Folded Plate Structure

September 9, 2017 | Author: Sonal Sharma | Category: Prestressed Concrete, Building Technology, Concrete, Architectural Design, Mechanical Engineering
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Folded Plate Structure a thin-walled building structure of the shell type. Folded plate structures consist of flat components, or plates, that are interconnected at some dihedral angle. Structures composed of rectangular plates are said to be prismatic. In modern construction practice the most widely used folded plate structures are made of cast-in-situ or precast reinforced concrete (including prestressed and reinforced-cement structures). The structures are used as roofs for industrial and public buildings. The main advantage of folded plate structures over other shells (such as cylindrical) is the simplicity of manufacture. Approximate static calculations of folded plate structures are based on the membrane theory of shells; more exact static calculations are based on limit equilibrium and on P. L. Pasternak’s and V. Z. Vlasov’s general theory of shells.

Folded plates : Folded plates are assemblies of flat plates rigidly connected together along their edges in such a way so as to make the structural system capable of carrying loads without the need for additional supporting beams along mutual edges. Types of folded plates : 1- Prismatic : if they consist of rectangular plates. 2- Pyramidal : when non-rectangular plates are used. 3- Prismoidal, triangular or trapezoidal. On the other hand, Folded plates can be classified as: 1- single. 2- Multiple. 3- Symmetrical. 4- Unsymmetrical. 5- Simple. 6- Continuous. 7- Folded plates with simple joints. 8- Folded plates with multiple joints.

9- Folded plates with opened cross sectional. 10- Folded plates with closed cross sectional.

Following are Advantages and Disadvantages of Folded Plate Roofs over Shell Roofs Advantages of Folded Plate Roofs over Shell Roofs are: (a) Movable form work can be employed. (b) Form work required is relatively simpler. (c) Design involves simpler calculations. Disadvantages of Folded Plate Roofs over Shell Roofs are: (a) Folded plate consumes more material than shells. (b) Form work may be removed after 7 days whereas in case of shells it can be little earlier.

Shell Roof Advantages : i) Economical for very large span roofs as material consumption would be less ii) Dead loading is much less compared to other alternatives iii)Drainage may not be a problem and gutter formations at edges is natural iv) Quite aesthetic in views v)Transversely, load transfer is through shell action requiring min steel/concrete vi)Longitudinally the arch cross-section (as a long span beam) with its Rise will be effective for catering large spans vii) Deflection for large spans--- not a problem Disadvantages: i) Shuttering is expensive.centring is also costly. ii) Mostly used for RCC roofing iii) Thin sections --- strict supervision desired

Design: Longitudinally the shell thickness must be checked for crippling stresses due to bending compression.

Reinforced concrete has reinforcing bars (called rebar) simply embedded in the pour. With prestressed concrete, reinforcing rods or cables are stretched (stressed) and then the concrete is poured around them. After the concrete hardens, the tension on the reinforcing members compresses the concrete, making it more resistant to failure where poor soil conditions or severe loads exist. Prestressed construction is usually done in-plant because of the equipment involved, and the completed assembly shipped to the site for installation. A similar method, called post-tension, is usually done on site, and involves the tensioning of reinforcing cables after the slab is poured, using a special hydraulic jack. The prestress in a structure is influenced by either of the two processes: Pre-tensioning, and Post-tensioning Pre-tensioning can be further classified into two categories: Linear pre-tensioning Circular pre-tensioning PRE-TENSIONING Pre-tensioning is accomplished by stressing wires or strands, called tendons, to predetermined amount by stretching them between two anchorages prior to placing concrete as shown in fig.1. the concrete is then placed and tendons become bounded to concrete throughout their length. After concrete has hardened, the tendons are released by cutting them at the anchorages. The tendons tend to regain their original length by shortening and in this process transfer through bond a compressive stress to the concrete. The tendons are usually stressed by the use of hydraulic jacks. The stress in tendons is maintained during the placing and curing of concrete by anchoring the ends of the tendons to abutments that may be as much as 200m apart. The abutments and other formwork used in this procedure are called prestressing bench or bed. Fig.1: Section for Pre-tensioning Most of the pre-tensioning construction techniques are patented although the basic principle used in all of them is common and is well known. POST-TENSIONING The alternative to pre-tensioning is post-tensioning. In a post-tensioned beam, the tendons are stressed and each end is anchored to the concrete section after the concrete has been cast and has attained sufficient strength to safely withstand the prestressing force as shown in fig.2. in post-tensioning method, tendons are coated with grease or a bituminous material to prevent them from becoming bonded to concrete. Another method used in preventing the tendons from bonding to the concrete during placing and curing of concrete is to encase the tendon in a flexible metal hose before placing it in the forms. The metal hose is referred to as sheath or duct and remains in the structure. Fig.2: Section for Post-tensioning

After the tendon has been stressed, the void between the tendon and the sheath is filled with grout. Thus the tendons become bonded to concrete and corrosion of steel is prevented. Post-tension prestressing can be done at site. This procedure may become necessary or desirable in certain cases. For heavy loads and large spans in buildings or bridges, it may be very difficult to transport a member from pre-casting plant to a job site. On the other hand, pre-tensioning can be used in pre-cast as well as in cast-in-place construction. In post-tensioning it is necessary to use some types of device to attach or anchor the ends of the tendons to the concrete section. These devices are usually referred to as end anchorages. There are a large number of patents for different types of anchorages. They may also differ n the details of construction. Some of the popular methods are: Freyssinet system Magnel system Leonhardt system Lee-McCall system Gifford-Udall system

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