The Wind Load Provisions of
ASCE 7:
From 2005 to 2010
Gary Chock
, P.E.Member, ASCE 7 Wind Load Subcommittee
Member, International Code Council Structural Committee Structural Engineers Association of Hawaii Member of the State
Upcoming Major Changes to the ASCE 7
Wind Design Provisions
• RE‐ORGANIZATION OF THE WIND LOAD CHAPTER • SIMPLIFIED METHOD FOR ENCLOSED SIMPLE DIAPHRAGM BUILDINGS ≤ 160 FT. • NEW WIND MAPS with LRFD Basis • EXPOSURE D in HURRICANE PRONE REGIONS • WINDBORNE DEBRIS CRITERIA • Figure 6‐1 Hawaii designation as a Special Wind Region
Other New Provisions listed per the present
Chapter 6 organization (not discussed here)
• 6.2 Definitions – Definition of mean roof height – Definition of simple diaphragm building • Figure 6‐2 revision pertaining to the Enclosed Low‐Rise Simple Diaphragm Building Method • Figure 6‐10 revision and rewording pertaining to the Low‐Rise Building Method • 6.5.6.3 Downwind Transition from Exposure D • 6.5.9.3 Roof aggregate • 6.5.11.4 Overhangs • Table 6‐4 Free‐standing walls and signs
Upcoming Changes to the ASCE 7 Wind
Design Commentary
listed per the present
Chapter 6 organization
(not discussed here)
• C6.5.4 Saffir‐Simpson Category • C6.5.6 reference • C6.5.6.6 Multiple roughness regimes • Table C6‐8 z0 for Exposure B • C6.5.6 z0 and alpha • Example of roughness transition calculations • C6.6 Wind Tunnel database
Re-organization of Chapter 6 Wind Load into
Multiple Chapters in ASCE 7-2010
Clarifying the Basis of the Different Methods
• DIRECTIONAL PROCEDURE: A procedure for determining wind loads on buildings and other structures for specific wind directions, in which the external pressure coefficients utilized are based on past wind tunnel testing of prototypical building models for the corresponding direction of wind. Formerly referred to as Method 2 – Analytical Procedure, All Heights • ENVELOPE PROCEDURE: A procedure for determining wind load cases on buildings, in which pseudo external pressure coefficients are derived from past wind tunnel testing of prototypical building models sucessively rotated through 360 degrees, such that the pseudo pressure cases produce key structural actions (uplift, horizontal shear, bending moments, etc.) that envelope their maximum values among all possible wind directions. Formerly referred to as Method 1 Enclosed Simple Diaphragm Low‐Rise and Method 2 Analytical Procedure Low‐Rise, but not explained as a separate methodologyExample of
Method 2 Analytical Procedure
Clarifying the Different Methods
• WIND TUNNEL PROCEDURE: A procedure for determining wind loads on buildings and other structures, in which pressures and/or forces and moments are determined for each wind direction considered, from a model of the building or other structure and its surroundings, in accordance with Chapter 31.Formerly referred to as Method 3
Chapter 26 General Requirements
26.1 Procedures 26.2 Definitions 26.3 Symbols and Notation 26.4 General 26.5 Wind Hazard Map 26.6 Wind Directionality 26.7 Exposure 26.8 Topographic Effects 26.9 Gust Effect Factor 26.10 Enclosure Classifications 26.11 Internal Pressure CoefficientGeneral Requirements – Velocity Pressure
The ASCE/SEI Standard 7-05 utilizes the following equation for velocity pressure:
q = 0.00256 Kz Kzt Kd V2 I
where:
Kz is the velocity pressure exposure coefficient that is defined according to system or component design cases and terrain category,
Kzt is the topographic speed-up factor,
Kd is the wind directionality factor which accounts for the fact that the probability that the maximum wind may not impact the structural component or system in its weakest orientation,
V is the peak gust windspeed associated with a 700-year return period, divided by √1.6, and
I is the Importance Factor of the building or structure, based on its occupancy type.
