asce 7-10 significant changes to the wind load provisions · 2015. 8. 20. · ncsea webinar –asce...

ASCE 7-10 Significant Changes to the Wind Load

ProvisionsWilliam L. Coulbourne, P.E. SECBApplied Technology Council (ATC)

[email protected]

NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 2

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


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


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!


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


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


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.


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.


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



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



Chapter 26 – General Requirements Scoping Definitions Wind speed map Exposure Gust factor Topographic factor


MWRFS Directional Procedure

Buildings of all heights (old Figure 6-6, new Figure 27.4-1)


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.”


MWRFS Envelope Procedure

Buildings 60 ft (old Figure 6-10, new Figure 28.4-1)


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.”



Chapter 27 – MWFRS Directional Procedure Part 1: Buildings of all heights method Part 2: New simplified method for simple

diaphragm buildings 160 ft



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



Chapter 29 – MWFRS Other Structures and Appurtenances Signs Rooftop structures Other structures



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



Chapter 31 – Wind Tunnel Procedure


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


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)


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


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


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


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


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


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


Site-SpecificWind Speed Example

Location:New Orleans, LA


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


New V700/√1.6 vs. ASCE 7-05

140

130

150

140

140

130

110

120130150

110

110


1700 Year RP Winds


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


300 Year RP Winds


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


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


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


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


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

NCSEA Webinar –ASCE 7-10 Changes in Wind Load Provisions 39 39


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


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.


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


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


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


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”


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


Simple Diaphragm Building

rigid or flexible floordiaphragms

Main Wind ForceResisting System

(MWFRS)

Vertically spanning walls


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


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)


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


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)


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!)


Wall Pressures


Internal pressure GCpi

• Affects M, V, uplift in one story rigid frame buildings

• Cancels out in simple diaphragm buildings


Wind Pressure Equation

pipf GCqCqGp

pf CqGp

plhpwzfz CqCqGp

- General Equation (27.4-2)

- For simple diaphragm buildings

- windward, leeward walls


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


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


Tabularized Wall Pressures


Story Shear Vs HeightExact vs Simplified

0102030405060708090

100110120130140150160

0 1000 2000 3000 4000 5000 6000 7000 8000Story Shear (pounds)

Hei

ght (

ft)


Story Moment Vs HeightExact vs Simplified

0102030405060708090

100110120130140150160

0 100000 200000 300000 400000 500000 600000 700000Moment (foot-pounds)

Hei

ght (

ft)


Roof Pressure Zones

Roof Shapes:• Flat• Gable• Hip• Monoslope • Mansard

Roof Pressures - MWFRS


Exposure B,C,DTables

Height h (ft)

Pressure (psf)(Two load casesfor sloped roofs)

V (MPH)Roof ZoneRoof Slope


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


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


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.


Component and Cladding

Wall and Roof Zones


Component and Cladding Pressures

V (mph)

ZoneLoad CaseRoof ShapeHeight h (ft)Exposure C

Effective Wind Area = 10 sf


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


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


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.


[email protected]

asce 7-10 significant changes to the wind load provisions · 2015. 8. 20. · ncsea webinar –asce...

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