When approaching a custom glass project, it is important to consider the type of heat treated glass that is best for your needs. This can depend on a variety of factors including application, budget, and safety. Annealed glass can be treated with a few different heat treatment methods to achieve various strength and safety levels. The three types of heat treated glass Bellwether most often finds specified are heat-strengthened, fully tempered, and heat soaked. Each of these heat treatments affect the overall cost of the glass, and are best for specific applications.

Annealed Glass

Annealed glass is the basic form of glass before it is treated. It is created by pouring molten glass onto molten tin and then cooling the glass in an annealing oven once it has solidified. This cooling process removes any residual stress. The thickness is controlled by increasing or decreasing the rate at which the glass flows on top of the tin. All glass must be fabricated in its annealed state, meaning it must be cut into the desired shape and all holes drilled before any heat treatment is performed. 

Bellwether typically does not use annealed glass for projects because it is fragile due to its low amount of surface heat compression. It is not typically strong enough to use with point supported glass fittings. In addition, it breaks into long, sharp jagged pieces that can pose serious safety threats. 

To meet strength and safety requirements, annealed glass can be heat-strengthened, fully tempered, or laminated. In some cases, glass lites are too large to fit into a tempering oven, and lamination can be used to achieve safety. When laminated glass is broken, the fragments adhere to the laminate, reducing the risk of injury. 

Heat-Strengthened Glass

Heat-strengthened glass is created by heating annealed glass to its softening point and then cooling it using water. Water cools the surface of the glass quicker than air, which creates a greater surface compression. Heat-strengthened glass is two times stronger than annealed glass, but not as strong as safety glass, and is more resistant to heat, wind, and flying objects. This type of glass is typically laminated and used in overhead applications like skylights or canopies. The heat treatment reduces the risk of breakage in the first place, and the lamination keeps broken pieces from falling if a breakage does occur. 

Fully Tempered Glass

Fully tempered glass is created in a similar manner to heat-strengthened glass, but the cooling process occurs at a much faster rate which produces a much higher surface compression. This makes glass that is four times stronger than annealed glass. When fully tempered glass breaks, it forms very small, square shaped fragments, that are much less dangerous than the large jagged pieces from annealed glass. Bellwether often uses tempered glass because it is strong enough for point supported structures, and because of the reduced safety threat it poses. This type of glass is especially good for areas where glass breakage is frequent such as storefronts and car windshields. 

Heat Soaked Glass

Heat soaking takes the temping process a step further by replicating the heating and cooling cycles glass ungeros from the sun over a long period of time. These heating and cooling cycles are achieved in a factory oven where the temperature is raised to 290°C. In the process of creating glass, it is possible for nickel to contaminate it, and nickel sulfide inclusions to form. Although it is a fairly rare phenomenon, it is possible for glass to spontaneously break from nickel sulfide inclusions. The heat soaking process will cause glass with these inclusions to break in the oven, eliminating the risk of it breaking once installed. This process adds cost and time to the project schedule, so the probability of spontaneous breakage needs to be weighed against these considerations. 

When choosing the best type of heat treated glass for your project, a combination of factors must be considered, including safety, cost, glass application, local code, and project schedule. Bellwether is here to help architects and engineers consider all of these factors and ensure the highest quality project while staying within budget and schedule.

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Our point supported, inverted fin wall at the Lord & Taylor flagship store in Manhasset, New York, was selected for the design innovation honor from a field of incredible work throughout the industry.

This was Bellwether’s second time winning the award for Most Innovative Curtain Wall, having first won the it in 2014.  That same year, Bellwether also won the award for “Most Innovative Specialty Glass Product” for its patented line of structural glass skylights, called Insight.

The entrance wall at Lord & Taylor was one of the most challenging projects that Bellwether has completed to date.  The 41’ tall fin wall is designed to be as transparent as a retail display case, while withstanding 111 mph wind speeds.  Our design strategy was to match the architect’s design concept by creating large uninterrupted clear spans, with minimal sightlines to allow the visitor’s focus to be on the retail merchandise.  Even the 3’ tall splice plates became a unique design element with a mirror polished finish.