Chapter 27 WIND LOADS (MWFRS) – DIRECTIONAL
PROCEDURE FOR ENCLOSED, 4 PARTIALLY ENCLOSED, AND OPEN BUILDINGS OF ALL HEIGHTS
27.1 Scope PART I Enclosed and Partially Enclosed Buildings of All Heights 27.2 General Requirements 27.3 Velocity Pressure 27.4 Wind Loads PART II Enclosed Simple Diaphragm Buildings with h ≤ 160 Feet 27.5 General Requirements 27.6 Wind Loads PART II has been added to ASCE 7‐10 to cover the common practical cases of enclosed simple diaphragm buildings up to height h = 160 ft. Design wind pressures are tabulated directly using the Directional Approach of PART I
Frequency calculation to determine whether a building is
flexible, (listed per the present Chapter 6 organization)
6.5.8 Gust Effect Factor. 6.5.8.1 Frequency Determination. To determine whether a building or structure is rigid or flexible as defined in Section 6.2, the fundamental frequency of the structure, n1, in the direction under consideration shall be established for buildings greater than 60 feet in height, using the structural properties and deformational characteristics of the resisting elements in a properly substantiated analysis. As an alternative to performing an analysis to determine the frequency of the structure, n1, it is permitted to use the approximate building frequency, n a, for steel, concrete, or masonry buildings less than or equal to 300 feet in height, directly calculated in accordance with Section 6.5.8.2. Buildings up to 60 feet in height are permitted to be considered rigid.
Note, per 6.2 Definitions, RIGID BUILDINGS AND OTHER STRUCTURES: The defining criteria for rigid, in comparison to flexible, is that the natural frequency is greater than or equal to 1 Hz
Approximate Fundamental Frequency
(lower-bound)
6.5.8.2 Approximate Fundamental Frequency. The approximate lower‐bound fundamental frequency (n a), in Hertz, is permitted to be determined from one of the following equations: • For steel moment‐resisting‐frame buildings: • na= 22.2/ H0.8 (6‐4) • For concrete moment‐resisting frame buildings: • na= 43.5/ H0.9 (6‐5) • For steel and concrete buildings with other lateral‐force‐resisting systems: • na= 75/ H (6‐6) • For concrete or masonry shear wall buildings, it is also permitted to use: • n1 = 385(Cw)0.5/H ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ = ∑ = 2 2 1 83 . 0 1 100 i i i n i i B w D h A h H A cAB = base area of the structure (ft2)
Ai = area of shear wall “i “(ft2)
Di = length of shear wall “i”(ft) hi = height of shear wall “i” (ft) n1= building natural frequency (hertz)
H = building height (ft)
n = # of shear walls in building effective in resisting lateral forces in direction under consideration
What is an Enclosed Simple Diaphragm Building?
BUILDING, SIMPLE DIAPHRAGM: A building in which both windward and leeward wind loads are transmitted by vertically spanning wall elements only through the floor and roof diaphragms to the same vertical elements of the MWFRS. New PART II Enclosed Simple Diaphragm Buildings with h ≤ 160 Feet provides tables of design pressure based on calculations that assume lower bound frequencies (75/H) and account for non‐rigid buildings up to 160 ft. tall. This new simplified method also assumes that the building is enclosed, so that the internal pressurization effect GCpi is reduced. Accordingly, this would requireChapter 28 WIND LOADS (MWFRS) - ENVELOPE
PROCEDURE FOR ENCLOSED AND PARTIALLY ENCLOSED LOW-RISE BUILDINGS
28.1 Scope PART I Enclosed and Partially Enclosed Low‐Rise Buildings 28.2 Scope 28.3 Velocity Pressure 28.4 Wind Loads – Main Wind Force Resisting System PART II Enclosed Simple Diaphragm Low‐Rise Buildings 28.5 General Requirements 28.6 Wind Loads – Main Wind Force Resisting System
Chapter 29 WIND LOADS (MWFRS) – BUILDING
APPURTENANCES AND OTHER STRUCTURES
29.1 Scope 29.2 General Requirements 29.3 Velocity Pressure 29.4 Design Wind Loads – Solid Freestanding Walls and Solid Freestanding Signs 29.5 Design Wind Loads – Other Structures 29.6 Rooftop Structures and Equipment for Buildings with h ≤ 60 ft. 29.7 Parapets This is a Directional Procedure
Chapter 30 WIND LOADS COMPONENTS AND CLADDING 1) Part 1 applies to enclosed and partially enclosed low rise buildings, buildings with h ≤ 60 ft, and buildings with 60 ft < h 90 ft having flat roofs, gable roofs, multi‐span gable roofs, hip roofs, monoslope roofs, stepped roofs and saw tooth roofs. Wind pressures are calculated from a wind pressure equation. (This is an envelope procedure) 2) Part 2 applies to enclosed low‐ rise buildings and buildings with h ≤ 60 ft having flat roofs, gable roofs and hip roofs. Wind pressures are determined directly from a table. (This is an envelope procedure) 3) Part 3 applies to enclosed and partially enclosed buildings with a mean roof height h > 60 feet having flat roofs, pitched roofs, gable roofs, hip roofs, mansard roofs, arched roofs and dome roofs. Wind pressures are calculated from a wind pressure equation. (This is a directional procedure) 4) Part 4 applies to enclosed buildings having a mean roof height h ≤ 160 feet having flat roofs, gable roofs, hip roofs, monoslope roofs and mansard roofs. Wind pressures are determined directly from a table. (This is a new directional procedure)
Chapter 30 WIND LOADS COMPONENTS AND CLADDING
5) Part 5 applies to open buildings of all heights having pitched free roofs,
monoslope free roofs and trough free roofs. (This is a directional procedure)
6) Part 6 applies to building appurtenances such as roof overhangs and parapets.
