Protection Beyond Glass

When a tornado is bearing down, a window is more than a piece of glass in the wall.  Windows, doors and fenestration systems are crucial components of the building envelope. This is why terms such as impact glass, tornado-rated, hurricane-rated and storm-shelter glazing should not be treated as interchangeable. They refer to separate performance requirements. 

People should also be wary of the phrase “FEMA-rated” when referring to glazing systems. FEMA does not rate or certify glazing products; products may instead be tested or listed for compliance with applicable FEMA P-361 criteria. 

For architects, glaziers, and building owners, the starting point is simple: glass alone does not make an opening storm rated. The building community should fully understand how to specify the correct products, and how standards shape the test methods required for storm ratings. 

Navigating the Testing Standards 

Tornado and hurricane fenestration systems are intended to help building envelopes remain unbreached after windborne-debris impacts and fluctuating wind pressures. 

ASTM E1996 sets qualification requirements for exterior windows, glazed curtain walls, doors and impact-protective systems designed for use in areas subject to severe windstorms that generate windborne debris. ASTM E1886 is the companion test method for missile impact followed by cyclic positive and negative pressure differentials. The required missile and test conditions used depend heavily on the application type and classification.  

A nominal nine-pound 2-by-4 wooden post is used in common hurricane impact test large-missile protocols, even though the required impact velocity varies with applicable criteria. 

Storm shelters are a different category, though. ICC 500 establishes minimum design and construction requirements for tornado and hurricane storm shelters. FEMA P-361 builds on this framework with additional guidance for residential and community safe rooms and includes funding criteria for projects receiving FEMA grant assistance.  

For tornado shelters designed for a 250-mph design tornado speed, ICC 500 uses a 15-pound 2-by-4 lumber missile traveling at 100 mph against vertical surfaces and 67 mph against horizontal surfaces. This is a much more demanding impact than the familiar hurricane large-missile test and shows why simply specifying “impact glass” is not enough, given how different the testing conditions are. 

The same principle applies to ballistic, blast and forced-entry glazing. A laminate tested for one hazard does not automatically satisfy another. Multithreat systems are available, but every required performance needs to be supported by the appropriate testing or listing. 

The Glass Is Only One Part of the Test 

A common misconception is that a tested laminated glass makeup can be installed in any sufficiently robust frame and still provide the same rated performance. For hurricane and tornado impact, the fenestration assembly and its installation must be qualified under the applicable standard, listing, or product approval. The glazing, retention components, frame, mullions, sealants, fasteners and anchors work together to create impact resistance. Impact and pressure loads are resisted by those components and transferred through the frame and anchorage into the supporting structure. If the frame, glass retention, or anchorage fails, the glass makeup itself will not preserve the opening. 

This is also why substitutions deserve complete scrutiny. The NGA Technical Paper FB24-24, “Hurricane Product Substitution,” summarizes the ASTM E1996 approach to component changes. Depending on what is being changed, a substitution may be automatically permitted, require engineering analysis or additional testing, or require complete retesting. Changes involving interlayers, insulating glass unit components, glazing sealants or framing details should never be assumed to be equivalent simply because they look similar on paper.  

For glaziers, “or equal” deserves special attention on an impact-rated project. Any proposed substitute must maintain compliance with the applicable testing standard and the qualified assembly. A component with similar published properties is not automatically equivalent if the change takes the system outside the tested configuration.  

Understanding Design Pressure 

Debris impact gets most of the attention, but wind pressure is equally important. Hurricane-impact systems qualified under ASTM E1996/E1886 must withstand cyclic positive and negative pressure loading after impact. 

Required design pressures are project-location specific. Site wind speed, building height, exposure, building geometry, risk category and the opening’s location on the envelope can all affect the loads. The tested or approved positive and negative design pressures of the selected assembly need to meet the project requirements. “Impact-rated” tells only part of the story; the pressure rating and approved size range matter too. 

Anchorage Is Part of the System 

The load path continues beyond the glass and frame into the anchors and supporting construction. Anchorage must follow the tested or listed configuration, manufacturer requirements and project-specific engineering. The supporting structure also must be capable of accepting the loads delivered by the fenestration. 

This requires coordination among the architect, structural engineer, manufacturer and glazing subcontractor. Changing an anchor, spacing, substrate or perimeter condition in the field may affect the system’s qualified performance. 

Specify Early, Then Build What Was Approved 

Address the hazard and performance requirements during design, not when the glass is ready to release. Waiting until late in the process can trigger revised engineering and submittals, additional testing, refabrication and schedule delays. Identify whether the project needs hurricane impact resistance, tornado-shelter protection, another security performance or a combination. Confirm the applicable code, product approval or listing, design pressures, impact classification, approved sizes and installation requirements. 

Specify the complete system to match the tested configuration, including glazing, framing, anchors, sealants, approved sizes and installation details. This helps ensure the installed assembly performs to the qualified test standards. For FEMA-funded safe-room projects, the system must also meet the applicable ICC 500 and FEMA P-361 requirements to support grant eligibility. 

Appearance is a separate consideration from storm-impact qualification. Laminated glass makeups with multiple plies and interlayers can exhibit more tint or optical distortion than conventional glazing. On projects where appearance is especially important, the NGA recommends full-size mockups to evaluate the glazing under actual project, lighting and surrounding conditions.  

Most importantly, install what is approved. Verify the glazing makeup, framing, sealants, fasteners, anchors, and installation details before work begins. Storm-rated performance belongs to the complete assembly. If one link in the load path changes, the tested performance may change with it.