Chapter 11: Floors - PDF

# Chapter 11: Floors Floors are the structural part of horizontal supporting elements as distinct from the wearing surface. A good ground floor should be able to meet the following functional requirements: - To withstand the loads that will be imposed upon it, which could be persons, furniture, (d...

# Chapter 11: Floors Floors are the structural part of horizontal supporting elements as distinct from the wearing surface. A good ground floor should be able to meet the following functional requirements: - To withstand the loads that will be imposed upon it, which could be persons, furniture, (domestic buildings), machines or equipment (factories), and books (libraries). - To prevent the growth of vegetable matter inside the building by providing concrete oversite. - To prevent damp penetration by inserting damp proof membrane (dpm) or damp proof course (dpc) as the case may be, below the floor. - To meet the prescribed heat and sound insulation standards. - To be reasonably durable and thus reduce the amount of maintenance of replacement work to the minimum. - To provide an acceptable floor finish to meet the needs of users with regard to appearance, comfort, safety, cleanliness and other associated matters. However, upper floors should meet the following additional functional requirements: - To restrict the passage of fire, which is particularly important in high-rise buildings and in buildings where parts are of different ownership, many occupants, and large quantities of combustible materials. - To bridge the specific span economically and be capable of fairly quick erection. - To accommodate services where necessary. - To support their own self weight in addition to the super imposed loads on them. - To resist transmission of sound from one floor to another. ## Ground Floors ### Solid Ground Floor The components of a solid ground floor include the following: - **Hardcore:** This consists of broken pieces of any of the following materials: bricks, blocks, concrete and stones which is to form a base and make up levels where the ground has been stripped of vegetable soils and turf. On wet sites it may be used to provide a firm working surface and to prevent contamination of the lower part of the wet concrete during placing and compaction. It also reduces the amount of rising ground moisture but it cannot eliminate the need for a damp proof membrane. Hardcore should be well graded and free from impurities and should be properly consolidated in layers not more than 225mm thick. - **Oversite concrete:** The oversite concrete bed should not be less than 100mm thick although it is often 150mm thick. The concrete mix should be at least 1:3:6 (C:S:A) with a maximum size of coarse aggregate of 38mm. Adequate curing is necessary and the top of the oversite concrete must be above the finished ground level. Generally the thickness of the concrete over-site is determined by the anticipated load on it. - **Damp proof membrane:** This is a layer of impervious membrane on top of the Hardcore after blinding with sand or on top of the concrete oversite before screeding. It can be laid either as damp proof course (dpc) along the wall perimeter or as damp proof membrane (dpm) over the whole area of the floor. The position of dpm varies depending on the site conditions and the other building materials. Suitable materials include mastic asphalt, bituminous sheets with properly sealed joints; hot-applied pitch or bitumen, coal tar, rubber emulsion or polythene film sheets. - **Floor screed:** Floor screed is used to provide smooth surface to receive the floor finish (PVC, wood block, terrazzo, parquet etc); to provide falls for drainage purposes, thermal insulations and to accommodate service pipes and cables. It is normally of cement and sand mixed in the ratio of 1:3 (C:S) and up to 40mm thick. A mix with a lower cement content will be subject to less shrinkage. ### Construction of Ground Floors This is otherwise called "German" floor in Nigeria. It consists of a layer of concrete not less than 100mm thick laid on a hardcore bed of at least. 100mm thick compacted in layers of 50 - 75mm. The compaction provides a good and solid base for the concrete floor. To prevent cement grout loss from the superimposed concrete layer, or protect the damp proof membrane from fracture, the hardcore is blinded with a 25mm layer of sand or ashes. The damp proof membrane may be placed below the oversite concrete which is compacted using a tamping bar before the cement and sand (1:3) screed is finally laid on it to a thickness of not less than 19mm. However where the ground floor is subjected to vibration as in factories or in weak soils, it may be necessary to provide mesh reinforcement to assist in resisting tensile forces. **Advantages of solid ground floor:** - It is comparatively cheap. - It allows for a wide selection of floor finishes. - It is not susceptible to fungi and termite attack as in timber ground floor provided the foundation is treated with anti termite solutions. - The extra cost of providing underground ventilation as in timber floor is avoided. ### Suspended Timber Ground Floor This type of floor can be a little bit more expensive than the solid ground floor and if incorrectly or not carefully constructed, the timber members may decay due to fungal attack. Hence they are treated with preservatives. This form of construction has the following merits: - The floor can absorb small ground movements. - It provides facilities for underground storage. - It can accommodate service pipes and the likes. - It may become cheaper in sloppy sites. The floor construction consists of an oversite concrete laid on well graded and compacted hardcore. Dwarf walls (sleeper walls) are then built off the concrete surface to support the floor structure. The height of the sleeper wall should be either 150mm or 225mm from the concrete surface to the underside of the timber floor joists. Adequate ventilation is ensured with this spacing between the top of the floor and the underside of the joists, the laying of the sleeper walls in a honeycomb manner and the insertion of air bricks in the external wails of the floor at a spacing range between 2.0m and 3.0m. Damp proof courses of impervious sheet materials are provided on each sleeper wall before the commencement of the timber floor structure to prevent passage of moisture which can cause defects in the timber. The timber floor consists of the wall plate which spreads load from the floor to the sleeper wall. Timber joists are then laid to bear on the plates, their sizes and spacing depending on the floor loading, strength and the span. The joists are often of 50 X 100mm size and their ends are cut back some distance from the external wall to prevent any moisture being absorbed through contact with the walls. Tongued and grooved or butt-jointed boards are placed on the joists to form the floor surface. The edges of the floor boards are kept at 13mm (1/2") away from the surrounding wall to allow for movement