TOC (ASCE/SEI 7-22)
Provisions
CommentaryServiceability Considerations
This appendix is not a mandatory part of the standard but provides guidance for design for serviceability to maintain the function of a building and the comfort of its occupants during normal usage. Serviceability limits (e.g., maximum static deformations or accelerations) shall be chosen with due regard to the intended function of the structure.
Serviceability shall be checked using appropriate loads for the limit state being considered.
Floor systems supporting large open areas free of partitions or other sources of damping, where vibration caused by pedestrian traffic might be objectionable, shall be designed with due regard for such vibration.
Mechanical equipment that can produce objectionable vibrations in any portion of an inhabited structure shall be isolated to minimize the transmission of such vibrations to the structure.
Building structural systems shall be designed so that wind-induced vibrations do not cause occupant discomfort or damage to the building, its appurtenances, or contents.
Special camber requirements that are necessary to bring a loaded member into proper relation with the work of other trades shall be set forth in the design documents.
Beams detailed without specified camber shall be positioned during erection so that any minor camber is upward. If camber involves the erection of any member under preload, this shall be noted in the design documents.
Dimensional changes in a structure and its elements caused by variations in temperature, relative humidity, or other effects shall not impair the serviceability of the structure.
Provision shall be made either to control crack widths or to limit cracking by providing relief joints.
Commentary
Serviceability Considerations
CC.1 Serviceability Considerations
Serviceability limit states are conditions in which the functions of a building or other structure are impaired because of local damage, deterioration, or deformation of building components, or because of occupant discomfort. Although in general safety is not an issue with serviceability limit states (one exception would be for cladding that falls off a building because of excessive story drift under wind load), they nonetheless may have severe economic consequences. The increasing use of the computer as a design tool, the use of stronger (but not stiffer) construction materials, the use of lighter architectural elements, and the uncoupling of the nonstructural elements from the structural frame may result in building systems that are relatively flexible and lightly damped. Limit state design emphasizes the fact that serviceability criteria are essential (as they always have been) to ensure functional performance and economy of design for such building structural systems (Ad Hoc Committee on Serviceability Research 1986, NBCC 1990, West and Fisher 2003).
In general, serviceability is diminished by
Excessive deflections or rotation that may affect the appearance, functional use, or drainage of the structure or may cause damaging transfer of load to non-load-supporting elements and attachments;
Excessive vibrations produced by the activities of building occupants, mechanical equipment, or the wind, which may cause occupant discomfort or malfunction of building service equipment; and
Deterioration, including weathering, corrosion, rotting, and discoloration.
In checking serviceability, the designer is advised to consider appropriate service loads, the response of the structure, and the reaction of the building occupants.
Service loads that may require consideration include static loads from the occupants and their possessions, snow or rain on roofs, temperature fluctuations, and dynamic loads from human activities, wind-induced effects, or the operation of building service equipment. The service loads are those loads that act on the structure at an arbitrary point in time. (In contrast, the nominal loads have a small probability of being exceeded in any year; factored loads have a small probability of being exceeded in 50 years.) Appropriate service loads for checking serviceability limit states may be only a fraction of the nominal loads.
The response of the structure to service loads can usually be analyzed assuming linear elastic behavior. However, members that accumulate residual deformations under service loads may require examination with respect to this long-term behavior. The service loads used in analyzing creep or other long-term effects may not be the same as those used to analyze elastic deflections or other short-term or reversible structural behavior.
Serviceability limits depend on the function of the building and on the perceptions of its occupants. In contrast to the ultimate limit states, it is difficult to specify general serviceability limits that are applicable to all building structures. The serviceability limits presented in Sections CC.2.1, CC.2.2, and CC.2.3 provide general guidance and have usually led to acceptable performance in the past. However, serviceability limits for a specific building should be determined only after a careful analysis by the engineer and architect, in conjunction with the building owner, of all functional and economic requirements and constraints. It should be recognized that building occupants are able to perceive structural deflections, motion, cracking, and other signs of possible distress at levels that are much lower than those that would indicate that structural failure was impending. Such signs of distress may be taken incorrectly as indications that the building is unsafe and may diminish its commercial value.
CC.2 Deflection, Vibration, and Drift
CC.2.1 Vertical Deflections
Excessive vertical deflections and misalignment arise primarily from three sources: (1) gravity loads, such as dead, live, and snow loads; (2) effects of temperature, creep, and differential settlement; and (3) construction tolerances and errors. Such deformations may be visually objectionable; may cause separation, cracking, or leakage of exterior cladding, doors, windows, and seals; and may cause damage to interior components and finishes. Appropriate limiting values of deformations depend on the type of structure, detailing, and intended use (Galambos and Ellingwood 1986). Historically, common deflection limits for horizontal members have been 1/360 of the span for floors subjected to full nominal live load and 1/240 of the span for roof members. Deflections of about 1/300 of the span (for cantilevers, 1/150 of the length) are visible and may lead to general architectural damage or cladding leakage. Deflections greater than 1/200 of the span may impair operation of movable components such as doors, windows, and sliding partitions.
