1609.1.2 Protection of Openings - Exception 3

[Homer Guilliams]

Exception 3 allows glazing in building over 60 feet above the ground to be unprotected. Does anyone know why 60 feet was selected?

 

[RLGA]

My guess is that most debris is concentrated at lower elevations (from where they typically come) which increases the potential of glass impact. As debris is carried up to higher altitudes, they are more dispersed and the potential of glass impact diminishes.

The specific elevation was probably selected based on empirical evidence gathered after major wind events.

 

[Homer Guilliams]

If an opening above 60 feet were to fail without debris impact, (say from just wind and pressure) the wind and water consequences would be the same on the building. So why allow this exception?

 

[cda]

Over 60 feet?

 

[RLGA]

If an opening above 60 feet were to fail without debris impact, (say from just wind and pressure) the wind and water consequences would be the same on the building. So why allow this exception?

True, but all windows (even those above 60 feet) are required to resist wind load pressures. If they fail, then they either did not comply with the code minimums or the wind loads exceeded code minimums.

 

[RLGA]

Over 60 feet?

Yes, but that was from a tornado and it was located in Ft. Worth, Texas. Wind-borne debris protection is required for hurricane-prone areas--Ft. Worth is not in a hurricane-prone area. Wind speeds for tornados are much higher (200 mph + for EF-5) than hurricanes (Cat-5 157 mph).

 

[Homer Guilliams]

Are all windows required to meet hurricane wind speeds and pressures or just typical?

Sounds like the only purpose for the 60 foot exception may be financial.

Would like to know if there has been any written documentation on the subject.

Thanks for the discussion.

 

[RLGA]

Are all windows required to meet hurricane wind speeds and pressures or just typical?

Sounds like the only purpose for the 60 foot exception may be financial.

Would like to know if there has been any written documentation on the subject.

Thanks for the discussion.

All buildings and structures (including the windows) are required to be designed for the wind pressures created by the wind speeds provided in IBC Figures 1609.3(1) through (8) as applicable. The pressures will vary (for the same wind speed) depending on the configuration of the building. Building corners will have higher pressures than the field area of walls. Pressures will be positive on the windward side and negative on the leeward side.

A good source for understanding the basics of wind design can be found at the Whole Building Design Guide (https://www.wbdg.org/resources/wind-safety-building-envelope).

I'm sure there are academic and scientific papers prepared on the subject. I found one paper that focuses on windborne debris risk assessment using probability statistics based on the Poisson distribution theory. The article is centered on residential developments and not necessarily tall buildings. Another article, published by the "Journal of Wind Engineering and Industrial Aerodynamics," presents a model for windborne debris damage addressing wind events (typhoon/hurricane) in urban areas of Southeast Asia along the Pacific Rim.