ASCE 7-10 Significant Changes to the Wind Load...
Transcript of ASCE 7-10 Significant Changes to the Wind Load...
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ASCE 7-10 Significant Changes to the Wind Load
ProvisionsWilliam L. Coulbourne, P.E. SECBApplied Technology Council (ATC)
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Acknowledgements
Ron Cook, Univ. of Florida, Wind Load Task Committee Chairman
T. Eric Stafford, T. Eric Stafford & Associates Peter Vickery, Applied Research Associates Larry Griffis, Walter P. Moore and Associates ASCE 7 Wind Load Subcommittee
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Today’s Agenda
Classification of buildings Reorganization of wind provisions New wind maps MWFRS and C&C New Simplified Procedure for
buildings with h≤ 160 ft. Example problem
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ASCE 7-05 Chapter 6 Design Methods
Simplified Method (low-rise) Analytical Method
Low-rise buildings Buildings of all heights Other Structures
Wind Tunnel Procedure All in one chapter Confusing? Yes!
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ASCE 7-10 Significant Changes
Changes to Building Classification (Chapter 1) Complete reorganization of wind provisions New wind speed maps New wind-borne debris region Re-introduction of Exposure D for water
surfaces in hurricane-prone regions Simplified procedure for buildings < 160 ft
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Classification of BuildingsUse or Occupancy of Buildings and Structures Risk CategoryBuildings and other structures that represent a low risk to human life in the event of
failure:I
All buildings and other structures except those listed in Risk Categories I, III, and IV IIBuildings and other structures, the failure of which could pose a substantial risk to
human life.Buildings and other structures, not included in Occupancy Category IV, with potential
to cause a substantial economic impact and/or mass disruption of day-to-daycivilian life in the event of failure
Buildings and other structures not included in Risk Category IV (including, but notlimited to, facilities that manufacture, process, handle, store, use, or dispose ofsuch substances as hazardous fuels, hazardous chemicals, hazardous waste, orexplosives) containing toxic or explosive substances where the quantity of thematerial exceeds a threshold quantity established by the authority havingjurisdiction and is sufficient to pose a threat to the public if released.
III
Buildings and other structures designated as essential facilities. Buildings and other structures, the failure of which could pose a substantial hazard to
the community.Buildings and other structures (including, but not limited to, facilities that
manufacture, process, handle, store, use, or dispose of such substances as hazardous fuels, hazardous chemicals, or hazardous waste) containing sufficient quantities of highly toxic substances where the quantity exceeds a threshold quantity established by the authority having jurisdiction to be dangerous to the public if released and is sufficient to pose a threat to the public if released.a
Buildings and other structures required to maintain the functionality of other Category IV structures.
IV
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Classification of Buildings1.5.1 Risk Categorization. Buildings and other structures shall be
classified, based on the risk to human life, health and welfare associated with their damage or failure by nature of their occupancy or use, according to Table 1-1 for the purposes of applying flood, wind, snow, earthquake, and ice provisions. Each building or other structure shall be assigned to the highest applicable risk category or categories. Minimum design loads for structures shall incorporate the applicable Importance Factors given in Table 1-2, as required by other Sections of this Standard. Assignment of a building or other structure to multiple risk categories based on the type of load condition being evaluated (e.g., wind or seismic) shall be permitted.
When the building code or other referenced standard specifies an Occupancy Category, the Risk Category shall not be taken as lower than the Occupancy Category specified therein.
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Classification of Buildings
Elimination of the specific examples of buildings that fall into each category has the benefit that it eliminates the potential for conflict between the standard and locally adopted codes and also provides individual communities and development teams the flexibility to interpret acceptable risk for individual projects.
Guidance on what types of buildings might fall into each Risk Category is now gone from the body of the standard. Examples are still provided in the Commentary.
