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1、Serviceability Limit States Under Wind Load LAWRENCE G. GRIFFIS INTRODUCTION The increasing use and reliance on probability based limit states design methods, such as the recently adopted AISC LRFD Specification,1 has focused new attention on the problems of serviceability in steel buildings. These
2、methods, along with the development of higher-strength steels and concretes and the use of lighter and less rigid building materials, have led to more flexible and lightly damped structures than ever before, making serviceability problems more prevalent. The purpose of this paper is to focus attenti
3、on on two important serviceability limit states under wind loads; namely, deformation (including deflection, curvature, and drift) and motion perception (acceleration). These issues are particularly important for tall and/or slender steel and composite structures. A brief review of available informa
4、tion on these subjects will be presented followed by a discussion of current standards of practice, particularly in the United States. Finally, proposed standards will be presented that, hopefully, will focus attention, debate, and perhaps new research efforts on these very important issues in desig
5、n. IMPORTANCE OF SERVICEABILITY LIMIT STATES 12,31 Every building or other structure must satisfy a strength limit state, in which each member is proportioned to carry the design loads to resist buckling, yielding, instability, fracture, etc.; and serviceability limit states which define functional
6、performance and behavior under load and include such items as deflection, vibration, and corrosion. In the United States, strength limit states have traditionally been specified in building codes because they control the safety of the structure. Serviceability limit states, on the other hand, are us
7、ually noncatastrophic, define a level of quality of the structure or element, and are a matter of judgment as to their application. Serviceability limit states involve the perceptions and expectations of the owner or user and are a contractual matter between the owner or user and the designer and bu
8、ilder. It is for these reasons, and because the benefits themselves are often subjective and difficult to define or quantify, that serviceability limit states for the most part are Lawrence G. Griffis is Senior Vice President and Director of Structural Engineering for Walter P. Moore and Associates,
9、 Houston, TX. not included within U.S. building codes. The fact that serviceability limit states are usually not codified should not diminish their importance. Exceeding a serviceability limit state in a building or other structure usually means that its function is disrupted or impaired because of
10、local minor damage, deteriorations, or because of occupant discomfort or annoyance. While safety is usually not at issue, the economic consequences can be substantial. Interestingly, there are some serviceability items that can also be safety related. For instance, excessive building drift can influ
11、ence frame stability because of the P- effect. Excessive building drift can also cause portions of the building cladding to fall and potentially injure pedestrians below. Serviceability limit states can be grouped into three categories as follows: 1.Deformation (deflection, curvature, drift). Common
12、 examples include local damage to nonstructural elements (e.g., ceilings, cladding, partitions) due to deflections under dead, live, wind, or seismic load; and damage from temperature change, moisture, shrinkage, or creep. 2.Motion perception (vibration, acceleration). Common examples include human
13、discomfort caused by wind or machinery, particularly if resonance occurs. Floor vibrations from people or machinery and acceleration in tall buildings under wind load are usual areas of concern in this category. 3.Deterioration. Included are such items as corrosion, weathering, efflorescence, discol
14、oration, rotting, and fatigue. The focus on this paper will be items one and two. CURRENT TREATMENT OF SERVICEABILITY ISSUES IN U.S. CODES A review of the three model building codes3,29,35 in the United States reveals a somewhat inconsistent and haphazard approach to serviceability issues. For insta
15、nce, it is implied that the codes exist strictly to protect life safety of the general public. Yet, traditionally they have contained provisions for deflection control of floor members while ignoring provisions for other member types (columns, walls, mullions, etc.). No mention is made of limits for
16、 wind drift, vibration, expansion and contraction (expansion joint guidelines), or corrosion. The authors work in professional committees and code bodies, coupled with a review of recent surveys of the profe- FIRST QUARTER / 19931 2003 by American Institute of Steel Construction, Inc. All rights res
