ASM Application Guides and Notes

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ASM Application Guides and Notes

WIND RESISTANCE IN ASM DESIGN

(Reference NRC/IRC Client Report No. B1040-3)

1 OBJECTIVE

To develop a simplified procedure for the wind design of roof assemblies with metal coverings located in the province of British Columbia.

2 BACKGROUND

The National Research Council, Institute for Research in Construction (NRC/IRC) was approached by the Roofing Contractors Association of British Columbia (RCABC) to investigate typical 24 ga metal roof covering systems under dynamic wind load conditions. These investigations were carried out in two Phases.

In Phase I, April 2000, six typical systems with three metal panel products (KR-12, PROLOK and SNAP LOCK 2) were tested with two different deck conditions (air-permeable and airtight). Simulated deck air-leakage ratios, AL (leakage area / deck area), were about 2.42% for air permeable deck conditions and about 0% and 0.05% for airtight deck conditions.

In Phase II, September 2000, four additional systems were investigated with three panels (KR-12, PROLOK and SNAP LOCK 2) configurations with different deck conditions. Simulated deck air-leakage ratios, AL, were about 0% for the airtight deck condition and 0.09% and 0.27% for air permeable deck conditions.

3 EXPERIMENTAL DATA

For both Phases, systems were installed at the Dynamic Roofing Facility (DRF) at NRC/IRC and exposed to wind gusts until system failure was observed or maximum load cycle was reached. Details of the experimental setup and collected experiment data were documented under two separate reports as follows:

  • Wind Uplift Resistance of Metal Roofing System – Phase I by Hee Ham and Bas Baskaran.
  • Wind Uplift resistance of Metal Roofing System – Phase II by Hee Ham, Bas Baskaran, and William Lei.

4 SIMPLIFIED DESIGN PROCEDURE

In this report, a simplified procedure has been developed for designing flat, mono slope and sawtooth roof assemblies with metal coverings located in the province of British Columbia. Designers are directed to use building codes for detailed wind load calculation. This simplified procedure has conservative assumptions, and it is based on the following:

  • National Building Code of Canada, 1995
  • Experimental data
  • Engineering directives, and
  • Input from RCABC

4.1 CALCULATE THE DESIGN WIND PRESSURE

Step 1

Figure 1 divides the province of British Columbia into three regions:

Region 1 – High wind zone
Region 2 – Moderate wind zone
Region 3 – Low wind zone

Locate the proposed building location in Figure 1 and identify the one of the following three regions.

8.1.5.1.jpg

Figure 1. Wind Pressure Zone for the Province of British Columbia

Note: It is clear that Region 1 is located near the coast while most of Region 3 is located at the inland area.

Step 2

Low-rise buildings with flat roof: For building height less than building width with flat roofs design pressure are given in Section 1. Based on the building height and region, select the design pressure for roof covering design.

Building Height feet (m) Region R1 Region R2 Region R3
psf (kpa) psf (kpa) psf (kpa)
30 (9.2) -80 (-3.82) -57 (-2.72) -40 (-1.91)
35 10.7) -82 (-3.92) -59 (-2.82) -42 (-2.01)
40 (12.2) -84 (-4.02) -61 (-2.92) -43 (-2.06)
45 (13.7) -86 (-4.11) -62 (-2.96) -44 (-2.10)
50 (15.3) -88 (-4.21) -63 (-3.01) -45 (-2.15)
55 (16.8) -90 (-4.30) -65 (-3.11) -46 (2.20)
60 (18.3) -91 (-4.35) -66 (-3.15) -46 (-2.20)
65 (19.8) -93 (-4.45) -67 (-3.20) -47 (-2.25)
70 (21.4) -94 (-4.49) -68 (-3.25) -48 (-2.29)
75 (22.9) -96 (-4.59) -69 (-3.30) -49 (-2.34)
80 (24.4) -97 (-4.64) -70 (-3.35) -49 (-2.34)
85 (25.9) -98 (-4.68) -70 (-3.35) -50 (-2.39)
90 (27.5) -99 (-4.73) -71 (-3.39) -50 (-2.39)
95 (29.0) -100 (-4.78) -72 (-3.44) -51 (-2.44)
100 (30.5) -101 (-4.83) -73 (-3.39) -51 (-2.44)
Section 1. Flat Roof Cladding Design Pressure


Step 3

Low-rise buildings with mono slope roof: For building height less than building width with roof slopes that are greater than 3° and less than 10° :

Design Pressure = 0.90 X Design pressure reported in Section 1. Step 4

Low-rise buildings with sawtooth roof: For building height less than building width with the roof slopes that are greater than 10° and less than 30° :

Design Pressure = 1.34 X Design Pressure reported in Section 1.

Step 5

Medium rise buildings with flat roof: For building height greater than building width:

Design Pressure = 1.05 X Design Pressure reported in Section 1.

Step 6

Wind Uplift Pressure = Design Pressure X Factor of Safety.

