STRC17 Lateral Forces Wind Loads 0716
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Descripción: PPI2PASS SE Exam Review Course Fall 2016 Lecture 17 Structural Engineering Course...
Description
Structural Engineering Exam Review Course
Lateral Forces: Wind Loads
Wind Loads Structural Engineering Review Course
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Structural Engineering Exam Review Course
Lateral Forces: Wind Loads
Wind Loads
Lesson Overview Chapter 7: Lateral Forces • Lateral‐Force Resisting Systems • Seismic Design • Wind Design
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Learning Objectives You will learn • simple approximations for fundamental period of vibration of typical structures
• how to navigate ASCE/SEI7 and IBC design codes for wind loads
• how to calculate wind loads using methods from ASCE/SEI7 and IBC.
• use of tables and figures
• how to distribute wind loads to typical building structures
• choose variables • apply minimum load limits • interpret of important text
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Prerequisite Knowledge and Skills You should already be familiar with • layout of ASCE/SEI7 and IBC • load application by tributary areas
• typical LFRS (braced frames, moment frames, shear walls, etc.) • typical building components (braces, beams, trusses, etc.)
• linear interpolation • calculating weighted averages
• roof types (flat, gable, hip, etc.)
• common terms for wind loading (ex: windward, leeward, etc.)
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Referenced Codes and Standards • Minimum Design Loads for Buildings and Other Structures (ASCE/SEI7, 2010) • International Building Code (IBC, 2012)
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Design Procedures envelope procedure • wind loads determined independent of direction • external pressure coefficients envelop minimum and maximum values for all possible directions directional procedure • wind loads determined for specific directions • external pressure coefficients chosen based on wind tunnel testing
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Important Terms main wind force‐resisting system (MWFRS) • assemblage of structural elements assigned to provide support and maintain stability of overall structure, e.g., moment frames, shear walls, etc. • generally receives wind loading from more than one surface components and cladding (C&C) • elements of building envelope that do not qualify as part of MWFRS, e.g., façade components, roof framing, fasteners, etc.
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Important Terms building cladding C&C elements that receive wind loads directly, e.g., wall and roof sheathing, windows, doors, etc. building components C&C elements that receive wind loading from the building cladding and transfer the load to the MWFRS, e.g., purlins, studs, girts, fasteners, roof trusses, etc.
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Poll: MWFRS and C&C In the two‐story braced frame structure shown, the braced frames resist all lateral loads on the structure. How should the diagonal braces on the first floor level be classified? (A) main wind force‐resisting system (B) components and cladding
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Poll: MWFRS and C&C In the two‐story braced frame structure shown, the braced frames resist all lateral loads on the structure. How should the diagonal braces on the first floor level be classified? (A) main wind force‐resisting system (B) components and cladding The answer is (A).
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Poll: MWFRS and C&C In the two‐story braced frame structure shown, the braced frames resist all lateral loads on the structure. How should the spandrel beam on the first floor level be classified? (A) main wind force‐resisting system (B) components and cladding
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Lateral Forces: Wind Loads
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Poll: MWFRS and C&C In the two‐story braced frame structure shown, the braced frames resist all lateral loads on the structure. How should the spandrel beam on the first floor level be classified? (A) main wind force‐resisting system (B) components and cladding The answer is (B).
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Enclosure Classification enclosure classification (ASCE/SEI7 Sec. 26.2) • required to determine structure type and for several wind load calculations • three enclosure classifications based on number of openings in building envelope openings (ASCE/SEI7 Sec. 26.2) • apertures or holes in building envelope that allow air to flow through envelope (cladding, roofing, exterior walls, glazing, doors, etc.) • Exterior doors, windows, skylights, and other apertures or holes that can be open or closed should be considered as both open and closed for the purpose of wind load analysis.
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Enclosure Classification open building area of each wall is at least 80% openings
Figure 7.32 Building Openings
A0 0.8 Ag
Ao = total area of openings in a wall receiving positive external pressure Ag = gross area of wall in which Ao is identified
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Enclosure Classification partially enclosed building Figure 7.32 Building Openings
must fulfill two conditions
Aoi = sum of areas of all openings in building envelope except Ao Agi = sum of gross areas of building envelope, excluding gross area of wall represented by Ag
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Enclosure Classification enclosed building • does not comply with requirements for open or partially enclosed buildings
Figure 7.32 Building Openings
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Building Types simple diaphragm building wind loads are transferred to the MWFRS by either vertically spanning wall assemblies or continuous floor and roof diaphragms, for both windward and leeward walls regular‐shaped building no unusual geometric or special irregularities
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Building Types low rise building (ASCE/SEI7 Sec. 26.2) • enclosed or partially enclosed • mean roof height, h ≤ 60 ft • h ≤ least horizontal dimension (plan width or length)
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Example: Low‐Rise Buildings Does the enclosed building shown qualify as a low‐rise building per ASCE/SEI7 requirements?
