Types of Loads, Load Combinations & Design Loads
A practical and detailed guide for Civil Engineers, Structural Engineers, Site Engineers, Students and Construction Professionals.
Structural loads are forces or actions imposed on a structure during construction and throughout its service life. The designer identifies the applicable loads, determines their magnitude and distribution, combines them using the relevant code provisions and designs structural members for the most critical effects.
1. Introduction
Every structure is subjected to various types of actions during its construction and service life. A building may experience its own self-weight, occupancy loads, wind pressure, earthquake effects, temperature variation, equipment loads and several other actions.
The purpose of structural design is not simply to design a member for one maximum load. Instead, the engineer must determine which combinations of actions can realistically occur together and which combination produces the most unfavourable structural response.
Indian Standards provide separate provisions for different categories of loads. For example, IS 875 covers several non-earthquake loads, while earthquake-resistant design is covered under the applicable parts of IS 1893.
2. Types of Loads Acting on Structures
| Load | Symbol | Typical Source | Nature |
|---|---|---|---|
| Dead Load | DL | Self-weight and permanent components | Permanent |
| Live / Imposed Load | LL / IL | People, furniture, storage, vehicles | Variable |
| Wind Load | WL | Wind pressure and suction | Environmental |
| Earthquake Load | EL | Ground acceleration | Seismic |
| Snow Load | SL | Accumulated snow | Environmental |
| Temperature Load | TL | Thermal expansion/contraction | Environmental / imposed |
| Earth Pressure | EP | Retained soil | Geotechnical |
| Hydrostatic Pressure | HP | Water pressure | Hydraulic |
| Construction Load | CL | Temporary construction activity | Temporary |
3. Dead Load (DL)
Dead load is the permanent load resulting from the self-weight of structural members and permanently attached components.
Typical Dead Loads
- RCC slabs
- Beams
- Columns
- Footings
- Structural steel members
- Masonry walls
- Floor finishes
- Waterproofing
- Permanent partitions
- Fixed services and equipment
Basic Calculation
For a uniform RCC slab:
For example, for a 150 mm thick RCC slab and an assumed concrete unit weight of 25 kN/m³:
4. Live Load / Imposed Load (LL)
Live load is the variable load associated with the use or occupancy of the structure. Unlike dead load, it may change with time, magnitude and location.
Examples
- Occupants
- Furniture
- Movable partitions
- Stored materials
- Vehicles
- Temporary equipment
- Maintenance activity
For buildings, imposed loads should be obtained from the applicable provisions of IS 875 (Part 2).
Live load should not be selected merely from an assumed generic value. The occupancy category, usage, storage requirements and applicable code provisions must be checked.
5. Wind Load (WL)
Wind load is generated by the interaction of moving air with the external and internal surfaces of a structure.
Wind Effects
- External pressure
- External suction
- Internal pressure
- Uplift
- Lateral force
- Overturning moment
- Torsional effects
- Local cladding forces
Wind loading for buildings and structures is addressed in IS 875 (Part 3).
For tall, slender or flexible structures, wind may become a governing design action even when the structure has substantial dead load.
6. Earthquake / Seismic Load (EL)
Earthquake load is an inertia force generated by the acceleration of the structure during ground motion.
When the foundation moves with the ground, the mass of the superstructure tends to resist that movement. This creates inertia forces within the structural system.
Typical Seismic Effects
- Base shear
- Storey shear
- Storey drift
- Overturning moment
- Column axial-force variation
- Beam bending
- Joint forces
- Torsional response
Earthquake-resistant design must use the applicable provisions of the current IS 1893 series and the relevant material design standards.
7. Snow Load (SL)
Snow load is the load caused by accumulation of snow on roofs or other structural surfaces.
Snow loading is particularly important in regions where significant snowfall occurs and may depend upon altitude, climatic conditions, roof geometry and snow accumulation characteristics.
The applicable provisions of IS 875 (Part 4) should be used for snow loading.
8. Temperature Load (TL)
Temperature changes cause structural materials to expand and contract. When free movement is prevented, internal forces and stresses can develop.
where:
- α = coefficient of thermal expansion
- ΔT = change in temperature
Structures Where Temperature Effects Are Important
- Long-span bridges
- Continuous bridges
- Large industrial buildings
- Long pipelines
- Water tanks
- Large concrete structures
- Restrained frames
Temperature-related effects, along with other special loads, are covered by the relevant provisions of IS 875 (Part 5).
9. Other Important Loads
Depending upon the structure, location and function, several additional loads may become critical.
Earth Pressure
Important for retaining walls, basements, abutments, underground structures and similar systems.
