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Workshop on Vertical Shelter from Tsunamis

CIAPR June 18-20 2012

Calculation of Wind Loads

According to PR Building

Code 2011

Dr. Ricardo R. López Rodríguez,

Ph.D., P.E.

Professor of Civil Engineering

University of Puerto Rico at

Mayagüez, [email protected]

Earthquake Commission of CIAPR

1


mailto:[email protected]


CIAPR June 18-20 2012 2

Some slides taken and adapted from FEMA presentation

by Adam Reeder, PE, CFM from presentation

Wind Provisions of the 2009 International Building

Code and International Residential Code


CIAPR June 18-20 2012 3



P646 on Wind Loading

4



Agenda

• Wind effects on structures

• About the PR Building Code

• Wind Load Provisions for Buildings

Definitions and terms used in all Procedures

Alternative Procedures of Analysis

– Simplified

– Analytical (Low rise or high rise)

– Wind Tunnel

• Examples of calculation of horizontal wind pressures

• Cylindrical Structures

5



Wind effects on structures

6

VIENTO

Rectangular Buildings

The wind generates pressure in windward wall and

suction in leeward wall, lateral walls, and part of the roof.



Internal Pressures

7

There is always some

internal pressure, unless

the building is completely

open. Partially enclosed

buildings have the highest

internal pressures.

It is important for doors and

windows to resist the wind

pressure and impact of

flying objects.



Effect of openings on internal pressure

Windward or leeward opening

8



Wind effects on structures

9

Multistory Rectangular

Buildings

The wind generates

pressure in windward wall

and suction in leeward wall,

lateral walls, and part of the

roof.



PR Building Code is Based on

International Codes

10



Adoption Dates

11

March1, 2011



About the International Codes

The First Edition of the I-Codes was released in 2000

Input from five organizations which included:

• Building Officials and Code Administrators International,

Inc. (BOCA),

• International Conference of Building Officials (ICBO)

• Southern Building Code Congress International (SBCCI).

Intended to provide a comprehensive set of codes

New editions on a 3 year cycle

12



Organization of the IBC

Chapter 1: Administration

Chapter 2: Definitions

Chapter 3: Use and Occupancy Classification

Chapter 5: General Building Heights and Areas

Chapter 6: Types of Construction

Chapter 14: Exterior Walls

Chapter 15: Roof Assemblies and Rooftop Structures

Wind Loads

13



Organization of the IBC

Chapter 18: Soils and Foundations

Chapter 19: Concrete

Chapter 21: Masonry

Chapter 22: Steel

Chapter 24: Glass and Glazing

Chapter 25: Gypsum Board and Plaster

Chapter 34: Existing Structures

Chapter 35: Referenced Standards

Appendix H: Flood-Resistant Construction

14



What Code to Use – ASCE 7-05

15

PR Building Code 2011 refers the user to the International

Building Code 2009.

International Building Code 2009 refers the user to the

ASCE 7-05.

ASCE 7-05 contains the provisions for obtaining wind

pressures expected from hurricanes.

Use the wind pressures to calculate wind loads

Note: There is an ASCE 7-2010 code available. It is not the

currently mandated code for PR.

ASCE 7-2010 wind pressures differ from ASCE 7-05

pressures. (Earthquake loads also differ).



What code to use (2)

16

Once loads are obtained using ASCE 7-05, use the design

codes in accordance with the building material

For example, for design of reinforced concrete structures,

the IBC 09 has Chapter 19.

In PR the user is allowed to use ACI 318-08 =

American Concrete Institute Building Code and

Commentary.



Designing for Wind

Loads using the IBC

and ASCE 7-05

The primary source for

wind loads in the IBC

is ASCE 7. The

following is a

summary of important

aspects of ASCE 7-05.

17



Wind-borne debris region

Portions of hurricane-prone regions

that are within 1 mile of the coastal

mean high water line where the

basic wind speed is 110 mph or

greater; or portions of hurricane-

prone regions where the basic wind

speed is 120 mph or greater

18

Definitions of Wind Related Terms in

the International Codes

Basic wind speed

Three-second gust speed at 33 ft above the ground in Exposure C –

145 mph for Puerto Rico (IBC Figure 1609)

The ASCE 7-1998 had 125 mph for PR.



