changes from asce 7-05 to asce 7-10: wind provisions

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S. K. Ghosh Associates Inc.

www.skghoshassociates.com

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Changes from ASCE 7-05 to ASCE 7-10: Wind Provisions


Palatine, IL and Aliso Viejo, CA


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ASCE 7-10

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2012 International Building Code

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CHAPTER 6(Wind Loads in ASCE 7-05)

Reserved for Future Provisions

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Chapters 26 - 31Wind Loads

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Reorganization of Wind Provisions

ASCE 7-05: Chapter 6 contained all wind provisions

New:

• 6 new Chapters (Chapters 26-31)

• User Notes added

• Intent is to clarify the applicability of the wind provisions

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Reorganization of Wind Provisions(ASCE 7-05: Dark Gray; ASCE 7-10: Orange)

Method 1 – Envelope Procedure MWFRS, C&C (Simplified Method 2 Low-Rise)

Method 2

All-Heights - Directional Procedure MWFRS, C&C

Low-Rise – Envelope Procedure MWFRS, C&C

Method 3 – Wind Tunnel Procedure

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Chapter 26 – General Requirements

Chapter 27 – MWFRS Directional Procedure Method 2 All-Heights, new simplified version - MWFRS

Chapter 28 – MWFRS Envelope Procedure Method 2 Low-Rise, Method 1 - MWFRS

Chapter 29 – MWFRS Other Structures and

Appurtenances

Chapter 30 – C&C Method 2 Low-Rise, Method 1, Method 2 All-

Heights, new simplified version of Method 2 All-Heights – C&C

Chapter 31 – Wind Tunnel Procedure Method 3

Reorganization of Wind ProvisionsASCE 7-10: Dark Gray; ASCE 7-05: Red)

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Wind Loads

Load factor on W is now 1.0 in Strength Design and 1/1.6 or 0.6 in ASD.

Thus, the design wind speed maps in ASCE 7-10 produce strength-level design wind forces. The mapped design wind speeds are, therefore, higher than in ASCE 7-05.

W SD

0.6W ASD

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Ch 2 Load Combinations with WindStrength Design

2.3.2 Basic Combinations

3. 1.2D + 1.6(Lr or S or R) + (L or 0.8 0.5W )

4. 1.2D + 1.6 1.0W + L + 0.5(Lr or S or R)

6. 0.9D + 1.6 1.0W + 1.6H

Load factor on W in SD load combinations is now 1.0 because mapped design wind speeds will produce strength-level loads

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Ch 2 Load Combinations with WindAllowable Stress Design

2.4.1 Basic Combinations

5. D + H + F + (0.6W or 0.7E)

6a. D + 0.75L + 0.75(0.6W) + 0.75 (Lr or S or R)

7. 0.6D + 0.6W + H

Load factor on W in ASD load combinations is now 0.6 because mapped design wind speeds will produce strength-level loads

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Chapter 26 – General Requirements

• Scoping

• Definitions

• Wind speed map

• Directionality factor

• Exposure

• Gust effect factor

• Topographic effect factor

• Enclosure classification

• Internal pressure coefficient

ASCE 7-10Reorganization of Wind Provisions

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I

Wind Loads

No Importance Factor is used in wind design any more.

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Wind Importance Factor

Importance Factor No Longer Used

• ASCE 7-05: qz = 0.00256KzKztKdV2I

• ASCE 7-10: qz = 0.00256KzKztKdV2

Basic Wind Speeds: 3 Maps replace need for Importance Factor

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Wind Loads

The Standard includes three design wind speed maps:

A 300-year return period wind speed map for design of Risk Category I structures

A 700-year return period wind speed map for design of Risk Category II structures

A 1700-year return period wind speed map for design of Risk Category III, IV structures

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Occupancy/Risk Categories

Occupancy Category (ASCE 7-05)Risk Category (ASCE 7-10)

Description

I Miscellaneous Occupancy

II Standard Occupancy (not I, III, or IV)

