By Alfonso Alzamora
Engineering calculations are necessary to determine the structural design loads of windows and doors. According to ASCE 7, a structural loads design document, the basic wind speed applicable to a project is used with a series of calculations to develop specific positive and negative design pressures for a building and the windows and doors. These resulting design pressures are used as part of the data to select or design appropriate windows and doors.
The wind load calculations provide the maximum wind load values that a building will experience within the wall interior (Building Zone 4), and at the wall edge (Building Zone 5). Thus, Building Zones 4 and 5 are based on location at the building exterior. Building Zone 4 comprises the window and door assemblies located in the center of the façades, while Zone 5 comprises the assemblies located near a corner of a building. Both zones experience positive and negative pressures as the wind moves around and over a building. While one side of a building experiences positive pressure as the wind blows against it, the other sides and roof experience negative suction pressure.
Pressure values for corner zones are usually higher since wind speeds up and swirls at the corners of a building causing high negative pressures to occur at these locations. Consequently, fenestration products located near the building corners (Zone 5) will experience the highest design wind pressures and sometimes may require a different product selection from those located at the center of the building (Zone 4) to meet these pressures.
It is common within the industry for vendors to test their products to maximum design pressure values, which in some cases may be beyond Florida Building Code and/or Miami-Dade requirements for a specific project. This allows them to use the same products for different pressure zones within a building and/or for different projects without having to undergo the testing and approval process for each job they bid. For example, if the wind load calculations for a specific project indicate that the building is subject to maximum wind load values of 70/-70 p.s.f. at Wall Interior Zone 4 and 70/-90 p.s.f. at Wall Edge Zone 5 and the proposed window is rated for a maximum design pressure of 100/-100 p.s.f., the same product will be quoted for both building zones. Furthermore, the same window can be proposed for other projects where the maximum calculated wind loads do not exceed the window maximum design pressure of 100/-100 p.s.f.
However, when the conditions on a particular project exceed the maximum design values for the product proposed, additional development and testing is required to meet the project requirements. This results in additional costs for the vendor that will most likely be passed on to the client.
The design pressure values are also used to establish water performance requirements of fenestration products. There is a direct relationship between the design for positive wind pressure and the design for water performance. AAMA/WDMA/CSA 101/I.S.2/A440-05 “Standard/Specification for Windows, Doors, and Unit Skylights” is the industry standard that addresses this relationship.
ASTM E331 is a water test that simulates the effects of a wind driven rain storm. Air pressure difference is part of the test that simulates winds that can drive water through any gaps in the windows and doors or the window/door to wall interfaces. The direction of the flow is from the side with higher pressure to the side with lower pressure (outside to inside). Thus, the industry standard specification ties the water penetration resistance test pressure to the positive design wind pressure requirements.
The industry standard recommends that water performance requirements be established as 15% of the maximum positive design wind pressure. Therefore, using the previous example, if the building has a maximum positive design pressure of 70 p.s.f., the water test pressure requirement would be 10.5 p.s.f. It is important to note that the water resistance of windows and doors can be exceeded during high wind events such as hurricanes. Therefore, based on our experience, we generally recommend that a minimum water test pressure of 15 p.s.f. be specified for high risk projects in hurricane prone areas.
In addition to the laboratory water resistance testing, we strongly recommended that water performance capabilities be verified through field testing using ASTM E1105. Typically, water resistance field testing is performed on a representative sampling of the installed products. The AAMA 502-08 industry standard provides guidelines for field testing of windows and doors. This standard allows for a 2/3 reduction of the tested and rated laboratory performance test pressure and specifies a minimum water performance requirement of 1.9 p.s.f. for any given fenestration product.
Our firm has provided design assistance services to scores of owners, architects and contractors, and it has been our experience that the appropriate selection of windows and doors is an important first step towards a successful and watertight project and should be followed up with field testing to verify in-service performance.
If you have questions or comments, please contact us at info@glazingconsultants.com. Find out more about GCI at http://www.glazingconsultants.com or on Twitter @glazingconsult, and join our Building Envelope Matters LinkedIn group to discuss building envelope issues.
Alfonso Alzamora has been a consultant at GCI since 2005 and has provided construction consulting services including design, investigation, project management, inspections and testing of new or existing installations of glass, windows, doors, curtain walls and wall assemblies for building hurricane resistance and water integrity, as well as all aspects of waterproofing these and related products at office buildings, hospitals, schools, condominiums, municipal buildings and private residences. His clients include product manufacturers, attorneys, contractors, architects, developers and owners. He has a bachelor’s degree in civil engineering and a graduate degree in construction management from the North University of Colombia. Mr. Alzamora is a member of the Colombian National Board of Professional Engineers and is currently seeking his licensure with the Florida Board of Professional Engineers.
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