Main vs Components



ASCE 7-05 – Wind Loads

Chapter 6: General Requirements

Basic parameters for determining wind loads:

• Basic wind speed, V, Figure 6-1 (Puerto Rico = 145 mph)

• Wind directionality factor, Kd , Section 6.5.4.4

• Importance Factor, I, Section 6.5.5

• Exposure category, Section 6.5.6

• Topographic factor, Kzt, Section 6.5.7

• Gust Effect Factor, G, Section 6.5.8

• Enclosure classification, Section 6.5.9

• Velocity Pressure, qz, Section 6.5.10

• Pressure coefficients, (GCp), Section 6.5.11

20






Wind directionality factor, Kd

22

Structure Type Directionality Factor, Kd

Buildings

Main Wind Force Resisting System

Components and Cladding

0.85

0.85

Arched Roofs 0.85

Chimneys, Tanks and Similar Structures

Square

Hexagonal

Round

0.90

0.95

0.95

Solid Freestanding Walls and

Solid Freestanding and Attached Signs 0.85

Open Signs and Lattice Framework 0.85

Trussed Towers

Triangular, square, rectangular

All other cross sections

0.85

0.95



Occupancy Category

23

I

Low hazard to human life in the event of failure, including but not limited to

• Agricultural facilities

• Certain temporary facilities

• Minor storage facilities

II Not category I, III or IV

III

Substantial hazard to human life in the event of failure, including but not

limited to

• Covered structures for public assembly (>300 occupants)

• Schools (>250 occupants)

• College and other adult-education buildings (>500 occupants)

• Heath care facilities, non-emergency (>50 resident patients)

• Jails and detention facilities

• Any other facility with occupant load >5000

• Public utility facilities not included in Category IV

IV

Essential facilities, including but not limited to

• Hospitals and other health care facilities having surgery or emergency

facilities

• Fire, rescue and police stations

• Emergency shelters and response centers

• Public utility facilities for emergency backup of other Category IV

structures





Table 6-1 Importance Factor

24



Surface Roughness – based on topography, vegetation

and structures

25

Category B Terrain with numerous closely spaced obstructions

having the size of single-family dwellings

• Urban areas

• Suburban areas

• Wooded areas

Category C Open terrain with scattered obstructions having heights

generally less than 9144 mm

• Flat open country

• Grasslands

• Water surfaces in hurricane-prone regions

Category D Flat, unobstructed areas

• Smooth mud flats

• Salt flats





Exposure Category – based on Surface Roughness in the

upwind direction

26

Category B Applies where Surface Roughness category B prevails

in the upwind direction for at least 2,600 ft or 20 times the

building height, whichever is greater

Exception: Where roof height ≤ 30 ft, upwind distance

may be reduced to 1,500 ft.

Category C Not Category B or D

Category D Applies where Surface Roughness category D prevails

in the upwind direction for at least 5,000 ft or 20 times the

building height, whichever is greater





Exposure Categories

27

Exposure Category B



Exposure Categories

28

Exposure

Category C

Exposure

Category D




Exposure category

• Surface Roughness Category B, C or D

• Exposure Category B, C or D

• Wind Direction and Sectors

For each direction at which the wind loads are to be

evaluated, the exposure shall be determined for the two

upwind sectors extending 45° either side of the selected

wind direction.

The exposure resulting in the highest wind loads shall be

used to represent winds from that direction.

7-05 Does not use Exp. D in Hurricane Prone Coasts, 7-10 does

29




Topographic factor, Kzt

Wind Speed-Up over Hills, Ridges and Escarpments

30

H Height of hill or escarpment relative to the upwind terrain, in meters.

Lh

Distance upwind of crest to where the difference in ground elevation is half the

height of hill or escarpment, in meters.

x Distance (upwind or downwind) from the crest to the building site, in meters.

z Height above ground surface at building site, in meters.





