Battering winds

One of the most vivid examples of the use of advanced polymer interlayers in laminated glass applications is meeting strict requirements for hurricane resistance. The Wilkie D. Ferguson United States Courthouse in Miami [Figure 1], designed by Arquitectonica and the Miami office of Helmut, Obata + Kassabaum (HOK) and completed in 2005, is a leading example of the need for strength in key glass elements. The $163 million, 14-story, 577,000-square-foot facility is adjacent to six courthouse-related buildings constructed between 1910 and 1975 in a downtown area covering two blocks. The new building houses 14 courtrooms, 16 chambers for the U.S. District Court, space for the U.S. Marshals Service, the federal public defender, the U.S. attorney, and the building's owner, the General Service Administration.

The primary architectural feature is comprised of two limestone towers, said to represent the two sides to every argument, connected by a single, curved glass prism that houses the public circulation and waiting spaces. The breezeway marks the entrance to the new courthouse and the entire two-block campus, and it needed to be light and open and transparent. But it also needed to be strong.

Wilkie D. Ferguson, Jr. U.S. Courthouse, Miami, Florida. The structural glass interlayer provides combined benefits of blast and hurricane protection. Architects: Arquitectonica and HOK, Miami. Photo credit: Julio España, Arquitectonica

The interior atrium prism is about 130 feet tall, starting at the seventh floor and terminating in a skylight at the top of the structure. The design team conducted wind-tunnel studies using a scaled mock-up of the building to establish wind loads and impact that were to be accommodated on each part of the exterior wall system. As a result, the building envelope uses laminated glass with a structural interlayer to provide greater strength and to protect against large storms.

The design challenge was met in the context of evolving building codes and strict standards for withstanding hurricanes in regions of the country most prone to catastrophic weather.

Tests for missile impact and pressure cycling are spelled out in the Florida Building Code, which includes the high velocity wind zone that applies to Miami Dade and Broward counties. ASTM test method E1886, Standard Test Method for Performance of Exterior Windows, Curtain Walls, Doors, and Impact Protective Systems Impacted by Missile(s) and Exposed to Cyclic Pressure Differentials is referenced in both the Florida Building Code and the International Residential and Building Codes.

The large missile impact test, consisting of a 9 lb. 2" x 4" fired from an air cannon at 50 feet per second, is conducted for areas of the building below 30 feet. The small missile impact test consists of a two-gram steel ball fired from an air cannon at 130 feet per second, and is conducted for elevations above 30 feet. Either missile impact test is followed by 9000 cycles of positive and negative pressure; additional tests for air, water, and structural integrity are required for product certification.

According to the 2004 Florida Building Code, all Florida counties within the 110-150 - m.p.h. wind zones as defined by American Society of Civil Engineering ASCE 7-98, have mandatory impact standards. With the adoption of the 2000 International Residential and Building Codes, other states have begun to enforce windborne debris protection requirements in windows.

Following Hurricane Wilma in October 2005, a commission of glass-industry experts surveyed the damage in Miami-Dade and Fort Lauderdale, Fla. They reported that several high-rise buildings had sustained glass-related damage from the hurricane. The buildings with blown out glass used tempered or insulating glass, rather than laminated glass installed in certified window systems. Buildings that were constructed with impact resistant glazing systems withstood Wilma's 120-mile-per-hour winds.