In certain long-span floor systems, it may be necessary to place a limit (independent of span) on the maximum deflection to minimize the possibility of damage of adjacent nonstructural elements (ISO 1977). For example, damage to non-load-bearing partitions may occur if vertical deflections exceed more than about 3/8 in. (10 mm) unless special provision is made for differential movement (Cooney and King 1988); however, many components can and do accept larger deformations.
Load combinations for checking static deflections can be developed using first-order reliability analysis (Galambos and Ellingwood 1986). Current static deflection guidelines for floor and roof systems are adequate for limiting surficial damage in most buildings. A combined load with an annual probability of 0.05 of being exceeded would be appropriate in most instances. For serviceability limit states involving visually objectionable deformations, reparable cracking or other damage to interior finishes, and other short-term effects, the suggested load combinations are
D
Dead load;
L
Live load; and
Sser
Design roof snow load determined using the provisions in Chapter 7 with pg = 20-year MRI ground snow load from Figure CC.2-1. These 20-year MRI ground snow loads are available in the ASCE Design Ground Snow Load Geodatabase (https://asce7hazardtool.online/).
It is appropriate for serviceability checks using Equation CC.2-1(b) to include consideration for all applicable snow loading conditions outlined in Chapter 7. The 20-year MRI value is, on average, 80% of the 50-year MRI ground snow load. The selection of the 20-year MRI for this purpose was based on judgment and review of snow load serviceability criteria in other standards.
For serviceability limit states involving creep, settlement, or similar long-term or permanent effects, the suggested load combination is
The dead-load effect, , used in applying Equations (CC.2-1) and (CC.2-2) may be that portion of dead load that occurs after attachment of nonstructural elements. Live load, , is defined in Chapter 4. For example, in composite construction, the dead-load effects frequently are taken as those imposed after the concrete has cured; in ceilings, the dead-load effects may include only those loads placed after the ceiling structure is in place.
CC.2.2 Drift of Walls and Frames
Drifts (lateral deflections) of concern in serviceability checking arise primarily from the effects of wind. Drift limits in common usage for building design are on the order of 1/600 to 1/400 of the building or story height (ASCE Task Committee on Drift Control of Steel Building Structures 1988, Griffis 1993). These limits generally are sufficient to minimize damage to cladding and nonstructural walls and partitions. Smaller drift limits may be appropriate if the cladding is brittle. Larger drift limits may be acceptable when the connections between the structural and non-structural elements are detailed to accommodate the relative movement without damage. West and Fisher (2003) contains recommendations for higher drift limits that have successfully been used in low-rise buildings with various cladding types. It also contains recommendations for buildings containing cranes. An absolute limit on story drift may also need to be imposed in light of evidence that damage to nonstructural partitions, cladding, and glazing may occur if the story drift exceeds about 10 mm (3/8 in.) unless special detailing practices are made to tolerate movement (Freeman 1977, Cooney and King 1988). Many components can accept deformations that are significantly larger.
It is excessively conservative to use the basic wind speed for the project’s risk category to check serviceability because the corresponding MRI for design winds is very large. The following load combination, derived similarly to Equations (CC.2-1a) and (CC.2-1b), can be used to check short-term effects:
Here is wind load based on the serviceability wind speeds in Figures CC.2-2 through CC.2-5. Some designers have used a 10-year MRI (annual probability of 0.1) for checking drift under wind loads for typical buildings (Griffis 1993); others have used a 50-year MRI (annual probability of 0.02) or a 100-year MRI (annual probability of 0.01) for more drift-sensitive buildings. As a point of reference, the 2005 edition of this standard specified a 50-year MRI for determining the strength-level wind pressures in regions that are not hurricane-prone. It was then suggested [Equation (CC-3) of the 2005 edition] that the strength-level wind pressures could be multiplied by 0.70 when determining wind pressures for serviceability drift calculations. This 0.70 multiplication of the pressure resulted in an approximate 10-year MRI. The selection of the MRI for serviceability evaluation is a matter of engineering judgment that should be exercised in consultation with the design team and building owner.
The maps included in this appendix are appropriate for use with serviceability limit states and should not be used for strength limit states. Because of its transient nature, wind load need not be considered in analyzing the effects of creep or other long-term actions.
Deformation limits should apply to the structural assembly as a whole. The stiffening effect of nonstructural walls and partitions may be taken into account in the analysis of drift if substantiating information regarding their effect is available. Where load cycling occurs, under wind effects such as vortex shedding or ratcheting from inelastic frame behavior, consideration should be given to the possibility that increases in residual deformations may lead to incremental structural collapse.
