Engineering Fundamentals | PA-EF-002
What Is Forced-Entry Resistant Glass?
Forced-entry resistant glass is an engineered transparent glazing system designed to delay or resist unauthorized entry through a protected opening.
Overview
Unlike ballistic glazing, which is designed around resistance to specific projectile threats, forced-entry glazing is intended to remain an effective barrier after repeated physical attack. Depending on the application and test standard, that attack may involve blunt-force impacts, sharp tools, repeated strikes, or other forms of mechanical assault.
The purpose of the glazing is not necessarily to remain unbroken. In many successful forced-entry systems, the glass fractures early in the attack. The critical engineering objective is for the laminated assembly to remain intact and continue resisting the creation of an opening large enough to permit passage.
How Does Forced-Entry Resistant Glass Work?
Forced-entry resistant glazing is typically constructed from multiple layers of glass bonded together with high-performance interlayers.
During an attack, the glass may crack or break, but the interlayers continue to hold the fractured material together and help the laminate remain in place.
As the attack continues, the glazing must resist repeated attempts to tear, displace, penetrate, or create an opening through the assembly.
This is one of the most important differences between ordinary glass and engineered forced-entry glazing:
The glass does not need to remain visually intact in order for the security system to continue functioning.
The laminate is designed to remain a barrier even after significant visible damage has occurred.
Engineering Principle
In forced-entry glazing, fracture is not necessarily failure. The system fails when the attacker is able to create the opening or passage defined by the applicable test method.
Forced Entry Is Different from Ballistic Resistance
Forced-entry resistance and ballistic resistance address different threats.
A ballistic event involves a very short-duration, high-energy impact from a projectile. Forced-entry attack generally involves repeated mechanical assault over a longer period of time.
Because the loading conditions are different, a glazing system designed for one threat should not automatically be assumed to provide equivalent performance against the other.
Some laminate constructions can be engineered to provide both ballistic and forced-entry resistance, but the required performance must be evaluated independently against the applicable standards.
A ballistic rating does not automatically establish a forced-entry rating, and a forced-entry rating does not automatically establish ballistic performance.
The Importance of the Interlayer
Interlayers play a particularly important role in forced-entry resistant glazing.
Once the glass begins to fracture, the interlayer helps retain broken glass, maintain continuity across the laminate, and resist the attacker’s attempts to create an opening.
Different interlayer technologies provide different mechanical characteristics.
Depending on the construction and performance objective, forced-entry laminates may use:
- Polyvinyl butyral (PVB)
- Urethane interlayers
- Ionoplast interlayers
- Other engineered interlayer systems
The selection of interlayer type, thickness, and overall laminate construction influences how the system responds to repeated impact and deformation.
For this reason, forced-entry performance should not be evaluated by glass thickness alone.
Why Broken Glass Can Still Provide Protection
To someone unfamiliar with security glazing, a heavily fractured laminate may appear to have failed.
That visual assumption can be misleading.
Laminated security glazing is designed so that fractured glass remains bonded to the interlayer system. Even after extensive cracking, the assembly can continue to resist penetration, tearing, displacement, and passage.
This is why forced-entry testing focuses on the ability of the attacker to create a defined opening rather than simply on whether the glass breaks.
The condition of the glass surface after impact is therefore not, by itself, an indication of whether the security system has passed or failed.
Forced-Entry Testing Evaluates Delay
Forced-entry resistant glazing is intended to create time.
The objective is to delay an attacker long enough for occupants to react, security procedures to be implemented, or law enforcement and other responders to arrive.
Different standards evaluate this resistance using different attack sequences, tools, impacts, and pass/fail criteria.
ASTM F1233 evaluates security glazing through defined sequences of physical attacks using specified tools and methods.
ASTM F1915 addresses glazing used in detention facilities, where resistance to physical attack, containment, and the demanding conditions associated with correctional environments are important design considerations.
ASTM F3561 evaluates forced-entry resistance of fenestration systems following simulated active-shooter weakening. Under that standard, projectiles are used to weaken the system before mechanical impact testing begins. The standard is not a ballistic-resistance rating; the projectile impacts are intended to simulate weakening before the subsequent forced-entry assault.
This distinction is important because different forced-entry standards address different threats, environments, and performance objectives. A rating should therefore always be understood in the context of the specific test method used.
Glazing and the Complete System
Forced-entry resistance is especially dependent on the complete opening.
The glazing may be extremely resistant to repeated attack, but the overall system can still fail if the frame, glazing stops, anchorage, hardware, or surrounding structure does not provide comparable resistance.
