Understanding structural loads in buildings is the foundation of safe structural design. Every building must withstand various forces such as its own weight, the weight of occupants, environmental effects like wind and earthquakes, and accidental or dynamic actions. Structural loads determine the size of beams, columns, slabs and foundations, and form the basis of calculations in IS 875 and IS 1893. This guide explains the different types of structural loads, their definitions, engineering relevance and how they influence building design.
1. Introduction to Structural Loads
Structural loads are forces or actions that a building or structural element must safely resist throughout its life. Loads may act vertically, horizontally or dynamically, depending on their nature.
Key objectives of load analysis include:
- Ensuring safety against collapse
- Preventing excessive deflection or cracking
- Minimising vibrations
- Maintaining stability under wind and seismic events
- Designing economical structural members
Indian standards specify load requirements primarily through:
- IS 875 (Part 1–5) – Dead, Live, Wind, Snow and Special loads
- IS 1893 – Earthquake loads
- IS 456 – Structural design provisions for concrete
- NBC – General building code guidance
2. Basic Definitions Used in Load Analysis
Load
A force acting on a structure.
Dead Load (DL)
Permanent static loads from the structure’s own weight.
Live Load (LL)
Imposed loads due to occupancy, usage and movable elements.
Wind Load (WL)
Horizontal pressure exerted by wind on building surfaces.
Earthquake Load (EL)
Inertial force induced by ground motion during seismic events.
Impact Load
Sudden dynamic loads from movement, machinery or shock effects.
Load Combination
Various load cases applied together using safety factors.
Factor of Safety
Multiplier applied to loads to ensure conservative design.
3. Dead Load (IS 875 Part 1)
Dead load refers to the self-weight of the structure and other permanently attached components.
Dead loads include:
- Self-weight of beams, slabs, columns, walls
- Weight of finishing materials (tiles, plaster, screed)
- Weight of parapets, partitions
- Roofing systems
- Fixed equipment
Typical unit weights used in DL calculations:
| Material | Unit Weight (kN/m³) |
|---|---|
| RCC | 25 |
| Brick masonry | 18–20 |
| Steel | 78–80 |
| Timber | 5–8 |
| Soil | 18–20 |
Dead load governs structural sizing for major elements like beams and slabs.
4. Live Load (IS 875 Part 2)
Live loads (also called imposed loads) represent occupancies and movable loads.
Examples:
- People
- Furniture
- Stored materials
- Equipment
Live loads vary by building type:
| Occupancy | Live Load (kN/m²) |
|---|---|
| Residential floors | 2.0 |
| Staircases | 3.0 |
| Offices | 2.5–3.5 |
| Classrooms | 2.5 |
| Storage rooms | 5.0 |
| Corridors (public buildings) | 4.0 |
Live loads ensure safety for fluctuating and unpredictable usage.
5. Wind Load (IS 875 Part 3)
Wind load affects all medium to high-rise buildings. It applies horizontal pressure and suction.
Wind effects depend on:
- Basic wind speed zone (India has 33–55 m/s zones)
- Terrain category
- Building height
- Topography
- Importance factor
- Building shape
Wind pressure formula from IS 875:
Pz = 0.6 × Vz²
Where Vz = design wind speed at height z.
Wind load influences:
- Lateral stability
- Column and shear wall design
- Cladding design
- Foundation design
Wind is a dominant factor for buildings above 15–20 storeys.
6. Earthquake Load (IS 1893)
Earthquake load is a dynamic load caused by ground shaking.
The seismic design philosophy:
- Buildings should not collapse, even in major earthquakes
- Controlled damage is acceptable
- Ductility is essential
Key parameters for seismic analysis:
- Zone factor (India: Zones II–V)
- Importance factor
- Response reduction factor (R)
- Soil type (hard, medium, soft)
- Fundamental time period
- Base shear calculation
Earthquake effects lead to:
- Lateral forces
- Torsional effects
- Drift requirements
- Ductile detailing (IS 13920)
7. Snow Load (IS 875 Part 4)
(Not applicable in most regions of India but essential in northern hill states.)
Snow load depends on:
- Snowfall intensity
- Roof geometry
- Exposure
- Thermal conditions
Snow load formula:
S = μ × So
Where So = ground snow load.
Snow load can cause roof collapse if underestimated.
8. Special Loads (IS 875 Part 5)
Special loads include:
- Impact loads
- Vibration loads
- Crane loads
- Temperature effects
- Earth pressure load
- Hydrostatic pressure
- Explosion or blast loads
- Settlement loads
These are essential in industrial, offshore and infrastructure projects.
9. Load Combinations (LSM)
Load combinations ensure safety under various scenarios. IS codes specify factors for:
- Dead load
- Live load
- Wind load
- Earthquake load
Typical limit state combinations:
Without wind/earthquake
1.5 (DL + LL)
With wind or earthquake
1.2 (DL + LL + WL/EL)
1.5 (DL + WL/EL)
0.9 DL ± 1.5 WL/EL (stability check)
These combinations ensure that a structure performs safely in extreme events.
10. How Loads Affect Structural Elements
Slabs
Resist DL + LL primarily through bending.
Beams
Transfer loads from slabs to columns, experiencing bending and shear.
Columns
Carry axial loads from beams and floors; also resist lateral loads.
Shear Walls
Essential for tall buildings to resist wind and earthquakes.
Foundations
Transfer all vertical and lateral loads safely into the ground.
11. Load Path in a Structure
Understanding load paths ensures correct detailing.
Load moves as follows:
Slab → Beam → Column → Foundation → Soil
Any weak link in this chain can lead to excessive settlement or failure.
12. Example Load Calculation Summary Table
| Parameter | Example Value |
|---|---|
| Dead Load (slab 150 mm) | 3.75 kN/m² |
| Floor Finish | 1.0 kN/m² |
| Live Load | 2.0 kN/m² |
| Total Unfactored Load | 6.75 kN/m² |
| Factored Load | 1.5 × 6.75 = 10.12 kN/m² |
Such calculations form the basis of structural design.
13. Importance of Structural Loads in Design
Structural loads ensure:
- Safe design margins
- Serviceability (deflection, cracking)
- Durability
- Stability against lateral forces
- Compliance with codes
Ignoring or miscalculating loads is one of the main causes of structural failures.
Conclusion
Structural loads in buildings form the backbone of all structural design work. A strong command of dead loads, live loads, wind loads, earthquake loads and load combinations is essential for designing safe and economical buildings. Understanding load paths, code requirements and engineering behavior allows civil engineers to create structures that perform reliably throughout their service life.
Reference
IS 875 Parts 1–5 – Bureau of Indian Standards
https://bis.gov.in
IS 1893 – Earthquake Load Guidelines
https://bis.gov.in
NPTEL – Structural Dynamics & Earthquake Engineering
https://nptel.ac.in/courses/105102
NBCC Canada – Structural Load Guidelines (International Reference)
https://nrc.canada.ca