17、erved. This publication or any part thereof must not be reproduced in any form without the written permission of the publisher. ssion36 seem to reveal a reluctance of engineers to codify serviceability issues. This reluctance probably stems in part on differences of opinion as to the purpose of buil
18、ding codes (i.e., protection for life safety exclusively or establishment of complete minimum design standards including strength and serviceability), but also a genuine concern for restricting design options, stifling creativity, and removing the all- important concept of “engineering judgment“ fro
19、m the solution to the problem. There is also the belief, rightly so, that too little hard data exists to justify rigid standards on most serviceability issues. It is important that engineers recognize these problems and begin to focus on the solution of serviceability related design issues. The reas
20、on for doing so is the large economic impact that serviceability items are having on the operational costs of buildings. MEAN RECURRENCE INTERVAL WIND LOADS FOR SERVICEABILITY DESIGN The first step in establishing a serviceability design criterion is to define the load under which it is to be checke
21、d. Wind loading criteria for strength limit states in the United States are normally based on a 50-year mean recurrence interval for normal buildings and a 100-year mean recurrence interval for critical structures. There seems to be a general consensus that basing serviceability criteria on such a s
22、evere loading that may occur only once, on the average, during the lifetime of the structure is unrealistic and too stringent a standard to apply. The average tenant occupancy in office buildings has been defined as eight years.26 It seems reasonable to base serviceability criteria on a mean recurre
23、nce interval more in this range of time because the consequences of exceeding a serviceability limit state are usually not safety related. Various researchers have suggested mean recurrence intervals of from five to ten years for serviceability issues.10,11,12,14,17,18,19,20,33,36 If no permanent da
24、mage results from exceeding the serviceability limit, some researchers have also suggested selecting serviceability criteria (such as floor deflection) on an annual basis.14 A wind load for a mean recurrence interval of 10 years is recommended for checking the two wind serviceability limit states de
25、fined herein (deformation and motion perception). This corresponds to a 10 percent probability of being exceeded in any given year. While it has become standard practice to base building accelerations under wind load on this mean recurrence interval, drift criteria typically have been formulated aro
26、und the same mean recurrence interval (50 years or 100 years) as the strength limit state.36 The proposed 10-year mean recurrence interval compares to five years as proposed in ISO Standard 6897-1984, 10 years as proposed by the National Building Code of Canada (1990), 20 years in the Australian Sta
27、ndard AS 1170.2-1989 and 0.1 years as proposed by the Japanese.28 BUILDING DRIFTSTANDARD OF PRACTICE Serviceability of buildings under wind loads has traditionally been checked in the design office by evaluation of the lateral frame deflection calculated on the basis of a statically applied wind loa
28、d obtained from the local building code. The magnitude of the wind load is usually the same as that used in proportioning the frame for strength and typically is based on a 50-year or 100-year mean recurrence interval load. Sometimes, an arbitrary wind load (i.e., 20 PSF above 100 ft, 0 (zero) PSF b
29、elow 100 ft as has been used in New York City on the design of some buildings15) is used in the serviceability check. This serviceability check, for all but the tallest and most slender of buildings (where wind tunnel studies are utilized), has been used to prevent damage to collateral building mate
30、rials, such as cladding and partitions, and also to control the perception of building motion. None of the three national building codes in the United States specify a limit to lateral frame deflection under wind load. The degree of this serviceability check is left to the judgment of the design eng
31、ineer. Lateral frame deflection is usually evaluated for the building as a whole, where the applicable parameter is total building drift, defined as the lateral frame deflection at the top-most occupied floor divided by the height from grade to the uppermost floor (/ H); and for each floor of the bu
32、ilding, where the applicable parameter is interstory drift, defined as the lateral deflection of a floor relative to the one immediately below it divided by the distance between floors (nn1)/ h). Typical values of this parameter (commonly called drift index) used in this serviceability check are H /
33、 100 to H / 600 for total building drift and h / 200 to h / 600 for interstory drift depending on building type and materials used. The most widely used values are 1 / 400 to 1 / 500.36 Lateral frame deflections have historically been based on a first order analysis. DRIFTA REVISED DEFINITION 7 Drif
