C-L1L1 General provisions

Serviceability limit states are conditions in which the functions of a building are impaired because of local damage, deterioration or deformation of building components, or occupant discomfort. While serviceability limit states generally do not involve collapse of a building, loss of life, or injury, they can seriously impair the usefulness of a building and lead to costly repairs and other economic consequences. Serviceability provisions are essential to provide satisfactory performance of building structural systems. Neglect of serviceability may result in structures that are excessively flexible or otherwise perform unacceptably in service.

The general types of structural behavior that are indicative of impaired serviceability in steel structures are as follows:

• (a) Excessive deflections or rotations that may affect the appearance, function, or drainage of the building, or may cause damaging transfer of load to nonstructural components and attachments
• (b) Excessive drift due to wind that may damage cladding and nonstructural walls and partitions
• (c) Excessive vibrations produced by the activities of the building occupants or mechanical equipment that may cause occupant discomfort or malfunction of building service equipment
• (d) Excessive wind-induced motions that may cause occupant discomfort
• (e) Excessive effects of expansion and contraction caused by temperature differences as well as creep and shrinkage of concrete and yielding of steel
• (f) Effects of connection slip, resulting in excessive deflections and rotations that may have deleterious effects similar to those produced by load effects

In addition, excessive local damage (local yielding, buckling, slip, or cracking) or deterioration (weathering, corrosion, and discoloration) may also affect the function and serviceability of the structure during its service life.

Serviceability limit states depend on the occupancy or function of the building, the perceptions of its occupants, and the type of structural system. Limiting values of structural behavior intended to provide adequate levels of serviceability should be determined by a team consisting of the building owner/developer, the architect, and the structural engineer after a careful analysis of all functional and economic requirements and constraints. In arriving at serviceability limits, the team should recognize that building occupants are able to perceive structural deformations, motions, cracking, or other signs of distress at levels that are much lower than those that would

indicate impending structural damage or failure. Such signs of distress may be viewed as an indication that the building is unsafe and diminish its economic value and, therefore, must be considered at the time of design.

Service loads that may require consideration in checking serviceability include (1) static loads from the occupants, snow or rain on the roof, or temperature fluctuations; and (2) dynamic loads from human activities, wind effects, the operation of mechanical or building service equipment, or traffic near the building. Service loads are loads that act on the structure at an arbitrary point in time and may be only a fraction of the corresponding nominal load. The response of the structure to service loads generally can be analyzed assuming elastic behavior. Members that accumulate residual deformations under service loads also may require examination with respect to this long-term behavior.

Serviceability limit states and appropriate load combinations for checking conformance to serviceability requirements can be found in ASCE/SEI 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, Section 1.3.2, Commentary 1.3.2, Appendix C, and Commentary Appendix CC (ASCE, 2022).