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Jun20
Diagnosing Glass Breakage
Filed under: About windows & glass; Tagged as: broken glass, glass breakage, glass impact breakage, glass stress cracksMark Meshulam is a consultant for glass breakage and
expert witness for personal injury due to glass breakage.Sometimes it’s easy to know why glass breaks.
Diagnosing the cause of glass breakage can be a no-brainer
For example, a ball on the floor combined with neighborhood kids running away generally can be considered symptoms of impact breakage.
Today we will look at four types of breakage:
- Impact breakage
- Stress cracks
- Edge damage
- Spontaneous breakage
Impact breakage
Typical impact break without puncture
If you know anything about glass, you know that it can break, and when it breaks, it’s not a good thing.
Back to the ball-through-the-window example, the breakage pattern will vary depending on the speed and mass of the ball, and the size, thickness and post-annealing treatments that were performed on the glass prior to the ballgame.
A very well-hit hardball, or a well thrown rock squarely hitting a piece of annealed glass will produce a circular puncture with cracks emanating outward from the point of impact.
Glass shards from broken annealed or heat strengthened glass are dangerous and must be handled with care.
The resulting shards between these cracks are dangerous! If they fall out on your arm (as often happens during clean-up) you will soon be in the emergency room. Experienced glaziers often tape the shards together with duct tape, then remove the entire panel. If you must remove these shards, remove the upper ones first, then the lower ones. Use heavy rubber gloves, protect your arms, eyes and feet, and place the shards in a cardboard box, not a garbage bag.
Blunt or Distributed Impact on Long, Narrow Lite of Annealed Glass
In this example, we see a horizontal crack at the center of the blunt impact, with cracks radiating away from the impact. Due to the aspect ratio (relationship between width and height), shards are long and narrow.
Blunt impact on long lite of annealed glass
Small Rock High Velocity
Congratulations to this vandal who was able to select just the right sized small rock, and hurl it at a speed righteous enough to completely puncture this glass. The combination of size and speed resulted in a localized pattern of damage.
This vandal chose quality over quantity. He selected a smaller stone, which he was able to accelerate to a greater velocity, sufficient to completely puncture the glass.
Larger Rock Less Velocity
This vandal’s eyes were bigger than his throwing arm. Although he scored a large area of glass damage, he failed to achieve the goal of full glass penetration. The impact was large enough, however, to break the interior lite of the insulating glass unit. In the picture below you can see two sets of impact breakage patterns. You can also see the rich source of projectiles: railroad tracks. One week after we finished installing windows in a new high school, local kids had a field day with the rocks and our new windows.
Vandal with big ideas hurled a sizeable rock and achieved a big breakage pattern, but failed to breach the fortress
23 Comments
Cracks in tempered glass
Tempered Glass Breakage
When tempered glass breaks, the energy retained in the glass due to internal tension/compression releases explosively and produces a breakage pattern sometimes called “cubes’.Seeing a cubist breakage pattern does not tell you why the glass broke, it only tells you that the glass was tempered. Generally, there are three reasons tempered glass will break: impact, edge damage or nickel sulfide inclusions.
Normally, when tempered glass breaks, it falls down into a pile of little cubes. Only the most patient window consultants with the most generous client would ever consider piecing the cubes together to determine the cause of breakage. However, occasionally the pieces of broken tempered glass will stay in the opening, locked to each other like blocks in a masonry arch. And just like in a masonry arch, if you remove the keystone, the arch – or glass in this case – comes tumbling down.
If you look closely, you can see the point of impact on this tempered glass. A laminated interlayer holds the cubes in place.
This picture shows broken glass which was a part of a laminated unit. The PVB (polyvinyl butyrate) interlayer held the pieces in place, giving us an opportunity to observe that impact damage is visible, even in tempered glass.Spontaneous Breakage in Tempered Glass
Glass, and especially tempered glass, sometimes breaks all by itself. This can be quite disconcerting when, as has happened in a public place which will go unnamed here in our great city, large, thick panes of tempered glass basically blew up fairly frequently. The unusual cause in this rare instance: the glazing contractor attempted to grind the edges of the glass after it was tempered, creating a series of time bombs. It is a very bad idea to modify glass after it is tempered!A more well-known, but also quite rare cause of spontaneous breakage is nickel sulfide inclusions. If you read the previous post, you will already be down with the fact that glass is made from melted powders. A nickel sulfide inclusion is a tiny rock of unmelted material that remains in the glass. Below see an artists graphic representation of a nickel sulfide inclusion.
Artists representation of nickel sulfide inclusion.
