International Code Requirements for Residential Windows and Doors 2016: Design Loads

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Provisions for design loads of all exterior cladding of residential structures—including fenestration—are set forth in Section R301 of the 2015 IRC. The design loads of concern for vertical glazing are design wind load and impact resistance. Skylights and sloped glazing are also subject to snow load and dead load.

Wind Loads

Tables R301.2(2) and R301.2(3) of the 2015 IRC give the design wind loads for glazed openings based on the design wind speed of the specific location where construction is to take place, the mean height of the building and its exposure to wind.

A significant change to the wind load provisions occurred between the 2012 and 2015 editions of the IRC. Specifically, the methodology used to calculate design wind loads in the IRC was changed from Allowable Stress Design to Strength Design. This change initially occurred between the 2005 and 2010 editions of the American Society of Civil Engineers 7 Minimum Design Loads for Buildings. It was brought into the 2012 IBC but not into the 2012 IRC.

This caused confusion between the 2012 editions of the IRC and IBC. Some states (such as Florida) amended the 2012 IRC, when they adopted it, for consistency with the 2012 IBC. However, the 2015 editions of the IRC and IBC both reference ASCE 7, meaning that the design wind speeds are consistent between the 2015 editions.

The standards that the fenestration industry relies upon for structural design of its products are all based upon the Allowable Stress Design method. This includes all predecessor and current editions of NAFS, ASTM E1300 and ASTM E330.

The change in methodology from Allowable Stress Design to Strength Design results in higher design-wind speeds and pressures. At first glance, this might give the appearance of placing more stringent requirements on exterior windows, doors and skylights.

In actuality, the 2015 editions of the IBC and IRC contain provisions to multiply this new, higher Strength Design Load by a factor of 0.6 for the purpose of conversion, and therefore can compare to Allowable Stress Design values. In most cases, this conversion results in required design pressure ratings for fenestration that are roughly comparable to the more traditionally determined values.

Therefore, the design wind pressure values obtained from the 2015 IRC are to be multiplied by 0.6 for the purposes of comparison to the Design Pressure rating of the fenestration product obtained by testing in accordance with NAFS-11.

This 0.6 factor is not to be applied when the design wind pressure values are obtained from the 2012 or earlier editions of the IRC. The design wind pressure values of the 2012 IRC have already been adjusted to be consistent with other design provisions within the document that are based upon Allowable Stress Design. This includes the DP rating of fenestration products under NAFS.

It is imperative that the builder, code official, manufacturer and anyone else involved in choosing or approving the windows, doors or skylights for a particular residence be aware as to whether the 0.6 conversion factor is appropriate or not. AAMA, WDMA, Fenestration Manufacturers Association and Door & Access Systems Manufacturers Association International published a technical bulletin (TB 11-1) on this topic. This bulletin can be downloaded from the AAMA online store, aamanet.org/store.

Dead Loads

The provisions for dead loads in Section R301.1 of the 2015 IRC are also based on ASCE 7-10; there are no significant changes to the dead load requirements for fenestration between the 2012 and 2015 editions.

Impact Resistance

Section R301.2.1.2 of the 2015 IRC outlines the locations where impact-resistant products are required; all exterior openings in wind-borne debris areas are required to be impact-resistant.

The geographical locations where impact protection of openings is required are similar to those given in ASCE 7-10 and are primarily defined by design wind speed.

Products that need to meet impact resistance requirements must be tested to one of a few different sets of standards. One option is testing in accordance with ASTM E1886-05 and ASTM E1996-2012a, which must be used together.

The 2015 IRC also recognizes the AAMA 506 certification label tab as evidence that a product has been tested appropriately. This tab provides a method for window manufacturers to demonstrate its product has been successfully tested in accordance with ASTM E1886 and ASTM E1996 by including the tab as part of its NAFS product certification label. The 2015 IRC also permits the use of “other approved tests.” This may include Miami-Dade County test protocols, if approved by the authority having jurisdiction.

The use of protective wood panels as an alternative to impact-resistant glazing or shutters continues to be permitted for one- and two-story, single-family dwellings, duplexes and residential care facilities.

See also

Main Article

Energy Performance