and as well reduce the risk of damp penetration. The 13mm (1/2") so created is later closed by a skirting well treated with preservative. The sleeper wall next to the external walls should be about 50mm away to enhance the circulation of air through the sleeper walls. ### Other forms of Timber ground floor - **Spring floor:** The base here is usually of oversite concrete laid normally on hardcore. Helical springs are then placed at about 450 - 900mm centres before .laying floor joists and boards. It is used in floors that require some degree of resilience as in gymnasium. - **Cavity floor:** The construction is similar to that of spring floors except that the helical springs are replaced with adjustable props. They are used mainly in laboratories for storage and other services. ## Upper Floors These are otherwise referred to as suspended floors and may be constructed of timber, reinforced concrete or steel. The functional requirements and other specific consideration stated above govern the choice of the floor type. ### Timber floor This consists of timber floor joists usually 50 x 225mm spanning between opposite wails with its two ends bearing on the wall plates (100 x 50mm) which distribute the loads, to the supporting walls. The joists are spaced at 400mm - 600mm centres with the ends built into the walls. Floor boards are then laid on the joists using any desirable method of jointing, the tongued and grooved method being the most effective. It is simple and economical to construct. ### Reinforced Concrete Floor Reinforced concrete floor consists of concrete and steel reinforcement. The combination of these two materials produces an effective structure that is able to resist tensile, compressive and shear stresses. Reinforced concrete floor is stronger than timber floor. ### In-situ Concrete Floors - **Monolithic reinforced concrete floor:** This type of floor consists of a simple span of concrete usually supported by walls. It is composed of concrete slab reinforced with either mild or high yield (high tensile) steel bars in both directions. Temporary support or formwork is required while the concrete is still wet and plastic. The steel reinforcement should be protected from rusting and fire by providing a reasonable cover of between 15 - 25mm above the formwork. This type of slab is usually of 100-300mm in thickness depending on the load. - **Hollow pot floor:** This consists of hollow blocks or terra cotta blocks laid on the formwork with spaces between them to form ribs. Steel reinforcement is then placed between the blocks before the concrete is poured to fill in these spaces and also form a topping of not less than 50mm thick. This type of floor is light in weight and has good thermal, sound and fire resistance. - **Beam and slab floor:** This is a reinforced concrete slab cast monolithically with the supporting beams and the reinforcements are arranged to ensure continuity. It results into T or L-beams as the case may be. - **Plate slab floor:** This is a slab cast directly on columns without any supporting beams. It is used for domestic buildings with a limiting imposed load of about 3.5KN/m². It has a thickness of about 125 - 150mm. A non-structural concrete topping of about 50mm thick is provided to form the floor surface. - **Flat slab floor:** This is similar to the plate but the head of the column is haunched or splayed and at times with a drop panel. This helps to reduce shear concentration or punching shear around the column heads. The limiting imposed load is 4.5KN/m² with a thickness of 225-300mm. It is suitable for warehouse, and industrial buildings. - **Lift slab construction:** All the floors required here for the high rise building are cast at the ground level on top of one another. As each slab hardens, a resilient membrane e.g. polythene is spread on top to prevent adhesion to subsequent layers. Lifting collars are inserted in the slab at column points. When the slabs mature, they are jacked to appropriate levels and are welded (through the collars) to columns using shear blocks. This is a highly specialized method of construction because it involves the use of hydraulic jacks. - **Precast concrete floor:** This consists of reinforced concrete slab whose structural members are cast off-site and brought to the site for assembly. Provisions are made for handling and coupling during the casting. - **Hollow beam floor:** This is a rectangular hollow section with reinforcement cast into the angles of the beam. It comes in different sizes with spans up to 5.5m. The beams are placed side by side with their ends built into the supporting walls. Insitu concrete topping of about 50mm is then cast on the beam. Some services may be accommodated in the hollow. - **'T' section beams and hollow concrete block:** This consists of precast inverted 'T' sections cast off site up to 6m length; usually placed at about 250 centres and the hollow concrete blocks inserted between the beams in respect of fire resistance (if the steel is given adequate cover), sound insulation and supporting loads. It is usually constructed either as in-situ or precast concrete floor. ## Steel Floors These are floors entirely constructed with steel but when combined with other materials like timber or concrete, the steel serve as structural members. Some examples are: - **The filler joist floors:** These are the forerunners of the modern reinforced concrete floors and consist of small rolled steel beams or joists at fairly close centres as structural members of the floors and thereafter surrounded by or carry the concrete slab. This is a system designed to eliminate shuttering in the construction of suspended floors. - **Lattice joist floor:** In this system, the lattice, or open web, joined with flanges j of light channels, angles and flats can be used instead of the universal beams. The open lattice web gives complete freedom in running services through the thickness of the floor. - **Cellular steel floor:** This is constructed with steel in the form of cellular units working in conjunction with a concrete topping to form a structural floor slab. Here the steel cellular units consist of top and bottom profiled sections spot-welded together along their length, the sections varying in depth to suit different spans and loading. The minimum thickness of concrete topping is 45mm. - **Open metal floors:** These are examples of entire steel floor constructions used mainly in industrial buildings particularly for service and operating platforms to machines, where the passage of light and air is required to be maintained. This type of flooring is made up in steel or aluminum alloy, in panels of varying widths and lengths as required. It can be formed by parallel flats spaced apart and braced either by similar flats or by bars intersecting at right-angles at intervals along the length of the panel. Perforated cast-iron plates of a similar nature are used externally for walkways and fire escape stairs because of their heavy weight and good resistance to corrosion.