9. wind design. wind design.pdf · load combinations specified in asce 7-16 section 2.3 and 2.4 are...

Dynamic Loads

Wind Design © Richard L Wood, 2018 of 31

9. Wind Design

Lesson Objectives:

1) Define basic definitions within wind design including: Main Wind Force Resisting System

and Components and Cladding.

2) Select appropriate building enclosure classification from open building, partially enclosed

building, and enclosed building classifications.

3) Determine the basic wind design speed from the mapped values within ASCE 7-16.

4) Compute design wind pressures from ASCE 7-16 through selection of wind directionality

factor, exposure category and surface roughness, any topographical effect, and the gust

effect factor.

5) Outline the process to analyze a general and regular building using the MWFRS Directional

Procedure as outlined within Chapter 27.

Background Reading:

1) Read _______________________________________________.

Wind Nomenclature:

1) _____________________________________________________________ or MWFRS -

assembly of structural elements that provide support and stability under wind loads.

a. This consists of the various components, namely: building foundation, structural

floor members (joists, beams, and framing systems) columns, roof trusses,

structural bracing, walls, and diaphragms that transfer loads.

b. MWFFRS experiences external pressures and forced induced from wind loads

which may cause:

i. _______________________________ pressures on windward walls

ii. _______________________________ pressures on leeward walls, side

walls, and roof systems

iii. May cause collapse of surfaces due to ____________________________

____________________________

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2) _______________________________________________________ include elements of

the building envelope that are not considered part of the MWFRS.

a. Select _____________________ examples include: purlins, studs, roof decking,

and roof trusses.

b. Select _____________________ examples include: wall coverings, curtain walls,

roof coverings, exterior windows, and/or doors.

c. Elements of components and cladding may be considered as part of the MWFRS

under certain design scenario.

d. Components and cladding experience both ____________and ________________

external pressures. At sharp edges (corners, wall and roof junctions, etc.), the air

flow separates which creates _______________________ and ________________

__________________ positive and negative pressures.

e. Complex aerodynamic effects exist and are dependent on the shape of the structure.

As a result, components and cladding are subject to failures resulting from

numerous factors.

3) ____________________________________ – are caused by a permeability in a building

and/or when the wind enters the building through a dominant opening.

a. When determining the total wind pressure, the internal pressure may be _________

______________________________ from the external pressures.

b. The ____________________________ of the internal pressure depend on whether

the building is open, partially enclosed, or enclosed according to the definitions

prescribed in ASCE 7-16.

4) _______________________ - building having each wall at least 80% open. One example

is a rigid frame building having no side walls.

5) ______________________________________________________ - building which

meets both of the following statements:

a. The total area of openings in a wall that receives positive external pressure exceeds

the sum of the areas of openings in the balance of the building envelope (walls and

roof) by more than 10%, and

b. The total area of openings in a wall that receives positive external pressure exceeds

four (4) square feet or 1.0% of the area of that wall, whichever is smaller, and the

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percentage of openings in the balance of the building envelope does not exceed

20%

6) _____________________________________ - building that does not comply with the

requirements for open or partially enclosed buildings.

Wind Loads by ASCE 7-16 and 7-10 (Similar Process):

1) The basic _____________________________________________ to determine wind

loads can be located in Figure 26.1-1 of ASCE 7-16 or shown here in Figure 1.

Figure 1. Outline for determining wind loads, from ASCE 7-16.

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General Requirements - Basic Design Wind Speed:

1) ASCE 7-16 has developed ______________________________________________ for

the United States and all its territories by occupancy category.

2) This can be found as Figures 26.5-1A - 26.5-1D. In these notes, these figures are shown

in Figures 2-7.

3) Note design wind speeds can be found by location using the ________________________,

shown in Figure 8.

4) The values provided are nominal design 3-second gust wind speeds in miles per hour (m/s

in the parenthesis) at 33 feet (10 meter) above ground for exposure C category

5) ________________________________ is permitted between contours.

6) Island and coastal areas outside the last contour shall use the last week speed contour of

the coastal area.

7) Mountainous terrains, gorges, ocean promontories, and special wind regions shall be

examined for ___________________________________________________.