Hydrostatic Pressure
Important for water-retaining structures, tanks, basements and submerged components.
Construction Loads
Temporary loads from construction equipment, materials, workers, formwork, falsework and erection operations.
Impact Loads
Relevant where moving vehicles, cranes, machinery or other moving masses can produce impact effects.
Crane Loads
Industrial buildings may experience crane vertical loads, horizontal surge forces, longitudinal forces and impact effects.
Accidental Loads
Special situations such as vehicle impact, accidental actions and other project-specific hazards may require consideration.
10. Load Combinations
A structure is rarely designed by simply adding the maximum value of every possible load. The probability that all loads reach their maximum values simultaneously must be considered.
BIS guidance emphasizes that load combinations should account for the probability of loads acting together and the severity of the resulting stresses or deformations. It also recognizes that simultaneous maximum wind, earthquake, imposed and snow loads are generally unlikely.
IS 875 (Part 5) specifically addresses special loads and load combinations.
Common Ultimate Limit-State Combinations for RCC Design
| No. | Load Combination | Purpose / Typical Application |
|---|---|---|
| 1 | 1.5(DL + LL) | Gravity load combination |
| 2 | 1.2(DL + LL ± WL) | Gravity + wind |
| 3 | 1.2(DL + LL ± EL) | Gravity + earthquake |
| 4 | 1.5(DL ± WL) | Dead load + wind |
| 5 | 1.5(DL ± EL) | Dead load + earthquake |
| 6 | 1.5(DL + SL) | Dead load + snow |
| 7 | 0.9DL ± 1.5EL | Stability / uplift-sensitive seismic condition |
The above combinations are useful for understanding the commonly encountered RCC limit-state combinations, but they should not be treated as a universal load-combination table for every structure. The governing code, structure type, material, design method, amendments and applicable loading standard must be checked for the actual project.
11. Service Load vs Design Load
Service Load
- Represents expected working conditions.
- Used for serviceability assessment.
- Important for deflection.
- Important for crack control.
- Important for vibration and deformation.
Factored / Design Load
- Used primarily for strength or ultimate limit-state design.
- Obtained using appropriate partial safety factors.
- Used to determine critical design actions.
- Used for member strength checks.
It is incorrect to assume that every load is always multiplied by 1.5. The applicable factor depends on the load combination and the relevant design standard.
12. Detailed Numerical Example
Consider a simplified structural loading situation with:
| Load | Value |
|---|---|
| Dead Load (DL) | 20 kN |
| Live Load (LL) | 15 kN |
| Wind Load (WL) | 10 kN |
| Earthquake Load (EL) | 8 kN |
| Snow Load (SL) | 12 kN |
Combination 1 — 1.5(DL + LL)
= 1.5 × 35
= 52.5 kN
Combination 2 — 1.2(DL + LL + WL)
= 1.2 × 45
= 54.0 kN
Combination 3 — 1.2(DL + LL ± EL)
Positive earthquake direction:
= 51.6 kN
Opposite earthquake direction:
= 32.4 kN
Combination 4 — 1.5(DL ± WL)
Combination 5 — 1.5(DL ± EL)
Combination 6 — 1.5(DL + SL)
= 48.0 kN
Combination 7 — 0.9DL ± 1.5EL
= 18 + 12
= 30.0 kN
= 18 − 12
= 6.0 kN
Result Comparison
| Combination | Result |
|---|---|
| 1.5(DL + LL) | 52.5 kN |
| 1.2(DL + LL + WL) | 54.0 kN |
| 1.2(DL + LL + EL) | 51.6 kN |
| 1.2(DL + LL − EL) | 32.4 kN |
| 1.5(DL + WL) | 45.0 kN |
| 1.5(DL + EL) | 42.0 kN |
| 1.5(DL + SL) | 48.0 kN |
| 0.9DL + 1.5EL | 30.0 kN |
| 0.9DL − 1.5EL | 6.0 kN |
For this simplified numerical example, the largest calculated value among the listed combinations is 54.0 kN. However, in real structural design, the critical combination is determined separately for each response quantity such as axial force, shear force, bending moment, torsion, reaction, uplift and overturning.
13. Practical Structural Load-Combination Workflow
Determine whether the structure is a building, bridge, retaining wall, industrial structure, water tank, tower or another structural system.
Prepare a complete load schedule including permanent, imposed, environmental and project-specific actions.
Determine the magnitude, distribution and location of each load.
Use the applicable Indian Standard and project specifications.
Determine critical axial force, bending moment, shear force, torsion, reactions, displacement and other relevant effects.
Design beams, slabs, columns, walls, foundations and other components for the governing design effects.
Verify deflection, crack control, vibration, drift and other applicable serviceability requirements.
