Experimental investigation of plain CLT and CLT-concrete composite slabs subject to punching shear

To date, cross laminated timber (CLT) slabs have been gaining widespread adoption in the construction industry. In many cases, the significant capabilities of CLT slabs have been further enhanced with the use of concrete overlays acting compositely with the CLT section. In real applications, both CLT and CLT-concrete composite (CLTCC) slabs can be subject to concentrated load which may initiate punching shear behaviour. Whilst research into the flexural behaviour of CLT and CLTCC slabs is well evolved, by comparison much fewer investigations have been conducted on punching shear of CLT and no readily available studies exist for CLTCC. The present work aims to address this research gap by undertaking an experimental investigation of both CLT and CLTCC slabs subject to punching shear. A total of twelve 2-way simply supported slabs were tested to failure, including both plain CLT and CLTCC slabs with varying concrete-to-timber depth ratios (CTDR). For the CLTCC slabs, vertical coach screws were used to connect the CLT and concrete layers. The main aims were to evaluate the structural performance of CLTCC as a composite floor system, focusing on the punching shear behaviour, the role of CTDR, and the underlying failure mechanisms. In all cases, punching shear failure was observed, however all specimens involved a degree of ductility pointing towards their practical suitability. The CLTCC slabs with a CTDR of 60 % exhibited the highest punching shear resistance with a peak load 50 % higher than the corresponding plain CLT panels and a comparatively larger initial stiffness. Similarly, the tests revealed the chosen method of shear connection to be both effective and practical. The failure mechanisms were evaluated by classifying the structural behaviour into different stages. It was found that rolling shear failure in the CLT laminae controlled the end of elastic behaviour while further insights into the failure process were revealed by the post-test sectional cuts. Lastly, a simplified calculation method was proposed to provide a conservative estimate of the punching shear capacity of CLTCC slabs which may be suitable for design purposes after further numerical investigation to account for changes in geometry and boundary conditions reflective of real-life scenarios. The method provided a consistent lower bound prediction of load capacity, within the range of ∼70–85 % of the corresponding experimental value across the test series presented in this study. • First readily available experimental study on punching shear in CLT-concrete slabs. • Concrete overlays increased the punching shear resistance by up to 57 %. • Rolling shear observed to be pivotal in determining punching shear capacity.