8) Wind speeds correspond to various values between on the mapped occupancy:

a. Category ______ – _____% probability of exceedance in 50 years, mean recurrence

interval of _________ years

b. Category ____________________ – _____% probability of exceedance in 50

years, mean recurrence interval of _________ years

c. Category ______ – _____% probability of exceedance in 50 years, mean recurrence

interval of _________ years

9) Regional climatic data is permitted for the estimation of basic wind speeds. Refer to section

26.5.3.

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Figure 2. Basic wind speeds mapped for occupancy category I (part one).

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Figure 3. Basic wind speeds mapped for occupancy category I (part two).

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Figure 4. Basic wind speeds mapped for occupancy category II (part one).

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Figure 5. Basic wind speeds mapped for occupancy category II (part two).

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Figure 6. Basic wind speeds mapped for occupancy category III.

Figure 7. Basic wind speeds mapped for occupancy category IV.

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Figure 8. Design wind speeds by latitude and longitude. (Available at: http://windspeed.atcouncil.org).

Note this is for ASCE 7-10 loads, the updated version has not been released to date.

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General Requirements – Wind Directionality Factor:

1) The wind directionality factor, ______, shall be determined from Table 26.6-1 from ASCE

7-16 (herein shown as Table 1).

2) Note this directionality factor shall only be included in determining wind load when the

load combinations specified in ASCE 7-16 Section 2.3 and 2.4 are used for design.

3) The effect of wind directionality in determining wind loads in accordance with Chapter 31

(wind tunnels) shall be based on an analysis for wind speeds that conforms to the

requirements of ASCE 7-16 26.5.3.

Table 1. Wind directionality factor.

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General Requirements – Exposure Category:

1) For each wind direction, the ____________________________________ shall be based

on the ground surface roughness that is determined from natural ____________________,

_________________________, and constructed ________________________________.

2) A ground surface roughness within each 45-degree (0.79 radian) sector shall be determined

for a distance upwind of the site as defined in 26.7.2 and 26.7.3 from the categories defined

below.

a. The highest wind load shall be used to represent the winds from that direction.

b. Surface Roughness B: Urban and suburban areas, wooded areas, or other terrain

with numerous closely spaced obstructions having the size of single-family

dwellings or larger.

c. Surface Roughness C: Open terrain with scattered obstructions having heights

generally less than 30 ft (9.1 m). This category includes flat open country and

grasslands.

d. Surface Roughness D: Flat, unobstructed areas and water surfaces. This category

includes smooth mud flats, salt flats, and unbroken ice.

3) An exposure category shall be determined in accordance with one of the following three:

a. Exposure B: For buildings with a mean roof height of less than or equal to 30 ft

(9.1 m), Exposure B shall apply where the ground surface roughness, as defined by

Surface Roughness B, prevails in the upwind direction for a distance greater than

1,500 ft (457 m). For buildings with a mean roof height greater than 30 ft (9.1 m),

Exposure B shall apply where Surface Roughness B prevails in the upwind

direction for a distance greater than 2,600 ft (792 m) or 20 times the height of the

building, whichever is greater.

b. Exposure C: shall apply for all cases where Exposures B or D does not apply.

c. Exposure D: shall apply where the ground surface roughness, as defined by Surface

Roughness D, prevails in the upwind direction for a distance greater than 5,000 ft

(1,524 m) or 20 times the building height, whichever is greater. Exposure D shall

also apply where the ground surface roughness immediately upwind of the site is B

or C, and the site is within a distance of 600 ft (183 m) or 20 times the building

height, whichever is greater, from an Exposure D condition as defined in the

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previous sentence.

d. For a site located in the transition zone between exposure categories, the category

resulting in the largest wind forces shall be used.

General Requirements – Topographical Factor:

1) The effect of wind forces due to topographic effects is presented in ASCE 7-16 Section

26.8.

2) ___________________________________ effects at isolated hills, ridges, and

escarpments constituting abrupt changes in the general topography, located in any exposure

category, shall be included in the design when buildings and other site conditions and

locations of structures meet all of the following conditions:

a. The ____________________________________________________ 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 mi (3.22 km),

whichever is less. This distance shall be measured horizontally from the point at

which the height H of the hill, ridge, or escarpment is determined.

b. The hill, ridge, or escarpment protrudes above the height of upwind terrain features

within a 2-mi (3.22-km) radius in any quadrant by a factor of two or more.

c. The structure is located as shown in Fig. 26.8-1 in the upper one-half of a hill or

ridge or near the crest of an escarpment.

d. H/Lh ≥ 0.2.

e. H is greater than or equal to 15 ft (4.5 m) for Exposure C and D and 60 ft (18 m)

for Exposure B.