NBC, 1995 recommends minimum design values. Selecting an appropriate factor of safety depends on the designer, and it should be 1.0 or higher.

4.2 REDUCTION FACTOR CALCULATION FOR AIR PERMEABLE DECK CONSTRUCTIONS

When deck construction permits air to pass through openings or joints (leakage), there will be a significant reduction in the wind uplift resistance of the roof covering. One can minimize this effect by using an air barrier that is adhered to the deck prior to the installation of the covering and by sealing perimeters, penetrations and junction flashing. Some air permeable deck constructions include the following:

  • reroofing applications
  • acoustical steel decking
  • unblocked (or) non-tongue and grooved wood decking
  • precast concrete and gypsum planks without grouted joints
  • cementitious wood fibre planks.


Step 1

Calculate the ratio of leakage area by using the following formula:

AL = {Leakage area / Deck area} *100,%

For example, convention deck constructions, i.e., new 5/8" (16mm) tongue and groove plywood has AL = 0%, and for ½" (13mm) plywood installed with H clips AL can be varied based on roof dimension, number of boards used, and clip locations. A sample calculation is shown in Appendix 1.

Step 2

Using Figure 2, quantify a reduction factor that can account for the air permeable deck constructions. The horizontal axis shows the AL in percentage and the vertical axis displays the reduction factors. The selected reduction factor should fulfil the following criteria:

Reduction Factor < 1.0

8.1.5.2.jpg

Figure 2. Reduction Factor vs. Air Leakage Ratio, AL

Note 1: These reduction factors were developed based on limited experimental data.

Note 2: It can be observed that the resistant wind pressure is increasing dramatically as the air leakage ratio is decreasing. It can also be estimated that a critical zone exists between 0.12% < AL < 0.27%

4.3 IDENTIFYING A SUITABLE PANEL CONFIGURATION

Using Figure 3, select a panel configuration and its wind uplift rating. This rating represents a conservative scenario of metal coverings installed over airtight deck configuration. Therefore, calculate the actual wind uplift resistance as follows:

Wind Uplift Resistance = Panel Wind Rating X Reduction Factor

8.1.5.3.jpg

Figure 3. Wind Rating vs. Panel Width

5 APPENDIX: SAMPLE CALCULATION FOR A

A typical calculation of AL is shown for the 1/2" (13mm) plywood deck with H clips. For a low-rise square or rectangular shape commercial construction, deck layout patterns are shown in Figure A1. Nominal gap for plywood joints is assumed to be 0.08" (2 mm). The joists – plywood junctions and flashing edges are assumed to be air sealed. Based on the roof geometry, one can select the appropriate AL from Figure A2. This can be used for the calculation of reduction factors as specified in Step 2.2

8.1.5.4.jpg

Figure A1. Source of air leakage for plywood decking with H clip installation.


8.1.5.5.jpg

Figure A2. AL for ½" (13 mm) plywood deck with H clip installation.


APPENDIX II – OTHER CONSIDERATIONS
Air Leakage of the deck
Wind uplift resistance of the tested systems increased dramatically as the air leakage ratio decreased. Therefore, one should use caution in using Figure 2.
Roof construction with building height greater than 100’ (31m)
Wind speed increases logarithmically from the ground to the height of the free stream in the atmospheric boundary layer. The obtained result can be applied for low and medium rise buildings [under 100’ (31 m)]. It is not recommended to use the simplified procedure for high-rise buildings [exceeding 100’ (31 m)].
Roof construction with unique terrain surroundings
The intensifying effects of valleys and other terrain features on wind pressure were not considered in the above procedure.
Wooden deck span requirement
Spacing of the joists should not be larger than 16" (406 mm) for 1/2" (13 mm) plywood decking with H clips and 24" (243 mm) for 5/8" (16 mm) tongue and grooved decking.
Eaves, overhangs and canopies
Eaves, overhangs, canopies, and nearby areas, by their design, may be subject to greater uplift forces than the roof surface because of pressure differential. These effects should be considered.
Edge conditions
The use of parapets of insufficient height, typically lower than two feet, may increase the effect of wind on the roofing system. The specific influence of parapet height on wind design should be considered.
Panel configuration
Experimental data indicate that systems with Prolock and Snap Lock panels exhibit higher wind uplift stability compared to systems with KR-12 panels. The higher stability may be attributed to the locking mechanism at the seam. Systems with Prolock and Snap Lock 2 panels were locked down lower on the seam giving lower pivot points than those systems with the KR-12 panel.
Panel thickness
The uplift resistance is not only a function of air permeability of the deck and spacing of joists and pullout value of the fastener but also a function of the properties of metal panels such as thickness and elastic and plastic strengths. The obtained results can be applied to 24 Ga or higher thickness (e.g., 20 Ga) metal roof covering systems. Using these results is not recommended for metal roof covering systems with lower thickness (e.g., 26 Ga).



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