III High Occupancy

IV Essential FacilityHazardous Facility

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NEW WIND SPEED MAPS

Fig. 26.5-1A: Risk Category II

Fig. 26.5-1B: Risk Categories III & IV

Fig. 26.5-1C: Risk Category I

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ASCE 7-10 Chapter 26

ASCE 7-05Chapter 6

New Wind Speed Maps

Risk Category II

Risk Category III+IV

Risk Category I

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ASCE 7-05 Figure 6-1

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ASCE 7-10 Figure 26.5-1A

Risk Category II - 7% probability of exceedance in 50 years, MRI of 700 years

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ASCE 7-10: Figure 26.5-1B

Risk Category III and IV - 3% probability of exceedance in 50 years, MRI of 1700 years

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ASCE 7-10: Figure 26.5-1C

Risk Category I - 15% probability of exceedance in 50 years, MRI of 300 years

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Wind Speed Website

Applied Technology Council

• http://www.atcouncil.org/windspeed/

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Wind Speed Website

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Comparative Wind Pressures forSan Francisco

qz = 0.00256KzKztKdV2I OC I OC II OC III, IV

ASCE 7-05

V, mph 85 85 85

I 0.87 1.00 1.15

25.75 psf 29.59 psf 34.03 psf

RC I RC II RC III, IV

ASCE 7-10

V, mph 100 110 115

25.60 psf 30.98 psf 33.86 psf

-0.58% +4.70% -0.50%

6.1dztz KKK

q

dztz KKK

q

05

1005

qz = 0.00256KzKztKdV2

qz = 0.00256KzKztKdV2I

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Comparative Wind Pressures forChicago

qz = 0.00256KzKztKdV2I OC I OC II OC III, IV

ASCE 7-05

V, mph 90 90 90

I 0.87 1.00 1.15

28.86 psf 33.18 psf 38.15 psf

RC I RC II RC III, IV

ASCE 7-10

V, mph 105 115 120

28.22 psf 33.86 psf 36.86 psf

-2.22% +2.05% -3.38%

6.1dztz KKK

q

dztz KKK

q

05

1005

qz = 0.00256KzKztKdV2

qz = 0.00256KzKztKdV2I

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Risk Category II Comparison

0.74-0.86

0.80-0.93

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Risk Category III and IV Comparison

0.71-0.95

0.79-0.95

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Risk Category I Comparison

1.06-0.97

0.84-1.03

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Section 26.2 Definitions

Changes in the mapped wind speeds necessitated recalibration of some definitions and requirements.

1 map referenced 3 maps referenced

Old wind speed New wind speed

Occupancy Category Risk Category

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Section 26.2 Definitions

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 basic wind speed for Risk Category II buildings is greater than 90 115 mph, and

2. Hawaii, Puerto Rico, Guam, Virgin Islands, and American Samoa.

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Section 26.10.3 Protection of Glazed Openings

Glazed openings in Risk Category II, III or IVbuildings located in hurricane-prone regions shall be protected as specified in this Section.

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26.10.3.1 Wind-borne Debris Regions. Glazed openings shall be protected in accordance with 26.10.3.2 in the following locations:

1. Within 1 mile of coastal mean high water line where basic wind speed is equal to or greater than 110 130 mph, or

2. In areas where basic wind speed is equal to or greater than 120 140 mph.

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26.10.3.1 Wind-borne Debris Regions (contd). For Risk Category II buildings and structures and Risk Category III buildings and structures, except health care facilities, wind-borne debris region shall be based on Fig. 26.5-1A. For Risk Category III health care facilities and Risk Category IV buildings and structures, wind-borne debris region shall be based on Fig. 26.5-1B. Risk Categories shall be determined in accordance with Section 1.5.

