STRC17 Lateral Forces Wind Loads 0716

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Wind Loads

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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 calculate wind loads using methods from ASCE/SEI7 and IBC. • how to distribute wind loads to typical building structures • how to navigate ASCE/SEI7 and IBC design codes for wind loads • choose variables • use of tables and figures • 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 • linear interpolation • calculating weighted averages • common terms for wind loading (ex: windward, leeward, etc.) • typical LFRS (braced frames, moment frames, shear walls, etc.) • typical building components (braces, beams, trusses, etc.) • roof types (flat, gable, hip, 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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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 A_o = total area of openings in a wall receiving positive external pressure A_g = gross area of wall in which A_o is identified Figure 7.32 Building Openings

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0 0.8 g A  A

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Enclosure Classification

partially enclosed building must fulfill two conditions A_oi = sum of areas of all openings in building envelope except A_o A_gi = sum of gross areas of building envelope, excluding gross area of wall represented by A_g Figure 7.32 Building Openings

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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. The building fulfills the first mean roof height requirement.

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80 ft 52 ft 160 ft 64 ft 240 ft 60 ft 60 ft, OK   

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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, T_a • two methods for calculating given in ASCE/SEI7 Sec. 12.8.2 method 1 • T_a = 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,

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structural height n h  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) < 1 Hz • may exhibit significant resonant response to wind gusts • require additional consideration for wind load • often require wind tunnel testing (ASCE/SEI7 Chap. 31)

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Example: Fundamental Period of Vibration

Example 7.10

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Example: Fundamental Period of Vibration

Example 7.10

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Example: Fundamental Period of Vibration

Example 7.11

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Commentary on Examples

• Methods 1 and 2 may produce approximate fundamental periods that are significantly different. • In the previous two examples, method 2 produces a value 71% greater than the value produced by method 1 with no change in available data. • Method 2 tends to be more accurate since it accounts more directly for construction type and building height.

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Wind Load Calculation Methods

• Select (but not all) wind calculation methods will be covered with examples provided to demonstrate the process • ASCE/SEI7 methods are summarized in Sec. 26.1.2.1. Refer to the applicable section for design (e.g., ASCE/SEI7 Chap. 27 for Directional Procedure). • Table in each section provides step‐by‐step guidance for applying specific design method. Use these tables as guide to quickly and efficiently solve problems. • IBC Sec. 1609.6: IBC alternate method

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Directional Procedure

analytical directional design method (ASCE/SEI7 Sec. 27.4) applies to enclosed, partially enclosed, and open buildings of all heights and roof geometries simplified method (ASCE/SEI7 Sec. 27.5) applies to enclosed simple diaphragm buildings of any roof geometry with h ≤ 160 ft

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Envelope Procedure for Low‐Rise Buildings

envelope procedure (ASCE/SEI7 Chap. 28) applies to • enclosed, partially enclosed and open buildings (all heights and roof geometries) • low‐rise buildings • structures with flat, gable and hip roofs

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Directional Procedure for “Other Structures”

directional procedure (ASCE/SEI7 Chap. 29) applicable for “other structures” • solid freestanding walls and signs • chimneys, towers, tanks • lattice frameworks, trussed towers • rooftop structures and equipment

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Wind Tunnel Procedure

wind tunnel procedure (ASCE/SEI7 Chap. 31) • applicable for any building or structure • used in cases where one or more atypical conditions are expected • across‐wind loading • vortex shedding • instability due to galloping or flutter • ASCE/SEI7 Sec. 31.2 gives test conditions. • most accurate method

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section design method applies to

ASCE/SEI7 Sec. 30.4 analytical envelope enclosed and partially enclosed low‐rise buildings ASCE/SEI7 Sec. 30.5 simplified envelope enclosed low‐rise buildings

ASCE/SEI7 Sec. 30.6 analytical directional enclosed and partially enclosed buildings with h > 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

Wind Load Calculation Methods (C&C)

Wind loads for components and cladding are determined by the procedures summarized in ASCE/SEI7 Chap. 30 and IBC Sec. 1609.6.

