Thomas Reynolds

An analytical model for embedment stiffness of a dowel in timber under cyclic load

The fundamental part of a dowel-type connection is the embedment of a steel dowel into the timber that surrounds it, and the stiffness of the timber in embedment is represented by the foundation modulus. A standard experimental method for identifying the foundation modulus under static load is modified to assess the secant stiffness exhibited under one-sided cyclic load. It is shown that the steady-state secant stiffness is significantly higher than the static stiffness under initial loading, and that, if the amplitude of the cyclic component of the load is sufficiently small, a simple analytical elastic model predicts the foundation modulus well. The analytical model is based on a complex stress function for the timber in embedment and the frictional interaction between the dowel and the timber. The foundation modulus calculated in this way can be used to predict the stiffness of complete connections for analysis of vibration in frames modelled with semi-rigid joints. Although the application of the model is limited to vibration about a non-zero mean load, with no load-sign reversal, this form of vibration encompasses various important types of in-service vibration of structures, such as that induced by turbulent wind or footfall.ZusammenfassungBei stiftförmigen Verbindungsmitteln ist die Bettung eines Stahldübels im umliegenden Holz von grundlegender Bedeutung. Die Steifigkeit des Holzes im Bereich der Lochleibung wird anhand des Bettungsmoduls ausgedrückt. Zur Beurteilung der bei zyklischer Schwellbelastung auftretenden Sekantensteifigkeit wurde ein Standardprüfverfahren zur Bestimmung des Bettungsmoduls bei statischer Belastung modifiziert. Es wurde gezeigt, dass die stationäre Sekantensteifigkeit signifikant höher ist als die statische Steifigkeit bei Anfangsbelastung und dass bei einer ausreichend kleinen Amplitude der zyklischen Belastungskomponente ein einfaches analytisches elastisches Modell zur Bestimmung des Bettungsmoduls gut geeignet ist. Das analytische Modell basiert auf einer komplexen Spannungsfunktion des Holzes im Bereich der Lochleibung sowie der Reibung zwischen Dübel und Holz. Der so berechnete Bettungsmodul kann bei den Schwingungsberechnungen in Tragwerken mit modellierten nachgiebigen Verbindungen zur Bestimmung der Steifigkeit dieser Verbindungen verwendet werden. Obwohl das Modell nur für eine Schwellbelastung gilt, deckt diese doch verschiedene wichtige Schwingungsbeanspruchungen in Bauwerken ab, wie zum Beispiel Wind- oder Trittbeanspruchung.

Viscoelastic embedment behaviour of dowels and screws in timber under in-service vibration

Dowel and screw connections in timber structures behave nonlinearly, even at loads which would be experienced in a structure in normal service. They exhibit hysteresis and creep as a result of both the viscoelastic behaviour of the timber itself and the frictional interaction between the timber and connecting elements, and stress concentrations are created which behave plastically, even at loads well below the nominal yield force of the connection. A fundamental process in the load transfer through such a connection is the embedment of the connector into the timber that surrounds it, and the frictional, nonlinear and time-dependent properties in that process are investigated here. A simple rheological model, a combination of Kelvin–Voigt viscoelastic elements, was fitted to the measured response of a block of timber in embedment by a plain dowel or screw. Experiments were performed in which an oscillating force was applied to the screw or dowel, representative of in-service vibration in a timber structure. The effects of plasticity and viscoelasticity were quantified by comparing equivalent linear stiffnesses for an oscillating load, a short-term change in static load, and an initial static loading. The results showed a stiffness, on average, 3.8 times higher under oscillating load than under initial static loading with the same peak force. By quantifying and modelling viscoelastic behaviour in timber around a connector, this work contributes to the development of damping and stiffness models for joints under oscillating load. Such models could be used to determine the contribution of connections to the dynamic response of long spans and tall buildings in timber.ZusammenfassungDübel- und Schraubverbindungen in Holzkonstruktionen weisen auch bei Belastungen, welche unter normalen Praxisbedingungen auftreten, ein nichtlineares Verhalten auf. Aufgrund des viskoelastischen Verhaltens von Holz als auch der Reibung zwischen dem Holz und dem Verbindungsmittel kommt es zu Hysterese und Kriechen, und es treten Spannungskonzentrationen auf, welche sich sogar bei Belastungen weit unter der nominalen Fließgrenze der Verbindung plastisch verhalten. Ein wesentliches Element der Kraftübertragung in solch einer Verbindung ist die Bettung des Verbindungsmittels im umliegenden Holz, und die reibungsbedingten, nichtlinearen und zeitabhängigen Eigenschaften in diesem Prozess werden hier untersucht. Ein einfaches rheologisches Modell, eine Kombination viskoelastischer Kelvin–Voigt Elemente, wurde an Lochleibungsversuche mit einem Bolzen oder einer Schraube angepasst. Es wurden Versuche durchgeführt, bei denen eine für Holzkonstruktionen unter Praxisbedingungen typische Schwellbelastung auf die Schraube oder den Bolzen aufgebracht wurde. Bestimmt wurde der Einfluss der Plastizität und der Viskoelastizität mittels Vergleich entsprechender Steifigkeiten bei Schwellbelastung, bei kurzzeitig wirkender Last und bei länger wirkender statischer Belastung. Die Ergebnisse zeigten, dass bei gleicher Spitzenbeanspruchung die Steifigkeit unter Schwellbelastung durchschnittlich 3,8 Mal höher war als unter länger wirkender statischer Belastung. Durch Bestimmung und Modellierung des viskoelastischen Verhaltens von Holz im Bereich eines Verbindungsmittels trägt diese Arbeit zur Entwicklung eines Dämpfungs- und Steifigkeitsmodells für Verbindungen bei dynamischer Beanspruchung bei. Solche Modelle könnten dazu dienen, um die Wirkung von Verbindungen auf das dynamische Verhalten hoher Holzgebäude und von Gebäuden mit großen Spannweiten zu bestimmen.