Read more about this year’s Glass Magazine Awards here, and for more information on the design process that goes into a glazing system like this, read our blog entitled Structural Glass Walls: Process, Design, and Engineering Options.

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While the design of truly custom wall systems is more costly than off-the-shelf systems, designers are freed up to avoid sizing and structural limitations that are often associated with pre-engineered products such as stick-built curtain wall.

Our recently completed point supported structural glass wall at the Lord & Taylor store in Manhasset, New York is a good example to illustrate our process. The main entrance wall measures approximately 41’ tall by 130’ long, with an additional “display wall” measuring approximately 24’ tall by 16’ wide, for a total of 5,700 SF.

Structural Glass Wall Design Process

Bellwether begins its work with a high level feasibility analysis of the conceptual design, to determine basic appropriateness of design, availability of materials, etc. We evaluate the requirements of the wall system from the outside inward. Wall glass make-up is reviewed first based on proposed structural support, windloads, and any other criteria such as seismic, blast, and ballistic. We then move on to the options for the structural framing system, always keeping the focus on creating a system that is as simple and visually uncluttered as possible.

High Level Feasibility Analysis

This first stage reality check identifies any major issues that would make the project impractical or impossible to build. For instance, if the Lord & Taylor fins project had been designed to feature one-piece, 40’ tall glass fins, they would carry a huge cost, and make transportation and installation difficult and very expensive. These are import factors to consider if cost is a driving criteria!

Generally, at this stage, we are doing more of an exception-check on the design, rather than setting hard limits. We consider the ways to adapt the materials to the design, rather than boxing the architect in and making them adapt their design to the material limitations.

Analysis of Wall Glass Materials

The appropriate make-up of wall glass is defined by factors ranging from the size of the largest glass lites, to the type and frequency of glass anchorage (e.g. point supports vs. patch fittings, vs. continuous two-sided structural silicone support), local windloads, etc. In the case of Lord & Taylor:

• Wall lites are large, with the largest measuring 10’-11 3/4” wide X 10’-4” tall.
  • At over 113’ SF each, and over 1,800 pounds each, these are much larger than what is found in most point supported applications.
• The specified glass was a laminated make-up, with a thickness to be confirmed by the supplier.
  • Glass thickness is often a function of limiting deflection due to wind loads. Wind loads are relatively high at the Manhasset site, at 111 MPH. We used a 1-1/16” thick make-up, utilizing two layers of 1/2” glass, with a .060 PVB interlayer.
• The conceptual design for the wall assumed just four fittings per lite of glass, with one at each corner. This was one area where we knew that the design would have to be adjusted from the start. The inclusion of a mid-span support along the 10’-4” span on the sides of each lite (for a total of six fittings per lite) was assumed to manage glass deflection. This addition of the mid-span fitting had a minimal impact on the overall aesthetic or cost.

Analysis of Glass Wall Backup Structure

The determination of the appropriate material depth, thickness, and hardware for a glass fin wall is determined by a number of standard factors, such as height of the wall, width of bays and whether the wall is hung from above, or dead loaded to the ground. In this case, due to the depth of the fins to allow for the inverted wall, the requirement to limit buckling of the fins also became the critical factor in their design.

• The height of the wall would require deep and thick fins. At 40’ tall, these are massive structural components, regardless of material.
• The depth of the fins were somewhat determined by:
  • The 2’ dimension at the base, which was driven by a desire to maximize retail floor space.
  • The degree of inversion at the head condition. We reviewed 3 degrees, 4 degrees and 5 degrees, ultimately setline on 4 degrees for engineering efficiency.

Since these dimensions were relatively settled, we evaluated the required thickness of fin glass to manage deflection and buckling. This turned out to be 2-3/8”, made up of 3 layers of 3/4” glass, with two .060 SGP interlayers.

