Capacity model of CLT walls with openings and timber plasticization

This paper investigates the effect of timber plasticization on the ultimate lateral capacity of rocking Cross-Laminated Timber shear walls with different characteristics. Simplified capacity models predict a CLT wall ultimate capacity by multiplying the hold-down ultimate resistance by its distance to an assumed pivot point, often coincident with the panel edge. However, timber plasticization at the interface with the foundation can significantly reduce the lever arm of the hold down reaction. This paper aims at assessing the effect of timber plasticization and panel characteristics (geometry, width, presence of openings, etc.) on the position of the pivot point, expressed by the dimensionless lever arm τ, namely the ratio between the distance of the hold-down geometrical axis from the compressed timber resultant and the distance between the hold-down geometrical axis and the panel edge. An extended covariance-based sensitivity analysis of several Finite Element (FE) models, representing all the CLT shear wall design variability was carried out. The FE model, developed in Abaqus, has been calibrated on the experimental cyclic response of a CLT wall panel. Among the chosen parameters, the sensitivity analysis proved that the most significant ones affecting τ are two: the ratio between the hold-down reaction and the maximum resistance of the compressed timber, and the width of the opening. Accordingly, the authors estimated an elementary empirical regression equation based on the FE model results for predicting the dimensionless lever arm (τ) in the capacity equation of CLT panels. The use of this elementary formulation allows the designer to estimate with considerable accuracy the lateral capacity of a CLT panel given the panel geometry, the ultimate resistance of the hold-down, the compression strength of timber perpendicular to the grain and the vertical load. Almost all points used for the linear regression fall within the confidence bounds, thus proving the accuracy of the proposed empirical formula for the dimensionless lever arm.