14. Common Mistakes in Load Calculation
- Ignoring self-weight: Structural members must account for their own weight.
- Forgetting finishes: Floor finishes and waterproofing can contribute significantly to permanent load.
- Using an incorrect live load: The occupancy category must be verified.
- Using wind load in only one direction: Critical wind directions and pressure/suction effects must be considered.
- Ignoring uplift: Wind and seismic actions can create uplift and overturning effects.
- Combining every maximum load: Load combinations must follow the applicable code rather than simply adding all maximum actions.
- Using 1.5 for every load: Partial safety factors vary with the load combination.
- Checking only bending moment: Axial force, shear, torsion, reactions, drift and stability may also govern.
- Ignoring construction stage: Temporary construction loads can sometimes govern members that are safe under final service conditions.
- Using outdated code provisions: Always verify the current edition and amendments applicable to the project.
15. Expert Tips for Civil & Structural Engineers
Tip 1 — Prepare a Load Schedule
Before starting structural analysis, prepare a separate schedule for DL, LL, WL, EL, temperature, equipment and other project-specific loads.
Tip 2 — Check Load Paths
Always understand how the load travels from slab → beam → column/wall → foundation → soil.
Tip 3 — Check Both Directions
For wind and seismic loading, consider the required positive and negative directions and structural response.
Tip 4 — Think Beyond Gravity Loads
A structure that is safe under gravity loading may still be vulnerable to wind, earthquake, uplift, sliding or overturning.
Tip 5 — Separate ULS and SLS
Do not confuse strength design combinations with serviceability combinations. Both checks are essential.
Tip 6 — Review the Critical Effect
The combination producing the largest total load is not necessarily the combination producing the maximum bending moment, shear, axial force or foundation reaction.
16. Important Indian Standard References
| Standard | Subject |
|---|---|
| IS 456:2000 | Plain and Reinforced Concrete — Code of Practice |
| IS 875 (Part 1) | Dead Loads — Unit Weights of Building Materials and Stored Materials |
| IS 875 (Part 2) | Imposed / Live Loads |
| IS 875 (Part 3) | Wind Loads |
| IS 875 (Part 4) | Snow Loads |
| IS 875 (Part 5) | Special Loads and Load Combinations |
| IS 1893 Series | Criteria for Earthquake Resistant Design of Structures |
BIS currently lists IS 456:2000 with amendments including the sixth amendment from 2024. BIS also lists IS 875 (Part 1):2026, IS 875 (Part 3):2015, IS 875 (Part 4):2021 and IS 875 (Part 5):1987. Engineers should therefore verify the latest applicable edition, amendment and project-specific requirements before final design.
17. Frequently Asked Questions
Q1. What is the difference between dead load and live load?
Dead load is generally permanent and comes from the self-weight of the structure and permanently attached components. Live load varies according to occupancy, usage and temporary conditions.
Q2. What is a factored load?
A factored load is a characteristic or nominal load multiplied by the appropriate partial safety factor specified for the relevant design combination.
Q3. Is every load multiplied by 1.5?
No. The applicable load factor depends on the load combination, limit state, design standard and type of structure.
Q4. Why are both positive and negative earthquake directions considered?
Earthquake effects are reversible. The direction of seismic action can change the sign and magnitude of member forces, reactions, overturning and uplift.
Q5. Why is 0.9DL used in some combinations?
A reduced dead-load factor can become critical in stability-related cases, particularly where dead load provides resistance against uplift or overturning.
Q6. Which code is used for wind load in India?
Wind loads for buildings and structures are generally determined using IS 875 (Part 3), subject to the applicable current edition and amendments.
Q7. Which code covers special loads and load combinations?
IS 875 (Part 5) covers special loads and load combinations for buildings and structures. Its scope includes temperature effects, soil and hydrostatic pressures, internally generated stresses and accidental loads.
Q8. Is the largest load combination always the governing combination?
No. Different combinations may govern different structural actions. For example, one combination may govern beam bending, another may govern column axial force, and another may govern foundation uplift or overturning.
Conclusion
Understanding structural loads and load combinations is one of the most fundamental skills required in structural engineering. A designer must first identify every significant load, calculate its magnitude and distribution, and then combine the loads according to the applicable code provisions.
The fundamental sequence is:
Dead load, live load, wind load, earthquake load, snow load and temperature effects represent only the principal categories. Depending on the project, earth pressure, hydrostatic pressure, construction loads, crane loads, impact loads, equipment loads and accidental actions may also become important.
Never design a structure merely for the maximum individual load. Design it for the governing code-compliant load combinations and the corresponding critical structural effects.
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