The Structural Power of Glass (Continued from p. 38 of the October 2009 issue of BD+C)
                    
                      

Regarding curtain wall anchoring systems, Petermann explains that “each bolt used increases the likelihood of thermal bridging through the thermal break, thereby reducing energy performance. However, limiting the number of fasteners may impact structural capacity.” One way to overcome this problem, according to Petermann and Bostrom, is to use fasteners that don’t span the thermal barrier. However, such fasteners can be more costly to install than conventional fasteners.

Framing materials can also be affected. For example, while aluminum and steel offer good load-bearing capabilities, thermal breaks for better thermal performance can somewhat reduce the structural capacity of the frames, note Petermann and Bostrom. Consequently, some applications may call for non-metal frame materials, such as fiberglass or reinforced vinyl, which offer lower thermal conductivity levels.

GLASS CURTAIN WALLS
With proper design to guard against thermal bridging, the unique properties of glass have lent the material to the development of glass curtain walls, which offer transparency, aesthetics, and structural and thermal performance benefits. Whether stick-built on site or unitized through shop prefabrication, curtain wall systems offer specifiers the choice of numerous interior- and exterior-glazed options. In some cases, the choice is dictated by accessibility to the site. For example, according to Vigener and Brown, high-rise construction tends to work best with interior glazing because of the improved logistics and access for replacing or repairing glass.

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In terms of overall thermal characteristics, the SGH engineers explain that curtain wall performance is dependent on the following factors: 1) the glazing infill panel, 2) the frame, 3) construction behind opaque areas, and 4) the perimeter details.

As for water penetration resistance, this is determined by 1) glazing details, 2) frame construction and drainage details, 3) weatherstripping and frame gaskets, 4) interior sealants, and 5) perimeter flashings and seals, they add.

A few additional moisture prevention design tips from Vigener and Brown in their WBDG primer:
• Select frames with wept glazing and pocket sills sloped to the exterior to collect water that penetrates the glazing and drain it to the exterior.
• Ensure key frame drainage features, including sloping to the exterior at surfaces that collect water, closely spacing large weep holes, and providing for drainage at every horizontal frame.
• Include flashings at curtain wall perimeters—sills, jambs, and heads—that are sealed to the air and water barrier at adjacent walls.
• Use perimeter sealants as a rainscreen for limiting air and water penetration through the outermost plane of the wall—but take care not to rely upon them as the sole penetration barrier for air or water (or both).
• Coordinate the placement of setting blocks with the location of weep holes to avoid blocking drainage paths.

On the structural side, AAMA’s Brenden and Ruth point out that ASCE 7 prescribes wind-load design pressure resistance requirements based upon climate zones. In hurricane-prone regions, the curtain wall must also be capable of resisting impact loads from wind-borne debris. “The IBC also requires special inspection of the installation of curtain wall in high-wind-speed areas,” says Ruth. “In some cases, this will be on the same buildings where the impact test is required, but that will not necessarily always be the case.”

One other region-specific requirement is designing to prevent glass breakage in earthquake-prone areas. “Often this does not so much entail the curtain wall being able to resist certain loads as it does the ability of the framing system to deflect in the plane of the glass, without imparting lateral loads onto the glass,” says Ruth.

Summing up the recommended features for designing glass and metal framing systems, curtain wall consultant Muller cites longevity, maintainability, ease of repair and replacement, damage from public access, and ease of cleaning.

FRAMELESS GLAZING: EYE-CATCHING, INNOVATIVE
Falling right in sync with growing interest in greater transparency and structural glass design are frameless glazing systems. Whether it’s structural glazing or point-supported glazing, designers have been given greater freedom to come up with some eye-catching, innovative glass façades.

The way structural glazing works is that the glass is bonded to the building’s structural framing members with a high-strength, high-performance sealant or tape, often silicone sealant. Dynamic wind loads are then transferred from the glass by the sealant to the perimeter structural support. Because such systems have less metal framing exposure, they also offer good water and air protection and improved thermal efficiency.

In terms of practical design advice, Dodd and Weber stress the importance of understanding how the glass will be supported and restrained, and how the movements of the building will be accommodated.

Further advice on this subject from Thomas and Davies:
• Many projects also require positive fastening of glass panels to supports in order to avoid reliance on adhesives during extreme loading conditions or for inverted or suspended glazing.
• Structural silicone glazing should be performed in the factory, where controlled conditions offer maximum opportunities for success.
• Whenever possible, reliable systems should include primary and secondary drainage paths to evacuate water penetration. Water-bearing surfaces should slope to drain into weep systems.
• To optimize durability, sealants should be considered hole-fillers, not waterproofing agents. Structural adhesives do not guarantee waterproofing.
• Some jurisdictions and government agencies limit their acceptance of structural silicone glazing systems.

