Design Philosophies in Structural Design | WSM, ULM and LSM

Over time, engineers have come up with different ways to design reinforced concrete structures. We started with the working stress method (WSM), then moved on to the ultimate load method (ULM), and now we use the limit state design theory (LSM), which is the best way to design structures today.

Let's discuss each method one by one in detail.

What is Design Philosophy?

The design philosophy of a structure may be regarded as the process of selecting proper materials, and proportion elements of a structure. To perform this, we need to understand the art of design, engineering science (strength of materials and structural analysis), and the latest technologies.

Every philosophy will have a few fundamental assumptions and Procedures (reflect the way of finding the strength of materials or design strength).

Let's trace the evolution of design philosophies.

We started with the Working Stress Method (WSM) in the early 1900s. Then came the Ultimate Load Method (ULM) in the USA in 1956, followed by the UK in 1957, and eventually India. Finally, the Limit State Method (LSM) was developed in 1955 but was fully adopted in India in 2000.

Main Parameters That a Design Philosophy Must Satisfy

Before designing any structure, we must ensure that it performs safely and efficiently throughout its life. Every design philosophy—whether it is Working Stress Method (WSM), Ultimate Load Method (ULM), or Limit State Method (LSM)—aims to satisfy a few fundamental requirements:

Safety (Against Collapse)

The structure must have enough strength to resist all possible loads (dead load, live load, wind, earthquake, etc.) without collapsing. It ensures that stresses or moments in the structure do not exceed the ultimate strength of materials. This is often called the limit state of collapse.

Serviceability

Even if the structure doesn’t collapse, it should remain comfortable and functional during everyday use. This includes controlling deflection, cracks, and vibration within permissible limits. It ensures the building looks and performs well over time.

Durability

The structure should resist weathering, corrosion, creep and shrinkage effects during its service life without major deterioration. This depends on material quality, cover to reinforcement, exposure conditions, etc.

Economy

A safe and serviceable design should also be economical — not overdesigned or wasteful.  Efficient use of materials and labor reduces cost while maintaining safety.

Stability

The structure must remain stable under all types of loading and deformation. For example, columns should not buckle, and the overall frame should not sway excessively.

In summary, a good design philosophy aims to balance:

• Safety (collapse prevention)
• Serviceability (comfort and usability)
• Economy (cost-effectiveness)

Before diving into the different design theories, let’s first explore a simple concept regarding how these theories were developed and how we ultimately arrive at the best method for designing RCC (Reinforced Concrete Structures) or Steel Structures that would finally achieve all conditions of safety, serviceability and economy. 

To design any structure using a particular material, it is essential to understand the material’s behavior before incorporating it into design, whether for RCC, steel structures, or any other construction material. This is where stress-strain analysis comes in.

Stress-Strain Analysis of Materials 

❓Why do we perform a stress-strain analysis?

✅It helps us understand how a particular material behaves under a specific load. 

We conduct stress-strain analyses for various materials like concrete, steel, and mild steel reinforcement to ensure their suitability in structural applications.

Now, if we look at a general stress-strain analysis curve, we observe that the applied stress creates strain within the material.

Let’s understand the graph:

• Horizontal axis → Strain (deformation)
• Vertical axis → Stress

Now, as the load increases on a structural member, the stress and strain increase, and the graph typically passes through these key stages:

1. O–A: Elastic Region (stress ∝ strain, obeys Hooke’s Law)
2. A: Elastic Limit (end of elasticity; beyond this, permanent deformation begins)
3. A–B: Yielding begins — material starts to deform plastically
4. B–C–D–E: Plastic Region (nonlinear; strain increases faster than stress)
5. E: Ultimate Stress (maximum strength point)
6. F: Fracture Point (failure)

Now Let's learn each design theory in detail based on this. 

1. Working Stress Method (WSM)

In WSM, the structure is designed assuming the material behaves elastically — i.e., stress is directly proportional to strain (Hooke’s Law applies). So, we design the member such that stresses never exceed the elastic limit (point A).

The design stress (called permissible stress) is kept well below point A, often around 30–40% of the material’s actual strength.

