Difference Between WSM, ULM and LSM | Simple Guide for Structural Engineers

In structural engineering, different design philosophies have evolved to ensure safety, stability, and economy of structures under varying loads. Among them, the Working Stress Method (WSM), the Ultimate Load Method (ULM), and the Limit State Method (LSM) represent three major milestones in the development of modern structural design concepts.

Each method is based on a distinct way of understanding how materials behave under stress and how safety is ensured — whether by limiting stresses, loads, or states of failure.

While WSM relies on elastic behavior and permissible stresses, ULM focuses on the ultimate strength before failure, and LSM combines both concepts to achieve a balanced and realistic design approach.

Read More On: Working Stress Method  (WSM) of Design of Structures

The following table provides a clear, side-by-side comparison of ULM, WSM, and LSM, helping students and engineers understand how these methods differ in terms of basic principle, safety factor, material behavior, accuracy, and practical application.

Read On: Working Stress Method (WSM) vs Limit State Method (LSM) in Structural Engineering

Working Stress Method (WSM), Ultimate Load Method (ULM) and Limit State Method (LSM) - Comparison

Parameter / Design Aspect	Working Stress Method (WSM)	Ultimate Load Method (ULM)	Limit State Method (LSM)
1. Basic Philosophy	Based on Elastic theory – assumes linear relationship between stress and strain.	Based on Ultimate strength theory – analyzes behavior at collapse (nonlinear).	Based on Limit state philosophy – ensures both safety (collapse) and serviceability (deflection, cracking).
2. Design Load Used	Service (actual) load – not multiplied by any factor.	Factored (ultimate) load – actual load multiplied by a load factor (e.g., 1.5).	Factored load with partial safety factors – separate factors for dead, live, and wind loads.
3. Material Strength Used	Reduced (permissible) stress = actual strength ÷ large FoS.	Ultimate strength – nearly full strength of material used.	Characteristic strength / partial FoS – realistic and statistically based design strength.
4. Factor of Safety Application	Applied entirely on material (e.g., 3 for steel, 3 for concrete).	Applied on both load and material, but globally, not differentiated.	Partial safety factors applied separately to both loads and materials, depending on uncertainty.
5. Stress Condition	Stresses are kept within elastic limit – no cracking allowed.	Stresses reach ultimate (plastic) stage – cracking allowed up to failure.	Both elastic and inelastic behavior considered; serviceability ensured under working loads.
6. Strength Utilization	Only about 30–40% of material strength utilized.	Up to 90% of ultimate strength utilized.	Optimized use – typically 70–85% depending on load combinations.
7. Load and Stress Relationship	Linear (Hooke’s law valid).	Non-linear (beyond yield).	Non-linear at ultimate state, linear under service conditions.
8. Type of Safety Ensured	Ensures no failure and no visible cracks (safe but conservative).	Ensures safety against collapse only (service issues ignored).	Ensures safety + serviceability + durability (balanced design).
9. Section Size Requirement	Large/heavy section – because material strength is reduced.	Small/light section – full strength utilized.	Optimized section – between WSM and ULM, practical for economy and serviceability.
10. Use of High-Strength Materials	Not much beneficial – high-strength material still reduced by same FoS.	Beneficial – full strength can be used.	Very beneficial – design uses characteristic strength effectively.
11. Deflection and Cracking Control	Automatically low (because of large sections and low stress).	Not controlled – structure may deflect or crack under service loads.	Specifically checked under serviceability limit state conditions.
12. Realism / Accuracy of Design	Unrealistic – doesn’t match actual failure behavior.	Partly realistic – reflects failure load but not real-life service conditions.	Most realistic – combines strength and service behavior, matches real-life performance.
13. Basis of Safety Factors	Empirical – fixed values from experience.	Arbitrary – same factors for all loads and materials.	Statistical / Probabilistic – based on variability of loads and material properties.
14. Economy of Design	Least economical (heavy and material consuming).	Most economical (small sections).	Balanced economy and safety (rational and cost-effective).
15. Major Limitation	Too conservative and uneconomical.	Ignores serviceability (cracks, deflection, durability).	Slightly complex, but conceptually comprehensive.
16. Code Usage / Status	Obsolete – not used in modern codes (IS 456:1978 still included for reference).	Transitional stage – historically used, now outdated.	Current standard (IS 456:2000, Eurocode, ACI).
17. Reason for Next Evolution	Too safe and costly.	Unsafe for service use (cracks, excessive deflection).	Solves both safety and usability together.
18. Overall Evaluation	Safe but wasteful.	Economical but impractical for service.	Balanced, safe, economical, and serviceable.

Continue Read:  Design Philosophies in Structural Design | WSM, ULM and LSM