Point-supported glazing. Systems such as bolt-fixed glass and patch-plate assemblies are generally known as point-supported glazing, meaning that the glass sections are supported at discreet locations rather than continuously along the glass panel edges. Many professionals are familiar with the highly articulated “spider” systems that use single structural mounts to support multiple glass points. In all cases, these façade types are visually compelling yet require carefully machined or cast fittings and early-phase analysis using computer finite-element analysis to ensure the systems work properly and safely.

Addressing point-supported systems from a structural perspective, the SGH engineers explain that the design team will establish load paths between the glass units and the specialized hardware, members, and connections which transmit these vertical and out-of-plane loads from the façade back to the main building system. The connections need to accommodate constructability tolerances in all directions, they add, and to allow for structural movements, including floor slab deflections and lateral motions caused by wind or seismic loads.

Of course, it goes without saying that only tested glass and hardware systems should be selected. The engineering team will require data documenting the chosen system’s ability to withstand vertical and horizontal loading—numbers that are essential for calculating the final architectural design.

In terms of the fundamentals, Arup’s Dodd and Weber list accurate hole shape, correct bearing materials, controlled assembly and a tested design methodology as the basics for good design. At the same time, “There are still new suppliers coming into the market, and always new things being learned about glass, such as how easy it is to get uneven residual stresses at the surface of toughened glass if it is an unusual shape,” Weber says.

SLOPED GLAZING: PUTTING SAFETY FIRST
The biggest concern when it comes to sloped glazing is safety. In addition to protecting occupants below from falling glass, “Sloped glazing also has to be safe for those working over or around it, which means not allowing anyone to fall through it,” says Dodd. “In our view, there is no reason nowadays to accept a glass roof that is fragile, and we should insist on glazing that will support a person even if all layers of the glass are broken.”

Included in this equation is the framing system. “The way the glass has to be restrained may change dramatically between when the glass is intact and resisting its regular wind pressure, and when the glass is broken and an injured person is lying on the glass on a hot day,” says Dodd.

In addition to safety, Thomas and Davies add that thermal control, condensation resistance, and, in many cases, acoustic performance be carefully considered.

One helpful resource is AAMA’s Glass Design for Sloped Glazing.

According to the SGH engineers, “Sloped glazing requires condensate gutters that control modest leakage and effectively transmit all water from cross-members to rafters, and from rafters to perimeter drains. The perimeter requires through-wall flashing that is not penetrated on its horizontal surface for effective waterproofing. And systems with irregular shapes or innovative long-span framing often require detailed structural analyses to coordinate support requirements, load distribution, system displacements, component sizes, and optimized structural geometry.”

Unit skylights. According to Vigener and Brown, the most common skylight application is insulated glazing inside an aluminum frame. However, ASTM E1825 – Standard Guide for Evaluation of Exterior Building Wall Materials, Products, and Systems can be helpful in evaluating different product choices based upon track record and application.

It’s also important to note that skylights experience significant summer solar heat gain and wintertime heat loss, and even the best sealed assemblies will experience some sort of water leakage. This being the case, Vigener and Brown offer a few moisture control best practice tips from their WBDG article:
• Provide a continuous system of gutters, integral with the skylight rafters and cross members, to collect leakage and condensation.
• Provide an exterior wet seal.
• Select a system with continuous rafters.
• Use a continuous metal sill flashing to collect leakage and condensation.
• Choose a system with snap-on rafter caps rather than exposed pressure bars.
• Specify and detail flush-glazed horizontal mullions without exterior applied pressure bars to avoid bucking water run-off.
In sum, Building Teams need to be familiar with all aspects of providing safe, durable glass building systems. Glass has been and will continue to be a popular exterior building feature, and for good reason: according to Arup’s Wurm, the “lucidity and good chemical resistance” of glass to most corrosive media make it an excellent building skin material.

As CDC’s Clift puts it, “All glass structures have the advantage of providing maximum transparency, uniformity of material, and unique architectural style.” Adding daylighting, solar control, and new structural support capabilities to the list, what could be better?
      
     
About the authors
C.C. Sullivan is a communications consultant and author specializing in architecture and construction. Barbara Horwitz-Bennett is a writer and contributor to construction industry publications.

Take the AIA Exam (one-time registration required) 

This BD+C continuing education program qualifies for 1 AIA HSW learning unit.

is a Registered Provider with the American Institute of Architects Continuing Education Systems. Credit earned on completion of this program will be reported to CES Records for AIA members. Certificates of Completion for non-AIA members are available on request.
       This program is registered with the AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.

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