with series of small closely spaced ribs. In order to obtain plain ceiling effect, the space between the ribs is often filled with hallow block made out of clay, fiber glass, PVC or concrete. This type of slab is also termed as hallow block slab. Hallow block slabs are economically used in buildings like hospitals, hotels, schools, offices etc.

  • Precast Concrete Floor

 With the development of precast concrete constructions technique, the precast units like floor slab floor slab beams or girders, lintels etc., are being successfully manufactured by various factories. The precast units of the designed depths are used in the floor construction. The members are made of such sizes can be conveniently be handled and transported.

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