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Wind Load Parameters

The following parameters are used in one or more procedures to determine wind loads. • surface roughness category • site exposure category • risk category and basic wind speed at location of structure • velocity pressure exposure coefficient • topographic factor • wind directionality factor • wind velocity pressure • minimum design wind loads • gust effect factor • enclosure classification • internal/external pressure coefficients • others for specific methods/situations

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Surface Roughness Category

surface roughness • categories defined in ASCE/SEI7 Sec. 26.7.2 • accounts for geometric effects that impede flow (more turbulence results in a less streamlined airflow) Table 7.10 Surface Roughness Categories

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Site Exposure Category

site exposure category • defined in ASCE/SEI7 Sec. 26.7.3 • illustrated in Sec. C26.7 • accounts for surface roughness and building height Table 7.11 Site Exposure Category

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Risk Category & Basic Wind Speed

Risk categories are defined in ASCE/SEI7 Table 1.5‐1. Use wind speed maps to determine basic wind speed. Table 7.12 Risk Category and Wind Speed Maps

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Velocity Pressure Exposure Coefficient (MWFRS)

• represented by K_z • reflects change in wind speed with height and exposure category • given in ASCE/SEI7 Table 27.3‐1 and Table 28.3‐1 • For values of height not listed, K_z can be calculated by linear interpolation. Table 7.13 Velocity Pressure Exposure Coefficients For Main Wind Force‐Resisting Systems

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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

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0.76 0.70 0.70 38ft 30ft 40 ft 30 ft 0.748 z K    _  _    

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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, K_zt • 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 • when topography effects need not be considered, K_zt= 1.0

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ASCE/SEI7 Eq. 26.8‐1

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Wind Directionality Factor

• represented as K_d • 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, K_d= 0.85

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Wind Velocity Pressure

• q_z = wind velocity pressure at an arbitrary height z • found using ASCE/SEI7 Eq. 28.3‐1 • velocity pressure varies as velocity pressure exposure coefficient varies with height above ground

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2 2 mi/hr ,lbf/ft 0.00256 z zt d z q  K K K V K_z = velocity pressure exposure coefficient K_zt = topographic factor K_d = directionality factor V = basic wind speed

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Example: Wind Velocity Pressure

Example 7.25

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Example: Wind Velocity Pressure

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Example: Wind Velocity Pressure

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Example: Wind Velocity Pressure

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Gust Effect Factor

• represented by G_f • 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, G_f = 0.85 • alternatively, G_fcalculated using procedure summarized in ASCE/SEI7 Sec. 26.9.5

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Internal Pressure Coefficients

• (GC_pi) = combination of gust effect factor and internal pressure coefficient • values of (GC_pi) tabulated in ASCE/SEI7Table 26.11‐1 for all three enclosure classifications Table 7.14 Values of External Pressure Coefficients

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Internal Pressure Coefficients

• p_i= pressure acting on internal surfaces • found from second term of ASCE/SEI7 Eq. 28.4‐1 • positive acting toward surface, negative acting away from surface (consider both cases to determine worst case) Table 7.14 Values of External Pressure Coefficients

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i h pi p  q GC

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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

• (GC_pf) = combination of gust effect factor and external pressure coefficient • values of (GC_pf) 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

Table 7.15 External Pressure Coefficients for Load Case A

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Figure 7.33 Load Case A and Load Case B (partial figure)

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External Pressure Coefficients

Table 7.16 External Pressure Coefficients for Load Case B

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Figure 7.33 Load Case A and Load Case B (partial figure) 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, external pressure coefficients given for two zones on each wall and roof surface • end zone width: given by ASCE/SEI7

Fig. 28.4‐1, Note 9 as 2a where a is lesser of • acannot be less than either of • interior zone given by ASCE/SEI7 Fig. 28.4‐1 • pressures act normal to wall and roof surfaces • (+) toward the surface and (‐) away from the surface – consider both cases

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ASCE/SEI7 Eq. 28.4‐1

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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

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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 < 30 ft • two or fewer stories with light frame construction • two or fewer stories with flexible diaphragms

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Summary of MWFRS Wind Design Procedures

ASCE/SEI7 provides several MWFRS step‐by‐step procedures.

