Wind Load Provisions ASCE7_05

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 Building Code Council

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 methodology

Method 2 Analytical All Heights Figure 6-6

Example of Method 2 Analytical Procedure  Low‐Rise (Envelope Procedure) Figure 6‐10

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 Coefficient

General 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. (ASCE 7-10 will revise how V is defined and will eliminate I)

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 100 n ⎛ H ⎜⎜ cw = ∑ AB i =1 ⎝ hi

2

⎞ Ai ⎟⎟ ⎠ ⎡ ⎛h ⎢1 + 0.83⎜⎜ i ⎝ Di ⎣⎢

n1 = building natural frequency (hertz) H = building height (ft) n = # of shear walls in building effective in resisting lateral forces in direction under consideration

⎞ ⎟⎟ ⎠

2

⎤ ⎥ ⎥⎦ AB = 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)

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 require  impact protective systems on glazing in windborne debris regions like Hawaii

Chapter 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  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.  (This is a directional procedure)

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.  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  5  where  z = 0 .6 h and  V z  is the mean hourly velocity  The reduced velocity  n1 B min at height  z .   

v.   The minimum effective width

Bmin is defined as the minimum value of 

∑ h B / ∑ h considering all possible wind directions.  The summations are over the  i

i

i

height of the building for each wind direction, hi  is the height above grade of level i  , and  Bi is the width at level i normal to the wind direction. 

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 eliminated

Reasons (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

142

Table C6-ZZc √1.6 * Bas ic Wind Spe e d Saffir /Sim ps on Hur r icane Cate gor y Cate gor y 1

Cate gor y 2

Cate gor y 3

Cate gor y 4

Cate gor y 5

Bar Harbor, Maine Hampton Beach, New Hampshire Boston, Massachusetts Hyannis, Massachusetts New port, Rhode Island New Haven, Connecticut Southampton, New York Brooklyn, New York Atlantic City, New Jersey Bow ers Beach, Delaw are Ocean City, Maryland V irginia Beach, Virginia Wrightsville Beach, North Carolina Folley Beach, South Carolina Sea Island, Georgia Jacksonville Beach, Florida Melbourne, Florida Miami Beach, Florida Key West, Florida Clearw ater, Florida Panama City, Florida Gulf Shores, Alabama Biloxi, Mississippi Slidell, Louisiana Cameron, Louisiana Galveston, Texas Port Aransas, Texas Haw aii Puerto Rico Virgin Islands 82 (36.7)

108 (48.3)

130 (58.1)

156 (69. 7) Wind Spe e d, m ph (m /s )

191 (85. 4)

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 1.0

Relation between Saffir - Simpson Hurricane Scale Winds to Peak Gust Speeds Over Open Terrain SaffirSimpson 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

Jim Rossberg, Structural Engineering Institute of ASCE, representing NCSEA Code Advisory Committee and ASCE/SEI

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

Exposure Category - Oahu

Exposure Category - Hawaii

Exposure Category - Maui

Exposure Category - Molokai and Lanai

Exposure Category - Kauai

Maximum Topographic Factor - Oahu

Maximum Topographic Factor - Hawaii

Maximum Topographic Factor - Maui

Maximum Topographic Factor - Molokai

Maximum Topographic Factor - Lanai

Maximum Topographic Factor - Kauai

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.

Effective Wind Speed based on 105 mph Basis for C&C

Effective Wind Speed based on 105 mph Basis for C&C

Effective Wind Speed based on 105 mph Basis for C&C

Effective Wind Speed based on 105 mph Basis for C&C

Effective Wind Speed based on 105 mph Basis for C&C

Effective Wind Speed based on 105 mph Basis for C&C

ASCE 7 Wind Provisions Update Status • Currently completing remaining final ballot items before these are  forwarded to the ASCE 7 Main Committee in August • Updates have moved to a 5 year revision cycle • ASCE 7‐10 will be referenced in the 2012 IBC, then presumably  adopted by 2014 in the State of Hawaii, and then by the counties by  2016.