• The fins are designed in 4 pieces, with the top components weigh over 1,100 lbs. each.
• The combination of high windloads, wide bay widths and a 40’ span introduced a need to address the potential for buckling of the fins under load. Bellwether addressed the concern through the use of the triple laminated/SGP make-ups, and splice plates that were designed in modified “H” shapes to greatly enhance stability.

Point Supported Wall Fitting Analysis

Fitting requirements on any point supported project are based on criteria such as articulation of the fitting head, bearing area (or glass bite), of the fitting head on the glass, and distance between fittings, as related to the thickness/stiffness of glass.

• Disk style fittings were the selected from the start, to provide far greater bearing area than countersunk fittings – especially important with lites of this size.
• The H shaped splice plates were developed and proposed by Bellwether during the design and engineering phase. Additional stainless straps were incorporated into splice plate design at the inverted corner, to manage deflection and buckling at this unique condition.

As with every custom project, the requirements of the base building design, and site specific environmental conditions like high wind loads, created some unexpected challenges along the way, changing material thickness and component design strategy. Bellwether’s approach through the process was to keep the project as closely centered on the architect’s original design concept as possible, with a high level of customized design and coordinated engineering.

The result is a stunning entrance with massive glass components, which completely transforms the entrance to Lord & Taylor’s flagship store, to celebrate its 75th anniversary.

Glass Magazine Award Winner

We were beyond pleased to have the Lord & Taylor project selected by Glass Magazine as the recipient of the 2017 Most Innovative Curtain Wall Project. Glass Magazine awards are given to projects that feature bold feats of glass and glazing innovation, fabrication, installation, and design. This was Bellwether’s second time winning the award for Most Innovative Curtain Wall, having first won the award in 2014 for our Hyatt Place Crinkle Wall project.

Credits
Architect:                     Highland Associates, New York, NY
Installer:                       Craftsman Storefronts & Glass, Inc., Bayshore, NY
General Contractor: EW Howell, New York, NY

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• 8-10 weeks for design, samples, and approvals from the architect and owner.
• 10-12 weeks for material production and delivery.

Compressed schedules can sometimes be accommodated, with the understanding that short timelines result in tradeoffs in design flexibility and material choices, sometimes also leading to higher costs.

Three key areas that are important to keep an eye on to manage tight schedules effectively are:

• Architectural approval turnarounds
• Material lead times
• Testing and mock-ups

We will review each of these items, using the example of the Harbor Point S4 vestibule that Bellwether designed and supplied for the developer. The schedule for this project was just 12 weeks from start to finish, accommodating an aggressive occupancy date.

Architectural Approval Turnarounds

On a typical design development timeline, we like to provide two rounds of drawings, with two approval cycles (for more, see Why Shop Drawings Should Not be the First Deliverable You Ask For):

• Profile drawings are created in 2-3 weeks to confirm basic design intent and to agree on typical details and interfaces. We typically schedule a 1-2 week architectural approval period.
• Shop drawings are created in about 2-3 weeks, with another 1-2 week architectural approval period.

The short timeline on Harbor Point S4 required profile drawings in 1 week, and shop drawings in 2 weeks, with 24 hour approval turnaround times. Since the developer was very hands-on with the design and process, and since time was the ultimate project driver, this turnaround was agreed to, at contract signing.

Material Lead Times

Material choices are limited when you have an 8 week production schedule. Two things helped on this system design:

• Patch fittings were used to anchor the glass, rather than point supported fittings. This meant that glass could be standard insulated glazing units without the need for holes which would have elongated production time.
• The patch fittings themselves were designed as simple square shapes from the initial architectural drawings, so they could go into production without changes once engineering calculations were complete. Given more time, additional shapes, materials, glass make-ups, etc. could have been considered.

