Mark Meshulam is a window and glass expert. He served as an expert witness for the defense in the murder trial of Amber Hilberling, where she was charged with causing the fatal window fall of Josh Hilberling. Amber was convicted of manslaughter and died under questionable circumstances while incarcerated.
Mark served as an expert witness for the defense in the murder trial of Amber Hilberling. This case garnered national attention due to its tragic circumstances and complex forensic details, particularly regarding the failure of the glass.
Façade Consultants
At Façade Consultants, we apply real-world experience in façade engineering, glass testing, and forensic investigation to uncover the truth behind building envelope failures. The following account was written by our founder, Mark Meshulam, a nationally recognized expert in windows and glass.
The apartment number and door to the apartment.
The University Club Tower
The University Club Tower is a unique 32-story cylindrical tower built in 1966. It was the first significant building in the United States designed using computerized structural calculations. The concern of the structural engineers was to be sure the building was strong enough to prevent swaying in the wind. The building, including its glass-and-panel curtainwall façade, was 46 years old when Josh fell to his death in the late afternoon.
The site and highlighted path of the tragic window fall.
The Building Exterior
- The window system is a very early example of a system called a “curtainwall”
- The entire framing system that clads the exterior of the building is all one system.
- It runs to the exterior of floor slabs and is glazed with glass in some areas and with panels elsewhere.
Curtainwall condition
Layers of caulk were unprofessionally troweled onto the façade. The façade was badly worn and had been caulked over multiple times, probably due to air or water leaks. Although some of the glass in the building had been replaced, the glass from the incident was almost certainly original. On the interior, the back of the curtain wall was exposed. Cracking and deterioration were visible from the interior.
At Façade Consultants, we frequently encounter this type of deferred maintenance. Overuse of caulk and patch repairs can conceal deeper structural or integrity problems within the curtainwall. Our work includes detecting inherent safety hazards and performance issues.
The Apartment Layout
The Glass
The glass edges were fused into a U-shape, an unusual configuration which is no longer in use.
The glass was an unusual insulated glass unit. It consisted of two layers of glass separated by an airspace – that part is regular – but the edges were actually fused in a U-shape rather than using separate metal or thermoplastic edge spacers, as is used today. Each layer of glass was only 0.092 inches thick (3/32 inch), less than 1/8 inch.
This “single strength” glass, nowadays, is used mostly for picture frames.
This thickness is known in the glass industry as “single strength” and is typically used nowadays for picture frames. Single strength glass is too thin and weak for use in windows.
The space between the glass was also thin, only .191″ (3/16″). This would provide a nominal amount of insulation. It would also, in the event the glass was pushed outward (deflected), allow contact with the outer lite with similar force.
Non-safety Glass
The window glass was annealed, non-safety glass. Had it been tempered or laminated safety glass, Josh Hilberling would likely not have been able to penetrate the glass. This is because tempered glass is four times stronger against wind loads and, according to a study conducted by Façade Consultants, 2.5 times stronger against blunt impacts than regular annealed glass. Laminated glass, used in car windshields and hurricane windows, is designed to hold the glass together if it does break. Laminated glass, if anchored well, can continue to act as a barrier after breakage. For more, see Chicago Window Expert’s article Safety Glass: Is It Really Safe?
Glass Strength
The glass opening measured 45 3/8″ x 51 1/4″ high, so it was 16.2 square feet. The size of the glass is a factor in its weakness. Larger glass must also be thicker to meet loads from wind and other sources.
According to the applicable building code, the 1961 Tulsa Building Code, the façade should have been designed to resist a wind load of 30 pounds per square foot, acting either inward or outward.
According to wind load charts used by the glass industry at that time, the glass was too thin and should have been at least 1/8″ thick. And that is before the glass was weakened due to weathering.
According to ASTM E1300-94, a more recent calculation method that factors in an allowance for 20 years of glass weathering, the incident glass is also found to be of insufficient strength. Furthermore, this standard predicts that under a 30 psf wind load, the incident glass would have deflected 0.82 inches.
we find that the incident single strength would not be considered strong enough by this standard.
Experts at Façade Consultants often perform such calculations during forensic analysis to determine whether glass failures stem from design flaws, material degradation, or installation issues. It is not unusual that multiple factors are found.
Glass Weathering
ASTM E1300, which sets standards for calculating glass strength, builds in a factor for weathering.
This factor only assumes weakening that occurs over 20 years of exposure.
