Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct Verified and validated design curves and strength reduction factors for post- tensioned composite steel-timber stiffened wall systems Kristopher Orlowski Centre for Advanced Manufacturing of Prefabricated Housing, Department of Infrastructure Engineering, The University of Melbourne, VIC 3052, Australia ARTICLE INFO Keywords: Composite Steel-Timber (CST) Post-Tensioned (PT) Design curves Cross-Laminated Timber (CLT) Automated panelised prefabrication Snap-through buckling ABSTRACT Viability of timber-based solutions for projects is often limited by lengthy modelling and preliminary design when compared to traditional systems such as precast concrete. However, sustainability principles are becoming more prevalent in construction with increasing demands for mid-rise buildings constructed through automated prefabrication via Design for Manufacturing and Assembly (DfMA) techniques. Considering this, this paper in- troduces and presents design curves for Post-Tensioned (PT) Composite Steel-Timber stiffened wall systems under axial loading. Notably material efficient, they consist of a stiffened engineered timber panel, having half the typical minimum thickness of Cross-Laminated Timber (CLT). Composite action is gained through integrally stiffening the panel with timber studs and steel Square Hollow Sections (SHS), which house a PT rod. The post- tensioning facilitates vertical panel to panel connections for quick on-site assembly, permanent tie-down and self-centering rocking mechanism functionality. A full-scale experimental testing program was conducted in conjunction with analytically verified and validated finite element models. Parametric changes include: level of post-tensioning, number of stiffeners, thickness of panel, height of the wall and applied load. Highly versatile yet simple design curves have been developed from an exhaustive set of incremental results. In addition, a strength reduction factor capturing the effects caused by post-tensioning is proposed. With these, a simple design pro- cedure has been outlined for quick feasibility analysis and preliminary design. That is, for a desired load capacity and level of post-tensioning, the optimal system configurations is given. Likewise, for a chosen configuration and level of post-tensioning the allowable axial load is given. 1. Introduction Prefabrication is progressively being adopted in construction, par- ticularly with timber-based systems [1-3] in which industry is fur- thering its establishment for mid-rise construction through innovation and development [4,5]. Traditionally, timber is used in the form of open-panel lightweight frame construction [6,7] which is generally suitable for low-rise, one to three storey, developments [8,9]. Funda- mentally, timber is a renewable material that stores carbon, is highly workable and is suitable for automated prefabrication under via Design for Manufacturing and Assembly (DfMA) [10-17]. This sustainable material can be paired with efficient land development in tall timber construction achieved through what the International Building Code terms as ‘massive timber systems’, or ‘mass wood construction’ [18-20]. These use large solid Cross-Laminated Timber (CLT) built-up panels which so far have achieved 18 stories, or 53 m in height, with structural design concepts that each up to 150 m [21,22]. However, a more ma- terial efficient timber-based system is required for the practical con- struction of a new generation of mid-rise buildings. The presented fully prefabricated Post-Tensioned Composite Steel-Timber (PT-CST) stif- fened wall system and corresponding design curves and methods may provide a means for this. Post-tensioning of timber walls suitable for mid-rise buildings was first proposed and studied in 2005 at the University of Canterbury with the development of PresLam technology [23-25]. This and future stu- dies were done showcasing PT based technologies merits in terms of its high seismic performance [26-31]. These studies focused on the lateral stability and seismic performance of basic monolithic timber walls, termed ‘mass timber walls’ much like precast concrete in terms of its geometrical dimensions. The self-claimed first commercial use of post- tensioning in mass timber walls was in 2012, this was a timber alter- native solution to post-tensioned concrete shear walls with rocking functionality akin to Precast Seismic Structural Systems (PRESS) [32- 35]. The PT in mass timber shear walls creates rocking walls of a similar nature [30,36]. These walls in earthquake events control the damage, provide lateral stability and also facilitate self-centering capability [37]. The timber shear wall used in this leading example was made from mass Laminated Veneer Lumber (LVL) [38]. This has been studied https://doi.org/10.1016/j.engstruct.2019.110053 Received 26 August 2019; Received in revised form 22 November 2019; Accepted 6 December 2019 E-mail address: kristopher.orlowski@unimelb.edu.au. Engineering Structures 204 (2020) 110053 Available online 13 December 2019 0141-0296/ © 2019 Elsevier Ltd. All rights reserved. T