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For Risk Category II buildings and structures and Risk Category III buildings and structures, except health care facilities

Fig. 26.5-1A

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For Risk Category III health care facilities and Risk Category IV buildings and structures

Fig. 26.5-1B

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Section 26.10.3.2 Protection Requirements for Glazed Openings

ASTM E1886 - 05 Standard Test Method for Performance of Exterior Windows, Curtain Walls, Doors, and Impact Protective Systems Impacted by Missile(s) and Exposed to Cyclic Pressure Differentials

ASTM E1996 - 09 Standard Specification for Performance of Exterior Windows, Curtain Walls, Doors, and Impact Protective Systems Impacted by Windborne Debris in Hurricanes

EXCEPTION: Other testing methods and/or performance criteria are permitted to be used when approved.

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Section 26.7.2 Surface Roughness Categories

Surface Roughness C: Open terrain with scattered obstructions having heights generally less than 30 ft. This category includes flat open country, andgrasslands, and all water surfaces in hurricane prone regions.

Surface Roughness D: Flat, unobstructed areas and water surfaces outside hurricane prone regions. This category includes smooth mud flats, salt flats, and unbroken ice.

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Water surfaces in hurricane-prone regions are moved from Surface Roughness C to Surface Roughness D.

Older research and modeling suggested roughness of ocean approached Surface Roughness C with increase in wind speed.

New research suggests otherwise - roughness of ocean does not continue to increase with increasing wind speed.

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Section 26.9.2 Frequency Determination

In ASCE 7-05, several expressions for computing approximate fundamental frequency , n1, of a building were suggested in C6.5.8.

Some of those expressions are now included within the body of ASCE 7-10

Expressions provide conservative lower-bound estimates of n1, which is needed to distinguish between rigid and flexible buildings.

Low-Rise Buildings, as defined in 26.2, are permitted to be considered rigid.

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26.9.2.1. As an alternative to performing an analysis to determine n1, the approximate building natural frequency, na, shall be permitted to be calculated in accordance with 26.9.3 for structural steel, concrete, or masonry buildings meeting the following requirements:

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26.9.2.1 May use na for n1 if

1. Building height is less than or equal to 300 ft, and

2. Building height is less than 4 times its effective length, Leff.

(26.9-1)

summations are over height of building wherehi is height above grade of level iLi is building length at level i parallel to wind direction

n

ii

n

iii

eff

h

LhL

1

1

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Section 26.9.3 Approximate Natural Frequency

Structural steel moment-resisting-frame buildings:

na = 22.2/h0.8 (26.9-2)

Concrete moment-resisting-frame buildings:

na = 43.5/h0.9 (26.9-3)

Structural steel and concrete buildings with other lateral-force-resisting system:

na = 75/h (26.9-4)

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Concrete or masonry shear wall buildings

na = 385(Cw)0.5/h (26.9-5)

where

n

i

i

i

i

iBw

Dh

A

h

h

AC

12

2

83.01

100

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h = mean roof height (ft)

n = number of shear walls in building effective in resisting lateral forces in direction under consideration

AB = base area of structure (ft2)

Ai = horizontal cross-section area of shear wall “i” (ft2)

Di = length of shear wall “i” (ft)

hi = height of shear wall “i” (ft)

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BREAK!

If you are encountering technical difficulties, please call (847) 991-2700

If you have any questions, please type them in

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Question and Answer Session



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Chapter 27 – MWFRS Directional Procedure

• Part 1 – “Buildings of all heights” method ASCE 7-05 Method 2 All-Heights

• Part 2 – New simplified method for buildings of height less than 160 ft.

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ASCE 7-10 Sections 27.4.7, 28.4.4, 28.6.4 Minimum Design Wind Loads

ASCE 7-05

ASCE 7-10

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Directional Procedure

CLASS 1 BUILDINGSh ≤ 60 ft

0.2 ≤ L/B ≤ 5.0

PART 1Applies to enclosed, partially enclosed, and open buildings

of all heights

CLASS 2 BUILDINGS60 < h ≤ 160 ft0.5 ≤ L/B ≤ 2.0

Chapter 27Directional Procedure for MWFRS

Same as Analytical Method of ASCE 7-05

New simplified procedure in ASCE 7-10

PART 2Applies to enclosed simple diaphragm buildings with

h ≤ 160 ft

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Chapter 27 Directional Procedure for MWFRS – Part 2

ENCLOSED BUILDING: A building that does not comply with the requirements for open or partially enclosed buildings.

