Structural design is process of determining the configuration (form and proportion) of a structure subject to a load carrying performance requirement.
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 20^{th}
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 �R>?X_{i}L_{i}
Where, � and ?
are partial safety factors, Here �<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.