You can well imagine that a little rock embedded in a slab of glass which is under high tension/compression forces, could weaken the glass and eventually cause breakage.Stress Cracks
A “stress crack” will usually only happen in annealed or heat strengthened glass. Stress cracks emanate from the edge of the glass and meander about apparently without purpose. But there is a purpose: to relieve stress. However, the term “stress crack” can be misleading.If annealed glass is subjected to thermal fluctuations beyond its capabilities, it will break in a way that will relieve the stresses induced by thermal changes. This type of failure is a design issue. Heat strengthened glass should have been specified for the application.
However, there can be a near-identical breakage pattern which emanates from damage in the glass edge that fails as stresses, such as thermal, are applied.
Classic meandering pattern of heat-related stress crack, accompanied by evidence of edge damage. In this case, the building had both: edge damage and underspecified glass. Reflective blinds and a South exposure combined to create a high frequency of this type of breakage.
To tell the difference between a true stress crack and a crack due to edge damage, look at the edge of the glass for a chip, which we window linguists call an “oyster”. You might have to look hard because the oyster could be buried in the primary seal on the #2 or #3 surface.
Another clue would be the distribution of glass breakage in the building. It would be normal to find stress-like cracks on elevations with greater temperature swings. But does the breakage also coincide with the use of reflective interior blinds, especially in a partially opened position? That would be indicative of a true stress crack, rather than a crack induced by edge damage.
Great additional resource:
Viracon Technical Information: Thermal Stress Breakage
Mark Meshulam observing that glass is not broken
Need to diagnose glass breakage?
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If you are in these areas and need help with your building, call now and save consultant travel expenses.23 Responses to “Diagnosing Glass Breakage”
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Pretty good post. I just found your site and wanted to say
that I have really liked reading your posts. Any way
I’ll be subscribing to your feed and I hope you post again soon! -
Thank you for your kind comment. Feel free to ask questions or suggest future articles.
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I know the family that ownes Florida Crush Stone. About 80% of the aggregate for cement for the whole US comes from them. Do you want me to help forward this site to people in related industries? Do you work with companies around the country? How small is too small?
Hampton
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I would be grateful for readers forwarding this site to other interested parties. Yes, we do work with companies around the country, despite the ChicagoWindowExpert name. In the consulting part of our business, there is no job too large or small. Thanks for writing!
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Victor June 29th, 2009 at 3:28 am
Nice post by smart M and I was enjoyed. Hope new posts would be on the site and serving Chicago.
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Thanks for the nice comment from my friend Victor Wei of Yuanda. Yuanda is the Chinese curtainwall company responsible for two of the most memorable buildings of the Beijing 2008 Olympic Games, the Bird’s Nest and the Water Cube. Congratulations to Yuanda for those two wonders!
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Hi Mark,
I came across your website and found it very helpful. I was curious if you had any photos of nickel-sulfide inclusions? My window spontaneously shattered one night and the window supplier is claiming it broke because of impact — I know I didn’t hit it! Based on the tempered glass breakage photo on your website, it would appear that my living room window has a point of impact. But would spontaneous breakage also have the same radial, “point of impact” pattern? I’ve read on other websites about butterfly patterns that appear because of nickel-sulfide inclusions — I think mine shows that, but I would like to get a professional opinion.
I would be very interested in having you diagnose, but I’m an American expat living all the way in China, so I’m not sure if you can help or not. I’ve attached photos of the broken window taken by the window supplier (it’s a curtain wall, floor-to-ceiling residential window). The outside window pane is intact, according to the supplier.
Sincerely,
Kim
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Hi Kim,
According to my glass experts, a nickel sulfide inclusion should be visible to the naked eye.
Attached please find an interesting photo I recently came across.
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Hi, Kim,
I am working in a window glass production plant. Recently I am getting more complaint from my customers regarding temperness. But I found very good cutting in size before packing. will you please be able to help me in this regard. what are the general breakages styles of different reasons in related to stress, strain vice versa.if you have any pictures related to that subject please enclose with details. your co-operation is highly obliged.with regards
kumar
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Kumar,
The pattern you see on the background of this page (scroll all the way down) is a clasic breakage pattern of tempered glass. I recently learned that it is possible for a lite of glass to be incompletely tempered, i.e. tempered in some areas and not fully tempered in other areas.
Although I have never seen it, this condition could result in a hybrid breakage pattern that transitioned from the tempered cubes to larger shards. Has anyone out there ever seen such a thing?
Since tempering occurs when glass is heated then rapidly cooled, I would imagine an uneven temper could result from uneven heating or cooling.
What is the breakage pattern your customers are complaining about, Kumar?