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table procedure type applies to

ASCE/SEI7 Table 27.2‐1 directional procedure enclosed, partially enclosed, and open buildings of all heights

ASCE/SEI7 Table 27.5‐1 directional procedure enclosed, simple diaphragm buildings with h < 160 ft

ASCE/SEI7 Table 28.2‐1 envelope procedure low‐rise buildings

ASCE/SEI7 Table 28.5‐1 envelope procedure simple diaphragm low‐rise buildings ASCE/SEI7 Table 29.1‐1 envelope procedure rooftop equipment and other structures

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Envelope Procedure (MWFRS)

• defined in ASCE/SEI7 Sec. 28.4.1 • applicable to low‐rise buildings meeting following requirements: • must be regular shape (ASCE/SEI7 Sec. 26.2) without irregularities, such as projections or indentations • must not have response characteristics making it subject to dynamic effects from vortex shedding, or instability from galloping or flutter • must not have site location where channeling effects or buffeting in wake of upwind obstructions require special consideration • simplified design wind pressure

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h pf pi pq GC  GC ASCE/SEI7 Eq. 28.4‐1

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Envelope Procedure (MWFRS)

The following information must be derived to determine wind loads for MWFRS using the envelope procedure. • risk category • basic wind speed • exposure category • velocity pressure exposure coefficient • topographic factor • directionality factor • enclosure classification • internal pressure coefficient • wind velocity pressure • external pressure coefficient • internal wind pressure • external wind pressure • combined internal/external wind pressure • minimum applicable design loads

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Envelope Procedure (MWFRS)

• wind pressures applied to each building corner in turn (ASCE/SEI7 Fig. 28.4‐1) • torsional effects evaluated as necessary • procedure simplified by combining gust factor with pressure coefficient (treat combination as single factor)

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Example: Envelope Procedure (MWFRS)

Example 7.26

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Example: Envelope Procedure (MWFRS)

Example 7.26

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Example: Envelope Procedure (MWFRS)

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Example: Envelope Procedure (MWFRS)

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Example: Envelope Procedure (MWFRS)

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Summary of C&C Wind Design Procedures

ASCE/SEI7 provides several step‐by‐step C&C wind design procedures.

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table applies to

ASCE/SEI7 Table 30.4‐1 enclosed, partially enclosed, and open buildings of all heights ASCE/SEI7 Table 30.5‐1 enclosed low‐rise buildings (simplified method)

ASCE/SEI7 Table 30.6‐1 enclosed and partially enclosed buildings, h > 60 ft ASCE/SEI7 Table 30.7‐1 enclosed buildings, h ≤ 160 ft.

ASCE/SEI7 Table 30.8‐1 open buildings ASCE/SEI7 Table 30.9‐1 parapets

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Envelope Procedure (C&C)

• defined in ASCE/SEI7 Sec. 30.4 • applicable to enclosed and partially buildings that meet at least one of the following requirements: • low‐rise building • mean roof height h ≤ 60 ft • building with flat roofs, gable roofs, multispan gable roofs, hip roofs, monoslope roofs, stepped roofs, or sawtooth roofs

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Envelope Procedure (C&C)

The following information must be derived to determine wind loads for components and cladding using the envelope procedure. • risk category • basic wind speed • exposure category • velocity pressure exposure coefficient • topographic factor • wind directionality factor • enclosure classification • internal pressure coefficient • wind velocity pressure • external pressure coefficient • internal wind pressure • external wind pressure • combined internal/external wind pressure • minimum design wind loads • effective wind area

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Envelope Procedure (C&C)

• procedure simplified by combining gust factor with pressure coefficient and treating combination as single factor • design wind pressure given by ASCE/SEI7 Eq. 30.4‐1

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Velocity Pressure Exposure Coefficients (C&C)