Required for the new simplified procedure of Chapter 27 Part 2

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Enclosed Building Example

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SIMPLE DIAPHRAGM BUILDING: A building in which both windward and leeward wind loads are transmitted by roof and vertically spanning wall assemblies, through continuous floor and roof diaphragms, to the MWFRS.

Required for the new simplified procedure of Chapter 27 Part 2

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Simple Diaphragm Building

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ASCE 7-10 Chapter 27 Directional Procedure for MWFRS – Part 2 (Both Class 1 & 2)

General Conditions (same as Part 1) per Section 27.1.2:

1. The building is regular-shaped

2. The building does not have response characteristics making it subject to across wind loading, vortex shedding, instability due to galloping or flutter; or does not have a site location for which channeling effects or buffeting in the wake of upwind obstructions warrant special consideration.

PART 2 (Both Class 1 & 2)Applies to enclosed simple diaphragm buildings with

h ≤ 160 ft

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ASCE 7-10 Chapter 27 Directional Procedure for MWFRS – Part 2 Class 1

Additional Condition for Part 2 Class 1 Buildings per Section 27.5.2:

Unless Kzt = 1.0, wind pressures determined from this section shall be multiplied by:

Kzt at each height z as determined from 26.8 or

one value of Kzt for the building calculated at 0.33h.

Alternatively, enter the pressure tables with V√Kztwhere Kzt is determined at 0.33h.

PART 2 CLASS 1 BUILDINGSh ≤ 60 ft

0.2 ≤ L/B ≤ 5.0

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ASCE 7-10 Chapter 27 Directional Procedure for MWFRS – Part 2 Class 2

Additional Conditions for Part 2 Class 2 Buildings per Section 27.5.2:

1. The fundamental natural frequency of building shall not be less 75/h where h is in ft.

2. Unless Kzt = 1.0, wind pressures determined from this section shall be multiplied by:

Kzt at each height z as determined from 26.8 or

one value of Kzt for the building calculated at 0.33h.

Alternatively, enter the pressure tables with V√Kzt where Kzt is determined at 0.33h.

PART 2 CLASS 2 BUILDINGS60 < h ≤ 160 ft0.5 ≤ L/B ≤ 2.0

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Design Steps Parameter Source

1. Determine Basic Wind Speed for appropriate risk category

V (mph) Fig. 26.5-1 Maps

2. Determine exposure category Exp B, C or D Section 26.7

3. Enter table to determine net pressures on walls at top and base of building respectively

ph , p0 (psf) Table 27.6-1

4. Enter table to determine net roof pressures

Pz (psf) Table 27.6-2

5. Determine topographic factor and apply factor to wall and roof pressures (if applicable)

Kzt Section 26.8


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V (mph) 110 115 ………… 200

L/Bh (ft)

0.5 or less

1.02.0 or more

0.5 or less

1.02.0 or more

0.5 or less

1.02.0 or more

0.5 or less

1.02.0 or more

160ph

p0

150ph

p0

140ph

p0

15ph

p0


Table 27.6-1 for Exposure Categories B, C, and D

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Net pressure (psf) distribution in along-wind direction

h

p0

ph

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Pressure (psf) distribution on windward and leeward walls

p0 − pleeward

ph − pleeward

pleeward =

0.38ph for 0.2 ≤ L/B ≤ 1.0

0.27ph for 2.0 ≤ L/B ≤ 5.0

Interpolate for 1.0 < L/B < 2.0

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Net pressure (psf) on parapet

pp = 2.25 ph ph calculated from Table 27.6-1 at top of parapet for L/B = 1.0

Accounts for pressures on windward and leeward surfaces of windward and leeward walls

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Net pressure (psf) distribution on sidewalls