The incident Hilberling glass was 46 years old, likely far weaker than it would be at 20 years of weathering.
It has long been known that glass exposed to weather loses strength over time. This is because wind, airborne pollutants, and water combine to degrade the exposed surface of the glass. We have all seen glass with a dulled outer surface on older buildings. This dulling coincides with weakened glass, especially in the outward direction, such as when it is pushed out from the inside or sucked outward by the wind.
Depending upon the amount of degradation, the breaking strength can be reduced as much as 50% (Merritt). In a laboratory study performed by Façade Consultants on 50-year-old window glass removed from a building where a small child broke through the glass and fell to his death, the breaking force was reduced 61% compared with new glass. That glass would break easily in the presence of moderate impact.
Glass Gasketing
Glass strength calculations are typically based on the assumption that the edges of the glass are firmly grasped and supported. The industry terminology is “glazed in a stiff, weathertight rabbet”. Such was not the case at University Club Tower. The interior glazing gaskets were loose and shrunken back, sometimes over an inch at each end.
Some of the glass fragments can be seen hinging outward, still clinging to the frame with flexible sealant.
This allowed the glass to rotate out of the glazing pocket during impact or pressure, further weakening the glass. The outer edge of the glass was bedded in old sealant or putty, and then later over-caulked again. This did not create resistance to the force, but instead allowed the glass shards to swing around the frame to the exterior, like an open door.
The Window Position
The top of the glass was 6 feet 5 inches above the floor. This would be just above Josh Hilberling’s head. Much of his center of mass would be squarely into the center of the glass. According to Amber, Josh weighed 220 lbs.
The stick is slanted to indicate a possible angulation of his body as he was progressing toward the window. Amber Hilberling told Mark Meshulam testimony which was not heard in court, “he tripped over his own big feet”.
The bottom of the glass was 26 1/4 inches above the floor. According to Amber Hilberling, Josh was 6 feet 4 inches tall. For such a tall man, the short wall beneath the window probably ended near his knee and amounted to a tripping hazard rather than a safety barrier.
When I visited Amber on December 10, 2012, I asked her how Josh could have traveled backward 4 to 6 feet before he hit the window. She answered with a resigned, ironic comment that I remember clearly to this day, “He tripped over his own big feet.”
The Window Blinds
and the blinds are being sucked out of the window and around the jamb, indicating the wind forces.
The PVC vertical blinds were fully closed at the time of the incident, which was in the late afternoon. The inner face of the glass was only about 2 inches to 3 inches behind the closed blinds. Once Josh’s trajectory brought him into contact with the blinds, he immediately went further into the glass and quickly fell through the window.
I believe the blinds were closed due to the heat wave (see next section) and the time of day. The afternoon sun would have been blinding in the apartment.
Heat wave and the greenhouse effect
In the days leading up to the incident, Tulsa was experiencing a heat wave. There was a rash of water main breaks that were attributed to the hot weather. When tall buildings get hot, the warm air rises through elevator shafts and stairwells, just like smoke in a chimney.
This “chimney effect” results in an outward pressure on the glass in the upper portions of the building. We don’t know how much outward pressure was applied to the glass, but given the heat, full sun exposure, and time of day, the chimney pressure could have been significant.
Wind pressures and building geometry
On the tragic day, the wind speed at 33 feet (10 meters) above ground level – this is how all official weather data is collected – was about 18 mph, gusting to 23 mph. At the incident 25th floor (over 200 feet above the ground), the winds were likely much faster and stronger. The wind came out of the SSE. In that direction, there is nothing to block the wind all the way to the Gulf of Mexico (450 miles).
You may know that negative air pressures can be strong enough to lift an airplane off the ground. These pressures occur when moving air contacts the rounded front edge of the airplane wing. The air deflects up and over the wing, creating a pocket of negative air (sometimes referred to as a vacuum) on top of the wing, which causes the wing to rise. This “lift” is how planes fly. “Lift” happens on buildings, too, around the edges.
Turn the rounded front edge of the airplane wing vertically, and you have the façade of the University Club Tower. I ran a very conservative wind simulation using only a 23 mph wind blowing at the building from the SSE. One trial I conducted showed a 46.5-pound outward load on the incident window glass.
We know the wind created outward pressures on the glass from a news photo. The remaining slats of the vertical blinds are blowing through the window glass to the outside. We also know that the wind was blowing around the façade to the north because the slats of the blinds that broke away were found on the ground about 60 feet north of the window. The wind even affected Josh’s descent, causing him to fall about 10 feet to the north of the window. (Apologies to all for this horrible detail.