Material Mock-ups and Testing

Material mock-ups and testing are typically casualties of a short project schedule. Laboratory testing and mock-ups can sometimes double the lead-time needed for a custom project: Mock-up materials used for aesthetic approvals and testing purposes can be created only after shop drawings are done. Then, if system changes are made after testing or mock-up creation, a new material production schedule must begin. In the case of the S4 vestibule, the design was based on a straightforward structural glazing system with silicone weatherseals. The performance of this design approach is well tested and proven in the field.

By approaching custom glazing system development with a motivated design team and a good understanding of project drivers and tradeoffs, it can be possible to get a very custom aesthetic in a relatively short amount of time. For more on the topic, see The Two Most Important Questions Architects Can Ask Themselves When Designing Custom Glazing Systems.

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The canopies at the Puck Building were designed to be simple yet functional, with a need to blend with the architectural elements of the historic building. Bellwether created aluminum T-shaped outriggers, with integrated concealed anchors. One of the locations changes slope and direction, requiring a reversal of the outriggers within the run, to accommodate the directional change.

The two custom canopies provide a bridge between the original architecture of the building, and a modern aesthetic, by incorporating traditional outrigger shapes with point supported fittings to support the glass.

Location
New York, NY
Architect
PKSB Architects, New York, NY
Products Used
Glass canopies with aluminum T outriggers, and point supported fittings, with laminated glass.

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When completing a renovation on an existing building, detailed as-built drawings seem to be the most efficient way to uncover what is hidden by various exterior cladding systems. The trouble is, these drawings are not always available or accurate and up to date. For instance, older buildings can undergo multiple renovations without updates to as-built drawings. Unfortunately, this lack of information can lead to uncertainty during the bidding stage and preliminary design stages of subsequent construction.

If accurate as-built drawings are not available, the strategy to obtain an accurate determination of existing conditions is based on access.

– Field dimensions: In cases where the area under renovation is accessible, simple field dimensions can often provide what is needed for renovation design and planning.

– 3D scanning: Scan technologies such as laser scanners allow for the creation of a detailed “point cloud” of information of existing conditions that can be converted into detailed 3D models of various formats. Building scans are particularly helpful when the project conditions are too large or inaccessible to measure manually.

– Demolition / coring: To avoid issues with unforeseen conditions later in the project, there is always a certain level of physical demolition and research that should be done to accurately document what is already in place. Coordinating with the engineer of record is a great way to get the background information and set the expectations for the demolition work. Keeping the physical demolition to a minimum is a key component, peeling back just enough to allow for access. Field measurements can then be pulled to set the parameters of the design, quantify materials, determine anchor points, etc.

The following project examples include a situation where the position of structural members was determined well after the start of construction (1350 Eye Street), and one where a laser scan was performed prior to material fabrication to insure proper coordination/fit of new system on existing structure (MIT Kresge Auditorium).

Project Examples:

1350 Eye Street

Bellwether completed a renovation to an existing building in Washington, DC consisting of an exterior glass wall with an integrated interior vestibule. During the preliminary design stages, the structural steel anchorage was believed to be within 12” of the finished ceiling based on original building architectural and structural drawings. This proximity of structure called for a simple face-mounted anchor.

When the GC proceeded to core out the existing finished to expose the structure, well after the start of material fabrication, it was discovered that the structural steel anchor point was over 3’ from the finished ceiling surface. Bellwether coordinated with the building engineer to develop a steel kicker/anchor that could handle a significant increase in torsional forces applied to the tall anchor.

Bellwether then analyzed the ability of the entire structure (wall and vestibule) to handle the imposed loads as a coordinated system. To reduce the torsion on the tall kickers and existing structure above, Bellwether used the stability of the base glass vestibule and walls to nullify the cantilever of the suspended fins. The end solution saved the installer, GC, and building owner money by looking at the complete picture and engineering a relatively low cost kicker versus finish materials such as extended glass fins or a more extensive supplemental steel support structure.

MIT Kresge Auditorium

Bellwether recently completed a renovation project at the historical Kresge Auditorium on the MIT Campus in Cambridge, MA. The main objective was to replace the existing aluminum skin system with a new laser fused stainless steel system while retaining the original curtain wall design, shapes, and dimensions.

