STRC17 Lateral Forces Wind Loads 0716

Structural Engineering Exam Review Course

Lateral Forces: Wind Loads

Wind Loads Structural Engineering Review Course

STRC ©2015 Professional Publications, Inc.

© 2015 Professional Publications, Inc.

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Wind Loads

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



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Wind Loads

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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Wind Loads

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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Wind Loads

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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Wind Loads

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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Wind Loads

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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Wind Loads

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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Wind Loads

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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Wind Loads

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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Wind Loads

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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Wind Loads

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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Wind Loads

Example: Low‐Rise Buildings Does the enclosed building shown qualify as a low‐rise building per ASCE/SEI7 requirements?



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Wind Loads

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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Wind Loads

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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Wind Loads

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) < 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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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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Wind Loads

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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Wind Loads

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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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. 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



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Wind Loads

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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Wind Loads

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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Wind Loads

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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Wind Loads

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, Kzt = 1.0



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Wind Loads

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 Loads

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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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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Wind Loads

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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Wind Loads

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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Wind Loads

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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Wind Loads

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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Wind Loads

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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Wind Loads

External Pressure Coefficients pressure acting on external surfaces,

• a cannot be less than either of


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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Wind Loads

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 < 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. table

procedure type

applies to

ASCE/SEI7 Table 27.2‐1

directional procedure

enclosed, partially enclosed, and open buildings of all heights


directional procedure

enclosed, simple diaphragm buildings with h < 160 ft


envelope procedure

low‐rise buildings


envelope procedure

simple diaphragm low‐rise buildings


envelope procedure

rooftop equipment and other structures



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Wind Loads

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



p  qh  GC pf    GC pi 



ASCE/SEI7 Eq. 28.4‐1 STRC ©2015 Professional Publications, Inc.


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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

• internal pressure coefficient • wind velocity pressure • external pressure coefficient • internal wind pressure

• basic wind speed

• external wind pressure

• exposure category • velocity pressure exposure coefficient

• combined internal/external wind pressure


• minimum applicable design loads

• directionality factor • enclosure classification



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Wind Loads

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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Summary of C&C Wind Design Procedures ASCE/SEI7 provides several step‐by‐step C&C wind design procedures. table

applies to


enclosed, partially enclosed, and open buildings of all heights


enclosed low‐rise buildings (simplified method)


enclosed and partially enclosed buildings, h > 60 ft


enclosed buildings, h ≤ 160 ft.


open buildings


parapets


roof overhangs



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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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Wind Loads

Envelope Procedure (C&C) The following information must be derived to determine wind loads for components and cladding using the envelope procedure.

• external pressure coefficient • external wind pressure

• basic wind speed • exposure category • velocity pressure exposure coefficient • wind directionality factor

• wind velocity pressure • internal wind pressure

• risk category


• internal pressure coefficient

• combined internal/external wind pressure • minimum design wind loads • effective wind area

• enclosure classification STRC ©2015 Professional Publications, Inc.


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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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Wind Loads

Velocity Pressure Exposure Coefficients (C&C) • represented by Kz • reflects change in wind speed with height and exposure category

Table 7.17 Velocity Pressure Exposure Coefficients for Components and Cladding Systems

• given in ASCE/SEI7 30.3‐1 • for values of height not listed in table, Kz can be calculated by linear interpolation



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External Pressure Coefficients (C&C) • (GCp) = combination of gust effect factor and external pressure coefficient • pressure acting on external surfaces obtained from ASCE/SEI7 Eq. 30.4‐1 • (GCp) values for walls tabulated in ASCE/SEI7 Fig. 30.4‐1 • (GCp) 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

Figure 7.34 Components and Cladding External Pressure Zones

• 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 STRC ©2015 Professional Publications, Inc.


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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

Figure 7.34 Components and Cladding External Pressure Zones

• a cannot be less than either of

• ASCE/SEI7 Fig. 30.4‐1 Note 5: (GCp) values reduced by 10% for walls where building roof slope ≤ 10°



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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 be = 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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Wind Loads

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, GCp • 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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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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Wind Loads

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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Wind Loads

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



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Wind Loads

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

Table 7.18 Wind Stagnation Pressure

qs ,lbf/ft 2  0.00256V 2



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Net‐Pressure Coefficient net‐pressure coefficient • represented as Cnet • equation given in IBC Sec. 1609.6.2

• wind directionality factor and gust effect factor taken as constants

• equation adds internal and external pressures; appropriate for simple diaphragm buildings where internal pressures cancel out



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Wind Loads

Net‐Pressure Coefficient • net pressure coefficient expression reduces to

• values of Cnet are 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 • MWFRS: Pnet ≥ 16 lbf/ft2 multiplied by area of building projected on plane normal to wind direction

• calculated from IBC Eq. 16‐35

• C&C: Pnet ≥ 16 lbf/ft2 acting normal to surface in either direction • values of Pnet applicable for both MWFRS and C&C • IBC Sec. 1609.6.3 requires minimum values of Pnet



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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 Kz 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.

• net‐pressure coefficients at walls and roofs • design wind pressure • minimum design wind pressure

• risk category • basic wind speed • exposure category • velocity pressure exposure coefficients • topographic factor



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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: 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 minimum design wind load requirements are met.

The net design wind load on the walls is Ptotal  Pnet,windward  Pnet,leeward lbf lbf  9.80 ft 2 ft 2  17.40 lbf/ft 2   16 lbf/ft 2 , OK   7.60



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Learning Objectives You have learned • 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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STRC17 Lateral Forces Wind Loads 0716

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