How do the numbers of factors differ?

FOS is one for all uncertainties, PSF are several for particular uncertainties

For a beam, what happens in WSM and in LSM?

WSM limits stress to 0.66fy; LSM multiplies load by 1.5 and design strength is fy/1.10

WSM multiplies load by 1.5; LSM limits stress to 0.66fy

Asked 2/21
2071 Bhadra · 5 marks
2070 Bhadra

Factor of safety and partial safety factor

Short answer

Factor of safety in WSM is a single number (e.g., 1.67) applied to material strength to get permissible stress, covering all uncertainties. Partial safety factors in LSM are separate factors for loads (γ_f) and material (γ_m), statistically derived for uniform reliability.

Write about (differentiate between) factor of safety and partial safety factor used in steel structures.

Point	Factor of safety	Partial safety factor
Used in	Working stress method	Limit state method
Definition	$f_y/\sigma_{perm}$ (ratio of failure stress to permissible stress)	Separate factors on loads ( $\gamma_f$ ) and on material ( $\gamma_m$ )
Number	One factor for all uncertainties	Several, each for a particular uncertainty
Applied to	Material strength only	Loads and strengths separately
Typical value	About 1.67 ( $\sigma=0.6f_y$ )	$\gamma_f=1.5$ , $\gamma_{m0}=1.10$ , $\gamma_{m1}=1.25$ , bolts 1.25, welds 1.25/1.5
Basis	Experience / judgement	Statistical, reliability-based
Result	Non-uniform reliability	More uniform reliability

Example: for a beam under working load, WSM limits the stress to $0.66f_y$ . In LSM the load is multiplied by 1.5 and the design strength is $f_y/1.10$ .

✓ Answer checked by Claude Opus 5.5 (high effort)

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Factor of safety and partial safety factor

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