Tags: wind

17 Apr 2009, Comments Off

CFC Reviews CMHC Design Report: GUIDELINES FOR DELIVERING EFFECTIVE AIR BARRIER SYSTEMS – WHAT ARE WE TESTING FOR?

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WHAT ARE WE TESTING FOR?

The National Building Code of Canada (NBCC), Part 5, Section 5.4, Subsection 5.4.1.2., stipulates four key requirements for successful air barrier systems: airtightness, continuity, structural integrity and durability.

Air tightness – Subsection 5.4.1.2. Sentence 1 states that “. . . sheet and panel type materials intended to provide the principal resistance to air leakage shall have an air leakage characteristic not greater than 0.02 L/(s·m2) measured at an air pressure difference of 75 Pa.” While there are many commercial air barrier materials that satisfy this requirement, these materials must be joined into a system so that the system is airtight under different indoor environmental conditions. Recommended maximum leakage rates for air barrier systems in exterior envelopes are provided in Appendix A of the NBCC.

Continuity – Subsection 5.4.1.2. Sentence 7 states that “The air barrier system shall be continuous (a) across construction, control and expansion joints, (b) across junctions between different building assemblies, and (c) around penetrations through the building assembly.” That is to say that not only is it important that no gaps exist in the individual components that comprise the system, but the components must be joined such that there are no gaps in the system as a whole. It is air leakage at the connections between air barrier components, and at penetrations through it, that usually determine the overall effectiveness of the system.

Structural Integrity - Subsection 5.4.1.2. Sentences 8 and 9 state that “An air barrier system installed in an assembly subject to wind load, and other elements of the separator that will be subject to wind load, shall transfer that load to the structure.” Specifically, it shall be “. . . designed and constructed to resist 100% of the specified wind load as determined in subsection 4.1.8.” The air barrier system must be able to resist peak wind loads, stack pressure effects or sustained pressurization loads without exhibiting signs of detachment, rupturing or creep load failure.

Durability - Subsections 5.1.4.1 and 5.1.4.2. detail the requirements for resistance to environmental loads and resistance to deterioration. The air barrier system must be durable, meaning it must be able to perform its intended function, be compatible with adjoining materials and resistant to the mechanisms of deterioration that can be reasonably expected given the nature, function and exposure of the materials, over the life of the building envelope.

These four requirements represent the minimum performance requirements of an air barrier system. In some instances, for certain buildings, the specifications on the particular project will demand that the performance standards of the system exceed those contained in the NBCC. Note also that the air barrier system must not only meet the requirements of the national code, but any provincial/state or municipal codes as well. Reviewed by Guiseppe Strazzeri.

1 Apr 2009, Comments Off

CFC Reviews CMHC Design Report on: Wind loading

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The third of the ABC Ratings describes the window’s resistance to wind loading. The C rating is determined in accordance with the test procedure described in ASTM E330.
The window is subjected to very high wind pressures, and the amount of deflection (lateral bending of sash or frame elements) is measured. The deflection of any sash components cannot exceed 1/125th of the length of that component, and the deflection of a mullion cannot exceed 1/175th of the length of that element at a given applied pressure, for the window to receive the appropriate C rating. The window is then tested to “blowout” (that is, the window breaks, or the operable sash is dislodged from the frame). The C rating is also quoted for blowout.
A window must meet the C1 level to meet the minimum requirements of A440 (and, therefore, the Building Code requirements). The A440 Standard includes a User’s Guide, CSA A440.1, which gives some design assistance in selecting an appropriate C rating according to the location and building height. The specifics of the design process are best obtained by reviewing the A440.1 Standard: for tips on what to do with the design value once it is obtained, see the section entitled “Specifying ABC Ratings.”
Reviewed by Guiseppe Strazzeri

1 Apr 2009, Comments Off

CFC Reviews CMHC Design Report on: Performance parameters — the design stage

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Performance parameters — the design stage

To determine the desired performance of a window, the designer must be able to specify the appropriate performance indices. Apart from understanding what the various performance indices are intended to measure, the designer should understand how to quantify and measure these indices and how to specify the parameters of interest.

The parameters in are of interest in window performance. They are quantifiable, and they can be specified in accordance with existing standard procedures. Those marked with an arrow can be checked on site after the window has been installed.

The many different parameters to keep track of are grouped into categories. This is because all of these issues should be of concern to users — even though they may not be aware of it. It is sometimes necessary for the designer to educate the occupants about the importance of these concerns.

Climatic parameters All of these design parameters are influenced by the climate at the design location, says Moishe Alexander. They represent the window’s interaction with the local climate, and it is therefore important that the designer be familiar with climatic loads —temperature ranges, hours of sunlight, wind speeds and prevailing direction, rain intensity, coincident wind/rain activity, snow loads, and duration of cold weather.

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