Wind speed-up effects shall be included in the design

when all of the following conditions are met:

31

1

The hill, ridge or escarpment is isolated and unobstructed upwind by other similar topographic

features of comparable height for 100 times the height of the topographic feature (100H) or 2

miles, whichever is less.

2 The hill, ridge or escarpment protrudes above the height of upwind terrain features within a 2 mile

radius in any quadrant by a factor of two or more.

3 The structure is located in the upper one-half of a hill or ridge or near the crest of an escarpment.

4 H/Lh ≥ 0.2.

5 H ≥ 4.5 m for Exposure C and D and H ≥ 18 m for Exposure B.





Wind Speed-Up over Hills, Ridges and Escarpments

Kzt = (1 + K1 K2 K3)2

32

K1

Factor to account for shape of

topographic feature and maximum

speed-up effect.

K2

Factor to account for reduction in

speed-up with distance upwind or

downwind of crest.

K3

Factor to account for reduction in

speed-up with height above local

terrain.





• K1 is determined using the table below

Factor to account for shape of topographic feature and

maximum speed-up effect.

33

Hill Shape

K1/(H/Lh)

Exposure

B C D

2-D Ridges

(or valleys with -

H) 1.30 1.45 1.55

2-D Escarpments 0.75 0.85 0.95

3-D Axisym. Hill 0.95 1.05 1.15

H

Height of hill or

escarpment relative to

the upwind terrain, in

meters.

Lh

Distance upwind of crest

to where the difference

in ground elevation is

half the height of hill or

escarpment, in meters.





• K2 = (1 - )

Factor to account for

reduction in speed-up with

distance upwind or

downwind of crest.

• K3 = e-γz/Lh

Factor to account for

reduction in speed-up with

height above local terrain.

34

hL

|x|

Lh

Distance upwind of crest to where the difference

in ground elevation is half the height of hill or

escarpment, in meters.

x Distance (upwind or downwind) from the crest to

the building site, in meters.

z Height above ground surface at building site, in

meters.

μ Horizontal attenuation factor.

γ Height attenuation factor.

Hill Shape γ

μ

Upwind of

Crest

Downwind

of Crest

2-D Ridges

(or valleys with -

H) 3 1.5 1.5

2-D Escarpments 2.5 1.5 4

3-D Axisym. Hill 4

1.5 1.5




Topographic factor,

Kzt = (1 + K1 K2 K3)2

35

Notes:

1.Linear interpolation

between values is

permitted.

2.For H/Lh > 0.5,

assume H/Lh = 0.5 for

finding K1 and

substitute 2H for Lh for

finding K2 and K3. 3.Multipliers assume

wind approaches along

the direction of

maximum slope.




Gust Effect Factor, G

• G = 0.85 for rigid buildings and other structures

• G is determined by formula for flexible or dynamically

sensitive buildings or other structures

Flexible: natural frequency < 1

36

Note:

Where combined gust-effect factors and pressure

coefficients (GCp), (GCpi) and (GCpf) are given in figures

and tables, the gust-effect factor shall not be determined

separately.




Enclosure classification

37

Open

Ao ≥ 0.8Ag

Ao = total area of openings in a wall that receives positive

external pressure, in m2

Ag = gross area of that wall in which Ao is identified, in m2

Partially

Enclosed

• Ao > 1.10Aoi AND

• Ao > 0.37 m2 OR Ao > 0.01Ag (whichever is smaller) AND

• Aoi / Agi ≤ 0.20

Aoi = sum of the areas of openings in the building envelope

(walls and roof) not including Ao, in m2

Agi = sum of the gross surface areas of the building envelope

(walls and roof) not including Ag, in m2

Enclosed Not open or partially enclosed.



Velocity pressure qz

Velocity pressure

Factor Kz includes the effect of height



ASCE 7-05 and 7-10 – Wind Loads

Internal Pressure Coefficient, (GCpi)

39




Internal Pressure Coefficient, (GCpi)

40

Enclosure

Classification (GCpi)

Open 0.00

Partially Enclosed +0.55

-0.55

Enclosed +0.18

-0.18

Note:

Plus and minus signs

signify pressures acting

toward and away from

the internal surfaces,

respectively.