Chapter 30 WIND LOADS COMPONENTS AND CLADDING 30.1 Scope 30.2 General Requirements 30.3 Velocity Pressure PART I Enclosed and Partially Enclosed Low‐Rise Buildings 30.4 Design Wind Pressures for Enclosed and Partially Enclosed Low‐Rise Buildings with h ≤ 60 ft. 30.5 Design Wind Pressures for Enclosed and Partially Enclosed Low‐Rise Buildings with 60 ft. < h < 90 ft. PART II Enclosed Low‐Rise Buildings 30.6 Conditions 30.7 Design Wind Pressures for Enclosed Low‐Rise Buildings h ≤ 60 ft.
Chapter 30 WIND LOADS COMPONENTS AND CLADDING PART III Enclosed and Partially Enclosed Buildings with h > 60 ft. 30.8 Design Wind Pressures for Enclosed and Partially Enclosed Buildings with h > 60 ft. PART IV Enclosed Simple Diaphragm Buildings with h ≤ 160 ft. 30.9 General Requirements 30.10 Wind Loads – Components and Cladding This section has been added to ASCE 7‐10 to cover the common practical case of enclosed buildings up to height h = 160 ft. Table 30.9‐1 includes wall and roof pressures for flat roofs (θ < 10 deg), gable roofs, hip roofs, monoslope roofs and mansard roofs. Pressures are derived from Fig.30.8‐1 (flat roofs), Fig. 30.4‐1B, C and D (gable and hip roofs) and Fig. 30.4‐2 (monoslope roofs) of Part 3. The GCp values from these figures were combined with an internal pressure coefficient (+ or – 0.18) to obtain a net coefficient from which pressures were calculated.
Chapter 30 WIND LOADS COMPONENTS AND CLADDING PART V Open Buildings of All Heights with Monoslope, Pitched or Trough Free Roofs 30.11 Design Wind Pressures for Open Buildings of All Heights with Monoslope, Pitched or Trough Fee Roofs PART VI Building Appurtenances (Roof Overhangs and Parapets) 30.12 Roof Parapets 30.13 Roof Overhangs
Chapter 31 Wind Tunnel Procedure
• 31.1 Scope • 31.2 Test Conditions • 31.3 Dynamic Response • 31.4 Limitations on Wind Speeds • 31.5 Wind‐Borne DebrisC6.5.2 Limitations of Analytical Procedure
The provisions given under 6.5.2 apply to the majority of site locations and buildings and structures, but for some projects these provisions may be inadequate. Examples of site locations and buildings and structures (or portions thereof) that may require special studies, either using applicable recognized literature pertaining to wind effects, or using the wind tunnel procedure of 6.6 include: 1. Site locations which have channeling effects or wakes from upwind obstructions. 2. Buildings with unusual or irregular geometric shape, including barrel vaults, and other buildings whose shape (in plan or vertical cross‐section) differs significantly from the shapes in Figures 6‐3 through 6‐8.C6.5.2 Limitations of Analytical Procedure
2. Buildings with response characteristics that result in substantial vortex‐ induced and/or torsional dynamic effects, or dynamic effects resulting from aeroelastic instabilities such as flutter or galloping. Such dynamic effects are difficult to anticipate, being dependent on many factors, but are likely to be important when any of the following apply. i. The height of the building is over 400 ft.ii. The height of the building is greater than 4 times its minimum effective width as defined below.
iii. The lowest natural frequency of the building is less than 0.25 Hz. iv.
