0DESIGN OF STRUCTURAL ELEMENTS
Construction is the ultimate objective of the design. An engineer is a crucial person for successful completion of any project undertaken. Hence, he should adopt all means to reduce the cost of the project to a minimum, without decreasing serviceability aspect of the project as engineering structure is an assemblage of members for elements transferring the load and providing from space, on the enclosure and or a cover to serve the desired function. The objective of structural design is to plan a structure that meets the 'essential requirements such as serviceability, safety, durability, economy, aesthetic beauty, feasibility and acceptability.
Design philosophies:
The following design philosophies have been evolved for the design of R.C structures,
- Working stress method
- Ultimate load method
- Limit state design method
(a) Working stress method:
This method is based on classical elastic theory and developed for purely flexible materials. It assumes that the structure is made up, viz, concrete and steel both obey HOOK's law. In this approach, the margin of safety is provided for the available stress. IS 456-2000 code of practice specifies, different permissible stresses for different materials.
(b) Ultimate load method:
This method is based on the strength capacity of the member just before the collapse stage. In this method, safety has been specified concerning the behaviour at the ultimate stage of member are proportioned, so that the full strength of concrete and steel are utilised. When the final load occurs on it, the ultimate load is obtained by multiplying the working load by a factor known as the load factor.
(c) Limit state design method:
This method is used in practice in the design of reinforced and pre-stressed concrete structures. In this method, various components of a structure are proportional such that the structure does not attain the specified limiting conditions during its lifetime. It shall satisfy the serviceability requirements such as limitations on deflection and cracking. The acceptable limit for the safety and serviceability requirements before failure occurs is called a limit state method. The aim of the design is to acceptable probabilities that the structure will not reach a limit state.
