Shot Bag Impact Testing for Curtain Wall Glass: Standards, Methods, and Safety Compliance

Shot bag impact testing is a fundamental safety performance evaluation for curtain wall glass, simulating the effect of a human body striking glazing under real-world conditions. This article explains the test principle, applicable standards such as GB 15763.2, GB 17841, JGJ 102, and EN 12600, specimen requirements, test procedures, and acceptance criteria used by accredited third-party laboratories. Understanding this test is essential for architects, facade engineers, and glass manufacturers seeking to verify that safety glazing in building envelopes resists impact or breaks in a non-hazardous manner.

Technical Overview

Shot bag impact testing, also known as pendulum bag impact testing, is a destructive safety test used to evaluate the impact resistance and post-breakage behavior of safety glazing products installed in curtain wall systems. The test simulates the loading condition of a relatively soft but heavy object, representative of a person falling or colliding against a glazed panel, by swinging a leather bag filled with lead shot against a rigidly mounted glass specimen. This is fundamentally different from hard-body impact tests using steel balls, because the soft, deformable impactor distributes load over a larger area and reproduces the dynamics of a human body impact more realistically.

For curtain wall applications, the test addresses one of the most serious safety risks in building envelopes: the possibility that glazing shatters and allows the impacting body to pass through, causing fall injuries at height or severe lacerations from sharp fragments. In China, the shot bag impact test is a core requirement of GB 15763.2 (Safety glazing materials in building – Part 2: Tempered glass), GB 17841 (Tempered and heat-strengthened glass used in curtain walls), and JGJ 102 (Technical code for glass curtain wall engineering), while GB/T 21086 for curtain wall systems references these safety glazing requirements. International equivalents include EN 12600 in Europe, which uses a twin-tire pendulum impactor, and ANSI Z97.1 and 16 CFR 1201 in the United States, which also employ shot bag impactors. Related heat-treated glass specifications such as ASTM C1048 reference these impact performance requirements.

• Impactor type: soft-body pendulum impactor (leather bag filled with lead shot)
• Primary purpose: verify safety glazing performance of tempered and laminated glass for curtain walls
• Key standards: GB 15763.2, GB 17841, JGJ 102, GB/T 21086, EN 12600, ANSI Z97.1, 16 CFR 1201
• Test type: destructive type test performed on representative specimens by accredited third-party laboratories

Testing Samples

Monolithic Tempered Glass

Tempered glass is the most common safety glazing product evaluated by this test. When properly tempered, the glass either withstands the specified impact energy or fractures into small, relatively harmless dice-shaped particles rather than large sharp shards. Specimens are typically cut to 610 mm × 610 mm square panels and taken from normal production to represent the actual tempering process.

Laminated Safety Glass

Laminated glass specimens, consisting of two or more glass plies bonded by a polymer interlayer such as PVB or SGP, are tested to confirm that even if the plies crack, the interlayer retains the fragments and prevents the impacting body from passing through. This is particularly important for overhead glazing, balustrades, and high-level curtain wall panels where glass fallout must be prevented.

Heat-Strengthened Glass and Composite Constructions

Heat-strengthened glass alone is generally not classified as safety glass, but it is widely used as a ply within laminated or insulating glass units for curtain walls. Test programs often evaluate the complete laminated construction, since the impact response depends on the combination of glass thicknesses, interlayer type, and heat-treatment level rather than any single component.

Insulating Glass Units and Project Assemblies

For insulating glass units, testing is normally performed on the individual glass lights or on representative mock-up assemblies when project specifications or designers require verification of the complete unit. Conditioned specimens should be free from edge damage and should be tested within a defined period after cutting and processing to avoid strength degradation from edge flaws or moisture exposure.

Testing Items

The shot bag impact test program typically covers the following evaluation items:

• Impact resistance at the specified drop height, commonly 1200 mm for tempered safety glass used in curtain walls under Chinese standards
• Breakage mode and post-impact integrity, including whether the impactor passes through the specimen
• Fragmentation characteristics, assessed as a companion test by counting particles within a 50 mm × 50 mm reference area after breakage
• Retention of glass in the frame or support rebates after impact, with no detachment of large fragments
• Performance classification according to the applicable standard, based on drop height and breakage behavior

Testing Methods

Test Apparatus

The test rig consists of a rigid steel support frame, a pendulum suspension system, and the shot bag impactor. Under GB standards, the impactor is a leather bag with a total mass of approximately 45 kg filled with fine lead shot, suspended by steel wires so that it can be hoisted to a controlled height and released to swing freely against the specimen center. The American ANSI Z97.1 method uses a similar 100 lb (45.4 kg) shot bag. The frame must be sufficiently stiff that frame deformation does not absorb a significant portion of the impact energy, and the drop height is measured as the vertical travel distance of the bag center of gravity.