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Example: Low‐Rise Buildings The building is enclosed, so it fulfills the enclosure requirement for a low‐rise building. Find the mean roof height.
80 ft 52 ft 160 ft 64 ft 240 ft 60 ft 60 ft, OK
The building fulfills the first mean roof height requirement.
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Example: Low‐Rise Buildings h ≤ shortest horizontal dimension. h = 60 ft The building’s shortest horizontal dimension is 80 ft. 60 ft ≤ 80 ft, so the building fulfills the second mean roof height requirement. Since the building satisfies all ASCE/SEI7 requirements, it qualifies as a low‐rise building.
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Fundamental Period of Vibration approximate fundamental period of vibration, Ta • two methods for calculating given in ASCE/SEI7 Sec. 12.8.2 method 1 • Ta = 0.1N
ASCE/SEI7 Eq. 12.8‐8
• applies only to structures that fulfill these requirements: • ≤ 12 stories above the base (ASCE/SEI7 Sec. 11.2) • average story height ≥10 ft • MWFRS consists entirely of concrete or steel moment frames
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Fundamental Period of Vibration method 2 From ASCE/SEI7 Eq. 12.8‐7 and Table 12.8‐2,
hn structural height
ASCE/SEI7 Sec. 11.2
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Building Types rigid building • fundamental frequency (fundamental period of vibration) ≥ 1 Hz • alternatively, building with ratio of height and minimum width ≤ 4 flexible building • fundamental frequency (fundamental period of vibration) 60 ft
ASCE/SEI7 Sec. 30.7
simplified directional
enclosed buildings with h ≤ 160 ft
ASCE/SEI7 Sec. 30.8
analytical directional
open buildings (all heights)
IBC Sec. 1609.6
alternate IBC method
simple diaphragm buildings with h ≤ 76 ft and height‐to‐least‐width ratio ≤ 4
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Wind Load Parameters The following parameters are used in one or more procedures to determine wind loads.
• wind directionality factor • wind velocity pressure • minimum design wind loads
• surface roughness category
• gust effect factor
• site exposure category • risk category and basic wind speed at location of structure • velocity pressure exposure coefficient
• enclosure classification • internal/external pressure coefficients • others for specific methods/situations
• topographic factor
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Surface Roughness Category surface roughness
Table 7.10 Surface Roughness Categories
• categories defined in ASCE/SEI7 Sec. 26.7.2 • accounts for geometric effects that impede flow (more turbulence results in a less streamlined airflow)
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Site Exposure Category site exposure category
Table 7.11 Site Exposure Category
• defined in ASCE/SEI7 Sec. 26.7.3 • illustrated in Sec. C26.7 • accounts for surface roughness and building height
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Risk Category & Basic Wind Speed Risk categories are defined in ASCE/SEI7 Table 1.5‐1.
Table 7.12 Risk Category and Wind Speed Maps
Use wind speed maps to determine basic wind speed.
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Velocity Pressure Exposure Coefficient (MWFRS) • represented by Kz • reflects change in wind speed with height and exposure category
Table 7.13 Velocity Pressure Exposure Coefficients For Main Wind Force‐Resisting Systems
• given in ASCE/SEI7 Table 27.3‐1 and Table 28.3‐1 • For values of height not listed, Kz can be calculated by linear interpolation.
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Example: Velocity Pressure Exposure Coefficient A low‐rise building with a 38 ft high roof is located in exposure category B. What is the velocity pressure coefficient at the roof level of the structure?
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Example: Velocity Pressure Exposure Coefficient A low‐rise building with a 38 ft high roof is located in exposure category B. What is the velocity pressure coefficient at the roof level of the structure?
Interpolate the exposure coefficients for a 38 ft high structure from ASCE/SEI7 Table 27.3‐1. The exposure coefficient for a building in exposure category B is 0.70 at a height of 30 ft and 0.76 at a height of 40 ft, so at a height of 38 ft above ground level, the exposure coefficient is 0.76 0.70 K z 0.70 38ft 30ft 40 ft 30 ft 0.748
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Topographic Effects • Increased wind speed effects are produced at abrupt changes in general topography (isolated hills, ridges, escarpments, etc.). • accounted for by multiplying velocity pressure coefficient by topographic factor, Kzt
• topographic factor is function of three criteria • slope of hill • distance of building from crest • height of building above local ground surface
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Topographic Factor • criteria represented by topographic multipliers, K1 , K2 , K3 (given in ASCE/SEI7 Fig. 26.8‐1) • topographic factor given by
ASCE/SEI7 Eq. 26.8‐1
• when topography effects need not be considered, Kzt = 1.0
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Wind Directionality Factor • represented as Kd • accounts for reduced probability of • extreme winds in any specific direction • peak pressure coefficient occurring for any specific wind direction • determined from ASCE/SEI7 Table 26.6‐1 • for building structures, Kd = 0.85
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Wind Velocity Pressure • qz = wind velocity pressure at an arbitrary height z • found using ASCE/SEI7 Eq. 28.3‐1 2 qz ,lbf/ft 2 0.00256 K z K zt K dVmi/hr
Kz = velocity pressure exposure coefficient Kzt = topographic factor Kd = directionality factor V = basic wind speed • velocity pressure varies as velocity pressure exposure coefficient varies with height above ground STRC ©2015 Professional Publications, Inc.
