A General Design Approach for Post-tensioned Timber Subassemblies

ABSTRACT

Structural systems with ductile connections have been proposed for multi-storey timber buildings based on research conducted over the last decade. The members designed with unbonded post-tensioning for recentering and energy dissipation through the mild steel elements exhibit almost complete re-centering capacity and significant energy dissipation without any structural damage. This paper describes the general design procedure developed following extensive experimental and numerical investigations of subassemblies. With similar features and geometric configurations, the moment capacities of different types of subassemblies subjected to either unidirectional or bidirectional loading condition can be calculated using the same scheme with appropriate values for connection interfaces.

KEYWORDS:

• Prestressed timber
• ductility
• connections
• seismic design

List of Symbols

c	=	Depth of neutral axis
θ imp	=	Imposed rotation at the rocking interface
${\epsilon }_{pt}$	=	Strain in the post-tensioning tendons
${\mathrm{\Delta }}_{pt}$	=	Deformation in the post-tensioning tendons
$l_{ub}$	=	Unbonded length of post-tensioning tendons
${\epsilon }_s$	=	Strain in the mild steel or energy dissipaters
${\mathrm{\Delta }}_s$	=	Deformation in the energy dissipaters
$l{ }_{ub}^{\prime }$	=	Unbonded length of energy dissipaters
$C$	=	Total compression force
$C_s$	=	Compression force in energy dissipaters
$T_s$	=	Tension force in energy dissipaters
$T_{pt}$	=	Tension force in post-tensioning tendons
λ	=	Re-centering ratio = (Mpt+MN)/Ms
$M_{pt}$	=	Moment contribution from post-tensioning
$M_s$	=	Moment contribution from energy dissipaters
$M_N$	=	Moment contribution from axial load
${\alpha }_0$	=	Steel reinforcement over-strength used as minimum value of re-centering ratio
$l_w$	=	Width of wall
$x_{pt}$	=	Distance of tendon from the centre of wall
${\epsilon }_{pt,t}$	=	Strain in post-tensioning tendons in tension
${\mathrm{\Delta }}_{pt,t}$	=	Deformation in post-tensioning tendons in tension
${\epsilon }_{pt,c}$	=	Strain in post-tensioning tendons in compression
${\mathrm{\Delta }}_{pt,c}$	=	Deformation in post-tensioning tendons in compression
${\epsilon }_{pt,i}$	=	Strain in post-tensioning tendon due to initial prestressing
${\epsilon }_y$	=	Yield strain in post-tensioning tendon
$T_{pt,i}$	=	Initial tension in post-tensioning tendons
$A_{pt}$	=	Area of post-tensioning tendons
$E_{pt}$	=	Modulus of elasticity of post-tensioning tendons
${\rho }_{pt}$	=	Ratio of initial prestressing to yield stress of tendons
${\epsilon }_t$	=	Strain in timber extreme fibre
L cant	=	Shear span of member
${\varphi }_{dec}$	=	Decompression curvature
$N$	=	Axial load on member
$E_{con}$	=	Modulus of elasticity of connection
$b$	=	Width of member section
$h$	=	Height of member section
$f_c$	=	Compressive stress of timber
${\epsilon }_{y,t}$	=	Yield strain of timber
$C_t$	=	Compressive force in timber
$A_s$	=	Area of energy dissipaters
$\varphi$	=	Reduction factor used in design
$M_n$	=	Nominal moment (capacity)
$M\ast$	=	Moment demand
$H_e$	=	Effective height of the structure

Acknowledgments

The research described in this paper was financially supported by Structural Timber Innovation Company (STIC). FPInnovations, Canada, currently holds the intellectual property rights of Post-tensioned Timber systems in North America. The LVL used for testing was supplied by Carter Holt Harvey, New Zealand. Structural testing was performed at University of Canterbury, Christchurch. Three unnamed reviewers helped improve the quality of the paper significantly. All help and supports are gratefully acknowledged.