The permissible (allowable) stresses are obtained by dividing the yield stress (for steel) or ultimate strength (for concrete) by a factor of safety (FOS).

Allowable Stress/Permissible Stress = Yield or Ultimate Strength/ Factor of Safety (FOS)

The design ensures that under working loads, no permanent deformation (plastic behavior) occurs.

Key Features of WSM

1. Safety-Oriented and Conservative

WSM gives primary importance to safety. It uses large Factors of Safety (FoS) to ensure that the working stress in materials (like concrete and steel) remains well below their actual strength.

Example: For steel, FoS ≈ 1.78; for concrete, FoS ≈ 3.0.

This means the structure operates far from its failure stress, making it very safe — but also overly cautious.

2. Serviceability Ensured

Because stresses are kept within the elastic region (O–A part of the stress–strain curve)

• Deflection remains small and recoverable
• Cracks are minimal or even absent and 
• The  structure returns to its original shape after unloading.

Hence, WSM automatically satisfies serviceability in terms of deflection, cracking, vibration, and durability — the structure remains comfortable and functional throughout its life.

3. Uneconomical Design

Since only a small portion of the material’s real strength is utilized, the resulting design sections are larger than necessary. 

• More concrete and steel are required.
• The structure becomes heavy and costly.

Thus, WSM ensures safety and serviceability at the expense of economy.

4. Linear Behavior Assumed

WSM assumes a linear stress–strain relationship for both concrete and steel under working loads.

• This means stress is directly proportional to strain (Hooke’s law).
• It simplifies design calculations but does not reflect actual material behavior once the material yields or cracks.

5. Not Realistic at Failure

Since WSM only considers the elastic range, it does not predict the true failure behavior of structures. Concrete and steel behave nonlinearly after yield, but WSM ignores this inelastic region completely. Hence, it cannot accurately estimate collapse loads or the real reserve strength of structures. So, WSM Ensures Serviceability but Lacks Economy!

Serviceability (deflection, cracking, vibration control) is naturally achieved in WSM because the structure always works well within the elastic limit.

However, the method is too conservative — it ignores the actual capacity of materials and leads to oversized, uneconomical sections.  As a result, although WSM designed structure is safe and serviceable, it is not cost-effective.

This limitation led engineers to search for a more realistic and economical approach — one that considers both ultimate strength and service behavior. That transition gave rise first to the Ultimate Load Method (ULM).

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

2. Ultimate Load Method (ULM)

In ULM design philosophy, the design is based on collapse load — the maximum load a structure can carry just before failure (at point E). Here, we do not limit the working stress of the material. Instead, we assume the structure will go all the way up to its ultimate strength (just before failure).

To ensure it doesn’t actually reach that condition in real life, we make the loads artificially higher during design. This is done by multiplying all actual (service) loads by a load factor (> 1). That’s why it’s also called the Load Factor Method.

Example:
Suppose the actual dead load = 100 kN and live load = 200 kN.
If we apply a load factor of 1.5,
then the design load = (100 + 200) × 1.5 = 450 kN.

We design the structure to resist 450 kN safely. But in real life, it will only face 300 kN — which means it will be safe. 
So here, safety is provided indirectly — we didn’t restrict the stress in materials; we simply increased the loads to make sure the structure is stronger than necessary.

Instead of using a small allowable stress (like WSM), we take the ultimate strength and apply load factors (like 1.5 or 2.0) to increase the actual loads. That means we are designing for a factored ultimate load, not for normal working conditions.

Key Features of ULM

1. Design Based on Ultimate (Collapse) Load

The method assumes that the structure will reach its ultimate strength — the point just before failure.  Hence, the design is based on the collapse condition, not on normal service conditions. 
It considers the plastic region of the stress–strain curve where materials are fully stressed.

2. Safety Ensured Indirectly

Unlike WSM, where safety is achieved by limiting stress, in ULM, safety is provided indirectly by factoring up the loads. All service loads (dead, live, wind, etc.) are multiplied by a load factor (>1) — usually between 1.5 and 2.0. This ensures that even if real-life loads are smaller, the structure remains safe.