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Francis July 14th, 2010 at 11:56 pm
I have experienced the incomplete tempering process. It is a laminated glass, the breakage happened at surface no.3 however surface no. 4 is intact. The breakage pattern is as shown in the background, just like a tempered.
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Hi Mark. I stumbled on your article while researching a problem I have. Perhaps you can help me? I recently broke a peice of safety glass in one of my homes french doors. There are 15 seperate glass panels and they are double paned. Only one panel/section broke and only 1 panel. I’ve been told the door can not be reglazed and they only option is to replace the entire door! It appears to me that there is about 1/8th to 1/4 inch between the broken pane and the second pane behind it. The glass that shattered remained in place entirely – just completely cracked. Now my question – I’m considering just trying to remove the broken pane and leaving it as a single pane of glass in this one area. I have no idea how to carefully get the broken piece out though without possibly breaking the pane behind it too. Should I just carefully tap on it with something like a screwdriver until it punctures it? Any thoughts/tips/advice would be greatly appreicated!
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Mark,
I cleaned the outsides of the exterior windows on a newly constructed large downtown building. The windows have the new external aluminum window louvers attached above in strategic locations on the east south and west sides of the building. This is a very time intensive job and after a month we are finally wrapping this up. (due to many days of heavy wind delays and carefully working around those louvers). at the outset we were instructed to not put any weight on these louvers whatsover as in the past there had been some cracking problems with the same set up but at a different office building (using the same glass installers) I noticed today that one of the windows that we had cleaned at the beginning has developed a large stress crack that goes from upper right hand corner side down to right corner and then up and across entire window to the left side. This just happened over the weekend. What are your thought on this?
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Hi David,
There can be many reasons for this type of breakage. First, there will always be a few lites that seem to break spontaneously during the first few years of building operation – usually this means that there was a hidden weakness, such as edge damage or an inclusion, that finally got around to breaking. Temperature fluctuations help bring out hidden defects.
If it is not edge damage or an inclusion, then the glass may not be designed for the conditions. In other words, if the louvers or other projections cause a pattern of triangular shading on the glass, if the glass is annealed only, it may break.
If they want to blame you for the crack, get pictures and ask them to show you the point of impact. If glass is broken by impact, there will be a very noticeable point from which many cracks emanate. If it’s not there, it is probably not your fault.
Good luck!
Mark -
Hi Ed,
Unfortunately too many manufacturers make their products difficult or impossible to reglaze. Shame on you, window manufacturers who do this. You are screwing the public!
The glass will be held in by two things: stops and sealant. If it is a wood door, the stops will be little strips of wood nailed just to the interior side of the glass to hold it in place, although with some of the high strength sealants out there, it is not really necessary, except for appearance.Gently and carefully remove the stop by sliding a sharp razor knife along the seam between the stop and the frame (and also between glass and stop) to separate the stop from the frame. You should now see the glass edge. Measure the actual glass size. Go order a piece of replacement insulated glass. When it arrives, remove the old glass by any means possible: cutting along the glass where it is adhered to the frame on the exterior, cutting away any sealant around the glass edges visible from the interior, prying and prying every witcha-way. If the glass is stubborn, and it is tempered, nick away at the face of the glass with the claw end of a hammer until the glass breaks, then peel and clean it away. Bed the new glass in 100% silicone or follow the manufacturer’s glazing instructions, then install the glass and the stops.
I am not a fan of removing the broken lite and leaving the intact lite. It will look strange and who needs that?
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pete parlow February 28th, 2011 at 2:13 pm
we put some mirrors in a work out room several months ago and 3 or 4 months later they crack at the corner and than went straight across the mirror. The owner said it was out fault , Now he wants new mirrors. He is a good customer . In front of these mirrors on the floor is a weight rack where they are picked up and put down. I told them that the floor was bouncing up and down. I think I am right.
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TJ Chana April 2nd, 2011 at 4:00 pm
I have recently visited your website (chicagowindowexpert.com) and have looked at the glass patterns that you have described. The pattern of my broken window glass is exactly as shown in the “Blunt or Distributed Impact on Long, Narrow Lite of Annealed Glass”. Can this pattern also be a result of thermal or temperature related stress? If not, can you tell me what kind of object or impact would cause such a pattern? Thanks a lot!
TJ Chana
Full width of lite
Closeup of center -
Hi TJ,
It looks like the glass is receiving some loading from above. Remove the interior glass stop at the top of the glass lite and see if the frame is touching the top of the glass (this would be bad).Then remove the bottom glazing stops and see if the setting blocks are at quarterpoints. I am going to guess that the setting blocks are at the transition between the cracked area and the middle bottom crescent.
