Burglar resistance of glazed building elements
Due to its aesthetic characteristics, glass is a material of choice for architects and widely used in the construction and renovation of buildings. Transparent and elegant, glass optimizes visibility and natural light, improving interior comfort for every building user.
In addition to its aesthetic appeal, glass can also fulfill other functions. Depending on its composition, a glazing solution1 can provide sound and thermal insulation, and control heat gains from solar radiation.
Glazing is capable of providing not only fire protection but also high security: bullet-resistant protection, resistance to explosive blasts, and resistance against forced entry. It is that latter resistance that we focus on here.
Due to the diversity of methods used by criminals (axes, screwdrivers, drills, hammers, ram cars, etc), how can a glazing system resist break-in? What standards are applicable?
First, we must understand one essential characteristic of glass, that it breaks. After an impact, a glazing system will show cracks, but may resist break-in by holding firmly in place with no openings through it. The breakages may be large and sharp, targeted around the impact, or tiny and non-cutting in the case of tempered glass (for more information: tempered section).
Second, a glazing system cannot be vandal-proof. This is physically and logically impossible because, after a certain time, a criminal with the right tools will manage to break through the glass. That notion of time is therefore important when talking about resistance to break-in.
There are two European standards for the burglar resistance of glazed building elements. EN 356, which measures only the glass, and EN 1627, which measures the complete glazed solution.
EN 356 subjects the glass to different levels of test to measure glass strengths. The lower levels provide only resistance to vandalism3 and, for breaking and entering, the appropriate resistance levels are P6B, P7B and P8B.
For the glass to reach level P6B, it must withstand at least 31 strikes from a hammer and axe. The glass layers may fracture or break but, to pass, the glass must keep its integrity and not develop an opening. For level P7B, the glass receives 51 strikes, and 71 for level P8B. In each case, it is a purely mechanical test, without any notion of resistance time.
But what happens if a burglar decides to attack the glass with a tool other than a hammer or axe?
The EN 1627 standard, which assesses the complete glazing system, defines several resistance levels from RC1 to RC6, with RC1 being the lowest level. In addition, there is a notion of resistance duration, which does not exist in EN356.
To reach RC1, the glazing system is tested against a whole tool package for a specified time, in this case the A1 package. To reach RC2, it is tested against two tool packages A1 and A2, and always for a certain duration. Each level is therefore more stringent, until the glazing system can resist the following six tool packages.
But there are two problems with this test regime. First, up to and including RC4, it is only the locks and frames that are tested, the glass itself is not touched!
How then can we ensure the reliability of the complete system?
The second problem is that, while the glazing is not tested up to class RC5, EN 1627 still combines a minimum required glass strength (EN 356) per level (RC1 to RC7).
Thus, to reach RC4 level, a minimum of P6B glass is required.
So, how can we be sure that the complete system is resistant to packages 1,2,3 and 4 below, when we have not tested the glass with these tools? Should hope that vandals will use only a crow-bar but won’t dare attack the glass directly?
These two problems are significant because it means we are offering “tested” solutions to the market, but which do not prove the reliability of the complete system. As any standard should be about providing complete reassurance to the building industry, we may inadvertently be installing unreliable glass solutions.
But what about classes RC5 and RC6?
To achieve these strength levels, the complete system is fully tested, including the glazed element. However, the standard recommends a minimum glazing resistance level, P7B and P8B (respectively for classes RC5, RC6). However, the P7B and P8B glasses have been tested with hammer and axe - tools different from the packages required by EN 1627 to achieve classes RC5 and RC6.
How could regulations be adapted to ensure the reliability of glass solutions?
To ensure optimal safety at each level, we should go beyond current standards and establish tests of all complete systems, including the glazed elements.
Like fire resistance, regulations should then only allow the sale of components that constitute a “complete system” with a Classification Report to certify the reliability of a glazed system, issued by an independent certification body, and following the successful completion of a resistance test of the entire system.
In this way, protection would be enhanced, better protecting both people and property.
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1 A glazing system consists of several glass panes
2 A "glass solution" or "complete system" consists of glass, a frame and a specific environment. The environment can be, for example, concrete, gypsum, or wood.
3 Vandalism is here defined as an act of unpremeditated destruction. Unlike breaking and entering, it is a spontaneous and improvised act.