Chicago Window Expert

Nobody knows more about windows.

  • By Mark Meshulam

    Breaking glass shower doors in the news…

    There has been news coverage lately about breaking glass shower doors and enclosures. Here are two, both of which included yours truly as a quoted resource:

    CBS news report on shattering shower doors ABC news item about shattering shower doors

    Recent news stories on this subject from CBS (left) and ABC (right) quoted Chicago Window Expert.
    Thank you CBS and ABC!

    Whereas TV news is obliged to hurry up and hit the high points of a story, your Chicago Window Expert can take his sweet time and discuss breaking glass shower doors at length. There is nobody telling him to talk fast because the commercial is fast approaching. Now you can get the full story.

    With that introduction, let’s get in the shower together and look around.

    I have been involved with three separate matters in which glass shower doors or enclosures have “spontaneously” broken, often while surprised, defenseless, naked people have been inside. All three of these breaking glass shower door matters involved hotels where multiples of such events occurred.

    You can imagine the shock experienced by a bathing hotel guest who suddenly finds herself standing barefooted in a sea of shattered glass. While you are busy imagining that, try imagining the chagrin of a hotel manager who works hard to create a positive guest experience, only to find that his nude guest was pelted with sharp broken glass while in the upscale tub enclosure.

    Bathtub after shower door shattered

    View looking down into a bathtub after the shower door shattered. In this case a bather was in the shower at the time of the breaking glass. Bloody footprints are indicated by arrows

    All in all, this is a bad situation. A bit of knowledge can reduce the chances of this occurring, and that is what I am here for.

    So what causes glass in shower doors and enclosures to suddenly shatter? It’s a number of things, and these things can interact with one another. So if you are looking for a sound bite to explain this phenomenon, take your ADHD tuchas to the top of this page and see what the networks have to say.

    Tempered glass shower and tub doors and enclosures

    Glass shower doors and enclosures are made from tempered glass. This is because in 1977, due to many horrific* glass injuries, the Consumer Products Safety Commission (CPSC – an agency of the Federal Government) mandated that their Safety Standard for Architectural Glazing Materials, 16 CFR Part 1201 become the law of the land.

    In the 16 CFR 1201 standard, the highest standard for strength (Category II) was applied to;

    Tempered glass breakage pattern

    Tempered glass breakage pattern

    • Shower doors and enclosures
    • Bathtub doors and enclosures
    • Sliding glass doors
    • Storm doors that contain any piece of glazing material greater than 9 sf
    • Doors that contain any piece of glazing material greater than 9 sf

    Tempered glass is defined by the CPSC as, “a piece of specially heat treated or chemically treated glass that cannot be cut, drilled, ground, or polished after treatment without fracture. When fractured at any point, if highly tempered, the entire piece breaks into small particles.” This renders the particles far safer than those of annealed or heat strengthened glass.

    So if your shower door breaks and you are nearby, be thankful to the CPSC for enacting 16 CFR 1201. Without this or similar regulations**, your injuries could have been much more serious because the glass shards would have been much more large and dangerous.

    Why does tempered glass break in small pieces?

    I’m glad you asked. Tempered glass is glass that has been subjected to the tempering process after it was originally formed at a float glass plant.

    In the float glass plant, the glass first becomes glass by melting a series of powders including silica sand at high temperatures and “floating” it out on a bed of molten tin to the desired thickness. Farther down the line, the ribbon of glass is slowly cooled, or annealed, to reduce and unify internal stresses in the glass. The glass is then cut into large sheets, crated and shipped to other factories.

    Float glass being made from melting powders, including silica sand

    Float glass being made from melting powders, including silica sand

    These secondary factories perform additional fabrications to the glass such as cutting, drilling, edge polishing, tempering or heat strengthening, coating and insulating. So when the CPSC and other agencies require tempering, this is done at secondary facilities that specialize in tempering. These are known as “tempering plants” or factories with “tempering lines”.

    Tempered glass for shower doors and enclosures must be cut to size, edge polished, drilled where holes are necessary before the glass goes into the tempering oven because once it is tempered, there can be no further fabrications. Any further fabrication will bring breakage, as the CPSC definition states.

    During tempering, the glass is heated to about 1300 degrees F, so that it is nearly in a plastic state, then quickly cooled (quenched) with the use of strong blowers which blow simultaneously at both side of the glass. It is this rapid cooling that creates the unique properties of tempered glass. One property is that the tempered glass is 4-5 times as resistant to breakage due to blunt impact as annealed glass. The other property is that when it breaks, tempered glass breaks into thousands of small cubes instead of large shards, which is desirable for safety.

