Mark E. Baker, P.E.¹ Structural Glazing Failure Five Case Studies Reference: Baker, M.E., Structural Glazing Failure Five Case Studies, Durability of Building and Construction Sealants and Adhesives, STP 1453, A.T. Wolf, Ed., ASTM International, West Conshohocken, PA, 2003. Abstract: Silicone sealant manufacturers produce extensive amounts of information instructing users of silicone structural glazing material on proper use of their material, including design guidelines, compatibility analysis, material storage, handling and application, including surface preparation and installation instructions. What are usually not shared are the results of not following the manufacturers' instructions. The following five case studies are examples of actual projects where structural silicone sealant failure occurred as a result of improper use of the material. The case studies are followed by commentary on what might be done to prevent failure in the future. Keywords: silicone structural glazing, curtain wall, adhesion failure. Silicone structural glazing is defined as a system in which glass, ceramic, metal or composite panels are bonded to framing members of a curtain wall utilizing an elastomeric silicone adhesive without the use of conventional glazing stops or retainers. Negative and positive structural loads are transferred from the panels to the perimeter framing via the sealant. Early in the author's career, while in London working at the Canary Wharf project, one of the glazing subcontractors proposed a four-sided structural glazed curtain wall system for an 18-story office building. Much to the author's surprise, the local District Surveyor, the equivalent of the local building official, rejected the plans stating that he was uncomfortable with four-sided structural glazing and ruled that the use of sealant as the sole method of attachment of the glass to the structure was unacceptable (Figure 1). The developer, the architect, the subcontractor and silicone manufacturer all submitted evidence of the adequacy of structural glazing, the extensive worldwide industry acceptance as well as successful laboratory test results. ¹Principal, IBA Consultants, Inc., 7104 NW 51 st Street, Miami, FL, 33166.
Figure 1 Typical Silicone Structural Glazing Detail The District Surveyor confirmed his position and stated that the Secretary of State had recently ruled that because the sealant manufacturer could not guarantee the sealant adhesion of his product for the full expected life of the building (100 years), the use of structural glazing did not meet the requirements of the Building Regulations and as a result would not be permitted. The glazing subcontractor submitted a revised drawing showing a continuous mechanical retaining clip around the entire glass perimeter and the project proceeded (Figure 2). Having recently arrived in London from New York, where structural glazing is a way of life, the author was puzzled by the skepticism of the British Secretary of State and the local District Surveyor and felt that their conservative views were standing in the way of technical innovation. Figure 2 Mechanical Retaining Clips added to Structurally Glazed Frames at the request of the Building Official
In early 1993, the author relocated to Miami to continue his curtain wall consulting career. Much to his surprise over the next 18 months he found himself involved in five structural glazing failures. The following are brief case studies of these five structural glazing failures that highlight the various risks of structural glazing. Material Failure A general contractor retained IBA Consultants, Inc. to help their struggling glazing subcontractor achieve a satisfactory level of performance. The subcontractor was the low bidder for the glazing package on a high-rise condominium on Miami Beach, which included sliding glass doors, single hung windows and a structurally glazed curtain wall. The subcontractor had no experience with structural glazing, was behind schedule and being pushed hard as the reality of liquidated damages was quickly approaching. One of the major sealant manufacturers sold the subcontractor two-part structural sealant, leased them a pump and trained them on how to use it. The problems started immediately. The sealant was not curing, as verified by both the snap tests and subsequent deglazing. The sealant manufacturer was contacted by IBA, and the salesman concluded that the problem was due to the subcontractor s improper maintenance of the pump. The pump was completely taken apart, cleaned and reassembled, but the problem continued. The sealant manufacturer was contacted again and it was requested that they send a technical representative to review the problem. Because the manufacturer knew the subcontractor was inexperienced in structural glazing, they again indicated that the problem was with the application. Meanwhile, the general contractor who retained IBA was upset because IBA's engineers wouldn t let any of the structurally glazed curtain wall panels leave the shop. IBA requested that the sealant be installed on a trial panel by the book and the sealant still did not cure. The subcontractor borrowed another pump, ran sample panels with the same results, the sealant still did not cure. Finally, the sealant manufacturer was contacted again and a technical representative appeared, who came only under threat of changing to another sealant manufacturer. It was determined that the particular batch of silicone material was bad. They duly provided new material, which cured properly so that the project was satisfactorily completed. Improper Joint Design Around the same time, on another high-rise condominium in Miami, a highly regarded window and sliding glass door manufacturer signed a contract that included a structurally glazed curtain wall at the elevator shaft.