The new system was to mount to the existing interior steel mullions. To accommodate the existing steel dimensions and tight tolerances of the new material to be fabricated, a laser scan was completed to ensure accuracy in the relationship of new to old when the pre-fabricated material arrived on site.

Materials were delivered pre-fabricated and pre-finished to accommodate efficient installation and a tight project schedule. The existing steel mullions outside tolerance were straightened and refinished, allowing the new system to fasten directly to the existing structure with minimal site work.

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When these costs are not well documented, they can become a source of disagreement further into the project regarding who is responsible for specific charges. It is critical that the design team understand the site conditions and installer limitations, and prioritize them against the parameters of the original design.

Site specific challenges posed by factors such as surrounding conditions, site logistics, material sizes, and even installation methods can vary greatly. The most efficient way to avoid these issues from arising once the project has begun is to engage in clear, up front communication between the design team and installer. Information such as glass panel weight, steel size and length, and even fabrication assumptions should be discussed in the design phase so that a strategically coordinated installation plan can be created. For instance, limited site access and equipment availability may be an argument for limiting material weights, sizes and fabrication methods.

Design Assistance Project Examples:

Edward M. Kennedy Institute

Bellwether’s structural glass wall at the EMK Institute in Boston includes glass lites that are 11’6” tall x 3’ wide. Designed to meet 105 mph wind speeds with minimal, two-sided structural support, the insulated glass units are 2-5/8” thick, and 24 lbs. per square foot. This yields a glass unit weight of 828 lbs. The final dimensions of the lite sizes was partially determined by a strategy to keep each lite below 1,000 lbs., based on the type of equipment needed to set the lites initially, and for potential re-installation after finished walkways were installed. Various design studies were undertaken during the shop drawing phase in order to fine tune the appropriate bay widths with these weight parameters.

Glass Conservatory – Private Residence

Bellwether recently completed an exterior glass conservatory project at a private residence that had very limited access to the install area. In addition to the limited access, the structure had to be designed so that it could be disassembled as needed in the future. A modular steel frame was designed with low profile connections to provide for easy assembly, without compromising the architectural design intent. Additionally, the installer was able to easily transport materials down a narrow walkway and maneuver them into place by hand.

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Diana Center at Barnard college

The Diana Center at Barnard college was designed to incorporate four walkable skylights – three at a rooftop garden level, and one at the plaza/ground level.  The skylights provide daylighting benefits to the building, while offering a unique architectural element at ground level.

A key challenge with any walkable skylight is to provide resilient weatherproofing perimeters that are flush with the surrounding surfaces, unlike the typical curb that a skylight is raised up on.  The plaza level skylight shown here integrates the weatherproofing details seamlessly within the pavers and grout joints.  The skylights at Barnard are also designed for energy performance, using insulated glass with laminated outboard and inboard lites for point load support of pedestrians.  Exterior walking surfaces incorporate a non-skid frit for safety.

The walkable skylights at Barnard College provide a unique accent that blends the exterior architectural elements with the interior.  Unique daylighting benefits also contribute to the building’s LEED Gold  rating.

Harvard Leverett House Walkable Skylight

The design of the walkable Gallery skylights at Harvard’s Leverett House, was part of the design assistance project for the McKinlock Hall Light Court skylights.  The goal was to provide natural, diffused light to a gallery space below ground within the building.

Bellwether refined the design of four long, narrow skylights that are positioned next to an exterior walkway.  The skylights were designed to support foot traffic, and to provide privacy above and below with opacified glass.

The Gallery skylights at Leverett House’s McKinlock Hall effectively convert a previously dark and narrow hallway into an engaging gallery space.