    During quenching, the outer layers of the glass, which are about 20% of the glass thickness on each sides, cool more quickly than does the core. As the outer layers cool, they shrink and solidify while the core is still nearly plastic. Then when the core cools, it also shrinks, but the outer layers are already solid, so they resist this shrinkage.

    The surface of tempered glass is being permanently compressed by a core that is permanently "trying" to shrink

    The surface of tempered glass is being permanently compressed by a core that is permanently "trying" to shrink

    By the end of the tempering process, and forever thereafter, a permanent tug-of-war is present inside the glass. The core is trying to shrink while the surface layers are unable to allow this. Thus it can be said that the core is permanently in a state of tension, being pulled apart by the outer layers. The outer layers, in turn are said to be in a state of compression, because the core is constantly pulling on them as it tries to shrink. The tensile zone, where this fight occurs, is at a depth of 21% of the glass thickness from each face.

    In order for glass to be considered fully tempered (FT), the residual (after tempering) surface compression must be at least 10,000 pounds per square inch. Many glass lites are even more pressurized because there is no maximum in the standard. This is a lot of compression, and is the reason tempered glass breaks into thousands of pieces when it breaks. The release of energy that occurs when tempered glass breaks is almost unimaginable.

    The speed at which the breakage occurs is also mind-boggling. Cracks in tempered glass propagate at speeds of approx. 4,900 feet per second. That’s 3,354 miles per hour, or 4.4 times the speed of sound. If you have a glass shower door that measures 3′ x 6′ and it is chipped at the bottom corner, the time that it takes for the shower door to be 100% broken will be a touch over 1/1000 of a second! That breakage could have happened 256 times during the time you blinked your eye only once!

    Let’s think about this. The glass shatters from a solid object to a flexible aggregation of thousands of cubic puzzle pieces in just over an instant. Then the glass has a leisurely second or two to decide what it is going to do next. If the glass was evenly supported at all edges like in a fully framed shower door, it might just stand there, suddenly festooned with an attractive cracked-glass pattern after first letting out a quick cracking noise. But more often, there are forces on the door that cause the glass to seem to explode. A towel bar with a towel hanging on it will impart a load on the glass that will encourage it to fall out of its standing position and possibly shatter in the direction of a naked bather.

    That most common type of high-end shower enclosures available today are called “frameless”. These beautiful bath enclosures – I am having some installed in my house soon – utilize minimal framing. At corners, for example, the only hardware present might be a small angled patch fitting. The rest of the structure is the glass itself. Hinges and door handles are frequently attached to the shower door through the use of drilled holes. All of these attachments impart loads to the glass that influence its breakage pattern and particle distribution when a glass break occurs.

    Since frameless shower enclosures rely on the strength of the glass for support, thicker glass is used. Many frameless shower enclosures here in the U.S. utilize 3/8″ thick tempered glass. This gives a hefty feel, but is more weight that might fall on you in a glass shattering event.

    What causes the glass to break?

    Another good question! Here is Chicago Window Expert’s top ten causes of tempered glass shower door and tub enclosure breakage. They are not in a particular order because I simply don’t have enough data to order them.

    1. Edge damage under normal operation

    Edges are by far the most vulnerable part of glass. Our frameless bath enclosures leave these exposed. The glass edge can not be allowed to hit anything other than a soft bumper when going through its travel. I have seen frameless glass doors be allowed to hit a ceramic tile baseboard edge. Bad idea. Ceramic is a hard material that can equal the hardness of the glass. In a collision, both will be damaged.

    Cracks at top and bottom of a frameless shower door

    Cracks at top and bottom of a frameless shower door usually result in complete breakage

    2. Edge damage under stressed operation

    Glass shower door guide allows too much in-out movement

    Glass shower door guide is too flexible. It allows too much in-out movement which can result in the opposite glass edge hitting the frame

    It’s not enough to make sure a sliding door hits the bumper when it travels normally. What happens if someone leans or pushes on the door while sliding it? Try to push the door into and also out of the enclosure while opening and closing it. Does it hit the frame or anything else?
    A child or senior might inadvertently push the glass door leaf out of alignment during closing so that it hits the frame instead of the bumper, breaking the glass.

    Shower door out of alignment can hit the frame and break

    This shower door was easily pushed out of alignment so that it was capable of hitting the frame while being closed.

