AECT360 Lecture 25

8/8/2019 AECT360 Lecture 25

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Lecture 25 - Page 1 of 15

Lecture 25 Wind Loads (cont.)

As previously mentioned, the IBC dictates that the determination of wind loadsmay 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 asAn assemblage of structural elements assigned to provide support andstability for the overall structure. The system generally receives windloading from more than one surface.

For the design of MWFRS systems, the building must meet all ofthe 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 thebuilding 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 notesc) Determine exposure category from IBC Section 1609.4.

d) Determine height and exposure adjustment factor, from below:

Adjustment Factor, for Building Height and ExposureMean Roof Ht.(feet)

Exposure

B C D15 1.00 1.21 1.4720 1.00 1.29 1.5525 1.00 1.35 1.61

30 1.00 1.40 1.6635 1.05 1.45 1.7040 1.09 1.49 1.7445 1.12 1.53 1.7850 1.16 1.56 1.8155 1.19 1.59 1.8460 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 = KztIwPs30

where:ps = revised wind pressure, PSF

= adjustment factor from aboveKzt = topographic factor (usually 1.0 if fairly flat terrain)Iw = importance factor from Lecture 24 notesPs30 = pressures given in ASCE 7 Figures 6-3

2) Determination of Wind Loads for Components & Cladding of aBuilding:

Pnet = KztIwPnet30

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

based upon wind zones and effective wind area of thespecific component (or cladding) of interest


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Example 1GIVEN: 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 onthe MWFRS of the roof.

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

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

Roof angle =

12

4tan 1

= 18.40

Roof pitch = 4:12

100-060-0

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 from above:From Table above = 1.45 (Exp. C and roof ht. = 35)

(4/12)(30)= 10

30 Mean roof ht, h = 30 + (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 = KztIwps30

= (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 perFigs. 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 = KztIwps30

= (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 forroofs 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 = KztIwps30

= (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.:

MR1 = 0-47.6 PLF(30)(15) 75.4 PLF(30)(45) + R2(60) = 0

Truss end Uplift at R2 = 2054 lbs.

R2R1

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)

30-030-0 Wind

w = 2(-23.8 psf)= -47.6 PLF

30-030-0

Concrete wall (typ.)


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Example 2GIVEN: The truss end uplift from the previous example. Assume the truss is tobe anchored into a poured-in-place concrete wall. Assume the truss is single-plyand 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 thefollowing suggested connector types:

META connector

Double - METAconnector

HETA connector


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

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

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AECT360 Lecture 25