Procedures for determining wind loads for Main Wind-

Force Resisting System (MWFRS)

41

Procedure Application Location

Simplified Procedure Low-rise buildings Section 6.4

Analytical Procedure

Buildings of all heights

Distinguish low rise and

High-rise

Section 6.5

Wind Tunnel Procedure All buildings and other

structures Section 6.6



ASCE 7-05 – Simplified Procedure

MWFRS and C&C Simplified Procedure (Sec 6.4)

for Enclosed Low-Rise Buildings

• Applicability

Building has simple diaphragm and low-rise (h<60’)

Enclosed and meets WBD requirements

Building not classified as flexible

Building is not subject to the following:

– Across-wind loading

– Vortex shedding

– Instability due to galloping or flutter

– Channeling effects

– Buffeting in the wake of upwind obstructions

Building has a symmetrical cross section

Building exempted from torsional load cases

42



MWFRS and C&C Simple Procedure (Sec 6.4)

Enclosed Low-Rise Buildings


Step 1 Basic wind speed, V Sec 6.5.4

Step 2 Importance Factor, I Sec 6.5.5

Step 3 Exposure Category Sec 6.5.6

Step 4 Height and Exposure Adjustment, λ Fig 6-2

Step 5 Design pressure at 30 ft ps30 shall be

determined Fig 6-2

Step 6 Topographic amplification factor calculated. Sec 6.5.7

Step 7

Wall and roof pressures calculated. Check

Minimum

Sec 6.4.2.1.1 or

Sec 6.4.2.2.1

43



Fig 6-2 Simplified Procedure

44





Simplified Method

Fig 6-2



Fig 6-2 Simplified Design Wind

pressure ps30

47

p



𝒑𝒔 = 𝝺𝑲𝒛𝒕𝑰𝒑𝒔𝟑𝟎

Design pressure, Simplified Procedure




Analytical Procedure

MWFRS and C&C Analytical Procedure (Sec 6.5) Enclosed, Partially Enclosed and Open Buildings of All Heights

• Applicability

Building is regular-shaped

Building is not subject to the following:

– Across-wind loading

– Vortex shedding

– Instability due to galloping or flutter

– Channeling effects

– Buffeting in the wake of upwind obstructions

49




MWFRS and C&C Analytical Procedure (Section 6.5) Enclosed, Partially Enclosed and Open Buildings of All Heights

50

Step 1 Basic wind speed, V, Kd Sec 6.5.4

Step 2 Importance Factor, I Sec 6.5.5

Step 3 Wind load parameters (Exposure and height Kz,

topographic effect, Gust factor, Enclosure)

Sec 6.5.6 thru

6.5.9

Step 4 Internal pressure coefficient, GCpi Sec 6.5.11.1

Step 5 External pressure coefficient, Cp Sec 6.5.11.2 or

6.5.11.3

Step 6 Velocity pressure qz Sec 6.5.10

Step 7 Design Wind Load, F Sec 6.5.12 thru

Sec 6.5.15



1. V and Kd (directionality)

2. Importance, I

3. Exposure and height, Kz, Kh

4. Topographic Effect

5. Gust effect G

6. Enclosure

7. Internal pressure GCpi

8. External Pressure Cp

9. Velocity pressure qz

10. Design wind load p

Analytical Method - Procedure



Exposure Coefficient Kh and Kz



Internal pressure coefficients

53



Fig 6-6 Monoslope roof

External wall pressures

54



Fig 6-6 Cp for Analytical Method

55



Fig 6-10 Low-Rise Analytical Procedure

56

Wall pressures

Windward: 1, 1E

Leeward: 4, 4E



Low Rise Force coefficients GCpf

57



Velocity pressure and wall pressures

Velocity pressure

Wall pressure, analytical method



Pressures for Analytical Procedure




Wind Tunnel Procedure (Section 6.6)

• Permitted for any building or structure

• Tests shall meet the following conditions:

Atmospheric boundary layer modeled for wind speed variation

Atmospheric turbulence modeled to match the same scale as

the building model

Modeled building, surrounding building and topography scaled

properly

Modeled building and surrounding area is less than 8 percent

of the test section

Longitudinal pressure gradient accounted for

Reynolds number effects on pressure minimized

Response characteristics consistent with model

60



Cylindrical Structures

61

ASCE 7-05 does not consider buildings with shapes

different from rectangular or combinations of rectangular.