The condition of the glass in Amber’s words
I asked Amber if the window appeared to resist the impact. In her words, “No. It was like the glass was not even there.” She said the sound was that of the breaking of a “small glass bowl.” I asked about the condition of the windows. She said the windows were old. They leaked air. The glass rattled in the frame. She knew of no prior cracks in the incident glass. Considering the chimney effect and outside wind, the rush of air through the window opening could have carried some of the sound waves away from Amber, diminishing the sound that reached her ears.
The concrete floor and sound transmission
The resident who lived downstairs reportedly testified that he heard a noise in the ceiling that sounded like someone running toward the window, followed by the sound of glass breaking. The news report stated it this way: “a neighbor who testified he heard running and a loud crash, followed by a woman screaming.” This gave rise to a theory that Amber butted Josh into the window after a run across the floor. If the neighbor had his window open, I would find the perception of a crash and screaming to be credible. However, the part about hearing running has never rung true. Recall that the building design was of concern structurally.
It was the first building designed using a computer for structural calculations. There was concern about the building swaying. Now, take a look at the picture of the concrete beam surrounding the building, which supports the floor slab. The exposed beam below the floor slab is 18” deep and 13” tall, plus the floor thickness above the beam. By contrast, nowadays we see floor slabs that are 8”-9” thick with no added beam to support it. The floor was carpeted. Amber wore “slip-on tennis shoes”. I do not believe that, given all these factors, the downstairs neighbor could have heard anything from the domestic scuffle through the floor.
Summary
The Hilberling and Whitlock families suffered a fatal set of circumstances that came together on that fateful day. It started with a domestic dispute, which brought Josh Hilberling in direct contact with weak, unsafe, poorly supported glass in a poorly maintained building on a day when chimney pressures were already pushing the glass out, wind pressures were sucking the glass (and him) out, and the short kneewall beneath the window added to whatever tripping may have been in progress.
Josh Hilberling’s window fall was actually a domino effect of dangerous factors. As happens so often in death and injury cases, factors piled upon one another, creating a catastrophic result. The later death of Amber while she was incarcerated seems to be a continuation of the falling of evil dominoes that have plagued the families. I wish for all family members, and especially son Levi, that the horror and sadness will finally end and that healing and building can begin.
At Façade Consultants, we view such cases as reminders of why façade integrity and safety glass standards are essential. Weak or outdated glazing systems are silent hazards that must be addressed.
Our mission remains clear: to help clients, investigators, and attorneys understand the real causes of façade failures and prevent future tragedies through forensic investigation, evidence, and experience.
FAQs
What causes glass to break in high-rise buildings?
Glass breakage in high-rise façades is often the result of a combination of factors, such as weakened glass, aging materials, improper glazing, thermal stress, or structural movement. At Façade Consultants, our forensic investigations sometimes reveal that even sealant deterioration or worn gaskets can create vulnerabilities that can contribute to catastrophic failures over time.
How does weathering affect the strength of window glass?
Over the years, exposure to wind, pollution, and moisture, especially on façades facing harsh environmental conditions, causes glass to lose its strength. Façade Consultants has tested decades-old glass samples and found a reduction in strength of up to 60% compared with new glass. Glass weathering and degradation makes older buildings particularly susceptible to impact-related failures.
What role does curtainwall maintenance play in building safety?
Regular curtainwall inspection and maintenance are crucial to preventing façade failures. Building owners and managers may be unaware of cracks or failing sealants, which can compromise performance. Façade Consultants helps property managers identify these issues early, ensuring their façades remain safe, airtight, and structurally sound.
Can non-safety glass still be found in occupied buildings today?
Yes. Many older high-rise buildings still contain annealed, non-safety glass that does not meet modern impact standards. Façade Consultants frequently encounter such outdated glazing in buildings constructed before safety glass regulations became widespread, and we advise immediate assessment which may lead to retrofit with tempered or laminated glass, or the use of a safety film.
What should building owners do if they suspect façade or glass problems?
If you notice drafts, leaks, rattling windows, unusual glass breakage or visible façade deterioration, contact Façade Consultants for a professional evaluation. Our team provides testing, diagnostics, and expert reports that pinpoint the root cause, helping owners prevent accidents, reduce liability, and maintain the long-term safety of their buildings.
