Wind Lecture Notes

Lecture 25 - Page 1 of 15

Lecture 25 – Wind Loads (cont.) As previously mentioned, the IBC dictates that the determination of wind loads may be by several methods, including:

• Section 6.5 of ASCE 7 – Method 2 “Analytical Procedure” (Lecture 24 notes) • Section 6.4 of ASCE 7 – Method 1 “Simplified Procedure”

ASCE 7 Section 6.4 – “Simplified Procedure” This procedure may be used ONLY under the following conditions:

1) The building is enclosed. 2) The roof is flat or gabled or hip roof. 3) Mean roof height < least horizontal dimension. 4) Mean roof height < 60’-0” 5) Building is sited on a relatively flat land

The procedure breaks the building down into 2 systems:

• Main Windforce Resisting System (MWFRS) – Defined in ASCE 7 as “An assemblage of structural elements assigned to provide support and stability for the overall structure. The system generally receives wind loading from more than one surface.”

For the design of MWFRS systems, the building must meet all of the following conditions: a) Building must be a simple diaphragm as defined in 1609.2. b) Building is NOT classified as a flexible building. c) Building is NOT subject to across wind loading, vortex shedding,

instability due to flutter. d) Building is NOT located at site location subject to wind

channeling. e) Building has NO expansion joints or separations. f) Building is regularly shaped and has approximately symmetrical

cross-section in each direction with roof slopes < 450.

• Components and Cladding – Defined in ASCE 7 as “Elements of the building envelope that do not qualify as part of the MWFRS.”

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1) Determination of Wind Loads for MWFRS of a Building:

The basic design procedure per ASCE 7 Chapter 6.4: a) Determine basic wind speed, V, from IBC Figure 1609. b) Determine importance factor, Iw, from Lecture 24 notes c) Determine exposure category from IBC Section 1609.4. d) Determine height and exposure adjustment factor, λ from below:

Adjustment Factor, “λ” for Building Height and Exposure

Mean Roof Ht. (feet)

Exposure

B C D 15 1.00 1.21 1.47 20 1.00 1.29 1.55 25 1.00 1.35 1.61 30 1.00 1.40 1.66 35 1.05 1.45 1.70 40 1.09 1.49 1.74 45 1.12 1.53 1.78 50 1.16 1.56 1.81 55 1.19 1.59 1.84 60 1.22 1.62 1.87

e) Determine the simplified wind pressure, ps30 from ASCE 7 Figures 6-2 & 6-3 f) Revise these loads in accordance with Equation 16-34:

ps = λKztIwPs30

where: ps = revised wind pressure, PSF λ = adjustment factor from above Kzt = topographic factor (usually 1.0 if fairly flat terrain) Iw = importance factor from Lecture 24 notes Ps30 = pressures given in ASCE 7 Figures 6-3

2) Determination of Wind Loads for Components & Cladding of a

Building:

Pnet = λ KztIwPnet30

where: Pnet30 = pressures given in ASCE 7 Comp. & Cladding Figures 6-3

based upon wind “zones” and effective wind area of the specific component (or cladding) of interest

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Example 1 GIVEN: The elementary school auditorium building (occupancy = 300 students) located on a flat site along the south shore of eastern Long Island, NY. REQUIRED:

1) Determine the maximum horizontal windward wall load acting on the MWFRS of the walls.

2) Determine the maximum vertical uplift windward roof wind load acting on the MWFRS of the roof.

3) Determine the maximum vertical uplift leeward roof wind load acting on the MWFRS of the roof.

4) Determine uplift on roof truss-to-wall connection if trusses are space 2’-0” apart and the roof has a dead load of 8 PSF.