CC.2.3 Vibrations
Structural motions of floors or of the building as a whole can cause the building occupants discomfort. In recent years, the number of complaints about building vibrations has been increasing. This increasing number of complaints is associated in part with the more flexible structures that result from modern construction practice. Traditional static deflection checks are not sufficient to prevent annoying vibrations of building floor systems or buildings as a whole (Ad Hoc Committee on Serviceability Research 1986). Control of stiffness is one aspect of serviceability, but mass distribution and damping are also important in controlling vibrations. The use of new materials and building systems may require that the dynamic response of the system be considered explicitly. Simple dynamic models often are sufficient to determine whether there is a potential problem and to suggest possible remedial measurements (Bachmann and Ammann 1987, Ellingwood 1989).
Excessive structural motion is mitigated by measures that limit building or floor accelerations to levels that are not disturbing to the occupants or do not damage service equipment. The perception and tolerance of individuals to vibration depend on their expectation of building performance (related to building occupancy) and to their level of activity at the time the vibration occurs (ANSI 1983). Individuals find continuous vibrations more objectionable than transient vibrations. Continuous vibrations (over a period of minutes) with acceleration on the order of 0.005 g to 0.01 g are annoying to most people engaged in quiet activities, whereas those engaged in physical activities or spectator events may tolerate steady-state accelerations on the order of 0.02 g to 0.05 g. Thresholds of annoyance for transient vibrations (lasting only a few seconds) are considerably higher and depend on the amount of structural damping present (Murray 1991). For a finished floor with (typically) 5% damping or more, peak transient accelerations of 0.05 g to 0.1 g may be tolerated.
Many common human activities impart dynamic forces to a floor at frequencies (or harmonics) in the range of 2 Hz to 6 Hz (Allen and Rainer 1976; Allen et al. 1985; Allen 1990a, b). If the fundamental frequency of vibration of the floor system is in this range and if the activity is rhythmic in nature (e.g., dancing, aerobic exercise, or cheering at spectator events), resonant amplification may occur. To prevent resonance from rhythmic activities, the floor system should be tuned so that its natural frequency is well removed from the harmonics of the excitation frequency. As a general rule, the natural frequency of structural elements and assemblies should be greater than 2.0 times the frequency of any steady-state excitation to which they are exposed unless vibration isolation is provided. Damping is also an effective way of controlling annoying vibration from transient events because studies have shown that individuals are more tolerant of vibrations that damp out quickly than those that persist (Murray 1991).
Several studies have shown that a simple and relatively effective way to minimize objectionable vibrations from walking and other common human activities is to control the floor stiffness, as measured by the maximum deflection independent of span. Justification for limiting the deflection to an absolute value rather than to some fraction of span can be obtained by considering the dynamic characteristics of a floor system modeled as a uniformly loaded simple span. The fundamental frequency of vibration, , of this system is given by
Flexural rigidity of the floor;
Span;
ρ
Mass per unit length = w/g,
where g = Acceleration due to gravity = (32.17 ft/s2) (9.81 m/s2), and
Dead load plus participating live load.
The maximum deflection caused by is
Substituting from this equation into Equation (CC.2-3), we obtain
This frequency can be compared to minimum natural frequencies for mitigating walking vibrations in various occupancies (Allen and Murray 1993). For example, Equation (CC.2-6) indicates that the static deflection caused by uniform load, , must be limited to about 0.2 in. (5 mm), independent of span, if the fundamental frequency of vibration of the floor system is to be kept above about 8 Hz. Many floors that do not meet this guideline are perfectly serviceable; however, this guideline provides a simple means for identifying potentially troublesome situations where additional consideration in design may be warranted.
CC.3 Design for Long-Term Deflection
Under sustained loading, structural members may exhibit additional time-dependent deformations caused by creep, which usually occur at a slow but persistent rate over long periods of time. In certain applications, it may be necessary to limit deflection under long-term loading to specified levels. This limitation can be done by multiplying the immediate deflection by a creep factor, as provided in material standards, that ranges from about 1.5 to 2.0. This limit state should be checked using the load combination in Equation (CC.2-2).
CC.4 Camber
Where required, camber should be built into horizontal structural members to give proper appearance and drainage and to counteract anticipated deflection from loading and potential ponding.
CC.5 Expansion and Contraction
Provisions should be made in design so that if significant dimensional changes occur, the structure will move as a whole and differential movement of similar parts and members that meet at joints will be at a minimum. Design of expansion joints to allow for dimensional changes in portions of a structure separated by such joints should take both reversible and irreversible movements into account. Structural distress in the form of wide cracks has been caused by restraint of thermal, shrinkage, and prestressing deformations. Designers are advised to provide for such effects through relief joints or by controlling crack widths.
CC.6 Durability
Buildings and other structures may deteriorate in certain service environments. This deterioration may be visible on inspection (e.g., as weathering, corrosion, and staining) or may result in undetected changes in the material. The designer should either provide a specific amount of damage tolerance in the design or specify adequate protection systems and/or planned maintenance to minimize the likelihood that such problems will occur. Water infiltration through poorly constructed or maintained wall or roof cladding is considered beyond the realm of designing for damage tolerance. Waterproofing design is beyond the scope of this standard. For portions of buildings and other structures exposed to weather, the design should eliminate pockets in which moisture can accumulate.
Appendix CC – References
Other References (not cited)