What is a Point-Supported Glass System?
When deciding upon the right glass enclosure for your building, it’s important to consider all of your options. In other blogs, we’ve discussed stick-built systems and unitized curtain walls. Today, we dive into the point supported structural glass system, which has recently been taking precedence throughout North America as the premier system of choice for highly transparent facades, entrances, atriums, and lobbies.
Point supported structural glass is not a new system on the market. In fact, it has actually been around since the 1960s. Its origins stem from St. Helens, United Kingdom at Pilkington (one of the world’s largest float glass producers). Point supported glass has gone through some changes over the years, but the basics of these systems have remained the same. It consists of tempered glass (which should be fully heat soaked tested for a minimum cycle of twelve hours held at 290 degrees Celsius to help limit the chances of nickel-sulphide spontaneous breakage when in service) with holes for attachment to the structure by using bolted fittings. The face glass is hung off of the back-up structure in most cases, which can be glass fins, steel members, or stainless steel cables. These fittings can be very small, usually ¾” countersunk flush bolt heads, allowing for ultimate transparency. The system that utilizes the smallest fittings in the industry is the Pilkington Planar™ System. These are professionally designed and highly engineered systems that offer many options to architects when detailing the glass and connections.
These systems are different from average structurally-glazed aluminum curtain wall designs in many ways. One of the biggest differences is in the way the glass is supported. Aluminum curtain wall designs must be held together on all sides by a cap or from behind with structural silicone bonded back to mullions, whereas the point supported structural glass systems are anchored only at specific points. Silicone is only used for a weather seal between the joints of point supported glazing. Aside from enhanced clarity with the change in structure from aluminum box mullions, there are many technical reasons why these systems are so different.
First, the glass is allowed to temporarily deflect more than aluminum curtain wall systems from wind loading. This is a major reason the glass thickness is larger than standard curtain wall units. It resists the uneven loading and stress at hole locations. It’s important to make sure you are using a fully tested and engineered system warranted by a “system manufacturer” in lieu of someone who just sells components, like those used in aluminum-framed curtain wall systems, so that these stresses are properly checked by the manufacturer of the structural glass. The fittings and glass must work together, and every case must be analyzed independently. The Pilkington Planar™ System offers a full design integrity, manufacturing, and installation warranty of twelve years. This is the most comprehensive warranty in the industry! Lastly, the glazing and installation of all glass, hardware, and perimeter metal for point supported systems is done entirely on site, which is different than most prefabricated unitized curtain wall systems.
There are two basic applications for this specific system: vertical glazing and sloped/overhead glazing. Vertical glazing can use tempered monolithic, laminated, insulating glass units, or even tempered insulated laminated units. The glass can be tinted, low-e coated or even painted with colored silk-screened ceramic frit patterns.
Sloped glazing, on the other hand, would require tempered laminated glass or insulated laminated glass units with the laminated glass facing the occupant side for protection. The biggest difference between the two applications is gravity. Sloped or overhead glazing is subject to permanent gravity load from its self-weight and depending on the location, long-term gravity load from snow drift. This snow load can increase the thickness of the glazing dramatically!
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