Volume 43, Issue 3 - March 2008

Architects' Guide to Glass


Designing Beyond the Code 

By Paul E. Beers
 
When Hurricane Wilma slammed into the Mexican Yucatan peninsula in October 2005, the popular tourist destination was devastated by wind speeds of more than 125 miles per hour. One of the hardest-hit hotels was the popular Marriott Casa Magna Hotel in Cancun where almost every door and window failed or was damaged.

During the turmoil of recovery, the hotel owners faced an unexpected roadblock—obtaining quality insurance coverage. The hotel’s coverage had been dropped promptly by its Mexican property insurers. An American insurer agreed to provide coverage if the hotel was retrofitted to a new standard, which exceeded even the toughest impact-resistant American codes–Miami Dade. My company began the process of developing an entirely new set of design criteria for the hotel’s glazing which could not be repaired properly. Today, the Marriott Casa Magna is designed to withstand wind speeds of 150 miles, exceeding the requirements of Miami Dade. All products had to be shipped in from the United States to ensure they met the design criteria, as Mexican products are unrated. We designed a completely new custom curtainwall system as well as a new skylight application. We also oversaw the installation of new windows and sliding glass doors throughout the project. Furthermore, we reduced the potential for water leakage by using glazing products with higher water ratings. The extra effort paid off, as the hotel withstood the outer winds of Hurricane Dean, a Category 5 hurricane that struck Mexico’s Yucatan peninsula in 2007. The story of the Marriott Casa Magna underscores a growing trend in the glazing industry–designing beyond the code. Building codes represent a minimum standard for protection. Unfortunately, this is often interpreted as the standard that will protect all buildings from all types of events. Codes provide for a basic level of protection, but there are instances where it is appropriate to exceed the code. Marriott Casa Magna illustrates how designing for improved performance during hurricanes can prevent future instances of major damage significantly.

When is it advisable to exceed the code minimums? There are the obvious candidates, which include buildings that will be used for shelters, hospitals, fire and police stations and other critical types of services. However, some architects, engineers, building owners, and even homeowners, are beginning to understand the importance of designing beyond the code in “non-essential” situations. Examples include corporate data centers, corporate headquarters, critical business locations, waterfront buildings and high-end luxury homes or condos that contain expensive furnishings and art. 

The Wake-Up
Prior to Hurricane Andrew in 1992, there was very limited discussion about the problems of windborne debris. The United States was emerging from a 20-year “lull” during which major hurricanes had not presented serious problems. Andrew changed all that and the resulting code changes to require hurricane-resistant materials has addressed the basic deficiencies with the codes. Since then, there have been steady improvements in the quality and enforcement of the impact-resistant standards. Adoption of windborne debris standards also has grown geographically, as many coastal states have adopted some form of the codes.

Interestingly, the next major anticipated change involves exceeding the code. The proposed new ICC–500 Storm Shelter Standard is expected to go into effect during the middle of this year. These types of buildings are designed to allow functionality both during the storm and in the days immediately following. The ICC-500 proposal is based on a 10,000-year mean recurrence interval. Glazing will need to withstand 200- to 225-mph, 3-second wind gusts in South Florida. The large-missile impact test applies, but with tougher requirements. A 9-pound, 2 x 4 missile must be fired at .04 x design wind speed for vertical surfaces. For horizontal surfaces, the requirement is .01. 

Most industry professionals are familiar with the large-missile impact test; firing a 9-pound 2 x 4 at 50 feet per second. However, there is another standard listed in ASTM E1996 that requires a higher standard of performance: Level E.

Level E was developed for critical facilities and requires that the fenestration or shutter resist the 9-pound 2 x 4 fired at 80 feet per second. While the cycling requirements are the same, the higher impact speed requires a different product choice when designing to Level E. Meeting it often requires the use of a double-laminated insulating glazing system. 

