What FEMA Says About Glass Behavior During an Earthquake

Southern California was rocked by two earthquakes during the holiday weekend, leaving many to assess how prepared other highly populated areas of the state are for a major earthquake. While many building codes and guides focus on the structure of a building, the Federal Emergency Management Agency’s (FEMA) guide titled Reducing the Risks of Nonstructural Earthquake Damage – A Practical Guide, gives insight into how glass behaves during an earthquake.

According to the document, which was released in January 2011, brittle materials such as glass “cannot tolerate any significant deformation and will crack when the space between stops or molding closes and the building structure pushes directly on the brittle elements. Once cracked, the inertial forces in the out-of-plane direction can cause portions of these architectural components to become dislodged and to fall far from their original location, possibly injuring passersby underneath them.”

FEMA’s guide shows an example of non-tempered glass that fell several stories from a multistory building in the form of large glass shards during the 1994 Northridge Earthquake.

Tempered glass will reduce the seismic hazard because it breaks into smaller fragments rather than large shards. According to the guide, tempered glass is required within 10 feet above a walking surface under some circumstances.

According to the guide, failures of this type can be very hazardous, especially if the glazing is located above exit ways. It also states that annealed, tempered, laminated and insulating glass units can pose a significant falling hazard if not designed to accommodate seismic forces and displacements. Glazing is especially vulnerable in flexible structures and in large storefronts because the windows occupy a large, structurally unsupported area at the ground floor. The guide recommends use of laminated glass for storefront windows to reduce seismic risk and to increase protection from burglary and vandalism in the aftermath of an earthquake.

“The design of glazing assemblies depends on the calculated inter-story drift for the building. Glazing generally performs better with stiffer structural systems that have lower inter-story drift or where larger edge clearances are provided at the mullions. The building code ASCE/SEI 7-10 and rehabilitation standard ASCE/SEI 41-06 Seismic Rehabilitation of Existing Buildings, (ASCE, 2006) include minimum requirements for Δfallout, the relative displacement that causes the glass to fall from the glazing assembly, as a multiple of the design displacement and the importance factor,” reads the guide.

The guide also includes a section about glazed partitions, which it says are particularly vulnerable in assemblies where there is insufficient clearance in the glazing pockets or insufficient isolation from the structure to accommodate inter-story drifts.

“The design of glazed partitions depends on the calculated inter-story drift for the building. Glazing generally performs better with stiffer structural systems that have lower inter-story drift or where larger edge clearances are provided at the mullions. The building code ASCE/SEI 7-10, Minimum Design Loads for Buildings and other Structures (ASCE, 2010), and rehabilitation standard ASCE/SEI 41-06 Seismic Rehabilitation of Existing Buildings, (ASCE, 2007) include minimum requirements for Δfallout, the relative displacement that causes glass to fall from the glazing assembly, as a multiple of the design displacement and the importance factor,” reads the guide.

To mitigate damage and harm from glass partitions during a seismic event, the guide recommends that glazing partitions be avoided in emergency exit corridors or stairways. Limiting the height and area of partition glazing or using multiple smaller lites of glass may be less hazardous than larger lites.

In regard to mitigating damage to curtainwall, retired architect Chris Arnold wrote in “Seismic Safety of the Building Envelope” that field installers should ensure that “adequate clearances are maintained, rather than using connection details that allow for movement of the curtainwall framing independent of the main building structure. Glass is retained within the frame by flexible gaskets and clearance between glass and frame is maintained by inserting small rubber block spacers. The flexible gaskets and rubber spacers allow for considerable movement of the glass within the frame and the rubber blocks must be compressed before the glass impacts the metal.”

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