What is Factor of safety and Partial safety factor? Mention difference between them?
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Factor of safety –
Factor of safety can be defined as the ratio by which structure is safe. The higher factor of safety means structure is more safe to the load coming on it. It can be defined as the ratio of ultimate stress to allowable stress.
It is also known as safety factor.
Factor of safety = Ultimate stress / Allowable stress
Partial safety factor –
The safety of structure depend on each of two principal design factor load and material strength which are not the function of each other hence, two different factors one for load and other for material strength are used each of two safety factor contribute partially to safety they are term as partial safety factor.
Different b/w FACTOR OF SAFETY AND PARTIAL SAFETY FACTOR
Partial factor of safety
1. Definition:The factor normally greater than unity by which either the loads(actions) are multiplied or the resistance are divided to obtain the design value.
2. Purpose: It’s basic purpose is to determine design value of load and permissible value of stress.
3. Application: It is generally used in Limit state design method.
4. It is of two types: a) Partial FOS for load b) Partial FOS for material strength
Example: Design load= partial FOS for load *service load; Permissible stress=Characteristic stress/partial FOS for material strength
Partial FOS for load ranges from 0.6 to 1.5 while partial FOS for material strength ranges from 1.10 to 1.50
Factor of safety
1. Definition: The factor by which the yield stress of the material of a member is divided to arrive at the permissible stress in the material.
2. Purpose:It’s basic purpose is to determine the permissible stress in the structure.
3. Application: These factor of safety is generally used in working stress method (WSM).
4. Example: Working stress(permissible stress in WSM)=Yield stress(fy)/FOS
FOS for steel =1.5 to 1.8, concrete=3, timber=3 to 5