Top Structural Glass Misconceptions

By Jeff Haber
February 14, 2016
COMMERCIAL, RETAIL, FABRICATION
A close-up of a Pilkington Planar 905 Fitting on a custom machined, longhorn rod in a highly polished finish connected to a stainless steel tension rod. Photo by W&W Glass.

There are numerous misconceptions about point-supported structural glass systems. Having over 30 years of experience in the structural glass business, we’ve heard a lot of reasons why structural glass systems are value engineered out of a project early on in the design phase, from architects and contractors alike. From cost concerns to specification confusion, this article presents three top misconceptions about structural glass.

NO. 1 “STRUCTURAL GLASS SYSTEMS ARE TOO EXPENSIVE.”

Cost is one of the first things that come to mind when a consultant, general contractor, or construction manager looks at a potential component for the building. It can be tempting to throw out a square foot number on something that scares off the architect/client from using a particular type of system. However, it is important to understand that there are many factors that determine the price of a structural glass system. Factors such as the module width and height, number of points of support, glass make-up (thickness and performance) required, and back-up structure are all important factors to understand.

These factors should be considered based on project requirements—wind load, live load deflections, seismic criteria, maximum snow drift load (if a canopy or skylight), acoustics, and any impact or blast resistance criteria—before any budget pricing takes place. Many of these items can be quickly vetted by a structural glass systems engineer before an arbitrary number is put on the table.

Additionally, glass types and makeups are rated at different optical and thermal performance levels. Finding the one that is right for a particular project depends on the unique requirements of the building. For example, the type of glass an architect chooses can strike a balance between visible light transmittance and thermal performance, which can have a direct impact on the type/ size of HVAC mechanical heating and cooling systems used. This trade-off may provide a cost savings in building overhead and maintenance by downsizing the HVAC system by using a slightly more expensive high-performance coated structural glass make-up.

The overall amount of structural glass can also have an impact on cost. The larger the amount of structural glass area, the greater the economy of scale that can be achieved on a cost-per -square-foot basis.

NO. 2 “I CAN SPEC THE SAME GLASS I HAVE IN AN ALUMINUM-FRAMED CURTAIN WALL FOR POINTSUPPORTED STRUCTURAL GLASS.”

Structural glazing components are inherently more expensive than standard curtain walls. The loads put on pointsupported glass to limit deflection and stress at specific points without risking breakage at the holes or fittings are much higher than the edge loads on a continuously supported unit backed up with aluminum mullions. As such, structural glass requires much thicker make-ups than typical one-inch insulating glass units.

Often, the outer lite needs to be stiffened using 3/8-inch or ½-inch glass, and there are special “bosses” that transfer the load through the holes. Additionally, the whole unit needs to be flexible to a certain degree while maintaining the air seal. In the event of over-deflection, the PIB sealant shears, allowing condensation to form inside. This is known as a seal failure. It’s very important to make sure the glass and fittings are manufactured and engineered together to help prevent these types of issues.

Each project requires different kinds of glass. Important factors to be considered when specifying structural glass are coating and size availability, required thicknesses, product sourcing, roller-wave flatness (optical distortion), fabrication tolerances, interactions with fittings, and system warranties.

At W&W Glass, we recommend using a separate spec section. Section 089446 or 089070 can be used for the structural glass wall system specification, which should include glass type and make-ups as it is integral to a complete system warranty.

A structural glass system supplier can provide a design team with the type of glass required for each project. Bringing the supplier in early will prevent major problems later in the bid phase due to lack of coordination with either the structural or mechanical engineer, or due to an unrealistic budget for a given design. Each vendor will do their own calculations in most cases, but it is important to get an idea going in from a budget and design standpoint.

NO. 3 “I WANT A COMPETITIVE BID, SO I NEED TO INCLUDE MULTIPLE GLASS SUPPLIERS IN THE SPEC.”

A common line of thinking is that one company handles all the design work and budgeting, and then chooses three or four structural glass suppliers as alternates that meet specifications. The problem with this thinking is that the vendors may not always be qualified to meet all of the same system requirements specified. There are many companies that only make components—glass, fittings, structure—and then subcontract out engineering services or vice versa. If this is the case, how can you know who is really responsible for manufacturing the “system” if there is an issue? Is it the glass manufacturer, the fitting supplier, the engineer or the installer?

Do your homework early to prequalify the vendors. There are several quality system providers that actually make the product and offer a warranty on it, as opposed to brokers or engineering firms that buy up the bits and pieces and try to make a system. Make sure the list of approved manufacturers specified meet all of the criteria of the basis of design. This isn’t as easy as picking names out of a directory or performing a Google search for “structural glass.” Just because a fabricator makes a high quality 1-inch insulating glass unit for a curtain wall does not qualify them to make point-supported structural glass.

When glaziers bid a specified structural glass system, they are usually looking to have a fully fabricated and engineered package to install. There is a high degree of risk in attempting to bid an unspecified product, let alone the additional time it takes to cobble together a system and get it tested to meet specifications in most cases. If a substitute was proposed and rejected, or the quality level installed was not acceptable, a glazier may not get paid until it is approved or replaced with an acceptable product. They bear the ultimate risk of being accountable for providing systems per the bid contract construction documents. Their livelihood is based on providing a properly warranted product installation that meets owner expectations to help them to secure the next project.

There is no doubt that there is a difference when it comes to the design and fabrication of standard glass in aluminum curtain wall versus custom tempered structural glass and fittings for point-supported glass systems. That’s why it is always important to discuss the options with a system supplier from the beginning. They can clear up any misconceptions about structural glass early on, and ensure a project is specified with the highest quality product that meets the exact project requirements. A good structural glass system vendor helps the glazing contractor avoid risk and maintain profit margin while satisfying their customer and the design team.

The author is managing partner for W&W Glass, a leading contract glazing firm and distributor of the Pilkington Planar point-supported glass system. He can be reached at 845/425-4000.