Volume 50, Issue 9 - September 2015
Guiding the Rails
by Mark Jacobson
Glass is used as an architectural element for railing and guard systems to enhance a building’s appearance while also providing safety for occupants and those walking in its vicinity. In recent years, several incidents of glass breakage, which resulted in glass fall-out, have sparked concern over the safety of glazing used in these systems. The Glazing Industry Code Committee (GICC) developed and proposed some changes to the railing section of the 2015 International Building Code (IBC). The IBC model building code is adopted in most jurisdictions in the country.
Changes to the 2015 IBC calling for the use of laminated glass in most railing applications represent a major adjustment for many industry companies.
Changes in IBC 2015
With two exceptions, the elimination of monolithic tempered glass from Section 2407.1 in the 2015 IBC represents a major shift for glass railing systems in North America. The revised model code (Section 2407.1) now states, “handrail, guardrail, or a guard section shall be laminated glass constructed of fully tempered or heat-strengthened glass ...” The two exceptions noted in the model code that permit monolithic tempered glass in railing systems are based on the absence of a walking surface beneath the railing or the permanent protection of the walking surface from the risk of falling glass. As the 2015 IBC is adopted by the states, it is clear that the use of laminated glass for this application will increase significantly, because most areas that have glass railings or guards are above non-protected walking surfaces.
Section 2407.1.1 of the IBC refers to Section 1607.8 of the model code to define the load requirements of handrails and guards. It specifies a linear load of 50 pounds per linear foot or a concentrated load of 200 pounds, both in accordance with Section 4.5.1 of ASCE 7, and specifies use of a design factor of four for safety.
Clarification of the safety factor is critical in designing glass for the system. In the 2015 commentary for the IBC model code, Section 2407.1.1 states, “It is not intended that an in-place glass guard or handrail system be tested for or capable of withstanding four times the design load.” So what is the safety factor? The safety factor is used to reduce a material’s limiting stress to an allowable stress in design calculations. For example, if the average modulus of rupture for fully tempered soda lime float glass is 24,000 psi, then applying the safety factor will give an allowable bending stress in the design of 24,000/4 = 6000 psi.
Focusing on Quality
As laminated glass is adopted for use in railing and guard applications, especially those with exposed edges, the glass quality will be critical. The standard tolerance for laminated glass edge mismatch according to ASTM C1172, Section 8.5.1 can be up to ¼ inch. A mismatch of this magnitude may, however, create issues with the performance of the glass, particularly when a load on the edge is concentrated on one lite.
ASTM C1172 addresses this concern in Section 8.5.3 as it states, “For some laminated applications such as point-supported glass and balustrades, where the edges of the laminate are exposed, tighter length and width tolerances may be requested by the customer. Consult the supplier to determine their capabilities.”
The laminated glass community should welcome this discussion with its customers to establish mutually agreed expectations for the final product, and the architectural community should fully pursue Section 8.5.3 of the ASTM C1172 when specifying its requirement for railings and guards.
Installation of Laminated Glass
Installing systems designed with laminated glass may not be accomplished in the same manner as systems that have traditionally used monolithic glass. With laminated glass, it is important to consider compatibility of the glass and interlayer with any sealant, grout or cement that may come in contact with the glass and interlayer.
There are two areas of principal concern. First, since the interlayer will come in contact with the other components, it is critical to make sure they are compatible chemically and that no interaction will occur with long-term contact. The second issue is mechanical and involves a consideration of the setting materials’ properties in terms of coefficient of thermal expansion and compression strength.
Portland Cement-based products traditionally have been used in the installation of railing systems that incorporate monolithic glass. Under certain conditions, these types of cements may initiate an interaction between the alkaline components of the cement with the glass. Interactions may include blemishes or delamination. Careful consideration of all relevant conditions, including conducting additional testing, should precede the use and installation of any cement-based products in railing systems. Many gypsum-based products are acceptable for use in interior applications. Other solutions to consider are based on structural silicone, polyurethane or epoxy technology. It is important to check with the respective component suppliers to verify compatibility of the component with laminated glass interlayers; or design in a way that limits direct contact with those components where delamination may be a concern.
Finally, when designing a railing system with laminated glass, it is important to allow for drainage in case water gets into the system. Interlayers, while resistant to occasional water, may not be resistant to prolonged exposure to standing water.
By requiring heat-strengthened or tempered laminated glass in railing systems, the 2015 IBC model code has taken a major step forward in terms of improved safety. Laminated glass offers increased likelihood of glass retention after breakage, thus preventing glass particles from falling onto people below. In some minimally supported railing systems, it may make sense to go a step further: to strive for post-glass breakage performance in which the glass remains upright, continuing to serve as a barrier until replacement can be made. ASTM E2353 suggests the following post-breakage performance evaluation of laminated glass involving classifying impact performance into three categories:
2. Broken and retained; or
3. Infill broken and not retained.
Currently, Section 2407.1 of the 2015 IBC model code grants a limited exception for glass railing installations without a top rail or guard, “where the glass balusters are laminated glass with two or more glass plies of equal thickness and the same glass type when approved by the building official.” With the adoption of post-breakage criteria, like category 2 of ASTM E2353, it may not be necessary to seek and secure building official approval on a case-by-case basis, expediting the review process and enabling more consistent enforcement of the model code.
Mark Jacobson is the North American architectural manager with Kuraray America and the chair of the GANA Laminated Glass Railing Code Task Group.