Design of Post - Tensioned Floors

Abstract

Concrete is strong in compression and weak in tension. Plain (unreinforced) concrete is placed when the structure must resist mostly compression forces and when tensile stresses are low. Reinforced concrete is used for structures that must resist significant tensile forces. Reinforcing materials (such as steel rebar or welded-wire fabric) that perform well in tension are embedded in the concrete. Prestressed concrete is concrete that is pre-compressed by stressing the reinforcement before loads are applied. This greatly increases its ability to resist tensile forces without excessive cracking. Concrete can be prestressed in a factory by tensioning the steel reinforcement first and then placing concrete around it-"pre-tensioned" reinforcement. Or concrete can be cast in place and the steel reinforcement tensioned after the concrete has reached a required strength-“post-tensioned" reinforcement. Structural engineers calculate the limits for tensioning. There are two types of Post-Tensioning systems: bonded and unbonded. Unbonded systems use strands surrounded with special corrosion-inhibiting grease and encased in waterproof plastic sheaths. This assembly is positioned, and then the concrete is placed, similar to standard reinforced concrete. With a bonded system, before the concrete is cast, empty steel or plastic ducts are positioned in the formed area and attached to the anchorages at either end. After the concrete is placed and gains strength, strands are threaded through the ducts, tensioned, and the ducts are filled with a special grout designed to prevent corrosion. Unbonded systems are nearly always used for building and slab construction, while bonded systems are mostly for bridge construction. As a method of reinforcing, post-tensioning is growing in popularity because it saves money, has many construction advantages, and contractors and designers no longer regard it as a mysterious method of reinforcement. Also, owners of structures are beginning to understand the process and its benefits. Tendon corrosion problems, an earlier issue, have been overcome by the development of corrosion-resistant tendons and new materials that electrically isolate tendons from the concrete eliminating the corrosion reaction. Load balancing method and the Equivalent frame method are the two methods which are generally used for the design of the post-tensioned slabs. As the popularity of the post-tensioning method is widely increasing day by day in the Indian construction the emphasis is given on the design of the post-tensioned flat slab in the present report. Along with this the parametric study of different parameters, such as thickness of slab, grade of concrete, shear, normal reinforcement, prestressing force, losses due to stressing and deflection etc. with the variation of span (7 to 12m at an interval of 0.5m) of flat slab is carried out. To understand the design procedure a case study of the office building is taken and it is designed by considering the different floor system such as post-tensioned flat slab, reinforced concrete flat slab, post-tensioned slab with reinforced concrete beams and reinforced concrete slab and beams. For the analysis of the post-tensioned and reinforced concrete building the ADAPT and STAAD PRO software are used respectively. The estimation and costing of the case study is carried out for the economic point of view as it is of great importance in any structure.