The only guidance for cylindrical structures is in the

section on Other Structures (Sec 6.5.15).

The total lateral load is calculated from equation 6-28



Other Structures

62



Fig 6-21 – Other Structures Coeff. Cf

63


CIAPR June 18-20 2012 64



Example

65

Rectangular building

Plan dimensions 70’ x 70’

Height = 60’

Plan View

70’

70’

Elevation

24’

12’

12’

12’

Story

Heights



Solution using simplified procedure

66

V = 145 mph from Fig 6-1

I = 1.15 from table 6-1 and occupation Cat IV (or III)

Exposure Category = C

λ = 1.62 from Fig 6-2 with h=60’ and Exp C

Width of special zone A = 2a

• a < 0.10 (B) = 0.10 (70) = 7’

• a < 0.4 (h) = 0.4 (60) = 24’

• Select the smallest = 7’ but not less than

• a > .04 (B) = .04 (70) = 2.8’

• a > 3’,

•Select a = 3’, 2a = 6’




67

Width of zone A = 6

Width of zone C = 70 – 2(6) = 58’

assuming zone A on both corners simultaneously

Horizontal pressures from Fig and roof angle ϴ = 0

Ps30 = 33.4 psf in zone A

Ps30 = 22.1 psf in zone C

Topographic factor Kzt =1 for flat terrain

Pressures 𝒑𝒔 = 𝝺𝑲𝒛𝒕𝑰𝒑𝒔𝟑𝟎

• psA = 1.62(1)(1.15)(33.4) = 62.2 psf

• psC = 1.62(1)(1.15)(22.1) = 41.2 psf



Fig 6-2 Simplified Design Wind

pressure ps30

68

p

PA = 33.4 psf PC = 22.1 psf




69

Apply uniform pressures in corresponding areas.

Use tributary wall area for each story level

Level Height,

ft

PsA,

psf

Width

A, ft

PsC,

psf

Width

C, ft

Force

Kips

1 30 62.2 12 41.2 58 94.1

2 12 62.2 12 41.2 58 37.6

3 12 62.2 12 41.2 58 37.6

4 6 62.2 12 41.2 58 18.8

Total 188.1



1. V and Kd (directionality)

2. Importance, I

3. Exposure and height, Kz, Kh

4. Topographic Effect

5. Gust effect G

6. Enclosure

7. Internal pressure GCpi

8. External Pressure Cp

9. Velocity pressure qz

10. Design wind load p

Analytical Method - Procedure



Solution using analytical procedure

71


Kd = 0.85 from Table 6-4



For Low-rise buildings use Fig 6-10 and q = qh

• Select a = 3’, 2a = 6’

• Select values of GCpf for zones 1, 1E, 4, 4E

• GCpf1 = 0.40, GCpf1E = 0.61

• GCpf4 = -0.29, GCpf4E = -0.43




Low-rise buildings

72

Kzt = 1

G = 0.85 for rigid structures, T< 1sec

Enclosure classification = enclosed

Internal pressure coefficients Fig 6-5

GCpi = ± 0.18

Kz = Kh = 1.13 from Table 6-3 and h=60’

Velocity pressure coefficient,

qz = qh = 0.00256(1.13)(1)(0.85)(145^2)(1.15) = 59.5 psf




Low-rise buildings

73

Design pressures

Use q = qh

Windward

p1 = 59.5(0.40 + 0.18) = 34.5 psf

p1E = 59.5(0.61 + 0.18) = 47.0 psf

Leeward

p4 = 59.5(-0.29 + 0.18) = -6.5 psf

p4E = 59.5(-0.43 + 0.18) = -14.9 psf




Low-rise buildings

74

Forces per level, calculated adding the windward and

leeward pressures because they act in the same

direction.

