Concrete: Design Philosophies
Structural design is process of determining the configuration (form and proportion) of a structure subject to a load carrying performance requirement. Form of a structure describes the shape and relative arrangements of its components. The determination of an efficient form is basically a trial and error procedure.
In the beginning of 20th century (1900 to 1960) to late 50's of this century, members were proportioned so that stresses in concrete and steel resulting from service load were within the allowable stresses. Allowable stresses were specified by codes. This method of design is called 'working stress method' (WSM). This method of design resulted in conservative sections and was not economical. This design principle satisfies the relation ___ > _.
Where R is resistance of structural element, RS is factor of safety and L is applied external load.
In 1950's ultimate load method or load factor method was developed. In this method, using non linear stress - strain curve of concrete and steel, the resistance of the element is computed. The safety measure in the design is introduced by an appropriate choice of the load factor (ultimate load/working load). Different load factors are assigned for different loads. Following equations are used for finding ultimate load as per IS456 - 1964
U = 1.5 DL + 2.2 LL
U = 1.5 DL +2.2LL to 5WL or 1.5 DL +0.5LL +2.2 WL
Here DL = Dead load, LL = Live load WL= wind load or earthquake load. The design principle should satisfy R?LF etc or R ? U, Where, R= Resistance, LF= Load factor, L= load. Ultimate load method generally results in more slender section, but leads to larger deformation. Due to the disadvantage of larger deflection, this method was discontinued. To over come the disadvantages of working stress method and ultimate method, a probabilistic design concept called as 'Limit state method, was developed during 1970's. IS456 -1978 recommended this method and is continued in 2000 version also. This method safe guards the risk of both collapse and unserviceability. Limits state method uses multiple safety factory format, which attempt to provide adequate safety at ultimate loads or well as a denote serviceability at service loads by considering all limit states, The acceptable limit for safety and serviceability requirements before failure or collapse is termed as 'Limit state' Two principal limit states are considered i.e 1. Limit state of collapse 2. Limit state of serviceability. The limit state of collapse include one or more of i) flexure, II) shear, III) torsion and IV) compression the limit state of collapse is expressed as µR>?XiLi Where, µ and ? are partial safety factors, Here µ<1 & ?>1. The most important limit state considered in design are of deflection, other limit state of serviceability are crack and vibration. For deflection ?max ? ?_ where deflection, l=span 4 ? is an integer numbers. For over all deflection ? is 250 and for short term deflection ? =350.