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In order to keep the structure as light and visibly transparent as possible, door openings with a simple door header and sidelites are often preferred to framed and clad portal structure. The design of the doors themselves can play a role in which components to choose for the opening:

Frameless glass doors

Frameless doors provide the lightest aesthetic overall, and can usually be anchored with simple patch fittings that attach to the header and sidelites. They do come with some important considerations, however:

• They require a gap of approximately 3/16” on all sides, which can allow air, water and dust inside. Options are limited for door sweeps and weatherproofing with acceptable aesthetics. See more about how to address this issue in our post on all glass vestibule conditioning.
• A frameless door and portal offers limited options for automated openers, typically requiring floor mounted equipment, which is more vulnerable to dirt and water infiltration along with other environmental issues.

Framed glass doors provide much better options for weatherproofing, but of course they are not as light aesthetically.

The glass doors at the portal on the CD Adapco headquarters building use simple patch fittings attached to the header and sidelites for a light aesthetic.

Floating headers

A floating header, which spans the door opening and anchors to the sidlites, can work with both door types, while providing a lighter look than a fully framed opening, with room for openers and associated electronics.

A simple header tube was used at this portal within the entrance wall of St. Joseph’s Drexel Library. Wiring for electronic openers, and strike plates for panic handles could be hidden with the header, to keep the opening relatively light and clean looking. The glass lites above the door opening are supported by floor to ceiling glass fins, and not the door opening itself.

Portal structure

The key design consideration in determining whether or not a structural portal is needed, is whether or not the opening must be self-supporting. In other words, when the door opening must support the load of the materials above (wall/framing/roof), a structural portal of one type or another (steel, aluminum, stainless steel, etc.) is needed to prevent loads from above from compromising the operation of the doors. Sizing the appropriate structural members is based on the width of the door opening, the weight of the load being supported, and the locations of where loads are being transferred to.

The framed portal at the Wistar Institute provides a dramatic stainless clad opening. This type of framed structural portal can provide the best weather seal at the doors, while also supporting structural loads above.

Initial design strategy recommendations

All glass vestibules are more about designing a signature look at the entrance than anything. Therefore, Bellwether recommends that the design process begin with conceptual aesthetic requirements, and then followed by an analysis of space, access (door opening size, electronics, etc.) and weatherproofing considerations. Once critical aesthetic and access requirements are set, a structural analysis can be undertaken to determine the options that will place the fewest limitations on each.

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Glass Vestibule Conditioning / Environmental Management

When properly executed, the vestibule’s design actively keeps outside elements from entering the building lobby. The double sets of doors keep heat loss and heat gain to a minimum, ultimately reducing the temperature swings that have to be made up by the HVAC system. The walk-off mats scrape sand, water and snow from visitor’s shoes to keep floors cleaner and safer.

During the design phase of a vestibule, we are often asked about the best ways to heat and cool them for comfort. This emphasis on glass vestibule conditioning (basically trying to create a mini lobby space) can be misplaced.

In terms of conditioning, think less about trying to mimic environmental conditions within the building, and more about designing a space that:

• Actively deflects its own heat gain / heat loss through low-e coatings, frits, tints, etc.
• Is pressurized properly to create an active curtain which keeps exterior elements in their place

Visitor Flow with a Vestibule

The size, configuration, and aesthetics of a jewel box vestibule serves to help visitors slow down and switch gears from the busy, unfiltered exterior world, to the quieter, and more orderly lobby space.

In terms of aesthetic design, we recommend an open and experimental approach. Make a strong design statement that sets the tone for the visitor experience within the building itself, and introduces the businesses that occupy it. An all glass vestibule is a excellent place to make that first architectural impression that occupants and visitors can sense tangibly. It is the first indication of the quality standards of the architecture and of the businesses within.

Again, the conditioning goal within a vestibule is to limit the intrusion of unwanted heat, cold, sand, water and snow, they keep the main building cleaner, safer, and more comfortable. And the primary design goal of an all-glass vestibule is to create a space that helps the visitor to experience the architectural intent, and transition from one mindset to another.

View our entire blog to learn more about Bellwether’s Design Development process, or visit our Glass Vestibule project page to learn more about Bellwether’s work with all glass vestibules.

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