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Reorganization of Wind Provisions
New organization (based on the more user-friendly multi-chapter seismic provisions): 6 new Chapters (Chapters 26-31) Flowcharts on how to use in each chapter Intent is to clarify the applicability of the
wind provisions
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Reorganization of Wind Provisions
Chapter 26 – General Requirements Chapter 27 – MWFRS Directional Procedure Chapter 28 – MWFRS Envelope Procedure Chapter 29 – MWFRS Other Structures and
Appurtenances Chapter 30 – Components and Cladding Chapter 31 – Wind Tunnel Procedure
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Reorganization of Wind Provisions
Chapter 26 – General Requirements Scoping Definitions Wind speed map Exposure Gust factor Topographic factor
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MWRFS Directional Procedure
Buildings of all heights (old Figure 6-6, new Figure 27.4-1)
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MWFRS Directional Procedure
“A procedure for determining wind loads on buildingsand other structures for specific wind directions, inwhich the external pressure coefficients utilized arebased on past wind tunnel testing of prototypicalbuilding models for the corresponding direction ofwind.”
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MWRFS Envelope Procedure
Buildings 60 ft (old Figure 6-10, new Figure 28.4-1)
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MWFRS Envelope Procedure
“A procedure for determining wind load cases onbuildings, in which pseudo external pressure coefficientsare derived from past wind tunnel testing of prototypicalbuilding models successively rotated through 360degrees, such that the pseudo pressure cases producekey structural actions (uplift, horizontal shear, bendingmoments, etc.) that envelop their maximum valuesamong all possible wind directions.”
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Reorganization of Wind Provisions
Chapter 27 – MWFRS Directional Procedure Part 1: Buildings of all heights method Part 2: New simplified method for simple
diaphragm buildings 160 ft
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Reorganization of Wind Provisions
Chapter 28 – MWFRS Envelope Procedure Part 1: Low-rise ( 60 ft) buildings method Part 2: Simplified method for low-rise ( 60
ft) simple diaphragm buildings
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Reorganization of Wind Provisions
Chapter 29 – MWFRS Other Structures and Appurtenances Signs Rooftop structures Other structures
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Reorganization of Wind Provisions
Chapter 30 – Components and Cladding1. Envelope method for h ≤ 60 ft2. Simplified method for h ≤ 60 ft3. Directional method for h > 60 ft4. Simplified method for h ≤ 160 ft5. Analytical method for open buildings of all
heights6. Building appurtenances and roof top
structures
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Reorganization of Wind Provisions
Chapter 31 – Wind Tunnel Procedure
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New Wind Speed Maps
New data and data analysis indicate current ASCE 7 hurricane wind speeds are generally conservative
Introduction of ultimate wind speed maps LRFD Wind Load Factor = 1.0 ASD Wind Load Factor = 0.6
Specific maps for each building category
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Facts About the ASCE 7-05 Wind Speed Map
In most of the non-hurricane US mainland, the mapped values represented a 50-year mean recurrence interval
In hurricane regions, the mapped values varied from 50 to 100 years along the hurricane coastline
Wind speeds along the hurricane coastline had been adjusted upward so that when incorporated with the wind LF, produce a wind load having a consistent hazard level with the interior US (700 MRI)
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 23
Wind Speed vs MRIASCE 7-05
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
2.00
1 10 100 1000 10000
MRI T
V(T)
/V50
= (L
F)0.
5
Non-HurricaneV(T)/V(50)=0.36+0.1ln(12T)
HurricaneV(T)/V(50)=0.167ln(12T)0.97
1.00For Non-HurrVmap = 50 Yr MRI
For Hurr, Vmap = 500 yr/(1.5)0.5
= 1.112 ; 97 MRI
97 50
1.112x
x
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Wind Pressure vs MRIASCE 7-05
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
1 10 100 1000 10000
MRI T
P(T)
/P50
= L
F
Hurricane
Non-Hurricane
Non Hurr: Vmap = 50 yr MRI
Hurr: Vmap = 97 yr MRI
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 25
Wind Pressure vs MRIASCE 7-05
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
1 10 100 1000 10000MRI T
P(T)
/P50
= L
F
Hurricane
Non-Hurricane
72050 97
1.6
Map V for hurr. (> 50 yr)
1.6
1770
With I=1.15
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 26
Why Change the Map?
