Serviceability Considerations for Beam Design : In designing structures, there are important serviceability considerations that must be addressed by the designer. These considerations are outlined in AISC Specification Chapter L and, although they may not affect the strength of the structure, they can impact the successful completion of a project. It is crucial for the designer to discuss the specific criteria in detail with their client to ensure that the final product meets the owner’s expectations in terms of quality.

Beams generally have two serviceability issues that must be addressed directly, deflection and vibration. With today’s high strength steels, deflection criteria may often control designs for typical building spans and loads. A third issue is overall building drift, where the beams in moment frames play a significant role in the response.

Deflection

Deflection is the expected bending or deformation of a beam when subjected to a load. While it’s impossible to completely eliminate deflection, designers can control and limit it through careful consideration. Deflection must be addressed for different load scenarios, including dead load and live load. Dead load deflection affects construction processes and requires sufficient concrete fill for a level floor. Live load deflection impacts finishes and should be limited to 1/360 of the span. Dead load deflection limits depend on the specific structural element and loading. AISC Design Guide 3 provides guidance on deflection and other serviceability design aspects for steel buildings.

Vibration

While floor system vibration is not a safety issue, it can be annoying and hard to rectify post-construction. Wide-open spaces with low damping, like jewelry departments, often face this problem. To minimize annoyance, a general guideline is to space beams or joists adequately apart, ensuring the slab thickness provides sufficient stiffness and damping. AISC Design Guide 11 focuses on designing steel-framed floor systems for human comfort by addressing vibration caused by human activity.

Drift

Under lateral loading, buildings experience sideways movement known as drift, which is not typically a safety concern but can be problematic and affect nonstructural elements by causing cracks in finishes. Beams and girders play a crucial role in reducing drift, and their size may be determined based on drift considerations. However, evaluating the impact of drift on beams requires assessing the entire lateral load-resisting system. Drift is discussed in AISC Design Guide 3.

Calculations for beam deflection, a serviceability consideration, involve specific loads applicable to the serviceability checks. These loads can be live load, dead load, or a combination, typically without load factors. Regardless of whether LRFD or ASD design methods are used, serviceability considerations are examined using the same loads. Various elastic analysis techniques are available to determine maximum deflection for a given beam and loading. Figure 1 illustrates some common loading conditions and their corresponding maximum deflections, while AISC Manual Table 3-23 provides additional examples.