    3. Stressed operation without impact

    If your sliding shower door allows too much in and out movement as discussed above, there may be an opportunity to overstress the door by twisting it, bringing about breakage. Much leverage can be developed in such movements, and strong forces can be transmitted to weaker parts of the glass door system, such as drilled holes. A chip near a drilled hole could be encouraged to creep toward the tensile zone under such stress, then crrraaaccck! Treat the doors gently. If they have too much slop, get thicker doors, better hardware or tighten the hardware.

    4. Edge damage under forceful operation

    What happens when you slam the shower door? Does it hit anything other than a bumper? When there are overhead rolling hardware systems that support the glass from overhead, sometimes they don’t effectively restrict the glass from upward movement. If you close the door with force, and it hits a bumper that is not positioned at the glass’ enter of mass, the glass door can hop upward and hit part of the overhead hardware. A few hops like this and you will be hopping out of the shower over a pile of shattered glass.

    5. Edge damage at hardware penetrations

    Shower door grommet is deformed, creating a potential for glass damage

    Shower door grommet is deformed, indicating the hole is too large for the penetration, creating a potential for glass damage

    It is obvious that any hardware penetrating a hole in the glass should be separated from the glass with a rubber grommet and washers. Metal should never touch glass! If the hole drilled in the glass is too big for the grommet, the grommet can become deformed during usage and eventually allow metal screw threads to contact the glass in the drilled hole. Likewise, if the hardware is loose, a catastrophic contact between metal and glass can occur. Grommets must fit the holes tightly and be maintained.

    6. Bad hole fabrication

    Studies have shown that the cleaner the hole, the stronger the glass. This is why ASTM, American Society of Testing and Materials (of which I am a proud member) has included in ASTM C1048 Standard Specification for Heat-Strengthened and Fully Tempered Flat Glass, provisions for drilled holes in glass:

    • ASTM C1048, 7.9.5 Chips and flakes at hole edges must not exceed 1.6 mm (1/16 in.).
    • ASTM C1048, 7.9.6.3 Inner surfaces of notches and cutouts must be smooth seamed or polished.

    If you ask your shower door supplier if they adhere to these standards, I assure you they will view you with surprise and a new level of respect. And some will tap dance and sing the “We’ve never had a problem” song.

    7. Hole misalignment

    Holes drilled in glass show alignment and chipping problems

    Holes drilled in glass show misalignment of holes drilled from each side but not meeting well (arrow at left) and chipping problems around the drilled hole (right). The right hole shows an additional problem: soap scum shows eccentric position of towel bar. Clearly the hole was too large for the hardware.

    In order to get a clean hole in the glass, fabricators often drill a hole into the glass from each side. These holes must meet smoothly in the middle, but sometimes they don’t. If there is too much of a misalignment between the holes, only the “higher” side of the hole receives the impact when, for instance, a door mounted bumper is used. Does glass like loads that are not evenly distributed? No! Now position that cusp at a depth of 21% of the glass thickness, where the tempered glass tensile zone is positioned. Does anyone besides me see a problem here?

    8. Nickel sulfide inclusions

    With thousands of tons of glass being produced 24/7 in a float plant, it’s not hard to imagine that impurities might get into the glass batch. The most famous of these impurities is the nickel sulfide inclusion, which is a tiny little sphere as small, or smaller than a grain of salt. In annealed or heat strengthened glass it sits benignly and unnoticed for years, even though it expands up to 4% in size after some time when exposed to moderate heat. In tempered glass, however, if positioned in the tensile zone, it can suddenly disrupt the glass and cause a true spontaneous break. These types of breaks are puzzling because there may have been nobody present for days, the glass just shatters by itself.

    Nickel sulfide inclusion at edge of tempered glass

    This nickel sulfide inclusion in tempered glass was the cause of a spontaneous breakage in a sliding glass door

    The other breakage categories may also show shattering at a later time than when the impact occurred. Cracks in glass can be small (fissures) or even smaller (microfissures), and they may take some time to creep over to the tensile zone. Vibration or lesser impacts help it along. This is why tempered glass with edge damage can be compared to a time bomb.

    9. Excessive tempering

    As mentioned earlier, in order for glass to be considered fully tempered, it must have a minimum of 10,000 psi residual surface compression. If a glass lite is tempered to a state that is far in excess of that, the glass may be less tolerant of imperfections and more prone to breakage. Think of a skittish thoroughbred horse. It is so tightly wound that it takes off like a rocket with little provocation.