v. The minimum effective width Bminis defined as the minimum value of
∑hiBi /∑hiconsidering all possible wind directions. The summations are over the
height of the building for each wind direction, hi is the height above grade of level i
, and Biis the width at level i normal to the wind direction. The reduced velocity 5 min 1 > B n Vz
where z = 0.6h and Vz is the mean hourly velocity
C6.5.2 Limitations of Analytical Procedure
4. Slender bridges, cranes, electrical transmission lines, guyed masts, telecommunication towers and flagpoles. When undertaking detailed studies of the dynamic response to wind forces, the fundamental frequencies of the structure in each direction of consideration should be established using the structural properties and deformational characteristics of the resisting elements in a properly substantiated analysis, and not utilizing approximate equations based on height.ASCE 7 2010 Determination of Design Wind
Speeds
• Incorporates a new probabilistic analysis with an improved windfield model for the continental U.S. and a separate analysis for Hawaii (ARA, 2001) • Load calculation at strength design point LRFD design Wind maps at 300‐1700 year recurrence Load Factor = 1.0 (versus 1.6 today) Other Wind maps provided at 50, 10 etc. years for serviceability and drift Allowable Stress available by using 0.8 factor • Design wind speed return period is based on occupancy, and the Importance Factor is thus eliminatedReasons (per ASCE)
• New data and research indicates that the current hurricane wind speeds given in ASCE 7 were conservative in the continental USA and needed to be adjusted downward. • A strength design wind speed map is more aligned with seismic design in that they both use a load factor of 1.0 for strength design. • Multiple maps eliminate the problem of having importance factors that vary with occupancy category and hurricane prone and non‐hurricane prone regions. • The use of multiple maps eliminates the confusion of engineers not understanding that the present map is not a 50‐year return period map. • Engineers have not understood that their design, after multiplication by the 1.6 load factor, was a roughly 700 year event with wind speeds √1.6 times that shown on the map. • Building owners have not understood that their buildings would not fail for wind speeds somewhat above the present map value. The revised maps give the owner a better idea of the wind speeds for which no damage or minimal damage is expected in an engineered structure.Basic Wind Speed, V
Occupancy Category Description Return Period Map Value for Hawaii
I (ASCE 7 -10) Agricultural, Temporary, and Minor Storage
300 113
II (ASCE 7 -10) “Normal Occupancies” 700 129
III and IV (ASCE 7 -10) High Hazard Occupancies (such as assembly, school buildings with > 250
occupants, Power, Telecom, Hazmat, Explosives,)
Essential Facilities
1700 143
I (ASCE 7- 05) 105 * √1.6 * √0.77 gives the LRFD equivalent of
117
II (ASCE 7-05) 105 * √1.6 gives the LRFD equivalent of 133 III and IV (ASCE 7-05) 105 * √1.6 * √1.15 gives the LRFD equivalent
of
Table C6-ZZc
√1.6 * Bas ic Wind Spe e d
Virgin Islands Puerto Rico Haw aii Port Aransas, Texas Galveston, Texas Cameron, Louisiana Slidell, Louisiana Biloxi, Mississippi Gulf Shores, Alabama Panama City, Florida Clearw ater, Florida Key West, Florida Miami Beach, Florida Melbourne, Florida Jacksonville Beach, Florida Sea Island, Georgia Folley Beach, South Carolina Wrightsville Beach, North Carolina Virginia Beach, Virginia Ocean City, Maryland Bow ers Beach, Delaw are Atlantic City, New Jersey Brooklyn, New York Southampton, New York New Haven, Connecticut New port, Rhode Island Hyannis, Massachusetts Boston, Massachusetts Hampton Beach, New Hampshire Bar Harbor, Maine
Wind Spe e d, m ph (m /s )
Cate gory 1 Cate gory 2 Cate gor y 3 Cate gory 4 Cate gor y 5 Saffir/Sim ps on Hur ricane Cate gor y
82 (36.7) 191 (85. 4) 156 (69. 7) 130 (58.1) 108 (48.3)
Hurricane Prone Regions
HURRICANE PRONE REGIONS: Areas vulnerable to hurricanes; in the United States and its territories defined as 1. The U.S. Atlantic Ocean and Gulf of Mexico coasts where the basic wind speed for Category II buildings is greater than 90 114 mi/h, and 2. Hawaii, Puerto Rico, Guam, Virgin Islands, and American Samoa. • Reasoning: Adjust the wind speed criteria by √1.6 to the LRFD level with a LF of 1.0Relation between Saffir - Simpson Hurricane Scale Winds to
Peak Gust Speeds Over Open Terrain
Saffir-Simpson category sustained (1-minute) wind speed
over open water (mph)
3-second Peak Gust over open terrain (mph) ASCE 7-05 [Vickery,
2000]
Updated 3-second Peak Gust over open terrain
(mph) ASCE 7-10 [Simiu, Vickery, Kareem, 2007] 1 2 3 4 5 74-94 95-110 111-130 131-155 >155 82-108 109-130 131-156 157-191 >191 81-105 106-121 122-143 144-171 >171
Exposure D will be back in Hurricane Prone
Regions
