EN 16613 Interlayer Stiffness Families Explained
What Is EN 16613?
EN 16613:2019 (Glass in building — Laminated glass and laminated safety glass — Determination of interlayer mechanical properties) is the European standard that defines how to characterise and classify polymeric interlayers for use in laminated glass design.
Its key purpose is to provide a standardised framework for specifying interlayer stiffness under design-relevant loading conditions. This allows structural engineers to select and verify interlayer materials without needing to run their own material tests for every project.
Key principle: EN 16613 recognises that interlayer stiffness is not a single number. It varies with load duration and temperature. The standard defines 11 specific (duration, temperature) pairs that represent the most common design scenarios for laminated glass in buildings.
The 11 Standard Loading Conditions
EN 16613:2019 Table 4 defines the following loading conditions. Each represents a realistic design scenario that engineers must check:
| # | Condition | Duration | Temperature | Category |
|---|---|---|---|---|
| 1 | Wind gust | 3 s | 35°C | Wind |
| 2 | Wind gust (cold) | 3 s | -20°C | Wind |
| 3 | Wind storm | 10 min | 35°C | Wind |
| 4 | Wind storm (cold) | 10 min | -20°C | Wind |
| 5 | Balustrade | 30 s | 30°C | Balustrade |
| 7 | Maintenance load | 30 min | 40°C | Imposed |
| 8 | Snow (3 weeks) | 3 w | 0°C | Snow |
| 9 | Snow (5 days) | 5 d | 20°C | Snow |
| 10 | Climatic (6 hours) | 6 h | 40°C | Climatic |
| 11 | Climatic (12 hours) | 12 h | 60°C | Climatic |
| 12 | Permanent load | 50 yr | 60°C | Permanent |
Note: Condition 6 (soft body impact on balustrade) is not included because it requires dynamic analysis rather than a quasi-static shear modulus evaluation.
Why these specific conditions?
Each condition represents a worst-case combination of duration and temperature for its loading category. For example, wind gusts are checked at 35°C (summer conditions when interlayers are softest under short loads) and at -20°C (winter, when glass is more brittle but the interlayer is very stiff). Permanent loads are checked at 60°C because roof-mounted or sun-exposed glass can reach high temperatures over decades of service.
Stiffness Family Classification
EN 16613 groups interlayer materials into stiffness families based on their shear modulus at the standard conditions. A material belongs to a given family if its stiffness meets or exceeds the family threshold at every relevant condition.
The classification system allows manufacturers to declare a stiffness family for their product, and engineers to use the family's guaranteed minimum values for design without needing full material characterisation data. This simplifies the specification process significantly.
Family 0 vs. Family 1 vs. Family 2
- Family 0: No shear transfer assumed. The interlayer is treated as having negligible stiffness. This is the conservative fallback for materials without characterisation data.
- Family 1: Partial shear transfer under short-term and medium-term loading (wind, balustrade, snow). The interlayer provides meaningful structural contribution for these scenarios.
- Family 2: Significant shear transfer across all loading conditions including long-term (climatic, permanent). Only the stiffest interlayers (typically ionomer/SGP) qualify.
Design Implications
The stiffness family determines how the laminated glass element is modelled in structural calculations per EN 16612:
- Family 0 interlayers require designing the glass as a layered system (no coupling between plies). This results in thicker glass and higher cost.
- Family 1 interlayers allow the engineer to account for partial composite action under wind and imposed loads, reducing glass thickness requirements for these load cases. However, permanent and climatic loads must still be checked without interlayer contribution.
- Family 2 interlayers provide composite action even under long-term loading. This enables the thinnest glass solutions and is essential for applications like structural glass beams, fins, and heavily loaded facades.
Cost impact: Moving from Family 0 to Family 1 can reduce glass thickness by 20-30% for wind-governed designs. Moving to Family 2 enables applications that are structurally impossible with lower-family interlayers.
Common Interlayer Performance
The table below shows typical shear modulus values for common interlayer types at selected EN 16613 conditions. These values illustrate the enormous range of stiffness behaviour across materials and conditions.
| Material | Wind gust (3 s, 35°C) | Maintenance (30 min, 40°C) | Permanent (50 yr, 60°C) | Typical Family |
|---|---|---|---|---|
| PVB Clear | 0.5 MPa | 0.2 MPa | 0.06 MPa | 0 |
| PVB Extra Stiff | 5.8 MPa | 0.6 MPa | 0.16 MPa | 0-1 |
| Ionomer (SGP) | 102 MPa | 11.4 MPa | 0.74 MPa | 2 |
| EVA | 4.2 MPa | 2.4 MPa | 0.88 MPa | 0-1 |
The difference between PVB Clear (0.5 MPa) and ionomer (102 MPa) under a wind gust represents a factor of 200 in interlayer stiffness. This difference directly translates to dramatically different structural behaviour of the laminated glass.
Practical Workflow
For a practising engineer, the typical workflow is:
- Identify the governing loading conditions for your application (e.g., a facade in Northern Europe: conditions 1-4, 8-9, 10-12).
- Obtain the interlayer shear modulus at each relevant condition. Use manufacturer datasheets, the EN 16613 stiffness family classification, or characterisation data.
- Calculate the effective thickness of the laminated glass per EN 16612, using the shear modulus to determine the degree of coupling between glass plies.
- Verify that stresses and deflections meet the requirements of EN 16612 for each loading condition.
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