0.54ph for 0.2 ≤ L/B ≤ 1.0

0.64ph for 2.0 ≤ L/B ≤ 5.0

Interpolate for 1.0 < L/B < 2.0

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0.5h

0.5h

h3

4

5

Wind

Flat Roof(θ < 10 deg)


Net pressure (psf) distribution on roof

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θ

1

2

Wind

θ

h Wind

0.5h

0.5h

3

4

554

3

Gable Roof



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1

2

3

4

5

5 4

3

h 0.5h

0.5h

Wind

1

2

3 4

5

5

4

3

h

0.5h 0.5h

Wind Hip Roof



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1

h θ

Wind

2

h θ

Wind

Monoslope Roof4

h

Wind

3

5

0.5h 0.5h



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3 4

Wind

5

h 0.5h

0.5h

θ

1

2

Wind

2

Mansard Roof



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Roof pressures from Table 27.6-2 for Exposure C

For Exposures B and D, use adjustment factors provided in the footnotes to Table 27.6-2

Where two load cases are shown in the table, both load cases shall be investigated

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V (mph) 110 - 200

h (ft) Roof SlopeLoad Case

Zone

1 2 3 4 5

15 - 160

Flat < 2:12 (9.46 deg)12

3:12 (14.0 deg)12

4:12 (18.4 deg)12

5:12 (22.6 deg)12

6:12 (26.6 deg)12

9:12 (36.9 deg)12

12:12 (45.0 deg)12


Table 27.6-2 for Exposure Category C

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Chapter 27 Directional Procedure for MWFRS – Part 2 Conversion from Exposure C to Exposures B and D

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Application of wind loadPressures on the windward, leeward and sidewalls, and on roof and parapet are applied simultaneously

h

ph

p0

θ

Wind

L

B

Plan

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The MWFRS’s in each direction shall be designed for the wind load cases defined in Figure 27.4-8.

The torsional load cases in Figure 27.4-8 (Case 2 and Case 4) need not be considered for buildings that meet the requirements for spatial distribution and stiffness of the MWFRS’s provided in new Appendix D.


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Load Cases

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The MWFRS in each direction needs to be designed for Fig. 27.4-8 wind load cases.

NEW APPENIDX D! The Fig. 27.4-8 Case 2 and 4 torsional load cases need not be considered for buildings that meet the requirements for spatial distribution and stiffness of the MWFRS’s provided in Appendix D.


What is Appendix D?BUILDINGS EXEMPTED FROM TORSIONAL

WIND LOAD CASES

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Requirements for spatial distribution and stiffness of MWFRS’s in new Appendix D are necessary to ensure that wind torsion does not control design.

In general, the designer should place and proportion vertical elements of the MWFRS in each direction so that center of pressure from wind forces at each story is located at or near center of rigidity of the MWFRS, thereby minimizing inherent torsion from wind on building.

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Chapter 28 – MWFRS Envelope Procedure

• Part 1 – Low-rise buildings method ASCE 7-05 Method 2 Low-Rise

• Part 2 – Simplified method for simple diaphragm buildings ASCE 7-05 Method 1

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Chapter 29 – MWFRS Other Structures and Appurtenances

• Signs

• Rooftop structures

• Other structures

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29.4 Solid Freestanding Walls & Solid Signs

The provisions for design wind loads on signs have been revised to clarify the applicability of the requirements for solid attached signs.

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29.4 DESIGN WIND LOADS – SOLID FREESTANDING WALLS AND SOLID SIGNS

29.4.1 6.5.14 Design Wind Loads on Solid Freestanding Walls and Solid Freestanding Signs

The design wind force for solid freestanding walls and solid freestanding signs shall be determined by the following formula:

F = qhGCfAs (lb) (N) (29.4-1 6-27)

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Where:

qh = the velocity pressure evaluated at height h (defined in Figure 29.4-1 6-20) as determined in accordance with Section 29.3.2 6.5.6.4.1

G = gust-effect factor from Section 26.9 6.5.8

Cf = net force coefficient from Figure 29.4-1 6-20

As = the gross area of the solid freestanding wall or freestanding solid sign, in ft2 (m2)

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29.4.2 Solid Attached Signs

The design wind pressure on a solid sign attached to the wall of a building, where the plane of the sign is parallel to and in contact with the plane of the wall, and the sign does not extend beyond the side or top edges of the wall, shall be determined using procedures for wind pressures on walls in accordance with Chapter 30, and setting the internal pressure coefficient (GCpi) equal to 0.

