Evaluation of the Structural Sealant for Use in a Four-Sided Structural Sealant Glazing Curtain-Wall System for a Hospital Building

ABSTRACT: Cathedral Hill Hospital (the California Pacific Medical Center) is a 15-story building designed to be constructed in downtown San Francisco. The curtain-wall system for this building is primarily of unitized design employing a four-sided structural sealant glazing (SSG) system. This paper initially introduces the building and its curtain-wall design. Next, the summary of the results of full-scale racking tests on stick-built mockups of the curtain – wall system developed to evaluate the structural sealant performance are presented. Stick-built mockup, instead of unitized system, was chosen to ini­tially evaluate the behavior of sealants under racking conditions. The mock – ups were designed to determine the behavior of the glass, framing, connections, and more importantly, the structural silicone under racking dis­placements following the AAMA 501.6 protocol. The test results (stress – strain relationships) on the structural sealant to determine tensile adhesion properties at selected temperatures and UV exposures are discussed and

comparisons to the finite-element analysis results are presented. The wind load and seismic design of the sealants, including the finite-element modeling and analysis of a typical panel to evaluate the stresses in the structural seal­ants, are presented as well. The allowable stress issue in structural sealants for seismic design of four-sided SSG systems is discussed in light of the new information generated for this project.

Introduction

Cathedral Hill Hospital (the California Pacific Medical Center) is a 15-story building that has recently been designed and is to be constructed in downtown San Francisco. The curtain-wall system for this building is primarily of unitized design employing a four-sided structural sealant glazing (SSG) system [1]. The four-sided SSG system is referred to a system where the glass panes are attached to the glazing frame on all four sides using structural sealant. The four-sided SSG system is known to be a curtain-wall glazing system that offers opportunities for shop-glazed construction quality, some reasonable savings, and expedited erection and installation. The unitized system consists of a shop – glazed framing that has the glass panes attached to the framing in the shop and the panels are then assembled at the job site. In particular, horizontal stack joints are provided that accommodate in-plane sliding between vertically stacked panels, which meet at the horizontal stack joint. Although four-sided SSG systems have been used in high seismic regions, such systems have not been used for healthcare facilities in California. This healthcare facility project will be the first of its kind to use this glazing system type.

To ensure satisfactory seismic performance of the curtain-wall systems, ASCE 7-05 [2] adopted by the International Building Code, IBC [3], requires dynamic racking tests to be carried out on mockups of the curtain-wall systems according to AAMA 501.6 procedure [4] when three or more sides of the glass panes are not mechanically captured. Besides such ASCE 7-05 requirements for this project, performing this test was also a requirement established by the Cali­fornia Office of Statewide Health Planning and Development (OSHPD) early in the design process so that they could be assured that the four-sided curtain-wall system would perform satisfactorily on a California hospital. This test proce­dure is intended to determine the drift associated with glass fallout. ASCE 7-05

[2] requires the drift capacity of the curtain wall, represented by the glass fallout drift, sufficiently exceed the design drift determined based on structural analy­sis of the building.

Because the aluminum glazing frame sections for this project are custom designed and will be extruded after design documents have received preliminary approval from OSHPD, it was decided to carry out preliminary AAMA 501.6 rack­ing tests on mockups constructed using available mullion sections with proper­ties similar to the final sections that will be extruded later. The objective was to show satisfactory performance of a four-sided SSG system to help ensure that the conditional design document received approval. The final approval of the design documents will require racking testing associated with the AAMA 501.4 [5] test­ing protocols, of mockups constructed using the custom extruded sections.

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The objective of this paper is to introduce the project, present the main results of the preliminary racking tests, discuss sealant properties, and finite – element modeling. In the following sections, some of the design and detailing aspects of the curtain-wall system and the AAMA 501.6 test method are explained. Then, the results of full-scale tests on preliminary mockups of the curtain-wall system carried out at Architectural Testing Inc. in York, PA are pre­sented. The mockups were designed to determine the behavior of the glass, framing, connections, and more importantly, the structural silicone under rack­ing displacements. The sealant coupon tests carried out by Dow Corning are dis­cussed, and the results of the finite-element modeling of a typical panel to predict sealant stresses are presented.