• represented by K_z • reflects change in wind speed with height and exposure category • given in ASCE/SEI7 30.3‐1 • for values of height not listed in table, K_z can be calculated by linear interpolation Table 7.17 Velocity Pressure Exposure Coefficients for Components and Cladding Systems

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External Pressure Coefficients (C&C)

• (GC_p) = combination of gust effect factor and external pressure coefficient • pressure acting on external surfaces obtained from ASCE/SEI7 Eq. 30.4‐1 • (GC_p) values for walls tabulated in ASCE/SEI7 Fig. 30.4‐1 • (GC_p) values for roofs tabulated in ASCE/SEI7 Fig. 30.4‐2 through Fig. 30.4‐7

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External Pressure Coefficients (C&C)

• building surface divided into 5 distinct zones for design • roofs divided into 3 zones for design: end zone, interior zone, corner zone • walls divided into 2 zones for design: end zone and interior zone • pressures act normal to wall and roof surfaces; positive for external pressure, negative for internal pressure Figure 7.34 Components and Cladding External Pressure Zones

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External Pressure Coefficients (C&C)

• ASCE/SEI7 Fig. 30.4‐1 Note 6: end zone width (5) and eave zone width (2) are lesser value of a • acannot be less than either of • ASCE/SEI7 Fig. 30.4‐1 Note 5: (GC_p) values reduced by 10% for walls where building roof slope ≤ 10° Figure 7.34 Components and Cladding External Pressure Zones

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Minimum Design Wind Loads (C&C)

minimum design wind loads for components and cladding (ASCE/SEI7 Sec. 30.2.2) net pressure of 16 lbf/ft2 _{applied in either direction normal to the surface}

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Effective Wind Area

• effective wind area defined in ASCE/SEI7 Sec. 26.2 as l = element span length and b_e = effective tributary width • for cladding fasteners, A ≤ area tributary to individual fastener • Per ASCE/SEI7 Sec. 30.2.3, C&C elements where A 700 ft2 _{may be designed using} main wind force‐resisting systems methods.

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Effective Wind Area

• Local turbulence may occur over small areas and at geometric irregularities of buildings (ridges, corners, etc.), and can cause higher wind loads on those areas. • components and cladding designed for higher wind pressures than main force wind‐ resisting systems to account for local turbulence • effective wind area, A, used to determine external pressure coefficient, GC_p • accounts for decreasing probability of elevated local loads with increasing loaded area

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Example: Envelope Procedure (C&C)

Example 7.27

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Example: Envelope Procedure (C&C)

Example 7.27

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Example: Envelope Procedure (C&C)

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Example: Envelope Procedure (C&C)

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Example: Envelope Procedure (C&C)

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Example: Envelope Procedure (C&C)

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IBC Alternate Procedure

IBC alternate all‐heights wind design provisions • also known as the “IBC alternate procedure” • specified in IBC Sec. 1609.6 • simplified version of directional design method (ASCE/SEI7 Sec. 27.4)

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IBC Alternate Procedure

may be used to determine wind effects on regularly shaped structures that meet all conditions given • h≤ 75 ft • either least‐height‐to‐width ratio ≤ 4 or fundamental frequency of vibration ≥ 1 Hz • structure not sensitive to dynamic effects • structure not located on site for which channeling effects or buffeting in wake of upwind obstructions warrant special consideration • structure can be classified as simple diaphragm building per ASCE/SEI7 Sec. 26.2 • no structural separations

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IBC Alternate Procedure

The following information must be derived to determine wind loads using the IBC alternate procedure. • velocity pressure exposure coefficient • topographic factor • wind stagnation pressure • net‐pressure coefficient • design wind pressure

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Velocity Pressure Exposure Coefficient (IBC)

• wind speed increases with height and exposure category (B  C  D) • velocity pressure exposure coefficient, K_z, determined from ASCE/SEI7Table 27.3‐1 • based on exposure category • windward wall: K_z based on actual height above ground level for each floor of building • leeward walls, side walls, roofs: K_z = K_h • evaluated at mean roof height only • constant value over height of building