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This procedure shall also be applicable to solid signs attached to but not in direct contact with the wall, provided the gap between the sign and wall is no more than 3 ft and the edge of the sign is at least 3 ft in from free edges of the wall, i.e., side and top edges and bottom edges of elevated walls.

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Solid Attached Sign with Small Gap Showing Limitations on Use of Component & Cladding Loads for Signs on Walls


This is a SOLID SIGN

Elevation Cross-Section

3 ft

Exterior Wall

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29.5.1, 30.11Rooftop Structures & Equipment

Design wind force for rooftop structures and equipment for buildings with h ≤ 60 ft has been revised and now also includes a vertical component.

New section has been added for determining component and cladding loads on rooftop structures and equipment for buildings with h ≤ 60 ft.

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29.5.1 6.5.15.1 Rooftop Structures and Equipment for Buildings with h ≤ 60 ft

The lateral force Fh, on rooftop structures and equipment with Af less than (0.1Bh) located on buildings with a mean roof height, h ≤ 60 ft shall be determined from Eq. 29.5-2 6-28. increased by a factor of 1.9. The factor shall be permitted to be reduced linearly from 1.9 to 1.0 as the value of Af is increased from (0.1Bh) to (Bh).

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Fh = qh (GCr) Af (lb) (29.5-2)

where:

GCr = 1.9 for rooftop structures and equipment with Af less than (0.1Bh). GCr shall be permitted to be reduced linearly from 1.9 to 1.0 as the value of Af is increased from (0.1Bh) to (Bh)

qh = velocity pressure evaluated at mean roof height of building

Af = vertical projected area of rooftop structure or equipment on plane normal to direction of wind, ft2

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Af

h

ELEVATION

B

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29.5.1, 30.11Rooftop Structures & EquipmentThe vertical uplift force, Fv, on rooftop structures and

equipment shall be determined from Eq. 29.5-3:

Fv = qh (GCr) Ar (29.5-3)

where:

GCr = 1.5 for rooftop structures and equipment with Ar less than (0.1BL). GCr shall be permitted to be reduced linearly from 1.5 to 1.0 as the value of Ar is increased from (0.1BL) to (BL).

qh = velocity pressure evaluated at mean roof height of building

Ar = horizontal projected area of roof top structure or equipment, ft2

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ArL

WIN

D

PLAN

B

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Rooftop Equipment MWFRS Loads

Horizontal Projected Area of Equipment, Ar

Vertical Projected Area, Af

Fv = qh (GCr)Ar

Fh = qh(GCr)Af

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30.11 ROOFTOP STRUCTURES AND EQUIPMENT FOR BUILDINGS WITH h ≤ 60 ft

The components and cladding pressure on each wall of the rooftop structure shall be equal to the lateral force determined in accordance with Section 29.5.1 divided by the respective wall surface area of the rooftop structure and shall be considered to act inward and outward.

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The components and cladding pressure on the roof shall be equal to the vertical uplift force determined in accordance with Section 29.5.1 divided by the horizontal projected area of the roof of the rooftop structure and shall be considered to act in the upward direction.

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Rooftop Equipment Component & Cladding Loads

(Design for Positive & Negative Pressures)

p = Fv /Ar

p = Fh /Af

(Design for Positive & Negative Pressures)

p = Fh /Af Rooftop Equipment

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BREAK!



