Monday, October 15, 2012

Self-tapping screws as reinforcement for timber structures

Modern self-tapping and continuously-threaded woodscrews enable new methods for
strengthening and joining timber and glulam construction elements. As these screws have a
high axial strength and an excellent continuous bond to the wood they can be used similar
to steel reinforcement in concrete construction.

Different from concrete the natural construction material timber shows a high grade of
anisotropy, resulting in low values of strength and stiffness for tension and compression
perpendicular to the woodfibre and for shear forces parallel to the woodfibre. Self-tapping
screws can be systematically used to reinforce these ‘weak’ directions either in sections of
load concentration, e.g. bearing areas, or to increase strength and stiffness of whole
structural elements.

In several series of tests conducted by the Chair of Structures and Structural Design at the
RWTH Aachen University, screws forming an internal truss system improved the shear
stiffness of glue-laminated timber beams. Similarly the reinforcement of the flexural
tension zone by steel plates attached with screws in a truss arrangement significantly
increased the bending stiffness and ultimate load of the beams.

A second field of the application of reinforcing screws are high-performance joints. Tests
on rigid frame corners connected with self-tapping screws proved a significant higher load
bearing capacity compared to conventional joints with dowels or glued finger-joints. The
layout for the configuration of the screws was done by modelling the internal flow of forces
with strut-and-tie-models in a related way to reinforced concrete design.

The presented reinforcing method with self-tapping screws opens new perspectives for
joining and upgrading all kinds of timber elements creating innovative engineering
solutions for challenging high-performance timber structures.

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