Graduate Thesis Or Dissertation
 

Cross-Laminated Timber-Concrete Composite Floor Systems for Tall Building Design

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/pn89df88z

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  • With growing interest in mass timber, especially mass timber panels (MTP), there has been a need to better understand their structural properties. One of the most versatile uses for MTP are as floor systems. Under new code provisions, these floors can be utilized in new and taller building types, but will require a noncombustible topping, generally concrete. This concrete topping is added for acoustic and fire performance but can be used to increase the structural performance as well by making the concrete composite with the MTP. Timber-concrete composites (TCC) have been investigated for almost a century, connecting together concrete and timber to form a stronger floor or deck system. The key to TCC is the shear transfer mechanism, which generates this composite action, shifting the neutral axis of the layers to increase the stiffness and strength. There are many different methods to transfer this shear and the characterization of existing connection types for new materials and development of new connections are an ever-present knowledge gap. Additionally, MTP are considered for use in two-way spans, rather than just spanning in one direction, and understanding their performance and methods for analysis and design are needed to realize their full potential. The first two phases of this project focused on shear connectors. The first portion involved characterizing the withdrawal response of different Self-Tapping Screws (STS) at varying angles to grain in MTPs and concrete. This was proposed as a rapid means to economically assess the withdrawal response of the STS for use in TCC, as STS are commonly installed at an angle to the interface, causing much of the response to be isolated into tension of the fastener. This tension loads the fastener in withdrawal from both the concrete and timber. One (1) fully threaded and two (2) partially threaded STS were characterized at both 45- and 90-degrees to the interface of three (3) different MTPs and concrete. The fully threaded fastener failed in tensile fracture of the screw at both angles for both V1 Douglas-Fir CLT and F16 MPP. All other tests with MTPS resulted in withdrawal failures of the fastener from the MTP. Withdrawing the head from the concrete resulted in cone pull-out failures of the concrete substrate, however the partially threaded STSs, with washer heads exhibited higher strengths. In all cases the withdrawal from the concrete was the limiting strength factor. The second phase involved the development and testing of three (3) alternative shear connectors. First, washer-screw, a grade 8 lag screw with an integral plate washer. Second, a built-up steel angle installed at the end of the MTP panel to arrest any interlayer slip. Third, a built up inverted-T steel section with stem perforations. An STS installed at 45-degrees was used as a baseline comparison. The washer-screw was tested at an individual fastener, push-off, and one-way bending level. The STS was investigated at the push-off and one-way bending level, while the other two connections were only tested in one-way bending. The washer-screw performed better than the STS in both push-off and one-way bending tests. The STS exhibited the lowest strength when compared to the alternatives in the one-way bending tests. The inverted-T section exhibited the highest stiffness in the one-way bending test, while the other two connections showed comparable stiffness to the STS. The third phase focused on assessing the two-way performance of an MTP-concrete composite floor system. From the second phase the washer-screw connection was selected as the shear connector and was used with CV3M1 Norway Spruce CLT and a reinforced concrete topping to form the floors. Three experimental programs were used to assess different performance characteristics of this floor. The first involved characterizing the orthotropic plate properties using six (6) different tests conducted in the linear elastic range. Second, the panels from the elastic testing were tested to failure using a concentrated patch-load. Finite element models were developed from these tests and analyses were run to assess the models for different loading cases. Finally, non-uniform bending tests were conducted on paired CLT-concrete composite beams with different longitudinal connections to characterize the load transfer across this longitudinal connection. The orthotropic panel property tests showed that the washer-screw increased all of the positive orthotropic panel properties when compared to previous tests, with the largest increase occurring in the weak-direction bending. The tests to failure exhibited an almost elasto-plastic response with high ductility. The panel oriented to span in the weak direction exhibited approximately half of the stiffness and lower strength and the panels in the strong direction. The load-sharing tests showed the ability that the concrete topping slab could effectively transfer load across the joint, but that this performance was further improved using a plywood spline and closely spaced screws in addition to the concrete. The results from these studies will assist with the implementation of these MTP-concrete systems into the built environment. The withdrawal tests can be used as a rapid assessment to compare new STS for use in TCC. The designed alternative connectors could be used or improved upon to help facilitate composite action in composite systems. The results from the two-way bending investigations can be used as a basis for further investigation into the two-way bending considerations of MTP-concrete composites.
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  • This work was financially supported by the USDA Agricultural Research Service in cooperation with the Tallwood Design Institute under Grant No. 58-0204-6-002.
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