Threshold Damage to Garage Doors Exposed to Explosion or Excessive Winds
An evaluation of building damage resulting from excessive air pressures generated by a nearby explosion or generated by excessive winds may include examination and evaluation of the garage door, especially if the garage door faces toward the site of the explosion or toward the direction of the excessive winds.
Garage doors are commonly one of the weakest components in a wall. The relative weakness is due to the relatively large surface area compared to the narrow width of its thickness. That is, a garage door is like a relatively large sail that catches the wind, but it is a relatively thin, flimsy wall component.
A garage door is usually held in place by rollers that are set into brackets on the door and extend in the metal tracks at each side of the door opening. Thus, a garage door usually spans side to side when exposed to inward or outward air pressures.
Before the inward or outward pressures on the door are transferred to the metal tracks at each side of the door opening, the garage door bends or flexes. To safely resist the bending or flexing the garage door must be strong enough and stiff enough. By strong enough I mean that the strength of the door material must be greater than the stresses generated within the material by bending. By stiff enough I mean that the door doesn’t deform or flex so much that the structural members, usually ribs or girts of the door, deform or buckle.
It can be shown mathematically that since the pressures on a door generate bending stresses proportional to the square of the span, for example, (162 = 256), and since the pressures on a door generate deformation or flexing proportional to the fourth power of the span, for example, (164 = 65536), stiffness is more critical than strength. Thus, one would expect that the components of a garage door would deform or buckle rather than the material fracturing or stretching when the door is exposed to excessive inward or outward pressures. In fact, deformation or buckling of garage door components is what we have commonly seen during our evaluations of garage doors exhibiting threshold damages (marginal, questionable, or minor damages) from exposure to excessive air pressures generated by an explosion or excessive winds.
We note here that the failure or damage to garage doors exhibiting obvious and severe damage from exposure to excessive pressures, such as, from direct exposure to an explosion or a tornado are another subject for another day.