    You can estimate the surface compression by the cube size. Industry sources state that 60-80 cubes in the 5cm x 5cm area would indicate that the desired compressive stress of 10,000 psi is present in the glass. In order to count the cubes, you need to find a piece where the cubes are still engaged with one another.

    Counting the cubes in tempered glass

    The chart at left indicates the number of cubes one might expect to find in a 5cm x 5 cm piece of broken tempered glass, by thickness and compressive stress. The glass lite at right, 1/4" thick, had 130 cubes in that area, far above the minimum.



    What can I do to prevent breakage of my shower doors and enclosures?

    You are full of good questions. Here are some tips:

    Shower door bumper that prevents glass from touching frame, even when pushed perpendicular to the direction of travel

    Shower door bumper that prevents glass from touching frame, even when pushed perpendicular to the direction of travel

    1. Treat the shower doors and enclosure gently. It’s glass!
    2. Don’t let kids hang on towel bars. Don’t slam the door.
    3. Tighten loose hardware. Replace any deformed or broken grommets or plastic washers.
    4. If there are more than one bumper that the shower door hits, adjust the door so they are impacted simultaneously. If only one, position it at the center of the door edge so that there is no “hop” when the door hits the bumper.
    5. Buy glass made in the U.S. Standards of cleanliness for float glass and cutting/drilling are higher here than some other counties and this reduces the chance of nickel sulfide inclusions and spontaneous breakage.
    6. If you are managing a facility that has many glass shower doors, such as a hotel, hospital or apartment building, consider using a film on the glass that holds the glass together in the event of breakage. If you go down this path, it is imperative that the film covers the entire glass and is not cut around hardware attachments such as overhead rollers. If you neglect this step, when the glass breaks, a large, very heavy “wet blanket” of glass may fall on the bather and definitely ruin their day.

    What should I do if I am showering and the glass shatters?

    The first thing you should do is stand still. If you move your feet, you might step on broken glass. Look around and assess the situation. If there is a clear path where you can escape, do so. If not, look for a towel or bathrobe that you can drape over the broken glass and walk above the fray. Brush a path with your loofa, squeegee or back-scrubbing brush. If you have a shower cap, put it on a foot.

    If you have no choice but to step on the glass, make no sudden movements but rather apply weight to your foot gradually to give the glass time to lay down beneath it. Hold faucets, shower heads or soap dishes to steady yourself. As soon as you are past the danger, sit down and carefully brush all glass from your feet.

    You may need the information to make a claim of some sort, so take pictures before cleaning up the broken glass. If it might be a big claim, whether because of multiple occurences or serious injury, leave the glass as is until an expert or claims agent can view the scene.

    If you are a manager of a hotel or other multi-unit facility, get an expert involved sooner rather than later. Many injuries from shattering shower doors are minor, but the next one can be big and it is your duty to expend reasonable efforts to avoid it. Call me, I will be happy to help make your hotel a safer place to bathe.

    * From the CPSC 16 CFR Part 1201:
    “The Commission estimates that 73,000 injuries associated with architectural glazing materials in the architectural products within the scope of this standard were treated in hospital emergency rooms during 1975, and that about 2,400 of these injuries required the patients to be hospitalized. Extrapolating to total injuries in the United States the Commission further estimates that approximately 190,000 injuries were associated with architectural glazing products covered by this standard.” (in just 1975)

    Laceration from broken glass shower door

    Laceration from broken glass shower door. This occurred in an upscale hotel

    “Although injuries occur at any age, children aged 14 and under appear to be at particular risk of injury since as a group they represent approximately half the injuries while comprising less than 30 percent of the population. Lacerations are the most common injuries associated with architectural glazing materials and account for 72 percent to 93 percent of the injuries associated with the architectural products identified in paragraph (a) of this section. These lacerative injuries span a broad spectrum of severity and extent of body part affected. During 1975, an estimated 200 injuries were treated in emergency rooms for lacerations over 25 to 50 percent of the victims’ bodies and over 7,000 persons were treated for lacerations to the head or face.”

    ** In the private sector, ANSI (American National Standards Institute) has a similar standard, ANSI Z97.1 upon which the CPSC 16 CFR Part 1201 was based. For years, building codes such as the International Building Code, have referenced one or both of these standards. And as tends to happen in the world of standards, they continue to evolve.

    Mark Meshulam inspecting broken shower door glass

    Mark Meshulam inspecting shattered shower door glass for clues to the breakage. Elementary, dear Watson.


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