Because the company had little experience with structural glazing, and the amount of structurally glazed curtain wall was small, it opted against using two-part structural silicone and decided instead to use a one-part structural silicone sealant. Due to limited warehouse space they used an accelerated cure material so that the curtain wall panels could be glazed in the plant, stacked and moved outside to their yard until needed on site. Figure 3 Discontinuities in Structural Silicone Sealant caused by Improper Detailing and Material Selection The subcontractor submitted and received approval on the typical details from the sealant manufacturer. However, when the first panels arrived on site large voids 10 to 30" (25 to 75 cm) long and 1/8-3/8 (3 to 9 mm) wide were observed along significant lengths of the split mullion halves on approximately 85% of the panels (Figure 3). Investigation of the problem revealed that it resulted from excessive sealant width and exposure of the frames to direct sunlight. The alcohol byproduct of the one-part material whilst curing, that usually migrates through the cured sealant to the atmosphere, had expanded from the heat, creating the voids and bubbles. Design, production and handling modifications for the balance of the project and remedial repair procedures, reviewed and approved by the sealant manufacturer, were implemented on the affected frames. Compatibility A glazier, replacing a broken lite of glass on a 15 year old, 10 story office building, discovered that the structural silicone was not adhered to the spandrel glass, and only the weather seal was retaining the glass. Subsequent investigation revealed that 90% of the spandrel glass and 30% of the vision glass exhibited lack of adhesion of the structural silicone to the glass and/or aluminum in substantial portions of their perimeter length. Analysis of the structural sealant, which was the acid cure silicone type, and of the glass and aluminum substrates revealed that a plasticizing oil was present in the
silicone at the unbonded areas and that the source of the plasticizer was the vinyl gaskets used at the interior perimeter of the glass. The sealant manufacturer confirmed that the presence of the plasticizer adversely affected the silicone adhesion and as a result they recommended against the use of vinyl gaskets in areas adjacent to their silicone. It is unknown whether the sealant manufacturer had reviewed the project details or performed material compatibility testing 15 years earlier. However, the result of material incompatibility was both potentially life threatening and proved to be costly to correct; all vision and spandrel glass was removed and reglazed. Surface Preparation The author was involved in an office building project in the Bronx, New York, consisting of structurally glazed vision glass and aluminum spandrel panels. While riding up the hoist of a nearly completed project, an aluminum spandrel panel was observed to be protruding from the building. A portion of the interior drywall and thermal insulation was removed and a complete lack of adhesion of the top of the aluminum spandrel panel was observed. The sealant had bonded to the aluminum mullion but had not adhered to a portion of the panel (Figure 4). A survey of the aluminum spandrels was performed and approximately 30 locations were found where silicone adhesion failure had occurred. Testing revealed adequate adhesion on three sides of the spandrel panel but that adhesion on the fourth side was nonexistent. Failure was attributed to improper cleaning and/or priming at the curtain wall manufacturer's plant. It was concluded that the workers had failed to clean/prime all four edges of the panels, resulting in adhesion failure. Poor workmanship and inadequate supervision/quality control measures resulted in serious adhesion failure. Figure 4 - Structural Sealant Adhesion Failure caused by Inadequate Surface Preparation
Inexperienced Workers IBA Consultants were retained to perform a due diligence inspection on a 13- story office building in Miami. The building was approximately 15 years old and had a combination of two and four-sided structurally glazed curtain wall. It was noted that mechanical retaining clips had been added at quarter points of the structural glazed lites. (Figure 5). The investigators were told that an engineer had noted adhesion failure of the structural glazing two years earlier (with one unconfirmed report of a piece of glass falling from the building to the roof of the attached parking garage) and had recommended the addition of the clips. Figure 5 Typical Field "Squish" Glazing Method of Structural Glazing During random inspection of the exterior wall, it was found that many of the clips were attached to the glass with double-sided tape because at many locations an attachment screw wouldn t fit in the joint between the glass lites. Because there was no failure, even without the clips, the building owner sought assurance that the clips were necessary. Several typical spandrel and vision lites were tested and deglazed. This confirmed a lack of adhesion between the glass and the silicone sealant. The investigation revealed that the project was field glazed using the squish method, where the glazier applied a bead of silicone sealant to the aluminum frame and squished the glass into the sealant (Figure 6). Too much time lapsed before the glass was installed, which allowed the sealant to skin over, preventing adequate contact/adhesion between the glass and sealant, thus causing the failure.
Figure 6 Remedial Mechanical Clips added after Silicone Structural Sealant Adhesion Failure noted. Conclusion The author is somewhat less confident of four-sided structural glazing as a result of these five failures. Evidence of the increase in structural sealant failure around the country is in the new standards recently developed by ASTM subcommittees: Standard Guide for In-Situ Structural Silicone Glazing Evaluation, ASTM C1394-98, ASTM WK2528, and Standard Guide for Evaluating Failure of Structural Sealant Glazing, ASTM C1392-00. Structural sealant is vulnerable to defective material, design and workmanship and the result of failure is potentially life-threatening. The author believes that its use should be governed, similar to welding, which can only be performed by registered/certified welders who are trained, tested and carry certification cards. Currently, sealant manufacturers informally police the structural glazing industry and although they do a relatively good job, they are subject to marketing pressures and client demands, which are sometimes contrary to published standards. The South Florida Building Code is unique in that it has specific requirements for structural glazing that may serve as a model for a more formal structural glazing certification program. These requirements are summarized in Table 1.
TABLE 1 South Florida Building Code Requirements for Structural Glazing Section 3515.5(a) 3515.5(c) 3515.5(d)(2) 3515.5(d)(3) 3515.7(a) 3515.7(b) 3515.7(d) Description Structural glazing systems shall be designed and bear the seal of a Professional Engineer. Mandatory review and approval of compatibility of all components and fabrication procedures in writing by the sealant manufacturer. Four-sided structural glazing is permitted only when units are shop glazed. Jobsite reglazing is permitted only when performed by firms approved or certified by the sealant manufacturer and in accordance with written procedures. A minimum of 1% of the structurally glazed production run panels shall be randomly tested for load carrying capacity. Mandatory inspection by a Professional Engineer of Structural Glazed panels for conformance with design and installation procedures prior to erection. Recertification of structural glazed curtain walls at six-month intervals for the first year and every five years thereafter. Recertification shall determine the structural condition and adhesion capacity of the silicone sealant. Were the British right? Probably not, but we should all be reminded of what can happen if good quality control procedures are not followed. Education is important, but enforcement is critical. Right now, unskilled, unsupervised workers are installing structural sealant around the country. There are plenty of signs that the current system is inadequate. The time for action is now, before someone is hurt or killed.