PRINCIPLE OFLIMIT STATE DESIGN

PRINCIPLE OFLIMIT STATE DESIGN

Design Requirements

1. Structure should remain fit with adequate reliability during its lifetime.

2. Should have adequate durability under normal maintenance.

3. Should not suffer overall damage or collapse disproportionately under accidents.

To achieve all the above it is not enough if the members are designed to take the expected internal

forces for load transfer. It is necessary that all structural elements are suitably connected and

anchored.

Limit States

* The major categories of limit states are limit state of strength and limit state of serviceability.

* Characteristic actions It is defined as the values of different actions which are not expected to be

exceeded with more than 5 per cent probability, during the life of the structure. In the absence of

statistical analysis, the loads presented in IS 875 and other codes may be considered as

characteristic actions.

* Design Actions (Loads) To take care of uncertainties involved in analysis, design and construction,

code specifies taking design actions as partial safety factor times the characteristic actions. Table 4

of IS 800–2007 gives partial safety factor gf

for limit state.

* Design strength To take care of manufacturing defects, IS 800–2007 recommends reduction in the

strength of material by a partial safety factor gm, where

gm =

where Su = ultimate strength and

Sd = Design strength

These values are given in Table 5 of IS. 800–2007.

* Deflection Limits

• Deflection limits are specified with the consideration that excess deformations do not cause damage

to finishing. Deflections are to be checked to adverse but realistic combinations of service loads.

• Table No. 6 of IS 800–2007 gives deflection limits.

Fire Resistance Fire resistance level is specified in terms of minutes depending upon the purpose for

which the structure is used and the line taken to evacuate in case of fire. For detailed specifications

refer to section 16 of IS. 800–2007.