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Wind Stagnation Pressure

basic wind speed may be converted to stagnation pressure, q_s, at standard height of 33 ft Table 7.18 Wind Stagnation Pressure

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2 2 ,lbf/ft 0.00256 s q  V

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Net‐Pressure Coefficient

net‐pressure coefficient • represented as C_net • equation given in IBC Sec. 1609.6.2 • equation adds internal and external pressures; appropriate for simple diaphragm buildings where internal pressures cancel out • wind directionality factor and gust effect factor taken as constants

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Net‐Pressure Coefficient

• net pressure coefficient expression reduces to

• values of C_netare provided in IBC Table 1609.6.2 (given for enclosed and partially enclosed buildings; and for windward walls, leeward walls, side walls, and roofs of varying slopes)

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Design Wind Pressure

• calculated from IBC Eq. 16‐35

• values of P_net applicable for both MWFRS and C&C • IBC Sec. 1609.6.3 requires minimum values of P_net • MWFRS: P_net ≥ 16 lbf/ft2 _{multiplied by} area of building projected on plane normal to wind direction

• C&C: P_net≥ 16 lbf/ft2 _{acting normal to} surface in either direction

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Design Wind Pressure

• IBC alternate method does not account for turbulence at wall corners or roof ridge and eaves. • leeward and side walls: wind pressure is constant over the surface • windward walls: wind pressure varies with height (since K_z varies with height) • roof: separate coefficients are given for the windward and leeward portions

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IBC Alternate Procedure (MWFRS)

The following information must be derived to determine wind loads for main wind force‐resisting systems using the IBC alternate procedure. • risk category • basic wind speed • exposure category • velocity pressure exposure coefficients • topographic factor • net‐pressure coefficients at walls and roofs • design wind pressure • minimum design wind pressure

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Example: IBC Alternate Procedure (MWFRS)

Example 7.28

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Example: IBC Alternate Procedure (MWFRS)

Example 7.28

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Example: IBC Alternate Procedure (MWFRS)

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Example: IBC Alternate Procedure (MWFRS)

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Example: IBC Alternate Procedure (MWFRS)

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Example: IBC Alternate Procedure (MWFRS)

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Example: IBC Alternate Procedure (MWFRS)

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Example: IBC Alternate Procedure (MWFRS)

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Example: Minimum Wind Loads (IBC)

IBC alternate procedure calculations are used to create a design load case for a main wind force‐resisting system, as shown. Does the design load case satisfy the IBC minimum design wind load requirements?

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Example: Minimum Wind Loads (IBC)

IBC alternate procedure calculations are used to create a design load case for a main wind force‐resisting system, as shown. Does the design load case satisfy the IBC minimum design wind load requirements? Per IBC Sec. 1609.6.3, the design wind load for a MWFRS cannot be less than 16 lbf/ft2 _{on any plane normal to the} assumed wind direction. The assumed wind direction is left to right, and the roof is parallel to this direction. Since the roof is not normal to the assumed wind direction, the minimum design wind load requirements are not applicable.

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Example: Minimum Wind Loads (IBC)

The walls are perpendicular to the wind direction, so the walls are normal to the wind direction, so the minimum design wind load requirements are applicable. The net design wind load on the walls is The minimum design wind load requirements are met.

total net,windward net,leeward

2 2 2 2 lbf lbf 7.60 9.80 ft ft 17.40 lbf/ft 16 lbf/ft , OK P  P P      _ _

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Learning Objectives

You have learned • simple approximations for fundamental period of vibration of typical structures • how to calculate wind loads using methods from ASCE/SEI7 and IBC. • how to distribute wind loads to typical building structures • how to navigate ASCE/SEI7 and IBC design codes for wind loads • choose variables • use of tables and figures • apply minimum load limits • interpret of important text

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Lesson Overview

Chapter 7: Lateral Forces • Lateral‐Force Resisting Systems • Seismic Design • Wind Design