3) The wind speed-up effect shall be included in the calculation of the design wind loads by

the factor Kzt. Note this is only done if the conditions and locations of the structures meet

all of the conditions specified above and within Section 26.8.1 as ASCE 7-16.

4) The values of the constants are provided in ASCE 7-16 Figure 26.8-1, shown here as

Figures 9 and 10.

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Figure 9. Topographic factor constant, Kzt – part one.

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Figure 10. Topographic factor constant, Kzt – part two.

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General Requirements – Gust Effect Factor:

1) The gust-effect factor for a rigid building or other structure is permitted to be taken as 0.85.

2) This accounts for the _______________________________________ effect of the

building and its resonance with the expected wind loading.

3) Section 26.11 outlines other methods to determine gust factors and combined gust-effect

factors and pressure coefficients. If a combined factor is determined, the gust-effect shall

not be determined separately.

a. This includes the determination of the frequency of a building or other structure.

4) One reference table used in this method is ASCE 7-16 26.11-1 (Figure 11).

Figure 11. Terrain exposure constants for determining gust-effects and pressure coefficients.

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General Requirements – Enclosure Classification:

1) For the purpose of determining internal pressure coefficients, all buildings shall be

classified as enclosed, partially enclosed, or open as defined in ASCE 7-16 Section 26.2.

a. Refer to section 26.12.

2) Multiple classifications note – if a building by definition complies with both the “open”

and “partially enclosed” definitions, it shall be classified as an “open” building. A building

does not comply with either the “open” or “partially enclosed” shall be classified as an

“enclosed building”.

General Requirements – Internal Pressure Coefficient:

1) Internal pressure coefficients (GCpi) shall be determined from ASCE 7-16 Table 26.13-1

based on the building enclosure classifications defined previously.

2) Note this is illustrated herein as Figure 12.

3) Note a reduction factor for a large volumetric building is permitted by ASCE 7-16 26.13.1.

a. For a partially enclosed building containing a single, unpartitioned large volume,

the internal pressure coefficient, (GCpi), shall be multiplied by the following

reduction factor, Ri:

b. Aog = total area of openings in the building envelope (walls and roof, in ft2)

c. Vi = unpartitioned internal volume, in ft3

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Figure 12. Internal pressure coefficient (GCpi).

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Wind Load Design MWFRS Directional Procedure:

1) Consider a general case for buildings of all heights. Detailed in ASCE 7-16 Chapter 27.

2) For this example, enclosed, partially, enclosed and open buildings can be considered.

3) The initial steps to determine the wind loads on the MWFRS can be outlined in the seven

steps below:

a. Determine the risk category of the structure (Table 1.5-1)

b. Determine the basic wind speed, V, for the applicable risk category, Figures 26.5.-

1 and 26.5-2.

c. Determine the wind load parameters.

i. Wind directionality factor, Kd, Table 26.6-1 and Table 26.6-1

ii. Exposure category, Section 26.7

iii. Topographic factor, Section 26.8 and Figure 26.8-1

iv. Ground elevation factor, Section 26.9

v. Gust effect factor, Section 26.11

vi. Enclosure classification, Section 26.12

vii. Internal pressure coefficient, Section 26.13 and Table 26.13-1

d. Determine velocity pressure exposure coefficient, Kz or Kh, Table 26.10-1 (Fig. 13)

e. Determine velocity pressure, Equation 26.10-1

f. Determine external pressure coefficient, Figure 27.3-1 through 27.3-7.

g. Calculate wind pressure on each building surface.

i. Use equation 27.3-1 and 27.3-2for rigid/flexible enclosed building or open

buildings, respectively.