Current hybrid pseudo MRI Map – confusing Designers, students, clients
Most Users Don’t Know LF =1.6: 700 yr MRI “ultimate wind speed” Importance Factor of 1.15: 1700 yr MRI
Update for new hurricane model Provide 3 Maps:
300 yr (Cat 1), 700 yr (Cat 2) 1700 yr (Cat 3,4) Eliminate Wind Importance Factors
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 27
New Hurricane Model Summary
New intensity model includes ocean mixing to limit hurricane intensity (defined by central pressure)
New statistical model for Holland B parameter (yields lower wind speeds than 2000 model)
New filling (storm weakening after landfall) model New wind field model 100,000 year simulation vs. 20,000 in the old map Map developed using 2851 pts vs. 208 in the old map
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 28
New Wind Speed Maps
Specific maps for each building category Category II buildings – 700 year return period wind
speed Category III and IV buildings – 1700 year return
period wind speeds Category I buildings – 300 year return period wind
speeds Importance Factor no longer required
Serviceability maps (10, 25, 50 and 100 year) to be included in Appendix C
Find wind speeds @ www.atcouncil.org/windspeed
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Site-SpecificWind Speed Example
Location:New Orleans, LA
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700 Year RP Winds
Notes:1. Values are nominal design 3-second gust wind speeds in miles per hour (m/s) at 33 ft (10m) above ground for Exposure C category.2. Linear interpolation between contours is permitted.3. Islands and coastal areas outside the last contour shall use the last wind speed contour of the coastal area.4. Mountainous terrain, gorges, ocean promontories, and special wind regions shall be examined for unusual wind conditions.5. Wind speeds correspond to approximately a 7% probability of exceedance in 50 years (Annual Exceedance Probability = 0.00143, MRI = 700 Years).
Location Vmph (m/s)Guam 195 (87)Virgin Islands 165 (74)American Samoa 160 (72)Hawaii Special Wind Region Statewide
Puerto Rico
110(49)
115(51)
150(67) 160(72)170(76)
115(51)
115(51)150(67)
140(63)120(54)
130(58)
170(76)160(72)
180(80)
180(80)
170(76)160(72)
150(67)140(63)
140(63)
150(67)
140(63)
130(58)
120(54)
115(51)
110(49)
150(67)
120(54)130(58)140(63)
160(72)
160(72)150(67)
140(63)
130(58)
120(54)
110(49)
Special Wind Region
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 31
New V700/√1.6 vs. ASCE 7-05
140
130
150
140
140
130
110
120130150
110
110
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1700 Year RP Winds
Notes:1. Values are nominal design 3-second gust wind speeds in miles per hour (m/s) at 33 ft (10m) above ground for Exposure C category.2. Linear interpolation between contours is permitted.3. Islands and coastal areas outside the last contour shall use the last wind speed contour of the coastal area.4. Mountainous terrain, gorges, ocean promontories, and special wind regions shall be examined for unusual wind conditions.5. Wind speeds correspond to approximately a 3% probability of exceedance in 50 years (Annual Exceedance Probability = 0.000588, MRI = 1700 Years).