34、t Measurement Index (DMI) If the goal in defining a drift limit is limited to only the control of damage to collateral building elements, such as cladding and partitions, and is separated from the problem of building motion, then frame racking or shear distortion (strain) is the logical parameter to
35、 evaluate. Mathematically, if the local x, y displacements are known at each corner of an element or panel, then the overall average shear distortion for rectangular panel ABCD as shown in Figure 1 may be termed the drift measurement index (DMI) and defined as follows: Drift measurement index (DMI)
36、= average shear distortion DMI = 0.5 (XA XC) / H + (XB XD) / H + (YD YC) / L + (YB YA) / L DMI = 0.5 (D1 + D2 + D3 + D4) 2ENGINEERING JOURNAL / AMERICAN INSTITUTE OF STEEL CONSTRUCTION 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof mu
37、st not be reproduced in any form without the written permission of the publisher. where, Xi= vertical displacement of point i Yi= lateral displacement of point i D1 = (XA XC)/H, horizontal component of racking drift D2 = (XB XD)/H, horizontal component of racking drift D3 = (YD YC)/L, vertical compo
38、nent of racking drift D4 = (YB YA)/L, vertical component of racking drift It is to be noted that terms D1 and D2 are the horizontal components of the shear distortion or frame racking and are the familiar terms commonly referred to as interstory drift. The terms D3 and D4 are the vertical components
39、 of the shear distortion or frame racking caused by axial deformation of adjacent columns. If it can be accepted that the DMI is the true measure of potential damage, then it becomes readily apparent that the evaluation of interstory drift alone can be misleading in obtaining a true picture of poten
40、tial damage. Interstory drift alone does not account for the vertical component of frame racking in the rectangular panel that also contributes to the potential damage, nor does it exclude rigid body rotation of the rectangular panel which, in itself, does not contribute to damage. It can be shown t
41、hat evaluation of the commonly used interstory drift can significantly underestimate the damage potential in a combined shear wall/frame type building where the vertical component of frame racking can be important; and significantly overestimate the damage potential in a shear wall or braced frame b
42、uilding where large rigid body rotation of a story can occur due to axial shortening of columns.7 Consider for example, the eight-story building shown in Figure 2. This frame represents a typical windframe that may be found in any office building with 36-ft lease depths (building perimeter to center
43、 core) and a central core containing elevator, stairs, etc. The frame shown consists of a combined moment frame and X-braced frame. Figure 3 shows a plot (exaggerated) of the deflected shape of the top level Fig. 1. Drift measurement index (DMI) under wind loads. Table 1 shows calculations for the t
44、raditional story drift and the revised drift definition DMI. The significant thing to note is that the potential damaging deformations, as represented by the DMIs, are more severe in the external bays (panels 1, 3) and much less severe in the internal bay (panel 2) than predicted by the traditional
45、story drift calculation. Most of the deformation in the center bay (panel 2) is simply rigid body rotation that, by itself, is not damaging to partitions. Drift Measurement Zone (DMZ) It is logical to identify the rectangular panel ABCD in Figure 1 as the zone in which the damage potential is to be
46、evaluated and define it the drift measurement zone (DMZ). From a practical standpoint, these zones will typically represent column bays within a building and would be incorporated as part of the building frame analysis. Drift Damage Index (DDI) Once the determination of the shear distortion or drift
47、 measurement index (DMI) is made for different column bays or drift measurement zones (DMZs), it must be compared to a damage threshold value for the element being protected. These damage threshold limits can be defined as the shear distortion or racking that produces the maximum amount of cracking
48、or distress that can be accepted, on the average, once every 10 years. It is logical to define these damage thre- Figure 2 Figure 3 FIRST QUARTER / 19933 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any for
49、m without the written permission of the publisher. Table 1. Drift Comparison D1D2D3D4DriftDMIDMI/Story Drift Panel 10.001010.001040.0002200.0002150.001010.00125001.23 Panel 20.001040.001040.0010300.0010200.001010.00001860.02 Panel 30.001040.001010.0002140.0002090.001010.00124001.22 shold shear distortions as the drift damage index (DDI). From the standpoint of serviceability limit states it is necessary to observe the following inequality: drift measurementindex drift damage index DMI DDI A significant body of information is available from racking te
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