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Chapter 30 – C&C

• Part 1 – Analytical method for h ≤ 60 ftASCE 7-05 Method 2 Low-Rise C&C

• Part 2 – Simplified method for h ≤ 60 ftASCE 7-05 Method 1 C&C

• Part 3 – Analytical method for h > 60 ftASCE 7-05 Method 2 All-Heights C&C

• Part 4 – New Simplified method for h ≤ 160 ft

• Part 5 – Analytical method for free roofs

• Part 6 – Building appurtenances

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Chapter 30 Components and Cladding

Part 4 applies to enclosed buildings having a mean roof height h ≤ 160 ft having flat roofs, gable roofs, hip roofs, monoslope roofs and mansard roofs. Wind pressures are determined directly from a table.

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Chapter 30 Components and Cladding

General Requirements:

30.2.2 Minimum Design Wind Pressures. The design wind pressure for C&C of buildings shall not be less than a net pressure of 16 lb/ft2 acting in either direction normal to surface.

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Chapter 30 Components and Cladding– Part 4

Design Step Parameter Source

1. Determine Basic Wind Speed for appropriate risk category

V (mph) Fig. 26.5-1 Maps

2. Determine exposure category Exp B, C or D Section 26.7

3. Enter table to determine net pressures on walls and roof for Exposure C

ptable (psf) Table 30.7-2

4. For Exposures B and D, use adjustment factors

ptable (psf)Notes to Table

30.7-2

5. Determine topographic factor and apply factor to wall and roof pressures (if applicable)

Kzt Section 26.8

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V (mph) 110 - 200

h (ft) Roof FormLoad Case

Zone

1 2 3 4 5

15 - 160

Flat Roof1

2

Gable RoofMansard Roof

1

2

Hip Roof1

2

Monoslope Roof1

2


Table 30.7-2 for Exposure Category C

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Chapter 30 Components and Cladding– Part 4 Conversion from Exposure C to Exposures B and D

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2

3

1

2a

a

a 2a

a

44

5

22

33

2

55

3

Flat roof (θ < 10)


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3

2

5

4

1

2 2

3

4

3

5

2a a

2a

2a

4a

a

Monoslope roof

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Gable roof

3

4

5

1

4 3

2

2

a a

a

3

3

1

3

2

5

2

3

5

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1

23

4

5

2

4

3

a a

a

a

3

3

5

1

2


Hip roof

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1

4

5

2 4

a

a a

3

5 3 5

1

2

3

Mansard roof

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Pressures shown in Table 30.7-2 are based on an effective wind area of 10 ft2. Reductions in wind pressure for larger effective wind areas may be taken based on reduction multipliers shown in notes to table.

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0.5

0.6

0.7

0.8

0.9

1

1.1

1 10 100 1000

Re

du

ctio

n F

acto

r

Effective Wind Area (sf)

20 50 200 500

0.8

0.7

0.6

1.0

0.9

A

B

C

D

E

Reduction Factor Based on Effective Wind Area

56



- 111 -


Pressure (psf) on parapet:

p1 = Positive wall pressure for Zone 4 or 5, as applicable, computed at top of parapet

p2 = Negative roof pressure for Zone 2 or 3, as applicable

p1 p2

- 112 -


Pressure (psf) on parapet:

p3 = Positive wall pressure for Zone 4 or 5, as applicable, computed at top of parapet

p4 = Negative wall pressure for Zone 4 or 5, as applicable, computed at top of parapet

p4 p3

57



- 113 -

SKGA Wind Simple Computer Program

Uses the new simplified directional (all-heights) procedure in ASCE 7-10.

Calculates MWFRS and C&C design pressures on walls, roofs, roof overhangs, and parapets.

Applies the effective area reduction factor for C&C pressures.

Provides the design wind pressures for each applicable zone of the building in clear and concise tables.

Documents the calculations in clear and attractive reports.

And more…..

http://skghoshassociates.com/wind-simple

- 114 -

30.10.1 Roof Overhangs

The determination and application of wind loads on roof overhangs has been revised to provide a more accurate description of how the loads are to be applied.

58



- 115 -


6.5.11.4.2 Components and Cladding. For all buildings, roof overhangs shall be designed for pressures determined from pressure coefficients given in Figs. 6-11B,C,D.

