Load Cases and Load Combinations

In structural engineering, the two main design criteria to consider are serviceability and capacity. Serviceability refers to the performance of a structure under load as perceived by its occupants. Due to the elastic nature of steel and wood, elements that are meeting loading demands can still feel uncomfortable if they deflect too much under live loads. Furthermore, floors and roofs must limit deflection in order to be safely serviced. Typically, serviceability is maintained by ensuring structural elements remain within a deflection limit. In WebStructural's Beam Designer module, these limits can be specified from the Design Specification button.

Capacity, as the name implies, refers to the structural element's ability to meet the loading demands it is expected to experience over its lifetime. While serviceability is based on human perception and general structural behavior, capacity calculations ensure structural elements remain in the elastic range to avoid catastrophic failure. All design specifications include some type of safety factor to modify capacity and demand, ensuring that beams and columns remain well within their elastic limits. These safety factors are built into the loads applied to the structure via Load Combinations (LRFD and ASD). Safety factors are also included in the design specifications used to determine capacity (AISC and NDS). In this article, we'll focus on Load Combinations. Specifically, Load Resistance Factor Design (LRFD) and Allowable Strength Design (ASD). In WebStructural, load combinations can be specified from the Design Specification button.

LRFD and ASD share the concept of Load Cases. Load Cases are a way to break loads up based on their type, allowing loads to be combined in different ways to simulate different interactions. These combinations, called Load Combinations, account for extreme events, the maximum load a member is likely to experience over its lifetime. It is important to remember that this is not the only place safety factors are applied. Safety factors are also applied to capacity checks to further ensure that even in extreme conditions, structural elements remain within the elastic limit.

So what's the difference between LRFD and ASD? In its simplest form, LRFD focuses on increasing load, accounting for uncertainty in dynamic loads like wind and snow. ASD, on the other hand, focuses on reducing load (and capacity) based on the probability of multiple load cases acting together at exactly the same time. WebStructural's Beam Designer module supports a comprehensive set of combinations for both LRFD and ASD.

A final important piece to all of this is understanding how each method adjusts capacity. The key difference becomes clear when we look at the controlling equations for each method:

So is one method better than the other? The short answer is no. For structures that experience more variable and dynamic loads, LRFD will typically result in a more conservative design. Conversely, if loads are less dynamic and predictable, ASD can result in a more conservative design. Both are still acceptable methods according to IBC. At WebStructural we've found that LRFD seems to be the more preferred method. That said, it's ultimately up to you, the engineer or designer, to decide which is best for your use case.