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  • By Mark Meshulam

    Mark Meshulam is an expert witness and exterior facade consultant located in Chicago, IL, USA

    Glass can be dangerous or deadly. Here are some legal matters I have been involved with as an expert witness:

    Woman falls through glass

    Woman trips and falls through glass. In an accident, the victim's life can go from normal to disastrous in an instant

    • A large bathroom mirror became detached from the wall, broke and injured the occupant who was unclothed
    • A storm window blew off a building and impacted and injured a city worker
    • A hotel guest was injured when a glass shower door shattered while she showered
    • A bicyclist fell through an untempered storefront window and died from his injuries
    • A woman fell against untempered storefront glass and was lacerated badly when the glass broke
    • Glass in a sports facility broke, presenting a danger to crowds below
    • A woman died when she fell through glass in a hotel window-wall

    In many of these cases, glass failed due to a pre-existing damage that expressed itself into a full blown failure in barely an instant. There is precious little research on this subject. How much does a bullet hole weaken the glass? How much more easily will a pane of glass break if an edge is chipped? Nobody really knows.

    I intend to find out.

    Join me and Quast Consulting & Testing of Mosinee, WI in this important study:

    Research Proposal
    Study to Determine the Effect of Pre-Existing Damage
    to Annealed Glass on Breakage Resistance

    Each year hundreds of people are injured or killed by broken glass, whether by falling through a sheet of glass, or by having glass fall on them, even while they shower when a glass shower door suddenly shatters. This glass may have been in windows, doors or storefronts near where people live, work and walk. In some of these instances, witnesses have reported that a seemingly intact lite of glass seemingly “gave way”, bringing about a dangerous condition.

    The author of this study, Mark Meshulam, has been involved as an expert witness and exterior facade consultant in legal matters involving two deaths and one severe injury due to glass breakage. In each of these cases, the glass broke suddenly without much force having been applied to it.

    Co-author Tim Quast of Quast Consulting and Testing says, “the importance of this study can not be understated. Glass can be very strong, yet sometimes it just simply gives way bringing about serious injury or even death. We are proud to work with Mark to develop a test method that will get at the root of this problem, and to generate data that will be of real value to our industry and community.” Quast’s window testing laboratory in Mosinee, WI will be the site of the glass testing.

    scratch in storefront glass

    Deep scratch in storefront glass. The pane next to this one broke under minor pressure, severely lacerating a woman

    It is well known in the glass industry that the resistance of glass to breakage can be compromised dramatically by the presence of a pre-existing scratch, hole or edge damage. For example, when glass is being cut as a part of the fabrication process, it is not really cut at all. Rather, it is being scored, or scratched to only a very shallow depth, then flexed to apply pressure at the score line. When a scratch is created, the glass readily and cleanly breaks along the line of the scratch.

    Equally known to those in the glazing industry is the propensity for glass to break if damaged at the edge. For this reason, almost universally, rubber setting blocks are placed beneath glass when it is installed. This avoids any chance that the glass edge will come in contact with a harder material that might chip the glass edge, thereby weakening the glass.

    Pressures which may be applied to the glass, whether by wind, impact, human contact or thermal variations, appear to greatly accelerate the propagation of cracks through glass. The purpose of this study is to determine if, and to what extent, pre-existing damage to glass reduces the ability of the glass to withstand pressure.

    To accomplish this, pressures will be applied to damaged and undamaged glass. Failure pressures will be observed, collected and compared.

    Methods for Testing Glass in Laboratory

    Bullet hole in glass

    Bullet hole in glass - conchoidal fracture

    Three damage types, found often in real life will be simulated:

    1. A low caliber bullet hole
    2. A chip at the glass edge
    3. A scratch on the glass surface

    Bullet holes will be simulated by dropping a steel ball from a height that will produce the familiar cone-shaped spall known as a Hertzian cone crack, a type of conchoidal fracture. This method of is known as the Hertzian fracture test. A clear opening of 1/8” diameter at the impact side will be created.

    Edge damage will be caused by using a spring-loaded hardened steel punch applied to the edge at a depth of approximately ¼ the overall glass thickness. This will produce a familiar chip fracture known to the trades as an “oyster”.

    Glass edge damage - oyster chip

    Glass edge damage - oyster shaped chip fracture

    The scratch damage will be simulated by scratching a uniform X on the glass face.

    In order to apply pressure the glass, it will be placed horizontally on the top of a test chamber. Using a regulated air blower, air will be extracted from the chamber. This will induce a downward load on the glass. A manometer will be used to measure the differential pressure that is applied to the glass. Undamaged and pre-damaged glass specimens will be brought to destruction through increasing chamber pressure. The pressure at which the failure occurs will be recorded.

    There are three additional variables that are of interest to this study:

    1. Deflection of the glass under load
    2. The side of the glass that has preexisting damage compared with the direction of pressure
    3. Speed of crack propagation.