One Example
Hurricane Andrew drew widespread attention for the need to protect hospitals, especially patient and operating rooms, during and after a storm. Miami Children’s Hospital has been a leader in addressing the need for physical protection during a hurricane. Because of the critical nature of a children’s hospital, we were asked to conduct a detailed site-specific risk analysis for this structure. This involved site-specific computer modeling of 20,000 years of theoretical hurricanes and then selecting a design wind speed based on a probability of return period. While the code used a 100-year event for critical facilities, the hospital selected a 300-year return period, which resulted in a 178-mile-per-hour design wind speed. We also selected the ASTM Level E missile for enhanced windborne debris design. New windows and exterior walls were designed for the entire facility and, today, the hospital enjoys enhanced protection in a hurricane-prone environment. Several hurricanes have passed over the property since completion of the work with no damage to the windows or walls.

Water Leakage
The Florida hurricanes of 2004 (Charley, Francis, Ivan and Jeanne) showed that there are still some areas where door and window improvements are needed–especially with regard to water leakage. While the potential for leaking doors and windows generally is understood, hurricane conditions often far exceed water-testing requirements. Many doors and windows that were developed for the hurricane environment have relatively low water-resistance ratings and were overwhelmed by hurricane-generated, wind-driven rains.

When designing glazing systems in hurricane prone areas, we often will recommend upgrading to the highest commercially-available water rating available. As a general rule, the industry standard for water infiltration ratings is 15 to 20 percent of the design pressure on the glazing system. Often this translates to wind speeds that are not even equivalent to a Category One storm (75 miles per hour). However, when designing systems for waterfront properties or other structures with high exposure to wind and water, the percentage should be higher, generally 25 to 50 percent. Consider the Omni Hotel at Champions Gate near Orlando. 

Product Selection Guidelines

It often is appropriate to select products that exceed code requirements, rather than just meet them. When considering product selection, keep the following factors in mind:

  • The code and industry standards are minimum standards and designing simply to meet them is designing to the minimum;
  • Low cost should not always be the deciding factor;
  • Make sure all the performance criteria meets or exceeds your specifications;
  • Quality installation of doors and windows is critical; and
  • Quality control programs, third-party monitoring and field tests help to verify that glazing system will perform well in future hurricane and high-wind situations.

GCI was retained to advise the owner and to provide site observation of the glazing, exterior wall system and roofing, along with field water infiltration tests. We advised them to require a water rating well above the industry standard of 15 to 20 percent and then provided field testing services to ensure the installed glazing systems complied. Several issues were discovered and corrected during construction. Shortly after construction was completed, the building successfully withstood the effects of three hurricanes in 2004 with no significant water leakage, while others in Florida that did not have higher water rating suffered major leakage.

In considering whether to exceed code, we not only look at windborne debris considerations but also the potential for water leakage problems. While water leakage does not garner the attention that impact resistance does, the effects of water damage can lead to devastating and long lasting problems with mold and moisture. 

While wind and water are problems that are beyond our control, there are tools that the construction industry can use to help mitigate these challenges from Mother Nature. External factors, such as the skyrocketing cost of insurance and our increasing inability to evacuate before a storm effectively, only serve to increase the risk to people and property. With that in mind, designing beyond the code minimums at least should be considered for important and critical buildings in storm-prone areas. Safer and thoughtful building practices today can help prevent tomorrow’s disasters. 

ASTM E1996 Test Criteria

Level   Projectile Impact Speed 
Level A 2 gm ball bearing  130 fps
Level B  2 lb 2 X 4 Lumber  50 fps
Level C  4.5 lb 2 x 4 Lumber  40 fps
Level D  9 lb 2 x 4 Lumber  50 fps
Level E  9 lb 2 x 4 Lumber  80 fps

Paul E. Beers established Glazing Consultants International in West Palm Beach, Fla., in 1988. He has more than 25 years of experience in the window and glazing industry and with building envelopes. 


USG
© Copyright 2008 Key Communications Inc. All rights reserved.
No reproduction of any type without expressed written permission.