Mistakes in Impact-Resistant Building Design

There are four critical impact-resistant design elements - avoid the typical mistakes made when implementing them

Extensive wind, storm, and hurricane damage have created major code changes to create building exteriors that are more weatherproof and impact resistant in mid-rise and high-rise structures.

Cyclone Tracy (Darwin, Australia, 1974) started the ball rolling toward safer building exteriors. Engineering studies showed that most of the damage from that cyclone was due to windborne debris and fluctuating pressure. 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, most of today’s codes and standards require the building envelope to be designed to resist impact from flying debris and cyclic (fluctuating) pressures. At the heart of impact-resistant design is creating window and door systems that can withstand these loads and prevent internal pressurization.

Code recognizes that, while there may be damage to the cladding as a result of flying debris, if the envelope isn’t breached, and wind isn’t permitted to enter the building, damage and danger to the public will be significantly reduced. 

When designing your next building or renovating an existing one, consider these four critical design elements and the typical mistakes that are made when implementing them:

1.    Establish Appropriate Design Loads. The SEI/ASCE-7 standard is the basis of design for most national codes, and it determines wind, snow, rain, and earthquake loads. It also establishes criteria for determining cladding design loads analytical calculations and wind tunnel testing.

If the design pressures are incorrect, the viability of an impact-resistant glazing system is greatly reduced. Often, the ASCE calculations are performed using large tributary areas (which is common for structures, but inappropriate for cladding loads), incorrect roof heights, exposure coefficients, or importance factors resulting in lower design pressures.

Alternatively, when wind tunnel studies are performed, the test laboratories often factor in large internal pressures that drive up the cladding design pressures. The internal pressures used by labs are based upon a projected “probability of glass breakage,” allowing wind to enter the structure. Since buildings in hurricane-prone regions are required to have impact-resistant glazing, the default “probability of glass breakage” is usually very conservative and can be reduced.

2.    Test to Maximum Load/Maximum Size. The ability of a glazing system to resist three impacts and 9,000 pressure cycles isn’t easily predicted (hence, the code requires systems to be tested at their maximum dimensions and peak design loads). This is contrary to curtainwall mock-up testing where typical modules are tested to the most common design loads. In places where the span, glass area, or design pressures exceed the tested area, engineers can demonstrate (through calculations) that the systems are adequate or validate reinforcement methods. While some jurisdictions allow rational analysis for impact-resistant glazing systems, most engineers agree that there’s not adequate data available to allow for accurate prediction of a systems performance.

3.    Selection of Glazing Infill including Type, Thickness, and Composition. 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.

A common problem with traditional laminated glazing infill products is optical distortion. Often, in large lites of traditional laminated glass, the planes of the glass don’t remain parallel during the autoclave process, resulting in a funhouse mirror effect. While the performance of the product isn’t impacted, the visual appearance can be disturbing. The potential for this phenomenon should be considered when selecting this product.

Other types of glazing infill products, like glass clad polycarbonates, aren’t as susceptible to the optical distortion; however, the polycarbonate layer is susceptible to cracking if exposed to water and, as a result, the glazing pocket must be carefully detailed to prevent prolonged contact of the edge of the glass with water. Usually, gasketed glazing systems should be avoided as well.

Another common mistake is for an insulated/laminated product to be configured so the laminated lite is to the exterior. After all, this is the component that provides the protection from flying debris; however, for the system to resist the cyclic loading, the laminate must remain bonded to the frame and, as a result, must be as close to the frame as possible, and located toward the interior. This results in the exterior lite being vulnerable to breakage from flying debris. While the exterior non-laminated lite often does break, if the interior lite stays in place, internal pressurization is avoided. 

4.    Attachment of Glazing Infill to Frame. Most impact-resistant glazing systems utilize structural silicone to adhere all edges of a mechanically retained infill to the framing system. One of the key elements of this detail is the glass bite into the glazing pocket. While impact-resistant performance may be achieved with sophisticated designs utilizing less bite, typically the structural silicone sealant placed on the inboard surface requires a glass bite of at least 5/8 inches. The most common problem with the increased bite is an inability to replace broken glass. Many systems just aren’t able to accommodate the installation of a new piece of glass with the original glass bite, negating the impact-resistant capacity of the window or door.

Mark Baker is president of IBA Consultants and serves as vice chair of the American Society of Civil Engineers’ (ASCE) Construction Quality Management and Inspection Committee. Baker can be reached at mbaker@ibaconsultants.com or at (888) 550-4422.

At the heart of impact-resistant design are window and door systems that withstand loads and prevent internal pressurization.

At the heart of impact-resistant design are window and door systems that withstand loads and prevent internal pressurization. View larger