Volume 42, Issue 2 - February 2007
The Devil is in the
Nobody wants to be in a building with leaks, drafts and indoor air-quality problems. That’s why the building envelope is so important. The performance of any building is affected in large part by the quality of the building envelope, which includes window and curtainwall systems.
If components within the window or wall system are not approached as a system, leaks, drafts, poor air quality, deterioration of the structural framing and excessive demands on the operational systems may result. To avoid this, contract glaziers must utilize proper design, testing, inspections and installation.
The first critical step in the development of an effective window or wall system is the design. A structural glazing system is comprised of structural silicone sealants, which may include compatible gaskets, tapes, spacers and blocks. The system’s adhesion, weatherability and durability over the long-term depend on all products being compatible throughout their productive service life. Some glazing gaskets and spacers have been shown to be chemically incompatible and may cause silicone to lose adhesion to the glass and/or metal surfaces. To make sure these components are compatible with the structural sealant, use ASTM C 1087: Standard Test Method for Determining Compatibility of Liquid-Applied Sealants with Accessories Used in Structural Glazing Systems.
Effectiveness also depends on how well the structural loading and thermal movement can be held by the sealant between the glass and metal surfaces. Testing by silicone sealant manufacturers has shown that high shear movements may cause the sealant to fail over time if the appropriate product has not been installed to meet the demands of the application. Structural silicone sealants should not experience more than 15 percent shear stress using one-component sealants and no more than 10 percent for two-component sealants.
Ultimately, performance is a function of the magnitude of windload, dead load, glass dimensions, sealant modulus and the sealants’ cross-sectional dimensions. Safe minimum design stresses of 139 kPa (20 psi) in the silicone sealant are based upon industry guidelines, which can be reviewed in ASTM C 1401: Standard Guide of Structural Sealant Glazing.
Specific windloads and the stresses for the proposed glass dimensions must be determined for a given project. When building codes specify hurricane/impact/blast-resistance, enhanced energy efficiency is required or design demands stretch the limits of component performance, the development of a complete, unified assembly is even more critical. The sealant supplier understands appropriate design to accommodate the movement of various materials and other performance requirements. Some even have their own design engineering staff to work with fabricators to develop components within the glazing pocket. Their goal: to ensure proper compression and sealant bite throughout the assembly, in-plant or onsite.
To test the adhesive strengths of structural sealants, ASTM C 1135: Standard Test Method for Determining Tensile Adhesion Properties of Structural Sealants is necessary. Compatibility testing must be performed using the ASTM C 1087 test method to ensure all the glazing components are compatible with the silicone sealant. Adhesion testing is performed using the structural sealant with the glass substrate and to the metal finish used on the frames, which will come into contact with the sealant.
To ensure the long-term performance of the structural glazing system, good quality assurance procedures must be followed. Silicone sealants used for in-shop structurally glazed and unitized curtainwalls typically are two-component products mixed and delivered by specialized plural component pumps. To ensure proper pump function (and on-ratio sealant mixing), three quality control tests should be conducted:
An additional flow rate test should also be run on a predetermined basis, depending on the type of pump used for the project, to verify the correct dispensing ratios and flow rates of the base and that curative components are constant.
To ensure success during the fabrication of unitized curtainwalls, sealant suppliers can provide glaziers with written guidelines about structural glazing. This includes recommended cleaning procedures for surfaces receiving the structural silicone sealant, proper handling guidelines to avoid surface contamination (as well as timely movement and shipping of assemblies), appropriate placement of spacers and backer rods for correct joint design, logs for recording batch numbers of materials used and procedures for performing quality control testing both during and after sealant application. But the key to success remains making certain that all parties are aware of these guidelines and adhere to the quality assurance program.
Many window leaks today result from improper perimeter details and installation practices. Onsite inspections can make the difference between success and failure of the job. Some glazing system suppliers have field technical sales representatives to offer on-site assistance throughout the job. They can help with site mock-up to verify installation procedures, establish a standard for the quality of the installation and perform follow-up visits.
Proper window and wall system design, integration and detailing can make the difference between a high-performance investment and a never-ending headache.