Why is the load factor of live load greater than the load factor of dead load?

Load combinations in structural design, Why is the load factor of live load greater than the load factor of dead load?

The load factor of live load in reliability-based design is usually higher than the load factor of dead load because of several inherent characteristics of the probabilistic modeling and analysis employed in this methodology:

1. Variability and Uncertainty:

Compared to dead loads, which are more predictable, live loads, such as occupancy, furniture, and equipment loads, are more variable and uncertain. By using probabilistic models to represent the variability in loadings and statistical distributions to account for these uncertainties, a reliability-based design seeks to address these issues. Because live loads may be more variable and uncertain, a larger safety margin must be provided, which is reflected in the higher load factor for live loads.

2. Safety Margin:

Reliability-based design aims to achieve a specific level of structural reliability, which is usually expressed as a probability of failure over the structure's design life. To achieve this target reliability level, higher load factors are applied to loads with greater variability and uncertainty, such as live loads. Using a higher load factor for live loads allows designers to increase the safety margin and reduce the probability of failure under extreme loading conditions.

3. Risk Management:

Reliability-based design entails identifying and managing the risks of structural failure. Live loads are frequently thought to pose a greater risk to structural integrity due to their dynamic and unpredictable nature. As a result, using a higher load factor on live loads helps to reduce this risk by accounting for potential variations and uncertainties in loading conditions.

4. Code Requirements:

Building codes and standards frequently specify minimum load factors for various types of loads to?ensure reliability. Engineering judgment, historical data, and probabilistic analyses all help to shape these code requirements. In many cases, the load factors for live loads are higher than those for dead loads to ensure that the structural design meets the required reliability and safety requirements.

Overall, in reliability-based design, the higher load factor for live load versus dead load reflects the need to account for the higher variability, uncertainty, and risk associated with live loads to achieve the desired level of structural reliability and safety.

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