IS7 - Slab PDF - Structural System

TOPIC 7 : BASIC STRUCTURAL SYSTEM SLAB 1.0 Structural Principle 1.1 Definition A set of structural elements which form the hor...

TOPIC 7 : BASIC STRUCTURAL SYSTEM SLAB 1.0 Structural Principle 1.1 Definition A set of structural elements which form the horizontal or inclined plane between beams or structural walls that allows the use, the traffic over them and provides the support of finishes and enclosures of a building. 1.2 Implication Floor framing is the most cost significant structural element of any building. Weight of floor determines the size of columns, walls and foundations. Floor structural thickness/depth determines the total building height and thereby the quantity of cladding. In seismic areas the floor weight also determines the member sizes of the lateral load resisting system. Construction time for floor will determine the commencement of fit-out work for mechanical and electrical services, thus the faster the floor system is completed the earlier those work could commence. 1.3 Slab / Plate Floors usually consist of slabs or plates. Ground slab: Slabs that rest directly on the ground. Suspended slab: Slabs that do not rest directly on the ground. They are required to resist vertical loads directly acting on them. Loads are transmitted via the beams to the columns. Floor slabs may be made from: In-situ concrete, Pre-cast concrete, Composite slabs using steel decking. Thickness: Minimum 100mm and up to 500mm. DTA30333 | BUILDING STRUCTURE | KOLEJ KEMAHIRAN TINGGI MARA PASIR MAS 28  TOPIC 7 : BASIC STRUCTURAL SYSTEM SLAB 1.4 Structural Arrangement The structural arrangement of floor slabs in a building is often inspired by the shape of the building plan. Depending on reinforcement design, structural floor slabs are categorised as: One-way slab Two-ways slab 1.4.1 One-way slab When the ratio of the length of long direction to short direction of a slab is greater than 2 it can be considered as a one way slab. Greater than 2 1 Needs moment resisting reinforcement only in its short-direction (moment along the long axes is so small that it can be neglected). Supported on four sides, still. May be designed as a beam spanning in the short direction. DTA30333 | BUILDING STRUCTURE | KOLEJ KEMAHIRAN TINGGI MARA PASIR MAS 29  TOPIC 7 : BASIC STRUCTURAL SYSTEM SLAB 1.4.2 Two-ways slab If the ratio of the lengths of long and short side is less than one then moment in both direction should be considered in design. Less than 1 1 Supported on four sides and with moment resisting reinforcing steel perpendicular to all sides. 2.0 Types of Slab 2.1 In-Situ Concrete Floor 2.1.1 Conventional In-Situ RC Slab In situ concrete slabs are built on the building site using formwork - a type of boxing into which the wet concrete is poured. For a ground slab, the hardcore acts as the base while the sides of the tray may consist only of sidewalls pushed into the ground. For a suspended slab, the form-work is shaped like a tray, often supported by a temporary scaffold until the concrete sets. DTA30333 | BUILDING STRUCTURE | KOLEJ KEMAHIRAN TINGGI MARA PASIR MAS 30  TOPIC 7 : BASIC STRUCTURAL SYSTEM SLAB Rebars are positioned within the formwork before the concrete is poured in. ▪ Steel mesh or deformed bars. Plastic tipped metal, or plastic bar chairs are used to hold the rebar away from the bottom and sides of the form-work, so that when the concrete sets it completely envelops the reinforcement. Ready-mixed concrete 2.1.2 Steel Decks Steel decks have long been recognized for their economy due to their light weight and high strength-to-weight ratio. Provide a durable and attractive system for all-weather construction. DTA30333 | BUILDING STRUCTURE | KOLEJ KEMAHIRAN TINGGI MARA PASIR MAS 31  TOPIC 7 : BASIC STRUCTURAL SYSTEM SLAB Designed to serve as a permanent steel base for poured reinforced concrete floor slabs. Installation is fast, easy, and economical: the need for temporary wood forming is eliminated. 2.1.3 Flat Slab When a slab is supported directly on columns, without beams and girders, it is called a flat plate or flat slab. Thicker and more heavily reinforced than slabs in beam-and-girder construction. DTA30333 | BUILDING STRUCTURE | KOLEJ KEMAHIRAN TINGGI MARA PASIR MAS 32  TOPIC 7 : BASIC STRUCTURAL SYSTEM SLAB Advantages of using flat slab: FLEXIBILITY IN ROOM LAYOUT: Allows Architect to introduce partition walls anywhere required, Allows owner to change the size of room layout. LOWER STOREY HEIGHT Reduce building weight due to lower internal partitions & façade cladding. SHORTER CONSTRUCTION TIME Big table formwork, Pre-fabricated steel mesh. 2.1.4 Waffle Slab If the column grid is increased from about 6.0 to about 12.0 square or near square it becomes economical to use a floor with intermediate cross beams supporting thin floor slabs. Intermediate cross beams are cast on a regular square grid that gives the underside of the floor the appearance of a waffle, hence the name. Advantage: support a thin floor slab and so reduce the dead weight of the floor as compared to a flush slab of similar span. DTA30333 | BUILDING STRUCTURE | KOLEJ KEMAHIRAN TINGGI MARA PASIR MAS 33  TOPIC 7 : BASIC STRUCTURAL SYSTEM SLAB Used where a widely spaced square column grid is necessary and floors support comparatively heavy loads. The waffle grid form of the floor may be cast around plastic or metal formers laid on timber/steel centering so that the smooth finish of the soffit may be left exposed. DTA30333 | BUILDING STRUCTURE | KOLEJ KEMAHIRAN TINGGI MARA PASIR MAS 34  TOPIC 7 : BASIC STRUCTURAL SYSTEM SLAB 2.2 Precast Pre-Stressed Concrete Floor Prefabricated concrete slabs are built in a factory and transported to the site, ready to be lowered into place between steel or concrete beams. Flat top and bottom surfaces provide for easy stacking during storage (at factory and on site) and transportation. It is vital that the supporting structure is built to the correct dimensions, or the slabs may not fit. Units are nominally 1200mm wide and of various depths placed side by side with a grouted joint. Customized Available – Can be sawn to smaller width than 1200. DTA30333 | BUILDING STRUCTURE | KOLEJ KEMAHIRAN TINGGI MARA PASIR MAS 35