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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There are three reasons that you should include as much detail as possible about your specific make-up, including glass thickness:

1. Aesthetics
2. To ensure that structural considerations have been met
3. To ensure comparable quotes from all bidders

Aesthetics
Glass thickness affects its tint, and thicker glass is greener in tone than thinner glass. As glass gets thicker, architects often choose a low iron option to reduce this green tint.

Unless glass tint is explicitly called out in the specification (clear, low iron, etc.), bidders will often quote clear glass to keep bids as competitive as possible. However, a laminated fin glass make-up of 3/4” over 3/4” clear glass will be relatively green. You will need to understand the exact thickness that your project requires during your design phase, in order to know how that thickness will impact project aesthetics.

Structural Considerations
By leaving glass thickness and sizing up to the bidder, you are assuming that they are doing the appropriate pre-engineering at bid time. Many do not engage in the step for the sake of time and cost. Instead, bidders often scale what is on the drawings for use in their bids. Conflicts can arise when thicker glass is indeed required, and the bidder does not have enough money in the job to meet requirements. The stage is set for disagreements before the project even starts.

Comparable Quotes
A well-defined glass make-up will ensure that all bidders are pricing the same exact make-up. This will make it easier for you to compare scopes, while ensuring that structural requirements are met by all bidders.

Determining the correct glass thicknesses during your design phase puts the engineering responsibility and costs on the architect. It is well worth the investment, however, since glass thickness is such a critical component of your project aesthetics and your structural requirements. For these reasons, glass thickness should be determined by the architect and included in the specification, and not be left up to bidders.

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Creating custom glazing systems requires time, planning, and process that are unique to the project. Too often, as construction schedules and budgets get compressed, there is an assumption that the custom materials can be sped up accordingly. That’s unfortunately not always the case.

The most important thing to consider when starting a custom project, is that the design team will be defining all aspects of the system, from design through structural tradeoffs. This happens from project development through design-build stages. When using standard product, you have the convenience of access to existing library testing, and your material choices, sizes and thicknesses are limited to what works within those test parameters. Basically, if you stay within product parameters, you will be safe. This “transactional” type of purchase and design is fast and easy to meet tight construction schedules, but it won’t win any design awards.

Custom glazing system development gives you complete flexibility to match architectural intent. Many system solutions can be developed based on proven design methodology, which maintain performance and structural integrity. However, it takes additional time to tweak things like material sizing, anchors, and coordination with adjacent systems. It is important to launch the design stage with a full discovery process that brainstorms, identifies, and budgets all of the potential options that should be considered, so that you don’t have to go all the way back to square one when it’s time to value-engineer the system. This is the point where the team must plan the design assistance schedule within the overall construction schedule.

Material choices can have a big effect on lead times. For instance, a glass fabricator may have limited quantities of 3/4” low iron glass, but a large inventory of 3/8” low iron glass. If you have the flexibility to use a laminate of 3/8” over 3/8” glass, and understand the tradeoffs, you may be able to get the same look in a lower price, with a much shorter lead time.

Structural considerations of custom systems can also have a significant impact on the construction schedule. Oversized, tall-span glazing systems impose unique loads on the surrounding structure. One of the common considerations during the design stage is to determine where these systems will be dead-loaded conventionally at the base, or hung from above. These decisions are based partially on the characteristics of the custom materials, and partially on the limitations of the surrounding structure. The analysis doesn’t always change the base building structure, but coordination of structures should be given appropriate time to consider impact.

In the end it comes down to the definition of your systems. Are you willing to accept existing off-the-shelf products for the convenience of design simplicity, or do you want to create something with your signature on it? Off-the-shelf systems can be delivered within shorter lead times, where customization and uniqueness of the aesthetic is not a key project driver. Signature projects incorporating custom glazing systems require a well thought out design-assistance stage, complete with a system-specific timeline that allows for analysis and experimentation that will get you as close to the design intent as possible.

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• Protect it from budget cuts?
• Maintain the accuracy of the design intent?

The establishment of project drivers (aesthetics, material quality, budget, etc.) is the critical first step in defining your signature design element. These drivers will guide design teams as well as the supplier of the custom glazing system to put them on the same page with you. And we find that the two most instructive questions that you can ask in identifying the project drivers are:

1. What are the “Need-to-have” components of the project?
2. What are the “Nice-to-have” components of the project?

What we’re getting at with these questions is: What design or material elements truly define what you are trying to express, and what elements would not have an impact on that core intent if they were altered?

The answers to these questions help custom system designers to identify what is critical to you, and not spend time working on items that are not as important. If we, as systems designers, know what you absolutely can’t or won’t change for the sake of the design intent, we can advise you of the associated costs and challenges, and discuss strategies to absorb the cost impacts in other, less critical areas of the design.

This is where our creativity works in tandem with yours. You are the expert in how the building design will translate your intent. We are the experts on how to interpret that intent in the form of products and systems that meet with your aesthetic, budget, performance criteria, etc. The more we know about your preferences, how the design came about, and what you can’t live without, the better we can be at giving you options to meet your goals.

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