Volume 37, Issue 5, May 2002
A Force of Interest
Revisiting Florida and the Impact-Resistant Requirements
by Mark Baker
On March 1, 2002, after several delays and modifications, the long-awaited Florida Building Code took effect. One of the major improvements in the new code is the identification of the high-velocity hurricane zone and wind-borne debris regions, which cover all or part of the coastal counties in the state. The code requires that new buildings constructed within these zones be designed to withstand full internal pressurization or be protected against fluctuating loads and flying debris by either approved shutters or impact-resistant glazing systems.
Implementation of the new code has expanded the market for impact-resistant glazing systems greatly. Manufacturers who have been reluctant to get into the market when it was limited to Dade and Broward Counties due to the perceived difficulties in obtaining product approval and the relatively small market are now taking a much longer look.
In addition to the expansion of hurricane-resistant glazing requirements to all of coastal Florida, other hurricane prone regions including coastal Georgia, the Texas barrier islands, coastal South Carolina and Charleston, N.C., Mobile, Ala. and the Delmarva Peninsula of Maryland have or are in the process of adopting hurricane codes.
As a result of many requests for copies of the article I wrote in the August 2000 issue of USGlass, “High Impact,” from manufacturers interested in developing an impact-resistant product line and those interested in the requirements of the new codes we have decided to re-run the article. The article follows:
Where It All Started
Following Hurricane Andrew in 1992, Florida’s Dade, Broward and Monroe counties adopted the first mandatory glazing impact-resistant standards in the United States, under the South Florida Building Code (SFBC).
Palm Beach County Fla., followed suit and adopted appendix (J) of the Standard Building Code for impact-resistant design as an option for local building departments.
In 1998, the Texas Department of Insurance (TDI) adopted Standard 1-98, Appendix E, Building Code for Windstorm Resistant Construction, requiring impact-resistant glazing for 14 Texas communities within 25 miles of the Gulf of Mexico, designated as a high-risk hurricane prone region.
Florida’s new statewide building code includes a Wind-borne Debris Protection Standard requiring impact-resistant glazing or shutters for all portions of 35 Florida counties falling in the hurricane prone wind-borne debris region per ASCE 7-98. Additionally, all coastal areas of the East Coast and the Gulf of Mexico are likely to adopt some form of impact-resistant standards.
South Florida’s Role
Contrary to popular belief, the SFBC was not a knee-jerk reaction to Hurricane Andrew by local officials responding to public criticism. The principles included in the current code, developed long before Hurricane Andrew devastated South Florida, were based upon Australian standards written in the late 1970s following Cyclone Tracy, which destroyed 90 percent of housing in Darwin, Australia, in 1974.
Engineering studies of Cyclone Tracey concluded that the vast majority of damage caused by hurricanes is due to wind-borne debris and fluctuating pressures. The experts concluded that the single-gust concept of design is inadequate to protect against sustained, turbulent winds that change direction slowly and carry debris.
Based upon these findings, original window protection codes and standards were written with an either/or provision requiring that either a building be designed to resist breaching of the building envelope by flying debris, or the structure must be designed for full internal pressurization.
Most current codes and standards have eliminated the option of designing the structure against full internal pressurization, instead requiring the building envelope to be designed to resist impact from flying debris and cyclic (fluctuating) pressures.
Small- and large-missile-impact testing requirements are the basis for designing against internal pressurization by protecting windows.
The following is based upon the SFBC test protocol PA 201. The SBCCI, TWIA, ASCE and ASTM standards closely follow the SFBC test protocol, although they do vary slightly, primarily with regard to the weight and speed of the projectiles.
All impact-test protocols have two basic components; impact with projectiles representing wind-borne debris; either large missile (projectile is 2- by 4-foot timber), or small missile (projectile is two gram steel balls); and cyclic structural loading representing fluctuating wind pressures.
Glazed openings below 30 feet from grade must be large-missile-resistant, while glazed openings above 30 feet must meet the requirements of either the small missile tests or the large missile tests.
What We Learned in Miami
I have witnessed many impact-resistant tests, have reviewed hundreds of test reports and have seen many systems pass and just as many fail. There are four major design elements that affect impact test results.
1. The size of the lite and magnitude of the design loads affect test results significantly. It is critical that reasonable test specimen sizes are used to maximize the benefits of the test results. Because impact-resistant systems are necessary in hurricane-prone regions, the design wind loads tend to be very high. In Miami, maximum design loads for 30-story-plus buildings are typically as high as 160 psf.
2. Once the size and wind-load requirements have been established, it is important to evaluate whether your existing frame structure can withstand the necessary pressure and fatigue of repetitive cycling. If your frame structure cannot, you must decide if it must be modified or if a new product is required. Often it is better to start from scratch than to attempt to modify existing products for impact testing.
3. After the structural framing system has been determined, you must select a suitable glazing infill. The thickness and composition of the infill are crucial in the performance of the system. Laminated glass and composite products are the items of choice currently. New products are introduced regularly and each has its limitations and varies in its ability to accommodate value-added products including coatings and frits.
4. Lastly, an appropriate glazing method must be selected. The glazing infill can (and usually will) break from the impact, but must remain in the frame during cycling. For large missile systems, this was initially achieved by mechanically attaching a laminate tail to the frame.
Over time, this method has been replaced by adhering all edges of a mechanically retained infill to the framing system, via a structural silicone sealant placed on the inboard surface with a glass bite of at least 5/8-inches. Mechanical attachment of laminate tails has become far less
For large-missile tests, impact at mid-span of a stopless structurally glazed framing member typically crushes the glass edges, resulting in failure during the structural cycling. Consequently, stopless structural glazing of large-missile impact-resistant systems has not been consistently achievable.
Conversely, shop-glazed, four-sided structural silicone systems, incorporating post-applied mechanically attached exterior elements proven by testing to protect glass edges from crushing, have been shown to pass the subsequent cycling phase.
We have been through a great deal in South Florida since the impact-resistant codes were introduced. What seemed impossible just a few years ago is now common practice. As the areas adopting impact-resistant glazing requirements grow it is important that we learn from the South Florida experience and continue to develop systems and materials that meet and exceed code requirements. n
Mark Baker, P.E., serves as principal of IBA Consultant Office in Miami. E-mail your questions to firstname.lastname@example.org.
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