Specimen Mounting and Conditioning

Specimens of 610 mm × 610 mm are conditioned to standard laboratory temperature, typically around 20 °C ± 5 °C, before being clamped securely on all four edges in the rigid test frame. A sufficient number of specimens, usually at least three to four panels, is prepared so that the statistical evaluation rules of the standard can be applied.

Test Procedure

The shot bag is raised to the specified drop height, held by a release mechanism, and then released so that it swings down and strikes the geometric center of the specimen in a single free-swing impact. The height is selected according to the product type, glass thickness, and required performance class. After each impact, the specimen is examined to determine whether breakage occurred, whether fragments detached or fell from the frame, and whether the bag penetrated the opening. If breakage occurs, the fragmentation pattern is documented and compared against the particle-count and fragment-size limits of the standard.

Acceptance Criteria

For tempered safety glass, acceptance generally requires that specimens withstand the specified drop height without breakage, or, where breakage is permitted under the statistical evaluation rules of the standard, that the glass fractures into small particles meeting the fragmentation limits with no large hazardous shards. For laminated safety glass, the specimen must not allow the impactor to pass through, and cracked glass must remain retained by the interlayer. Under EN 12600, results are classified using drop heights of 190 mm, 450 mm, and 1200 mm with a twin-tire pendulum, producing classifications such as 1C1 or 1B1 depending on breakage mode. Under ANSI Z97.1, performance classes are assigned based on impacts from 18 in (457 mm) and 48 in (1220 mm) drop heights.

Applications

Shot bag impact testing supports safety verification and regulatory compliance across a wide range of building applications:

• Unitized and stick-built glass curtain walls on high-rise commercial and residential towers, where safety glazing performance is a mandatory project requirement
• Safety glazing at critical locations, including entrance doors, sidelites, low-level vision panels, and glazing adjacent to walking surfaces
• Balustrades, guardrails, and full-height structural glazing where human impact loads are anticipated
• Overhead glazing, canopies, and skylights where glass fallout onto occupied areas below must be prevented
• Public buildings such as airports, stadiums, schools, hospitals, and transport hubs with high occupant density
• Factory quality control and product certification, where the test validates tempering furnace parameters, edge work, and lamination processes
• Forensic and failure investigations, comparing the impact performance of installed glazing against specification requirements after breakage incidents

FAQ

Why is the impactor a bag filled with lead shot instead of a solid mass?

The flexible shot bag deforms on contact and distributes the impact energy over a broad area, closely reproducing the loading characteristics of a human body. A rigid impactor would concentrate stress and produce fracture patterns unrepresentative of real accidental impacts.

What drop height is required for curtain wall glass?

Under Chinese standards, tempered safety glass for curtain walls is typically required to withstand impacts from a drop height of 1200 mm using the 45 kg shot bag. EN 12600 and ANSI Z97.1 use their own height series, such as 190 mm, 450 mm, and 1200 mm under EN 12600, to assign performance classifications.

What happens if the glass breaks during the test?

Breakage does not automatically mean failure. Tempered glass may be acceptable if it fractures into particles that meet the fragmentation limits under the statistical evaluation rules of the standard, while laminated glass must hold the fragments and prevent the bag from passing through. Large detached shards or penetration of the impactor constitute failure.

Can insulating glass units be tested as complete assemblies?

Yes, when project specifications require it, complete units or mock-up assemblies can be tested. However, type testing of the individual safety glass lights remains the standard approach for demonstrating compliance of the glass product itself.

How is this test different from hurricane or wind-borne debris impact testing?

Shot bag testing evaluates human-impact safety for glazing classification. Storm impact testing, such as the missile impact and cyclic pressure procedures of ASTM E1886 and ASTM E1996, uses different projectiles and loading protocols to simulate wind-borne debris, and the two should not be confused when specifying curtain wall performance.

Who should perform this test?

The test should be carried out by accredited third-party laboratories with calibrated pendulum impact equipment and documented competence in safety glazing standards, ensuring that results are accepted for product certification, tender submissions, and construction acceptance.