    Glass deflection is important because if a relationship can be established between deflection and failure, there may be an opportunity to communicate to the public that excessive deflection of a glass lite may be a precursor to a dangerous condition.

    The direction of pressure is important because of tension/compression in the glass under bending. Glass cutting practice involves bending the glass away from the score mark. There may be a difference in breakage behavior if the bending is toward the score mark.

    Glass Cutting Instructions

    Excerpt from glass cutting manual. The manufacturer points out that even annealed glass has internal tensions that aid in breakage. A fissure depth of only 8-10% is needed to break the glass by bending it. 8% of 1/4" typical glass thickness is only 20/1000 of an inch

    Speed of crack propagation is critical to the understanding of glass breakage and has critical safety implications. It is anticipated that when cracks propagate through glass, the propagation speed may be found to be quite fast. The dangerous condition is exacerbated by the surprise factor. The victim has little or no time to react as a pane of glass quickly transforms into a chaotic cloud of deadly shards of razor sharp glass.

    In order to quantify deflection, during the glass testing a dial indicator will be used at the center of the glass. Maximum deflection before failure will be recorded.

    In order to quantify the speed of crack propagation, a high-speed video camera will be utilized. With a known frame rate and glass dimensions, speed of propagation can be calculated by studying the videotapes after the testing.

    To reduce the variables, all damage applied to the test specimens will be located at centerlines of the glass. Bullet holes will be located as close to the glass center as possible. Edge damage will be located at the longitudinal center of the edge.
    Invest in research for the public good
    The Pitch
    We are seeking individual, corporate and government sponsors for this research.
    Minimum sponsorship is $1000. Sponsors will be invited to attend the glass testing in the laboratory and will be recognized when the research paper is released. If you or your organization are interested in participating, please call or write now.

    Please join this project if you can allocate resources for this life-safety cause.

    Thank you!
    Mark Meshulam
    Chicago Window Expert
    Expert witness and exterior facade consultant

    Want to be a research sponsor?

    Mark Meshulam, Chicago Window Expert invites you to join his research team

    Mark Meshulam, Chicago Window Expert invites you to join his research team

    No matter where you are,
    call me, Mark Meshulam,
    the Chicago Window Expert
    For the expert attention you deserve
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6 Responses to “Glass Breakage and Pre-Existing Damage”

  1. Dear Mark,
    We both follow the same route in glass pathology.
    I am based in the UK and travel extensively due to the lack of experts in the field of fenestration. Having read your note, it is refreshing to see others that recognize the limits of glass and the impacts that can cause failure. It is not just a pane of glass but much much more as it is now being used as an engineering component in buildings which bring a whole new set of issues.
    Would like to set up an exchange of ideas etc.

  2. Hi Rod,
    Thanks a lot for your comments on the website and at Linked-In.
    It’s always good to find kindred spirits.
    What are you working on now?

  3. Mark, I have some photos and video that you are welcome to use. One is my witness interview after a car window spontaneously exploded in a grocery store parking lot due to being bedded against debris from a prior piece of glass a few days earlier. The witness is a drunk bum and his description of the event is priceless. There are also some really scary glass situations currently in use in our area that might be of interest to you. Contact me privately sometime and I’ll be happy to discuss.

  4. I can’t sponsor as I have nothing to do with the industry.

    However, I can tell you as a kid I went through the glass portion of a screen door and
    and cut my jugular vein (I had about 2min left to live).


    Robert Blumenfeld
    Executive Director
    ACG New York

  5. Artist looking for old wood windows for new artwork. Contact Mark Meshulam if you have old wood windows to give away.

  6. There are many studies linking blemishes with statistical strength, which resulted in glass quality standards setting certain criteria, which In turn depend on location on a glass pane , and not just for aesthetic. I.e. this picture shows the tensioned areas (of four edge supported pane,) where the glass is most vulnerable to scratches. Some self-professed glass experts, whom I met on related insurance cases, universally pointed out to the glass center but seldom knew about the tensioned areas in glass corners. Glass tension areas are different depending on support mode. This can be determined by F EA simulations. Also, glass strength is specified by number of breakage per thousand. For example seven panes of the typical fully tempered glass is normally expected to break out of thousand. You would need to break a lot of glass for your research…

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Photo of the day

Varying the light in the picture, the light reflection reveals stress waves emanating from the inclusion. The stress is the cause of the glass failure. It developed slowly as the nickel sulfide inclusion grew over time, pressing against the unyielding encasing glass. One researcher estimated that this stress could reach 100,000 pounds per square inch. When the stress exceeded the ability of the glass to contain it, the glass failed catastrophically. Why catastrophically? Because tempered glass, by design, has a permanent internal tug-of- war to the tune of 10,000 pounds per square inch everywhere in the glass plate. Disrupt that structure, and you get...pop!