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Formwork is basically a mould which is used to attain a desired shape of desired size out of concrete and other building material. As far as concrete construction is considered the false work supports the shuttering moulds. Most of the buildings which are made to stand for a long time are made of concrete and mortar. These building materials are strongest and most suitable but at the same time they are a bit tedious to deal with. They do not acquire the desired shape and need help of some sort of support or any frame which can be detached as the concrete solidifies. Actually concrete is a material without any shape. For most applications the shape matters, and concrete has to be molded or formed.
Concrete has been in use for the thousands of years. The dome of Pantheon in Rome is made of lightweight concrete, and the under face shows the moulding effect of formwork used two millennia ago. But it was only at the end of nineteenth century that the use of concrete became common, with the invention of reinforced concrete. Some of the most previous examples of concrete slabs were built by engineers of Rome [Fig 1]. Concrete is quite strong in resisting the compressive loads but has poor Tensile strength. To mold these structures, temporary scaffolding and formwork are built as per the desired shape of the structure. These building techniques were not isolated to pouring concrete, but were and are widely used in Masonry. Because of the complexity and the limited production capacity of the building material, concrete's rise as a favored building material did not occur until the invention of Portland cement and reinforced concrete.
The construction industry forms the largest single sector in any economy. In the USA for example the construction sector is responsible for 14% of the GDP (US National Statistics Bureau). Formwork is the single largest cost component of concrete building's structural frame. The cost of formwork exceeds the cost of concrete or steel and in some situations the formwork costs more than the concrete and steel combined. For some structures, placing priority on the formwork design for a project can reduce the total frame cost by as much as 25%. This saving includes both direct and indirect costs. Formwork efficiencies accelerate the construction schedule, which can result in reduced interest costs during construction and early occupancy for the structure. Other benefits of formwork efficiency include increased job site productivity, improved safety, and reduced potential for error.
Sheathing is supported by horizontal members called joists or runners. Joists are made from dimension lumber spaced at constant intervals that are a function of applied loads and type of lumber. It is a recommended practice to round down the calculated joist spacing to the lower modular value. Joists are supported by another set of horizontal members perpendicular to the joists, called stringers. The stringers are supported by the vertical members called shores. In all wood conventional formwork [ i.e. 4 - 4 in. or 6 - 6 in.]. Shores are rested on heavy timbers called mudsills, to transfer the vertical loads to ground. In case where a slab on grade exits, shores are directly rested on them. Once the bottom of the beam is constructed and leveled, one side of the beam is erected first with the holes drilled in it for installing the tie roads. Tie rods are steel rods that hold the two sides of beam together. After the first side of beam form is erected, the reinforcement is placed inside the beam and then the other side of the beam is erected. Tie rods are then inserted into all holes on both side of the beam. The tie rods' function is to resist horizontal pressure resulting from the freshly put concrete and thus keep the sides of the beams in their proper location.[Fig 4]
Block board: Stripes of timber are made into a panel with veneers on both sides. The glue should be of high quality else the surface may not be flat enough as the stripes tend to show through. Block board is made up of softwood strips as a core. These strips may be up to about 25mm wide. The strips are placed edge to edge and sandwiched between veneers of hardwood. The sandwich is then glued under high pressure. Block board[Fig 9] is not suitable for outdoor use as the glues used are interior glues. It is important to make sure that the core runs lengthways in order to achieve maximum possible strength. The core can be joined by hands or by machine. There are only few small core gaps or even no core gaps in machine-made core. But core gaps are common in man-made cores. Machine-made core is much better than man-made core. Block board is sold in sheets of 2440 x 1220mm and are normally 30mm thick. Screws and nails may be used to attach block board but you have to ensure that you make contact with the strips of softwood and not the gaps between the softwood strips.
In this system, steel joists and stringers have the advantage of supporting greater spans and fewer joists and stringers. The main problem with using steel as joists and stringers for forming concrete slabs is their heavy weight, which makes it difficult for one person to handle. A standard steel W-section is used because its wide flange makes it easy to connect stringers with shore legs. Upper apex part of the steel joist get webbed through the upper chord part of the joist and through holes provided in the metal sheet formwork placed over such joists before to the filing of the concrete slab. Mixed action of open-web structure of steel joists supporting beams, girders and reinforced concrete slab. The improvement is related to a continuous round rod secured near the apex of each projecting web member parallel to the longitudinal axis of the joist and a reinforcing wire mesh draped between the rods thus giving greater spacing between the joists, whereby protruding apex parts, rods and draped wire mesh will be encased within the slab to act as shear interconnection and reinforcement devices therein, to secure the joist and formwork together, to enhance the locking of the concrete slab to the protruding joist apex portions, and to the supporting beams through the joist end connection welded to the beam or girder, and to reinforce said concrete slab. This improvement makes optional the use of wedge members forced between such protruding joist apex portions.
Now as far as the economical aspect of formwork is concerned let me make you aware of the fact that formwork share a considerable site budget. If we design the formwork intelligently and plan its use, then we can save a lot of money. As metal formwork is more durable and reusable as compared to the wooden formwork, it should be used. There are several advantages which make metal formwork a better option.