Let me know what happens.
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TJ Chana April 2nd, 2011 at 4:06 pm
Hi Mark,
I finally had a chance to take a look at the glass (weather is finally warm enough!). The frame was touching exactly where you thought it was, I suppose it is due to the settling of the frame.
The settling blocks were at quarterpoints and between the cracked area and the middle-bottom crescent as you said!
Thanks for your help!
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Flaws discovered in exploding glass balconies
Published On Thu Aug 18 2011
http://www.usgnn.com/fetch.php?url=http://www.thestar.com/news/article/1041662–flaws-discovered-in-exploding-glass-balconiesLiam Casey Staff Reporter
Microscopic imperfections buried within the pane are behind the spate of exploding glass balconies on downtown condo towers, an engineer says.
Mark Brook is a partner at BVDA Façade Engineering which is analyzing glass from the north tower of the Murano building, where at least nine panes have exploded since last September.
Brook recently discovered nickel sulfide crystals in two shattered panes. These crystals grow over time, which stresses the glass. If the imperfection is in the right place, the pressure builds until the pane explodes.
“It’s either bullets or inclusions,†Brook said. “And there’s no evidence someone’s shooting at the balconies.â€
To get a definitive answer, Brook needed at least one of two specific bits of glass where the explosion originated — the portion containing the nickel sulfide crystal. That’s nearly impossible when a pane explodes into hundreds of pieces strewn across a busy street.
But the engineering firm was lucky. It had been testing intact panels from the north tower of the Murano building as the developer took them down. One night, a pane resting against a wall in the testing facility exploded. They found their specific glass bit.
“It’s not poor installation,†Brook said. “It’s not wind, it’s not the weather, it’s these imperfections.â€
Brook first suspected the faulty glass was from a bad batch. It was actually several contaminated batches from manufacturing facilities in the United States that ended up in Toronto.
“Exploding glass isn’t that abnormal,†Brook said. “It’s just been making the news because it’s been exploding onto busy streets like Bay St.â€
Brook’s firm has been contracted by Lanterra Developments to examine the glass on three of its condos where balcony railings have shattered: the two Murano towers as well as One Bedford Rd. The developer has said it will replace the tempered panes with the safer laminated variety — an inner layer of tempered glass and an outer layer of heat-strengthened glass sandwiching a sheet of plastic — on these and all future projects.
Glass has also shattered on three Toronto buildings built by other developers.
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I have a 5/8″ OA Guardian RLE 70/36/Clear insulated unit 65x48x5″ (roughly) Rake Top glass pane that I was told has an oyster extending from behind the aluminum frame. When the weather got extremely cold this past winter, a crack started from the oyster and ran down the window about 18 inches. The house was just a few months old, and the Builder is refusing to replace it.
Can you provide me with a layman’s explanation of an ‘oyster’, how they happen, and why the crack happened during extreme cold. If you have any pictures, that would be very helpful.
Thanks for your blog. There’s almost no place for a layperson to get the real truth.
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Hi Shar,
An oyster is a clamshell-shaped chip in the surface of the glass, usually emanating from an edge. It is the result of damage to the glass edge, such as might occur if the glass were pried in place with a metal prybar during installation.
Glass edges are sensitive and brittle. They should never be allowed to contact metal. Wood is used frequently for standing a lite of glass onto prior to installation. Once installed, the glass should be sitting on hard rubber (neporene is often used) setting blocks at the bottom quarter points.
If your glass was already installed into the window when it arrived to your jobsite, the edge damage probably occurred at the factory. Bring this to the manufacturer’s attention and try to get them to provide material and labor to replace the glass. If they refuse, let me know. Maybe I will be able to persuade them to do the right thing.
The photo below is at the bottom of the glass just next to the setting block. There are a pair of oysters at the bottom of the glass. They are spaced apart about the width of a pry-bar used often by glaziers.
The glass cracked about a year after it was first installed. I was asked to become involved because a number of such breaks occurred about 90 feet up the side of an NFL football stadium and a serious safety hazard was of concern.
I asked to join the glaziers on the swing stage so I could inspect the glass edge as soon as it was uncovered. Edge damage from the original construction was evident on most of the broken lites.
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Mark – My sincerest thanks for your detailed response to my ‘oyster’ question and the attached picture. We have already been deposed over the issue(s), and this issue along with other defects is scheduled for trial in a month. The refusal of the builder to come out and look at the problems has necesssitated this action. The window has already been replaced because we are selling the house, but we would like to recoup sizeable expenses.
Again, I truly appreciate your time and efforts to help those of us who have little knowledge about these subjects.
Best wishes…
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