Location Vmph (m/s)Guam 210 (94)Virgin Islands 175 (78)American Samoa 170 (76)Hawaii Special Wind Region Statewide Puerto Rico
115(52)
120(54)
160(72) 170(76)
180(80)
120(54)
120(54)130(58)
140(63)150(67)160(72)
170(76) 180(80)
150(67)160(72)
170(76)180(80)
190(85)
200(89)
200(89)
160(72)
150(67)140(63)
130(58)
120(54)
160(72)
150(67)
165(74)
165(74)
160(72)150(67)
140(63) 130(58)120(54)
115(51)
115(51)120(54)
130(58)
140(63)150(67)
Special Wind Region
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 33
300 Year RP Winds
Notes:1. Values are nominal design 3-second gust wind speeds in miles per hour (m/s) at 33 ft (10m) above ground for Exposure C category.2. Linear interpolation between contours is permitted.3. Islands and coastal areas outside the last contour shall use the last wind speed contour of the coastal area.4. Mountainous terrain, gorges, ocean promontories, and special wind regions shall be examined for unusual wind conditions.5. Wind speeds correspond to approximately a 15% probability of exceedance in 50 years (Annual Exceedance Probability = 0.00333, MRI = 300 Years).
Location Vmph (m/s)Guam 180 (80)Virgin Islands 150 (67)American Samoa 150 (67)Hawaii Special Wind Region Statewide Puerto Rico
100(45)
105(47)
140(63) 150(67)160(72)
105(47)
105(47)
110(49) 140(63)
150(67)140(63)
130(58)
130(58)
150(67) 160(72)
130(58)
140(63)
120(54)110(49)
105(47)
170(76)
170(76)
130(58)
140(63)
120(54)
150(67)
150(67)
140(63) 130(58)120(54)
110(49)105(47)
105(47)
110(49)
120(54)130(58)
140(63)
Speical Wind Region
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 34
Wind speeds at selected locations
Location 6.1/700V
ASCE 7-05 Exposure C
Exposure C Exposure D Bar Harbor, Maine 97 95 103 Boston, MA 106 103 112 Hyannis, MA 117 112 122 New Port, RI 117 109 119 Southampton, NY 120 110 119 Atlantic City, NJ 114 102 111 Wrightsville Beach, NC 132 119 129 Folly Beach, SC 131 115 125 Miami Beach 145 136 148 Clearwater, FL 128 115 125 Panama City, FL 129 107 116 Biloxi, MS 138 129 140 Galveston, TX 131 119 129 Port Aransas, TX 134 117 127 Hawaii 105 103 112 Guam 170 155 168
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 35
ASCE 7-10Strength Design Load Combinations
1.4D 1.2D + 1.6L + 0.5(Lr or S or R) 1.2D + 1.6(Lr or S or R) + (L or 0.5W) 1.2D + 1.0W + L + 0.5(Lr or S or R) 1.2D + 1.0E + L + 0.2S 0.9D+ 1.0W 0.9D + 1.0E
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 36
ASCE 7-10Allowable Stress Design Load Combinations
D D + L D + (Lr or S or R ) D + 0.75L + 0.75(Lr or S or R) D + (0.6W or 0.7E) D + 0.75L + 0.75(0.6W) + 0.75(Lr or S or R) D + 0.75 L + 0.75 (0.7 E) + 0.75S 0.6D + 0.6W 0.6D + 0.7E
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 37
Windborne Debris Region Current Standard
V > 120 or 110 within one mile of coast Exact Mapping (new 700 year map)
120√1.6=152~150 110√1.6=139~140
New Standard V > 140 or 130 within one mile of coast Results in less area within WBD Region
than the existing standard
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 38
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 39 39
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 40
Reintroduction of Exposure D in Hurricane- Prone Regions
Older research and modeling suggested roughness of ocean approached Exposure C with increase in wind speed
Hence ASCE 7-98, -02, and -05 classified water surfaces in hurricane-prone regions as Exposure C
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 41
Reintroduction of Exposure D in Hurricane- Prone Regions
New research suggests hurricane coastline matches the exposure description for Exposure D
Roughness of ocean does not continue to increase with increasing wind speed.