Factors such as glazing-system size, frame bite, frame integrity, stop configuration, the glazing-to-frame interface, and anchorage to the surrounding structure can influence system performance.
For this reason, specifying forced-entry resistant glass without considering the surrounding framing system does not necessarily create a forced-entry resistant opening.
Common Forced-Entry Resistant Constructions
Forced-entry resistant glazing can be engineered in several ways depending on the required attack resistance, architectural requirements, weight, thickness, and other project objectives.
Common approaches may include:
- Multi-ply laminated glass
- Laminates incorporating thicker structural interlayers
- Glass and polycarbonate constructions
- Forced-entry systems combined with ballistic protection
- Forced-entry insulating glass units
- Application-specific laminated security constructions
The appropriate system depends on the threat, required delay, framing system, building use, and applicable test standard.
Forced Entry and Active-Shooter Glazing
The term active-shooter glazing is sometimes used broadly in the marketplace, but the engineering requirements should be understood carefully.
Some active-shooter security standards combine ballistic weakening with forced-entry attack rather than requiring the glazing to stop the projectiles.
ASTM F3561 is an important example. The test method is intended to simulate an attacker weakening a fenestration system with repeated shots before attempting mechanical forced entry. It is not used to establish a ballistic-resistant glazing rating.
Therefore, a system that performs successfully under an active-shooter forced-entry standard should not automatically be described as bullet-resistant unless it has also been separately tested and rated for the applicable ballistic threat.
Continue Learning: PA-EF-006 — Active-Shooter Glazing Explained
How Is Forced-Entry Performance Evaluated?
Forced-entry standards define the attack method and the condition that constitutes failure.
Depending on the standard, the evaluation may consider:
- Tool type
- Impact energy
- Number of strikes
- Attack sequence
- Duration
- Attack location
- Size of the opening created
- Ability to pass a defined test shape through the opening
- Performance of the complete frame-and-glazing system
The resulting rating therefore describes performance under a specific standardized test.
It should not be interpreted as a guarantee that the system will resist every possible real-world attack for a specific amount of time.
Where Is Forced-Entry Resistant Glass Used?
Forced-entry resistant glazing is used where maintaining a transparent barrier and delaying unauthorized access are important security objectives.
Applications can include:
- Schools and educational facilities
- Government buildings
- Courthouses
- Financial institutions
- Retail and transaction environments
- Healthcare facilities
- Detention and correctional facilities
- Corporate facilities
- Critical infrastructure
- Secure vestibules
- Doors and storefront systems
- Transaction and observation windows
The required level of resistance and complete system design vary significantly depending on the intended application.
Forced Entry Is About the Complete Security Objective
The most appropriate forced-entry glazing system is not necessarily the thickest or most complex construction.
Engineers must consider:
- Expected attack method
- Required resistance or delay
- Applicable test standard
- Door or window configuration
- Frame and anchorage design
- Panel size
- Optical requirements
- Weight
- Architectural appearance
- Thermal performance
- Maintenance
- Other security requirements, including ballistic protection where applicable
A successful design balances these objectives within a complete tested or engineered system.
Key Takeaways
- Forced-entry resistant glass is designed to remain an effective barrier after repeated physical attack.
- The glass itself may fracture while the laminated assembly continues to provide protection.
- Fracture does not necessarily mean failure.
- Interlayers play a critical role in retaining broken glass and maintaining continuity after impact.
- Forced-entry resistance and ballistic resistance address different threats and should not be treated as interchangeable ratings.
- Active-shooter forced-entry standards may use gunfire to weaken a system without requiring the glazing to stop the projectiles.
- The frame, anchorage, glazing retention, and surrounding structure are critical parts of forced-entry performance.
- Test ratings describe performance under defined laboratory conditions and should be interpreted in the context of the applicable standard.
Continue Learning
- PA-EF-005 — Bullet-Resistant vs. Forced-Entry Resistant Glazing Compare the different threats, engineering objectives, and testing approaches behind ballistic and forced-entry systems.
- PA-EF-006 — Active-Shooter Glazing Explained Understand how active-shooter forced-entry systems are tested and why ballistic weakening is different from ballistic resistance.
- PA-TS-002 — Understanding ASTM F1233 Explore one of the established test methods used to evaluate forced-entry resistant security glazing.
- PA-TS-003 — Understanding ASTM F3561 Learn how fenestration systems are evaluated after simulated active-shooter weakening and subsequent mechanical attack.
- PA-TS-004 — Understanding ASTM F1915 Learn about security glazing requirements for detention and correctional applications.
- PA-ED-002 — Every Layer Has a Job Learn how glass, interlayers, polymers, and other components contribute to the performance of a complete security glazing system.