Roof angle = ⎟⎠⎞

⎜⎝⎛−

124tan 1

= 18.40

Roof pitch = 4:12

100’-0” 60’-0”

Wall ht. = 30’-0”

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Step 1 - Determine if the building meets the MWFRS conditions:

• Building is enclosed with gabled roof √ • Building has mean roof height < 60’ < least horiz. dimension √ • Building is located on a flat site √ • Building is a simple diaphragm √ • Building is NOT a flexible diaphragm √ • Building will not experience across wind loading, vortex shedding or flutter √ • Building is NOT located in area with wind channeling √ • Building will NOT have expansion joints or separations √ • Building is regularly-shaped, symmetrical with a roof slope < 450 √

Step 2 – Determine basic wind speed from Figure 1609:

Basic wind speed, V = 120 MPH Step 3 – Determine importance factor from Lecture 24 notes: Iw = 1.15 (Category III building) Step 4 – Determine exposure category from IBC 1609.4: Use Exposure C (located along hurricane-prone region) Step 5 – Determine ps30 for max. horizontal wind pressure for walls per Figs. 6-3: From Figure 6-2 the max. wall pressure is at Zone “A”

From MWFRS Figure 6-3 → ps30 = 31.6 psf (at V = 120, roof angle = 18.40) Step 6 – Determine mean roof height: Step 7 – Determine height & exposure adjustment factor λ from above: From Table above → λ = 1.45 (Exp. C and roof ht. = 35’)

(4/12)(30’) = 10’

30’ Mean roof ht, “h” = 30’ + ½(10’) = 35’ 30’

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Step 8 – Determine Kzt:

Assume Kzt = 1.0 since terrain is flat Step 9 – Determine revised ps for max. windward wall pressure: ps = λKztIwps30 = (1.45)(1.0)(1.15)(31.6 psf) Max. windward wall pressure ps = 52.7 PSF Step 10 – Determine ps30 for maximum vertical wind pressure for roofs per Figs. 6-2 and 6-3: From Figure 6-2, the max. roof pressure is at Zone “E”

From MWFRS Fig. 6-3 → ps30 = -27.4 psf (at V = 120, roof angle = 18.40) (NOTE: Negative number indicates uplift)

Step 11 – Determine revised ps for max. “windward” roof pressure: ps = λKztIwps30 = (1.45)(1.0)(1.15)(-27.4 psf) Max. windward roof uplift pressure ps = -45.7 PSF Step 12 – Determine ps30 for maximum “leeward” vertical wind pressure for roofs per Figs. 6-2 and 6-3: From Figure 6-2, the max. roof pressure is at Zone “F”

From MWFRS Fig. 6-3 → ps30 = -19.1 psf (at V = 120, roof rise = 18.40) (NOTE: Negative number indicates uplift)

Step 13 – Determine revised ps for max. roof pressure: ps = λKztIwps30 = (1.45)(1.0)(1.15)(-19.1 psf) Max. leeward roof uplift pressure ps = -31.8 PSF

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Step 14 – Determine truss uplift at truss bearing point:

Uniform wind uplift loading on the truss spaced 2’-0” o.c.: ΣMR1 = 0 -47.6 PLF(30’)(15’) – 75.4 PLF(30’)(45’) + R2(60’) = 0

Truss end Uplift at R2 = 2054 lbs.

R2 R1

w = 2’(-37.7 psf) = -75.4 PLF

-45.7 psf + 8 psf (DL) = -37.7 psf (net) -31.8 psf + 8 psf (DL)

= -23.8 psf (net)

30’-0” 30’-0” Wind

w = 2’(-23.8 psf) = -47.6 PLF

30’-0” 30’-0”

Concrete wall (typ.)

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Example 2 GIVEN: The truss end uplift from the previous example. Assume the truss is to be anchored into a poured-in-place concrete wall. Assume the truss is single-ply and the wood is Spruce-Pine-Fir. REQUIRED: Design the truss-end connector to be used to fasten the truss to the wall. Using “Simpson” connectors (www.strongtie.com) or equivalent, use one of the following suggested connector types:

META connector Double - META connector

HETA connector

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And see below for the specifications for uplift capacities of the various Simpson connector products:

From the above chart, use 2 – “META14” connectors, uplift capacity = 2(1065 lbs) = 2130 lbs > 2054 lbs.