4) Velocity pressure, qz, shall be evaluated at height z using the following equation (26.10.2):

a. Kd = wind directionality factor

b. Kz = velocity pressure exposure coefficient

c. Kzt = topographic factor defined

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d. V =basic wind speed

e. qz =velocity pressure calculated using Eq. 26.10-1 at height z

f. qh =velocity pressure calculated using Eq. 26.10-1 at mean roof height h.

g. The numerical coefficient 0.00256 shall be used except where sufficient climatic

data are available to justify the selection of a different value of this coefficient for

a design application.

5) Design wind pressures for the MWFRS of buildings (all heights) that are enclosed and

partially enclosed rigid and flexible buildings shall be determined by:

a. q = qz for windward walls evaluated at height z above ground

b. q = qh for leeward walls, side walls and roofs evaluated at height h

c. qi = qh for windward walls, side walls, leeward walls, and roofs of enclosed

buildings and for negative internal pressure evaluation in partially enclosed

buildings

d. qi = qz for positive internal pressure evaluation in partially enclosed buildings where

height z is defined as the level of the highest opening in the building that could

affect the positive internal pressure. For buildings sited in wind-borne debris

regions, glazing that is not impact resistant or protected with an impact resistant

covering shall be treated as an opening. For positive internal pressure evaluation,

qi may conservatively be evaluated at height h (qi = qh)

e. G = gust-effect factor. For flexible buildings, use Gf from 26.11.5.

f. Cp = external pressure coefficient from Figs. 27.4-1 , 27.4-2 and 27.4-3

g. GCpi = internal pressure coefficient from Table 26.11-1

h. q and qi shall be evaluated using exposure defined in Section IV.C.2. Pressure shall

be applied simultaneously on windward and leeward walls and on roof surfaces as

defined in Figs. 27.3-1 , 27.3-2 and 27.3-3 (Figures 14 – 17).

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6) For flexible buildings, design wind pressures can be determined from a modified equation.

(23.4-2).

a. Refer to section 26.11.5 for additional details.

7) For open buildings with monosloped, pitched, and troughed free roofs, the net design

pressures can be determined using equations 27.3-2.

a. Note the pressure coefficients can be determined from Figures 27.3-4 through 27.3-

7 within ASCE 7-16 (Figures 18 – 21).

8) For roof overhangs, the positive external pressure shall be determined with a pressure

coefficient of 0.8 and combined with the top surface pressure from Figure 27.4-1.

9) For parapets, the design wind pressure of rigid or flexible buildings with flat, gable, or hip

roofs shall be determined using equation 27.4-4.

10) For MWFRS of buildings of all heights, the following design wind load cases to be

considered are defined in Figure 27.3-8 (Figure 22).

11) According to 27.1.5 Minimum Design Wind Loads:

a. Wind load to be used in the design of the MWFRS for an enclosed or partially

enclosed building shall not be less than 16 lb/ft2 (0.77 kN/m2) multiplied by the

wall area of the building and 8 lb/ft2 (0.38 kN/m2) multiplied by the roof area of the

building projected onto a vertical plane normal to the assumed wind direction.

b. Wall and roof loads shall be applied simultaneously.

c. The design wind force for open buildings shall be not less than 16 lb/ft2 (0.77

kN/m2) multiplied by the area Af.

d. Note these values are for Chapter 27 – MWFRS Directional Procedure. These

values will vary, note 28.3.4, 28.5.4, and 29.7.

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Figure 13. Velocity pressure exposure coefficients.

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Figure 14. Pressure coefficients for “enclosed and partially enclosed” buildings - walls and roofs.

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Figure 15. Pressure coefficients for “enclosed and partially enclosed” buildings - walls and roofs.

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Figure 16. Pressure coefficients for “enclosed and partially enclosed” buildings – doomed roofs.

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Figure 17. External pressure coefficients for “enclosed and partially enclosed” buildings and

arched roofs.

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Figure 18. Net pressure coefficients for open buildings and monoslope free roofs.

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Figure 19. Net pressure coefficients for open buildings and pitched free roofs.

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Figure 20. Net pressure coefficients for open buildings and troughed free roofs.

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Figure 21. Net pressure coefficients for open buildings and free roofs.

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Figure 22. MWFRS part 1 Design wind load cases.

9. wind design. wind design.pdf · load combinations specified in asce 7-16 section 2.3 and 2.4 are...

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