Performance under Seismic Loads

Buildings exposed to seismic loads pose a severe threat to life and safety of pedestrians as components of the cladding or curtain wall may fracture, dis­lodge, and fall down. The seismic performance of architectural glass installed in the fenestration section of curtain walls is of special interest, as glass is brittle and may crack, which increases the probability of catastrophic fail­ure, culminating in the fallout of the entire unit. In light of the extensive use of architectural glass in seismically active geographies, anecdotal evidence suggests that the actual performance of glazing during earthquakes is rela­tively good, as only few serious casualties associated with curtain wall prob­lems are reported. The U. S. National Institute of Building Sciences in their Seismic Safety of the Building Envelope Report (Arnold, 2009) attributes the relative good performance of glass and metal curtain walls to the inherent strength of glass, the flexibility of the framing assembly, the resiliency of the glass retention materials, and the relatively small size of the glass panels. However, historically the sizes of the glass panes have increased and novel methods of glass attachment, such as structural silicone glazing (SSG), have become commonplace. The fact that the load transfer between the glass and the framing system in a SSG curtain wall must occur through the sealant implies that the seismic response of SSG systems is most likely different from systems that are dry-glazed. Recent studies of the seismic performance of various SSG curtain wall configurations were focused on the identification of the failure limit states associated with glass in SSG assemblies. The seis­mic performance of curtain wall systems is generally assessed in dynamic racking tests on curtain wall mockups

In their paper, Broker, Fisher and Memari present the results of a study in which four-sided structural sealant glazing (SSG) insulating glass curtain

wall units were subjected to cyclic racking test methods in accordance with AAMA 501.6 testing protocols. The drift capacity of the system in terms of glass attachment and sealant performance is reported in detail for different levels of racking displacements and boundary conditions. The overall behav­ior of the system is characterized, and specifically the sealant performance at a corner condition during racking drift is discussed. The damage to the structural silicone sealant is evaluated using visual observation before and after cyclic racking. The authors discuss proposed acceptable sealant stress levels for seismic SSG design and present sealant test results, which show the modulus stability and durability of silicone sealants.

A law in California is mandating earthquake resistance of all hospitals by 2013. California Pacific Medical Center (CPMC) has been planning the new Cathedral Hill Hospital in Downtown San Francisco as a LEED Silver­rated building in conformance with this law. When complete, this 100,000 m2, fifteen-story, 555-bed hospital will fill a whole city block. The curtain wall system for this building is primarily of a unitized design employing a four­sided structural silicone glazing system. In order to ensure satisfactory seis­mic performance of the curtain wall system for this project, dynamic racking tests were carried out according to AAMA 501.6 procedure. In their paper, Memari, Fisher, Krumenacker, Broker and Modrich discuss the results of these dynamic racking tests carried out on curtain wall mockups with regard to the behavior of the glass, framing, connections, and the structural silicone. Tensile stress-strain test results on the structural silicone sealant at selected temperatures and after ultraviolet (UV) light exposures are dis­cussed, and comparisons to the finite element analysis results are presented. Finally, the allowable stress in seismic design of four-sided SSG systems is discussed in light of new information generated for this project.

The 8.8 Magnitude earthquake that shook Chile at 3:34 a. m. on Satur­day, February 27, 2010, was one of the most devastating in the history of the country. The earthquake was felt in most parts of Chile, Argentina and some parts of Bolivia, southern Brazil, Paraguay, Peru and Uruguay. The earthquake was followed by hundreds of aftershocks, the strongest measur­ing from 6.0 to 6.9 on the moment magnitude scale. In their paper, Bull and Cholaky report on the state of SSG systems in low, medium and high-rise buildings that were inspected in the aftermath of the event.