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Example: Wind Velocity Pressure Example 7.25
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Lateral Forces: Wind Loads
Wind Loads
Example: Wind Velocity Pressure
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Lateral Forces: Wind Loads
Wind Loads
Example: Wind Velocity Pressure
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Example: Wind Velocity Pressure
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Gust Effect Factor • represented by Gf • accounts for loading effects in direction of wind (along‐wind loading effects) caused by dynamic amplification in flexible structures, and for interaction between structure and wind turbulence • for rigid structures, Gf = 0.85 • alternatively, Gf calculated using procedure summarized in ASCE/SEI7 Sec. 26.9.5
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Internal Pressure Coefficients • (GCpi) = combination of gust effect factor and internal pressure coefficient
Table 7.14 Values of External Pressure Coefficients
• values of (GCpi) tabulated in ASCE/SEI7Table 26.11‐1 for all three enclosure classifications
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Internal Pressure Coefficients • pi = pressure acting on internal surfaces
Table 7.14 Values of External Pressure Coefficients
• found from second term of ASCE/SEI7 Eq. 28.4‐1 pi qh GC pi
• positive acting toward surface, negative acting away from surface (consider both cases to determine worst case)
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External Pressure Coefficients • Local turbulence at building corners and roof eaves produces local increases in wind pressures. • To account for this, envelope procedure subdivides building surface into distinct zones • 8 zones for transverse wind loads • 12 zones for longitudinal wind loads • external pressure coefficients tabulated for each zone
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External Pressure Coefficients • (GCpf) = combination of gust effect factor and external pressure coefficient • values of (GCpf) tabulated in ASCE/SEI7 Fig. 28.4‐1 • values given for two load cases • case A: wind acting transversely • case B: wind acting longitudinally
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External Pressure Coefficients Figure 7.33 Load Case A and Load Case B (partial figure) Table 7.15 External Pressure Coefficients for Load Case A
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External Pressure Coefficients Figure 7.33 Load Case A and Load Case B (partial figure) Table 7.16 External Pressure Coefficients for Load Case B
Adapted with permission from Minimum Design Loads for Buildings and Other Structures, Fig. 28.4‐1, copyright © 2010, by the American Society of Civil Engineers
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External Pressure Coefficients pressure acting on external surfaces,
• a cannot be less than either of
ASCE/SEI7 Eq. 28.4‐1
external pressure coefficients given for two zones on each wall and roof surface
• interior zone given by ASCE/SEI7 Fig. 28.4‐1
• end zone width: given by ASCE/SEI7 Fig. 28.4‐1, Note 9 as 2a where a is lesser of
• pressures act normal to wall and roof surfaces • (+) toward the surface and (‐) away from the surface – consider both cases
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Minimum Design Wind Loads minimum design wind loads for enclosed or partially enclosed buildings (ASCE/SEI7 Sec. 27.1.5; shown in Fig. C27.4‐1) • refer to minimum total load resisted by MWFRS (not minimum wind pressures) • given in pressures to be applicable to various building geometries • net pressure on windward wall areas ≥ 16 lbf/ft2 • net pressure on windward roof areas ≥ 8 lbf/ft2 (projected onto vertical plane normal to wind direction) • applied simultaneously to roof and walls as applicable • applied as separate load case in addition to normal load cases specified STRC ©2015 Professional Publications, Inc.
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Minimum Design Wind Loads Fig. 7.31 Minimum Design Wind Loads
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Design Wind Load Cases • in envelope procedure, building designed for all wind directions • Consider each corner of building as windward corner. • Consider wind acting in both the transverse and longitudinal direction. • eight basic load cases (4 windward corners × 2 wind directions = 8 cases)
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Design Wind Load Cases • External and internal pressures must be considered for each of these load cases. • 16 combinations should be considered. (8 basic load cases × 2 internal pressure directions = 16 combinations) • If building symmetrical about an axis, only two corners need be investigated. (8 basic load cases) • If building symmetrical about two axes, only one corner need be investigated. (4 basic load cases)
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Design Wind Load Cases • When torsion must be considered, each load case needs to be modified per ASCE/SEI7 Fig. 28.4‐1, Note 5. • Torsion need not be considered when any the following conditions apply. • one‐story building with h
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