3. No Limitation on Working Stress

ULM does not restrict the working stress in materials. Concrete and steel are allowed to reach their ultimate capacities, unlike WSM where stresses are kept within elastic limits. Therefore, the material’s full strength is utilized, making the design more realistic and economical.

4. Economy Achieved

Since the design uses the ultimate strength of materials, the cross-sections become smaller and lighter. 
Material usage is optimized, making the structure cost-effective compared to WSM.

5. Serviceability Not Considered

ULM focuses only on strength and collapse prevention. It ignores serviceability aspects such as:

• Deflection under normal load,
• Crack control,
• Vibration,
• Durability or appearance.

Hence, while the structure may be safe from collapse, it may crack, sag, or deform under daily use.

6. Behavior Considered Beyond Elastic Limit

ULM assumes materials behave non-linearly (plastic behavior) beyond the elastic limit until failure. It gives a more realistic picture of ultimate capacity, but does not represent how the structure behaves under service conditions.

In Short

• ULM focuses on ultimate strength and economy.
• Safety is ensured indirectly by factoring the loads.
• Serviceability is not satisfied, since working conditions (cracks, deflection) are ignored.

This limitation led to the evolution of the Limit State Method (LSM) — which combines the safety and serviceability control of WSM with the economy of ULM.

3. Limit State Method (LSM)

Limit State Method (LSM) is the modern and the most rational design philosophy used in structural design today. WSM was good at serviceability of structures and ULM was good at collapse prevention. LSM henceforth overcomes the limitations of both WSM and ULM by combining the advantages of both the method. And How is it done?

LSM ensures that the structure is safe under ultimate loads (as per ULM) and also serviceable under working loads ( as per WSM). Here too we make use different safety factors at ultimate loads and service. Let's now study the LSM method in detail:

Limit state means a boundary conditions beyond which a structure or any of its components no longer performs its intended function. As this methodology aims at preventing  the structure from reaching any of these limit states, it is called the limit state method. 

LSM design methodology is performed on structural design to satisfy two main conditions:

1. Limit State of Collapse
2. Limit State of Serviceability

Limit state of collapse or Ultimate Limit state is the stage at which the structure becomes unstable or fails. This can be due to flexure (bending), compression failure (crushing), shear or due to torsion.  Crossing  limit state of collapse means the structure looses structural safety.

Limit State of Serviceability (SLS) is the stage at which the structure no longer remain comfortable or functional even though it hasn't collapsed. It can be due to excessive deflection, cracking, vibration or corrosion.  Crossing  SLS means the structure looses its suitability or durability. 

Features of Limit State Design 

In LSM method, both the loads and material strength are treated as variables having uncertainties. Hence partial safety factors are applied to account for these variations:

• Loads are multiplied by load factors (> 1) to cover possible increases in actual loads.
• Material strengths are divided by partial safety factors (< 1) to cover possible weaknesses in materials.

This ensures that the structure remains safe and functional even under the worst combination of loads and material conditions. Hence, in short

• LSM combines the safety control of ULM and the serviceability assurance of WSM.
• It uses partial safety factors for both loads and materials to account for uncertainties.
• It gives economical, realistic, and durable designs that satisfy both strength and usability requirements.

Conclusion

We understood the evolution of design methods used in structural design- from WSM to ULM and finally LSM method. 

Today, the Ultimate Load Method (ULM) is completely obsolete and is no longer used in structural design practice. The Limit State Method (LSM) has become the most widely adopted and code-recommended approach across the world, including in IS 456:2000 for reinforced concrete design.

However, the Working Stress Method (WSM) still finds limited use in specific types of structures — particularly water-retaining structures, liquid storage tanks, swimming pools, and masonry or plain concrete members. These structures demand strict crack control, low deflection, and high durability, which WSM naturally ensures by keeping stresses within the elastic range.

In contrast, LSM is preferred for most modern structures like buildings, bridges, and industrial facilities because it balances safety, serviceability, and economy.

Hence, while LSM dominates present-day design, WSM continues to be used where serviceability and water-tightness are more critical than material economy.