Standard Practice for Determination of Laminated Glass Effective Thickness

Importancia y uso:

5.1 LG combines multiple plies of glass bonded with polymeric interlayers into a single lite. The structural response of the lite and the distribution of internal forces is dependent on the degree of shear coupling of each interlayer between the adjacent glass plies. Shear coupling in LG has a significant impact on structural performance and therefore requires rigorous engineering analysis.

5.1.1 Shear coupling is a function of the lite dimensions and LG section composition, applied loads, boundary conditions, and the interlayer shear relaxation modulus G_int. Evaluation of shear coupling is key to determining the flexural and torsional response of LG. Evaluation of shear coupling can be accomplished with the ET methods shown herein, or using other rigorous engineering analysis methods.

Note 1: This practice is not intended to restrict the use of finite element analysis (FEA) for LG evaluation.

5.1.1.1 LG with two plies, three symmetric plies, or n-equal plies each exhibit different shear coupling behavior, and therefore each case has different material properties and formulae for determining ET.

Note 2: Certain loading and boundary condition combinations included in this practice are limited to two-ply LG applications for calculation following the CBET method.

5.1.1.2 Interlayers are viscoelastic materials with load-duration and temperature-dependent shear relaxation modulus G_int determined from laboratory test data in accordance with Annex A4.

(1) Compliant interlayers with a low shear stiffness (G_int → 0) decrease composite performance and as a result the LG approaches the layered limit I_L for beams and D_L for plates.

(2) Non-compliant interlayers with a high shear stiffness (G_int → ∞) improve composite performance and as a result the LG approaches the monolithic limit I_M for beams and D_M for plates.

5.1.1.3 LG shall comply with Specification C1172.

5.2 The determined ET is used in a simplified engineering analysis for the evaluation of LG shear coupling in flexure and torsion. The process consists of defining effective section properties between bounding layered and monolithic limits and adopting ET results for use in a rational engineering analysis.

5.2.1 Different ET calculation methods are presented, and each method has been evaluated for each tabulated loading and boundary condition (Annex A1) to determine whether the method is appropriate for that loading and boundary condition within a margin. An appropriate method to calculate LG performance is not available in this practice for all loading and boundary conditions.

Note 3: Accuracy: If an ET model is not noted as applicable for a given loading and boundary condition, it is because the ET model yields inaccurate results when compared with detailed finite element modeling; however, the ET model may still be of aid for preliminary evaluation. The CBET method offers greater precision and is the only ET method specified for evaluation of certain loading and boundary conditions. Refer to Appendix X2 for comparison of ET methods with numerical analysis results in example problems and referenced literature.

Note 4: Ease of Use: The calculation may be completed with fewer terms using the ST and EET methods than the CBET method, where available.

5.3 ET models assume uniform transverse displacement across glass plies without crushing or extension of the interlayer. Low interlayer moduli influences transverse compression or extension of the interlayer that is not addressed by ET models and changes the distribution of forces within the section. This phenomenon has been observed in LG with cantilevered and point-supported boundary conditions in laboratory tests (19), and detailed finite element numerical analysis (3). The specifier shall validate that the interlayer has adequate stiffness to minimize axial deformation under loading for the application of this practice.

5.4 Critical buckling of in-plane loaded beams may be flexural or lateral-torsional in nature. Therefore, unique ET values for flexure and torsion shall be used for evaluation of critical buckling of an in-plane loaded beam.

5.4.1 Stress concentrations due to notches, holes, irregular and point supports are not addressed by this practice.

5.5 Geometrically non-linear plate behavior occurs where center-of-glass deflections exceed half of the deflection-ET ¹/₂ h_ω under uniform loading. Use of plate ET for deflections exceeding half of the deflection-ET is conservative.

Note 5: Practice E1300 is more efficient for evaluation of non-linear plate behavior and should be used to evaluate symmetric 2-ply LG plates supported on 2-, 3-, and 4-edges with uniform loading for design applications within its scope. Evaluation with FEA modeling may also be considered.

5.6 Plies are assumed to be free to slip relative to one another at lite edges for all loading and boundary conditions except at edges where a fixed-edge (encastre) restraint is indicated.

Subcomité:

E06.52

Volúmen:

04.12

Palabras clave:

annealed glass; conjugate beam; effective thickness; enhanced effective thickness; glass; heat-strengthened glass; laminated glass; structural glass; tempered glass;