Windows Shattering in Dubai - NiS Problems?

Shattered windows spark safety fears: DUBAI: Windows in high-rise apartment blocks are spontaneously breaking because of a combination of shoddy workmanship and inadequate building regulations, construction experts say. “I came home from work and it was smashed, and I’m not quite sure how or why,” said Nina, 38, a Briton, who found a half metre-long window damaged in her one-bedroom flat on the 36th floor of a tower in New Dubai. “On the ground there were a few splinters, but it was more the window being cracked while still in the frame.” According to experts, Nina was probably the victim of building contractors who had improperly fitted window panes. Large temperature fluctuations then caused the metal frames to expand and contract, straining the glass beyond its breaking point, they believe. The problem is compounded by insufficient building regulations with regards to window installation, they say, leaving standards to be decided by contractors and developers. “There are some incredibly bad practices – contractors who don’t know what they’re doing, putting things together in ways that are completely unacceptable,” said Tom Bell-Wright, founder and owner of Thomas Bell-Wright International Consultants, which specialises in building facades. “As far as regulations are concerned about installing windows and glass, there aren’t any.”



Asked if such regulations were maintained by Dubai Municipality, Kamal Azayam, a mechanical engineer who works in the qualifications and building studies section, said: “As far as I know, nothing.” Such decisions were not overseen by the Government, he said, but instead depended “on the requirements of the consultants”. Christine Stewart (pictured) is baffled as to why a glass panel in the lounge of her apartment suddenly shattered into dozens of pieces on Saturday. “Nothing had hit it, nothing had happened,” said Ms Stewart, 46, a Briton who works in the media. “It just literally went, and people were sitting nearby. It could have hit them, but luckily it didn’t. “What worries me is what caused such a thing to happen, is it going to happen again, and how safe are our apartments.”
When Barti Makhijani returned to her 28th-floor apartment in Dubai Marina recently, she found one of the five window panes in her bedroom, which run from floor to ceiling, was cracked in numerous places, but still intact. “It looked at first glance like it was raining on just that window,” said Mrs Makhijani, 30, an Indian who lives with her two children. “It was shattered, as if a large stone had hit and some sort of ripple effect all along that centre pane. From what I understand, it was the outside pane that was shattered. “I’ve got children around, and what if somebody throws something at that window like a ball because I’m not always in to guard the window?” The cost of replacing the window, about Dh12,000 (US$3,200), was borne by Mrs Makhijani’s insurance. But the men who did the repair told her “it wasn’t the first flat in the building that experienced it”. Paul Rogers, managing director at Eminent Surveyors and Loss Adjusters, described Mrs Makhijani’s window as “frosting up”. This happens when windows are installed without the necessary “wedges”, which act as shock absorbers. This exposes them to greater amounts of thermal pressure, making them more prone to buckling. “It’s mostly the outside pane that shatters,” he said. “If the inside shatters, that’s serious because it means the glass was fitted the wrong way.”

THERMAL BREAKS
Glazing that absorbs solar radiation is liable to break due to thermal shock. Thermal shock is caused by too great a difference in thermal expansion between one section of glazing and another (see diagram below). The center of the glass exposed to the sun heats up much more rapidly than the part of the glass which is shaded or enclosed within the frame. The resulting solar-induced thermal stress can, if the critical limit is reached, cause the glass to break. Various factors can cause thermal breaks, including:
• The geographical position of the building, facade orientation, window placement (vertical or slope);
• Interior factors, such as air conditioning;
• Glass-related factors, e.g. tinted glass;
• Shading patterns.
Thermal tempering and heat strengthening increase the resistance of glass to thermal breaks.

TEMPERED SAFETY GLASS: Glass, by its very nature, is considered a fragile material because there is always a certain number of virtually invisible surface imperfections. The most common imperfection is micro-cracking. When subjected to stress, the cracks open, expand and eventually cause the glass to break. To overcome this drawback, glass is subjected to a special heat-treating process that locks the surface in a state of compression to helps and prevent cracking. Heat treating consists in heating the glass to just below the softening point (620°C / 1200°F) and then cooling it rapidly and uniformly. During the cooling process, the surface layers of the glass harden and contract, while the central core, which is still warm, stays viscous and follows the movement of the contraction. As the core cools and hardens, the rigid surface layers lock it in, compressing the surface of the glass. When the core is completely cooled, the surface of the glass is locked in a state of compression and the central core in a state of tension. As a result, surface scratches or cracks will be subjected to compression, which in turn causes them to close. The glass is therefore stronger. The mechanical strength and thermal endurance of tempered tempered glass is four to five times higher than that of annealed glass. Thermally tempered glass shatters into a multitude of small fragments when broken and, therefore, cannot be cut or edged after being tempered. Because these processes would destroy the glass, they are carried out prior to tempering.

The process of tempering float glass can cause a nickel sulfide inclusion to change from its normal state (known as a low-temperature structure) to a different high-temperature, crystalline structure. When the glass is cooled quickly (as part of the process) the NiS particle is unable to change back to its original form. Over a certain period of time NiS will slowly convert to the original low temperature structure. This means the NiS increases in size, and the mechanical stresses caused by this cause the tempered pane to shatter, for no apparent reason (hence spontaneous glass breakage).

NiS and spontaneous breakage: In various situations fully-tempered glass may break for no reason. Many factors might cause such spontaneous breakages, but the most common are nickel sulphide inclusions. Nickel sulphide inclusion, also known as NiS, occurs during the manufacturing process for float glass. In the glass batch, nickel-rich contaminants such as stainless steel might be present, and then combine with sulphur form nickel sulphide inclusions. Glass manufacturers take extraordinary steps to minimize the potential for nickel sulphide inclusions. Considering that a large furnace may produce up to 600 tons of glass per day, total elimination of contaminants is extremely difficult. When glass is heat-treated during successive stages, in order to obtain fully-tempered glass, nickel sulphide inclusions change in size from what is known as a low-temperature structure to a high-temperature, crystalline structure. When cooled quickly the NiS particle is unable to change back to its original form. Over a certain period of time NiS will slowly convert to the original form (LT) phase with an increase in volume of about 2 - 4%. Such increase of NiS will cause glass breakage.

The Achille Heel of Toughened Glass: nickel sulphide inclusions- More here.