Load and Load Combinations

Why it matters

In civil engineering, understanding loads and their combinations is essential for designing structures that are safe, efficient, and economical. Proper load analysis ensures that structures can withstand various forces and conditions they will encounter during their lifespan, such as dead loads, live loads, wind loads, and seismic forces.

Key ideas

• Types of Loads:

  • Dead Load (DL): Permanent static forces due to the weight of structural elements.
  • Live Load (LL): Temporary or movable forces that the structure supports, such as people or furniture.
  • Wind Load (WL): Forces exerted by wind pressure on the structure.
  • Seismic Load (SL): Forces due to earthquakes.
  • Other Loads: Snow load, temperature effects, etc.

• Load Combinations:

  • Structures must be designed to withstand various combinations of these loads.
  • IS 875 (Part 5), the relevant material-design standard and applicable seismic provisions must be considered; the governing combination depends on the limit state and structure.

• Limit State Design:

  • Ensures safety and serviceability under the worst load combinations.
  • Involves partial safety factors for loads and materials.

Formulas

For each applicable combination, calculate the signed action effect E_d = Σ γ_i E_i where linear superposition is valid. Account for direction, favourable/unfavourable actions and the load pattern. Do not automatically sum the magnitudes of dead, live, wind and earthquake loads as though they all act simultaneously in one direction. Wind and earthquake combinations must follow the relevant standard.

Worked example

Given: A gravity-load exercise specifies dead load 10 kN and live load 5 kN acting vertically downward at the same location, and the design combination 1.5(D + L).

Unfactored resultant: 10 + 5 = 15 kN downward. Factored resultant: 1.5 × 10 + 1.5 × 5 = 22.5 kN downward.

Answer: 22.5 kN downward for the specified combination. Other applicable combinations, including uplift or load reversal, must be evaluated independently; the governing case may differ between members and checks.

Common mistakes

• Ignoring or misapplying partial safety factors.
• Incorrectly identifying or calculating load types.
• Failing to consider all relevant load combinations.

For GATE CE

• Questions often involve calculating design loads using given partial safety factors.
• Practice identifying different load types and applying IS 875 guidelines for combinations.

Quick check

1. What is a dead load?
2. Name two types of loads other than dead load.
3. What is the purpose of partial safety factors?

Answers: 1. Permanent static forces due to the weight of structural elements. 2. Live load, wind load. 3. To ensure safety and serviceability under worst load combinations.