Lateral-load resistance of cross-laminated timber shear walls

Cross-laminated timber shear wall systems are used as a lateral load resisting system in multistory timber buildings. Walls at each level typically bear directly on the floor panels below and are connected by nailed steel brackets. Design guidance for the lateral-load resistance of such systems is not well established and design approaches vary among practitioners. Two cross-laminated two-story timber shear wall systems are tested under vertical and lateral load, along with pullout tests on individual steel connectors. Comprehensive kinematic behavior is obtained from a combination of discrete transducers and continuous field displacements along the base of the walls, obtained by digital image correlation, giving a measure of the length of wall in contact with the floor below. Existing design approaches are evaluated. A new offset-yield criterion based on acceptable permanent deformations is proposed. A lower bound plastic distribution of stresses, reflecting yielding of all connectors in tension and cross-grain crushing of the floor panel, is found to most accurately reflect the observed behavior.

Predicting the pore-filling ratio in lumen-impregnated wood

Lumen impregnation, unlike most other wood modification methods, is typically assessed by the pore-filling ratio (PFR) (i.e. the fraction of luminal porosity filled) rather than by weight percentage gain (WPG). During lumen impregnation, the impregnants act on the voids in the wood rather than on the solid mass (e.g. cell walls), but the PFR cannot be measured as conveniently as the WPG during processing. Here, it is demonstrated how the PFR can be calculated directly from the WPG if the bulk density of the untreated wood is known. The relationship between the WPG and bulk density was examined experimentally by applying a pressured impregnation on knot-free specimens from Sitka spruce with a liquid mixture of methacrylate monomers. Based on the validated model, it was possible to further study the effect of different process-related parameters, such as hydraulic pressure, on lumen impregnation. Skeletal density is another key parameter in this model, which directly reflects the amount of inaccessible pores and closed lumens, and can be independently determined by helium pycnometry. The permeability can be qualitatively evaluated by PFR as well as skeletal density. For instance, poor permeability of knotty wood, due to the large extractives content around knots, was reflected by a lower skeletal density and inefficient lumen impregnation (low PFR). Although this model was examined on a laboratory scale, it provides guidance on the precise effect of different parameters on lumen impregnation, thereby improving the fundamental understanding of and enabling better control over the modification of wood by impregnation.

Cell geometry across the ring structure of Sitka spruce

For wood to be used to its full potential as an engineering material, it is necessary to quantify links between its cell geometry and the properties it exhibits at bulk scale. Doing so will make it possible to predict timber properties crucial to engineering, such as mechanical strength and stiffness, and the resistance to fluid flow, and to inform strategies to improve those properties as required, as well as to measure the effects of interventions such as genetic manipulation and chemical modification. Strength, stiffness and permeability of timber all derive from the geometry of its cells, and yet current practice is to predict them based on properties, such as bulk density, that do not directly describe the cell structure. This work explores links between micro-computed tomography data for structural-size pieces of wood, which show the variation of porosity across the woods ring structure, and high-resolution tomography showing the geometry of the cells, from which we measure cell length, lumen area, porosity, cell wall thickness and the number density of cells. High-resolution scans, while informative, are time-consuming and expensive to run on a large number of samples at the scale of building components. By scanning the same volume of timber at both low and high resolutions (high-resolution scans over a near-continuous volume of timber of approx. 20 mm3 at 15 μm3 per voxel), we are able to demonstrate correlations between the measurements at the two different resolutions, reveal the physical basis for these correlations, and demonstrate that the data from the low-resolution scan can be used to estimate the variation in (small-scale) cell geometry throughout a structural-size piece of wood.