- 116 -


30.10 ROOF OVERHANGS

The design wind pressure for roof overhangs of enclosed and partially enclosed buildings of all heights, except enclosed buildings with h ≤ 160 ft. for which the provisions of Part 4 are used, shall be determined from the following equation:

p = qh[(GCp) – (GCpi)] (lb/ft2) (30.10-1)

59



- 117 -


where

qh = velocity pressure from Section 30.3.2 evaluated at mean roof height h using exposure defined in Section 26.7.3

(GCp) = external pressure coefficients for overhangs given in Figures 30.4-2A to 30.4-2C (flat roofs, gable roofs, and hip roofs), including contributions from top and bottom surfaces of overhang. The external pressure coefficient for the covering on the underside of the roof overhang is the same as the external pressure coefficient on the adjacent wall surface, adjusted for effective wind area, determined from Figure 30.4-1 or Figure 30.6-1 as applicable

(GCpi) = internal pressure coefficient given in Table 26.11-1

- 118 -


The steps required for the determination of wind loads on components and cladding of roof overhangs is shown in Table 30.10-1.

60



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Roof Overhang C & C

Top Surface Pressure = Pressure Calculated using GCp for overhangs in Figures 30.4-2A to 30.4-2C minus bottom surface pressure calculated below

Bottom Surface Pressure = Pressure Calculated using GCp for walls from Figures 30.4-1 or 30.6-1 as applicable based on the effective wind area of the soffit

- 120 -


Chapter 31 – Wind Tunnel Procedure

61



- 121 -

31.4.3 Limitations on Loads

Lower limits on loads determined from wind tunnel testing are relocated from the Commentary directly into the standard.

- 122 -


Loads for the MWFRS determined by wind tunnel testing shall be limited such that the overall principal loads in the x and y directions are not less than 80 percent of those that would be obtained from Part 1 of Chapter 27 or Part 1 of Chapter 28.

The overall principal load shall be based on overturning moment for flexible buildings and base shear for other buildings.

62



- 123 -


Pressures for C&C determined by wind tunnel testing shall be limited to not less than 80 percent of those calculated for Zone 4 for walls and Zone 1 for roofs using the procedure of Chapter 30.

- 124 -


The limiting values of 80 percent may be reduced to 50 percent for the MWFRS and 65 percent for C&C if either of the following conditions applies:

1. There were no specific influential buildings or objects within the detailed proximity model.

63



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2010 Standard (contd.)

2. Loads and pressures from supplemental tests for all significant wind directions in which specific influential buildings or objects are replaced by the roughness representative of the adjacent roughness condition, but not rougher than exposure B, are included in the test results.

- 126 -


Appendix C – Serviceability Considerations

C. SERVICEABILITY CONSIDERATIONS

This appendix is not a mandatory part of the Standard but provides guidance for design for serviceability. . ..

C.1.2 Drift of Walls and Frames. Lateral deflection or drift of structures and deformation of horizontal diaphragms and bracing systems due to wind effects shall not impair serviceability of structure.

64



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Commentary to Appendix C – Serviceability Considerations

- 128 -

Commentary to Appendix C

Use of the nominal (700-year mean recurrence interval (MRI) or 1,700-year MRI) wind load in checking serviceability is excessively conservative. The following load combination can be used to check short-term effects:

D + 0.5L + 0.7Wa (CC-3)

in which Wa is wind load based on serviceability wind speeds in Figs. CC-1 through CC-4.

65



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Figure CC-1 10-Year MRI 3-sec Gust Wind Speed in mph (m/s) at 33 ft (10m) above Ground in Exposure C

- 130 -


66



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- 132 -


67



- 133 -

ASCE 7-10New Appendix

Appendix D – Buildings Exempted from Torsional Wind Load Cases

- 134 -

68



- 135 -

- 136 -

Closing Comments

Impact of Changes

Need for Changes

69



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- 138 -

Thank You!!

For more information…www.skghoshassociates.com

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