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 42
Exposure D
0.80
0.85
0.90
0.95
1.00
1.05
1.10
1.15
1.20
30 40 50 60 70
Gus
t Win
d Sp
eed
Rat
io (M
arin
e/La
nd)
Mean Wind Speed at 10m Over Water (m/sec)
Vickery et al. (2000a)Vickery et al. (2008a), RMW=20 kmVickery et al. (2008a), RMW=40 kmVickery et al. (2008a), RMW=80 km
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 43
Summary of Wind Maps
Use individual maps for structures with different life-safety concerns rather than using Importance Factors
A 1.0 load factor for LRFD (same as Seismic) A 0.6 load factor for ASD design The result is consistent with the intent of ASCE
7-98 and better represents life-safety objectives
Exposure D must be used on hurricane coastlines
Serviceability wind load maps are in Appendix C
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 44
New Simplified Procedure
Applicable to buildings less than or equal to 160 ft in height
Simple diaphragm buildings Frequency limitations and torsional
limitations Tabular loads for MWFRS and
Components and Cladding
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 45
Simplified Wind Design - MWFRSChapter 27 - Part 2
Simple diaphragm buildings Enclosed building (GCpi = + or – 0.18) h ≤ 160 feet Flat, monoslope, mansard roofs, gable
roofs Based on ASCE 7-10 Figure 27.4-1 – Part 1
Traditional “Directional Approach”
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 46
Simple Diaphragm Building
Building -with vertically spanning wall systemsin which both windward and leeward wind loads are transmitted through floor and roof diaphragms (rigid or flexible)to the same MWFRS
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 47
Simple Diaphragm Building
rigid or flexible floordiaphragms
Main Wind ForceResisting System
(MWFRS)
Vertically spanning walls
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 48
Simplified 160 ft MethodTwo Classes of Buildings
Class 1: Simple diaphragm building h ≤ 60 ft 0.2 ≤ L/B ≤ 5.0 Kzt = 1.0 or calculated
BL
h
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 49
Assumptions – Class 1 Bldgs
Rigid or flexible diaphragm enclosed buildings h ≤ 60 ft. 0.2 ≤ L/B ≤ 5.0 (interpolate between) No topographic effects (Kzt = 1) or calculate Symmetric placement MWFRS MWFRS placed so that torsion does not control
(guidance provided)
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 50
Simplified 160 ft MethodTwo Classes of Buildings
Class 2: Simple diaphragm building 60 ≤ h ≤ 160 ft 0.5 ≤ L/B ≤ 2.0 Kzt = 1.0 or calculated
BL
h
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 51
Assumptions – Class 2 Bldgs
Rigid or flexible diaphragm enclosed buildings h = 60 – 160 ft. Period T = h/75 seconds (upper bound) Damping = 1.5% (lower bound) L/B = 0.5. 1.0, 2.0 (interpolate between) No topographic effects (Kzt = 1) or calculate Symmetric placement MWFRS elements MWFRS placed so that torsion does not control
(guidance provided)
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 52
Why is Building Period Important?
Related to mass and stiffness of building Stiffness affects drift and motion perception Mass affects wind forces Mass affects seismic forces Mass affects motion perception
Period affects Gust Effect Factor, thus pressure p Buildings with high periods interact more with the
wind Note:
Higher Period T is conservative (opposite from seismic!)