Level Height

, ft

Ps1E+

4E, psf

Width

E, ft

Ps1+4,

psf

Width ,

ft

Force

Kips

1 30 61.9 12 41.0 58 93.6

2 12 61.9 12 41.0 58 37.5

3 12 61.9 12 41.0 58 37.5

4 6 61.9 12 41.0 58 18.7

Total 187.3




75

For comparison the solution was obtained neglecting the

low-rise classification


Kd = 0.85 from Table 6-4



Kzt = 1

G = 0.85 for rigid structures, T< 1sec

Enclosure classification = enclosed

Internal pressure coefficients Fig 6-5, GCpi = ± 0.18




76

Obtain Kz from from Table 6-3 and z = level height

Velocity pressure coefficient,

Height, z, ft Kz qz, psf

24 0.94 45.5

36 1.01 53.1

48 1.08 56.8

60 1.13 59.5




77

Design pressures. No special regions, but pressure varies

with height

Use q = qz for windward pressures

Use q = qh for leeward pressures

Use qi = qh

Shape factors Cp = 0.8 windward, Cp = -0.5 leeward,

obtained from Fig 6-6




78

Forces per level

Level Z qz,

psf

Windw

qzGCp

Internal

qhGCpi

Leew

qhGCp

Windw

p Leew

p

Total

p

Area,

sqft

Force

kips

1 24 45.5 30.9 -10.7 -25.3 41.6 14.6 56.2 70x30 118.0

2 36 53.1 36.1 -10.7 -25.3 46.8 14.6 61.4 70x12 51.6

3 48 56.8 38.6 -10.7 -25.3 49.3 14.6 63.9 70x12 53.7

4 60 59.5 40.5 -10.7 -25.3 51.2 14.6 65.8 70x6 27.6

Total 250.9



Solution for Cylindrical Plan Building

79

The code does not propose a solution for cylindrical

buildings.

Assuming a building with D = 70’, and height = 60’

Using z = h/2 = 60/2 = 30’

Kz = 0.98

qz = 0.00256(0.98)(0.85)(145^2)(1.15) = 51.6 psf

D√qz = 70√51.6 = 503 > 2.5

For h/D = 60/70 = 0.86 ≈ 1

Assuming moderately smooth surface

Cf = 0.5

F = 51.6(0.85)(0.5)(70x60) = 92 kips



Solution for Cylindrical Plan Building

80

Or assuming rough surface

Cf = 0.8

F = 51.6(0.85)(0.8)(70x60) = 147 kips



Summary of examples

81

Calculated Base Shear Forces

Procedure Calculated Base

Shear Force, kips

% Difference

from Simplified

Procedure

Simplified 188.1 0

Analytical, Low-

rise

187.3 -0.4

Analytical 250.9 33.4

Cylindrical, other

structures, note

plan area is not the

same

92 for smooth

surface,

147 for rough

surface

-51.1

-21.9




Basic wind speed, V, mph (m/s)

82

Figure 26.5-1A: Basic Wind Speeds for

Occupancy Category II Buildings and

Other Structures

Figure 26.5-1B: Basic Wind Speeds for

Occupancy Category III and IV Buildings

and Other Structures



ASCE 7-05 to ASCE 7-10 Wind Speeds

ASCE 7-05 uses an approximate 500-year return period

wind speed divided by the square root of the expect load

factor of 1.5. (This was approximately the 100-year wind

speed) – this was later multiplied by an importance factor

ASCE 7-10 uses maps per occupancy category (risk

category)

• Risk Category II (700 year)

• Risk Category III and IV (1700 year)

• No importance factor

Converting from ASCE 7-05 to ASCE 7-10

83

The preferred conversion of wind speeds is the



Questions?

84



Impacts of Wind Provisions on

Building Components

Load Paths and Connectors

Roof Systems

Windows, Doors and Openings

Wall Systems

Foundations

85

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