Doors & Windows

Doors and windows are important components of a building’s structural and environmental performance. They represent portions of a building’s envelope, which obviously separates interior spaces from the exterior environment. As such they are considered structural components. The building’s envelope must resist wind pressures, water intrusion, and air leakage while maintaining thermal performance. Openings are typically the weakest points in the envelope, sometimes requiring reinforced framing and always careful detailing. Loads from gravity, wind, etc. should be properly transferred around these openings through headers, lintels, or other adjacent wall elements.

Windows are commonly categorized by their operation and configuration. Fixed windows are non-operable units designed primarily for light transmission and views; structurally, they tend to be stronger because they lack moving parts and can be more tightly sealed. Operable windows include single-hung and double-hung types, where one or both sashes move vertically; casement windows that hinge at the side and swing outward; and sliding windows that move horizontally. Each type has implications for useability, structural performance, air infiltration, and resistance to wind loads. For example, casement windows can provide better sealing under pressure because wind forces tend to press the sash more tightly against the frame. The choice of window locations should also consider exterior reflections of the suns rays, as this can damage vinyl and other susceptible materials.

Doors likewise vary in type and structural behavior. Hinged doors, sliding doors, and overhead garage doors each respond differently to loads. Garage doors, especially large overhead sectional types, are particularly significant in structural design considerations because of their size and exposure to external pressures.

Wind loading is a primary design consideration in wind prone regions. The pressure exerted on a door or window is a function of wind speed, exposure, and the size and location of the opening. Interestingly, smaller openings typically require higher design pressures. This is primarily because the chance of a faster wind gust covering a smaller opening is greater than that same gust covering a larger opening. The primary design guide for wind pressures on windows and other components is ASCE 7 components and cladding.

One of the most critical structural concerns involving doors is the performance of garage overhead or other large doors during high-wind events such as hurricanes or tornadoes. These doors are large, relatively flexible, and typically more vulnerable to failure under high wind pressure. If a garage door fails, it can create a breach in the building envelope, allowing the interior of the building to be pressurized. The increase in internal pressure, combined with external suction forces on the roof and walls, can significantly amplify loadings on the structure. This can lead to catastrophic failures, including roof uplift, wall collapse, etc. For this reason, modern building codes in wind-prone regions require reinforced garage doors, improved anchorage systems, and sometimes impact-resistant designs.

Proper installation of windows and doors is important. Factors to consider are correct fastening, insulating, waterproofing and flashing. The latter two are particularly important in wood-framed construction. Over time, leaks around windows and doors can lead to rot, mold growth, and structural damage. Flashing directs water away from these openings, while waterproofing prevents infiltration.

Double and triple pane windows have improved thermal performance, but have a signficant potential drawback. This is their potential to develop fogging between the panes. Window bellowing occurs when pressure differences or temperature changes cause the glass panes in an insulated window unit to flex slightly. This repeated movement over time, or a severe wind event, can damage the pane seals. Once these fail, moisture enters the space between the panes. This can lead to condensation and permanent fogging. The fogging is often related to the breakdown and distribution of the silica dessicant between the window panes, and usually requires dissassembly to clean the fogged interior glass and replace the failed seals.

ADDITIONAL RESOURCES:

ASCE 7 Chapter 30

Field fenestration specifications AAMA 502 and AAMA 503

NAFS (AAMA/WDMA/CSA 101/I.S.2/A440)

ASTM E2112

AAMA Guidelines

Door and window manufacturer’s specifications