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 53
Wall Pressures
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 54
Internal pressure GCpi
• Affects M, V, uplift in one story rigid frame buildings
• Cancels out in simple diaphragm buildings
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 55
Wind Pressure Equation
pipf GCqCqGp
pf CqGp
plhpwzfz CqCqGp
- General Equation (27.4-2)
- For simple diaphragm buildings
- windward, leeward walls
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 56
Wind Pressure Vs HeightASCE 7-05
0
20
40
60
80
100
120
140
160
40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70Pressure (psf)
Hei
ght (
ft)
h=160 ft.V=120 MPHExposure CT=h/75Damping=1%
ASCE 7-05 ExactSimplified
p15
p = 1.04p160
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 57
Wind Load Equations
Pressure (psf):pz = p0 (1 - z / h) + (z / h) ph
Story Shear (pounds):vz = 0.5(h - z) [(p0 (1 - z / h) + ph (1 + z / h)]
Overturning Moment (ft.-pounds):mz = 1/3 (h - z)2 [0.5p0 (1 – z / h) + ph (1 + 0.5 z / h)]
zvz
zmz
p0
ph
zpz
Table values
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 58
Tabularized Wall Pressures
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 59
Story Shear Vs HeightExact vs Simplified
0102030405060708090
100110120130140150160
0 1000 2000 3000 4000 5000 6000 7000 8000Story Shear (pounds)
Hei
ght (
ft)
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 60
Story Moment Vs HeightExact vs Simplified
0102030405060708090
100110120130140150160
0 100000 200000 300000 400000 500000 600000 700000Moment (foot-pounds)
Hei
ght (
ft)
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 61
Roof Pressure Zones
Roof Shapes:• Flat• Gable• Hip• Monoslope • Mansard
Roof Pressures - MWFRS
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 62
Exposure B,C,DTables
Height h (ft)
Pressure (psf)(Two load casesfor sloped roofs)
V (MPH)Roof ZoneRoof Slope
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 63
Example Problem
Building is 100 ft. tall with flat roof The wind speed from ASCE 7-05 was 140
mph Ocean exposure Category II enclosed building Dimensions are B = 40 ft. and L = 200 ft. Determine pressures at 100 ft. using All
heights method of ASCE 7-05, ASCE 7-10, and the Simplified Method for buildings less than 160 ft. from ASCE 7-10
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 64
Comparative Results
Engineering Standard
Windward Wall (psf)
Roof Edge Zone (psf)
ASCE 7-05 Exposure C
73.94 -81.25
ASCE 7-10 Exposure D
83.85 -92.14
Simplified Exposure D
111.85 -95.82
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 65
Components and CladdingChapter 30 – Part 4
ph = qh [(G Cp) – (GCpi)] = qh (GCp)net
GCp values :• Flat roofs – Fig. 30.6-1 for zones 1-3• Gable roofs, mansard roofs – Fig. 30.4-2A, B, C for
zones 1-3 • Hip roofs – Fig. 30.4-2B for zones 1-3• Monoslope roofs – Fig. 30.4-5A, B for zones 1-3• Wall zones 4 and 5 for all cases from Fig. 30.6-1. • Table pressures based on h and qh
GCpi values for enclosed buildings are taken as +(-) 0.18.
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 66
Component and Cladding
Wall and Roof Zones
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 67
Component and Cladding Pressures
V (mph)
ZoneLoad CaseRoof ShapeHeight h (ft)Exposure C
Effective Wind Area = 10 sf
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 68
Exposure Amplification Factor
Roof and Wall Pressures - Components and CladdingExposure Amplification Factor
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25
Ratio Exposure B/C, D/C
Bui
ldin
g he
ight
h (f
t)
B/C D/C
Exp. Ampl. Factorh (ft) B/C D/C160 0.81 1.11150 0.80 1.12140 0.80 1.12130 0.80 1.12120 0.79 1.12110 0.79 1.13100 0.78 1.1390 0.77 1.1480 0.77 1.1470 0.76 1.1560 0.75 1.1550 0.74 1.1640 0.73 1.1730 0.71 1.1820 0.69 1.2015 0.68 1.21
Multiplier toTable pressures
Exp B, D
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 69
Reduction Factors - Effective Wind Area
Reduction FactorsEffective Wind Area
0.5
0.6
0.7
0.8
0.9
1
1.1
1 10 100 1000
Effective Wind Area (sf)
Red
uctio
n Fa
ctor
20 50 200 500
0.8
0.7
0.6
1.0
0.9
A
B
C
D
E
Roof type/case
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NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 70
Other Changes
30 items approved by the Wind Load Subcommittee
Examples: Improved “roughness” definitions and examples Revisions to low-rise “envelope” method Guidelines on wind-tunnel testing Reduced minimum load on projected area of roof etc, etc.