• Research since 2004 has showed that the drag coefficient over the ocean in high winds in hurricanes did not continue to increase with increasing wind speed as previously believed. The studies showed that the sea surface drag coefficient, and hence the aerodynamic roughness of the ocean, reached a maximum at mean wind speeds of about 30 m/sec (~70 mph peak gust). There is some evidence that the drag coefficient actually decreases (i.e. the sea surface becomes aerodynamically smoother) as the wind speed increase further, or as the hurricane radius decreases. The consequences of these studies are that the surface roughness over the ocean in a hurricane is consistent with that of exposure D rather than exposure C. Consequently, the use of exposure D along the hurricane coastline is now required.Windborne Debris
• WIND‐BORNE DEBRIS REGIONS: Areas within hurricane prone regions located: 1. Within 1 mile of the coastal mean high water line where the basic wind speed, for the building category under consideration, is equal to or greater than 130110 mi/h and in Hawaii or 2. In areas where the basic wind speed, for the building category under consideration, is equal to or greater than 140120 mi/h.Reasoning: adjust the windspeed to the LRFD design level with a LF of 1.0 instead of 1.6, and reference the new strength‐level wind maps of return periods that are occupancy category dependent
Differences between Content in the Standard and
Local Codes, per ASCE and NCSEA
“Material that is left in the building code conforms to one of the following criteria: Relates to local climatic, terrain, or other environmental conditions, which many building officials will wish to specify when adopting the model code by local ordinance. This includes specification of basic wind speeds, terrain, exposure and similar provisions. Relates to enforcement of types of construction which is often set by condition so local practice, materials availability and construction industry capabilities Is not presently covered in an adequate manner by a national consensus standard. This includes to material covering roofing materials, hurricane protection of openings, etc.” February, 2005
Hawaii Special Wind Region in ASCE 7-10
C6.5.4.1 Special Wind Regions. Although the wind speed map of Fig. 6‐1 is valid for most regions of the country, there are special regions in which wind speed anomalies are known to exist. Some of these special regions are noted in Fig. 6‐1. Winds blowing over mountain ranges or through gorges or river valleys in these special regions can develop speeds that are substantially higher than the values indicated on the map. When selecting basic wind speeds in these special regions, use of regional climatic data and consultation with a wind engineer or meteorologist is advised.Hawaii Special Wind Region in ASCE 7-10
C6.5.4.1 It is also possible that anomalies in wind speeds exist on a micrometeorological scale. For example, wind speed‐up over hills and escarpments is addressed in Section 6.5.7. Wind speeds over complex terrain may be better determined by wind‐tunnel studies as described in Section 6.6. Adjustments of wind speeds should be made at the micrometeorological scale on the basis of wind engineering or meteorological advice and used in accordance with the provisions of Section 6.5.4.2 when such adjustments are warranted. Due to the complexity of mountainous terrain and valley gorges in Hawaii, there are topographic wind speed‐up effects that cannot be addressed solely by Figure 6‐4. In the Hawaii Special Wind Region, there are special Kzt topographic effect adjustments to the Basic Wind Speed established by the authorities having jurisdiction. •Hawaii Design Maps
• Exposure based on Land‐cover data developed by the NOAA Coastal Services Center from Landsat Enhanced Thematic Mapper satellite imagery beginning in the year 2000 to provide land cover data for the coastal regions of the National Land Cover Database (NLCD). – (Subject to update for the Exposure D revision in ASCE 7‐10 which becomes effective locally by adoption of the 2012 IBC) • Topographic Factor giving the maximum topographic effect • Tables of Directionality Factor that take into account site directional probabilities of the occurrence of the maximum effect • Effective Wind Speed for Cladding and Components based on 105 mph basic wind speed – (subject to revision before LRFD wind speed maps of ASCE 7‐10 becomes effective locally by adoption of the 2012 IBC) • Therefore, the maps are good for the 2003 – 2009 IBC period of adoptions
Effective Wind Speed Maps
Algebraically-normalized maps of “Veffective”, i.e., V multiplied by
√( Kzt x Kd / 0.85 ) allow implicit consideration of topographic
effects for Cladding and Component design.
The Veffective values can be used for performance-specified
building components and cladding, as well as when using
prescriptive design tables and existing reference standards and simplified methods based on wind speed tables.
