Behaviour of bolted wood joints to traction tests.

Dates, Rodica Nicoleta ; Terciu, Ovidiu ; Baba, Marius 等

1. INTRODUCTION

Wood constructions represent an important area in the world's economy, because, nowadays, there is a tendency of using wood on a large scale. This tendency is justified by the technical, economical and technological advantages that it presents. The lack of fundamental research regarding wood joints in constructions and optimal joints, from a rheological point of view, imposes research that completes recent information and discovers the behaviour of wood, so much used and not yet fully discovered.

2. IN TIME BEHAVIOUR OF WOOD AND WOOD JOINTS

Due to its inhomogeneity, wood is considered an anisotropic material, having different physical and mechanical properties on its three principal directions of fiber orientation: longitudinal (L), radial (R), tangential (T) sections. The mecanical loads, under certain humidity and temperature conditions, determines the appearance of reversible elastic deformation and plastic irreversible, flowing deformations. The deformation takes until a new equilibrium phase begins. The plastic deformation process (flowing) develops in time (Curtu & Ghelmeziu, 1984).

Wood constructions are exposed to different loads: both short and long term types of loads. Joints, like other elements, take this load, and are subject to different ways of deformations, terms of load time and other external factors. Joints are the main areas where energy may be dissipated by the possibility of using the plastic capacity of these parts of the structure. It has been observed that wood, as an anisotropic orthotropic material, can carry, for a short period of time, a higher load than it can for a long period of time (Madsen, 1992). Joints are the main areas where energy may be dissipated by the possibility of using the plastic capacity of these parts of the structure. One of the limits of energy dissipation is wood cracking in the joining areas (Chaplain et al., 1994). The total deformation degree is determined by the value of the initial deformations, by the joint's type and stress intensity. Considered as a system, the joint cracks. This process related to the contribution of each element to total deformation, related to the local pressing intensity of the joining elements, shear (which determines important slipping) and the whole assembly deformation (Bocquet, 1997).

Bolts from bolted wood joints used at timber structures, are parts throughout the wood elements are stopping each other's displacement. They have usually bending stresses. The capacity of a single bolt depends on the bearing strength of the wood, the bending strength, and slenderness ratio of the bolt. As the slenderness ratio increases bolt stiffness is reduced and bending may occur before full bearing strength is achieved, reducing the capacity of the connection (ASCE, 1996).

The breakage of a wood bolted joint is due to the bolt's shear, or to the splitting of the wood elements, when bolts are stiff. When bolts have elastic properties, pressions occurs both in the assemblied elements, both in the bolt that rounds and bends. This happens under loads that tend to displace the assemblied wood elements (Curtu et al., 1988, 1993).

3. TRACTION TESTS

The aim of theese tests is to determine the influence of bolt's diameter upon mechanical strengths of joints. In order to test the bolted wood joints with the traction-compression testing machine, a clamping device was created.

For this type of joints, bolts of 6, 8, 10 millimeters diameter and 110 millimeters length were used. The bolt's diameter was the same as the joint element's hole. Nine bolted wood joints were made, three for each diameter.

The joints were made from spruce wood (radially cut) at small sizez: 434 x 50 x 40 millimeters for the centered element and 434 x 50 x 35 millimeters for the lateral elements (Fig.1). The joints were attached to the traction--compression testing machine through a clamping device (Fig. 2).

Joints were loaded till crack (Fig. 3). To all specimens, this occurred to the central element of the joint for the ones with 8 and 10 millimeters diameter bolts. For the joints with bolts of 6 millimeters diameter, the crack occurred only to one of the samples, due to their elasticity.

The behaviour of joints under load was the same for all types of bolts; the differences are due to the variation of diameters and materialize on the failure manners and load intensity.

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It has been noticed that wood deformation doesn't occur just like an instantaneous change of form immediately after load is applied, but there is a continuous process of deformation under load, which literature named slow flow. In certain conditions of humidity, under loads exercised over a long period of time, deformations increase until crack. First time, a primary flow appeared with a high velocity of deformation. In the second stage of deformations, a secondary flow appeared, where the velocity of deformation remained constant, the phenomena developing constantly, according to the degree of load. In the third area, a tertiary flow appeared, with a relatively high velocity of deformation, which accelerates as it approaches the rupture point. According to load-displacement diagrams, the limit of plastic flow was represented by the stresses at which deformations were irreversible. There where the plastic flow limits were exceeded, the wood remained in a special quality state. This is due to a fast increase of the elastic deformation as a result of the increasing loads applied and due to the inevitable crack, under loads, which occurs at the deformation limit of the plastic flow.

For the 6 millimeters bolts, the crack occurred at 14,57 kN maximum load with a displacement of 37,59 millimeters and 11,53 kN minimum load, with a displacement of 42,44 millimeters (Fig.4). For the 8 millimeters bolts, the crack occurred at 19,00 kN maximum load with a displacement of 33,63 millimeters and 15,49 kN minimum load, with a displacement of 21,69 millimeters (Fig.5). For the 10 millimeters bolts, the crack occurred at 23,46 kN maximum load with a displacement of 26, 4 millimeters and 19,32 kN minimum load, with a displacement of 14,61 millimeters (Fig.6).

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By overlapping load--displacement diagrams, results that at the same time with bolts diameter increasing, the maximum failure load increase and at the same time, the displacement decrease close to the failure point (Fig.7). By overlapping load displacement diagrams related to the humidity of joining elements it has been observed that at the same time with the increasing of humidity, the mechanical properties of wood decrease, this happens to the maximum failure load (Fig.8).

4. CONCLUSIONS

The intensity of forces and the displacement of the joining elements depend on the ratio between bolt's diameter and the dimensions of the elements. Applied perpendicullar to the grain, force determines longitudinal cracks in the wood elements, particularly to the central one, wich takes the whole applied load, while the lateral elements take this load both in equal manners. Deformation of joints under load begins with a linear increase of force in a short time, with small displacement. At the moment of the first internal crack, a jump into the force diagram, due to the decreasing of its intensity, without displacement of the element. After that, a hardening stage occurs, where loads increase slowly and displacements are high for a longer period of time. The last stage is the crack, with the strongly decrease of load intensity and small displacements.

5. REFERENCES

American Society of Civil Engineers, New York, ASCE (1996). Mechanical Connections in Wood Structures, ASCE Manuals and Reports on Engineering Practice No. 84, ISBN: 0-7844-0110-1

Bocquet, J.-F., (1997). Modelisation des deformations locales du bois dans les assemblages broches et boulonnes, PhD Thesis, University Blaise Pascal--Clermont--Ferrand, France

Chaplain, M. et al., (1994). Life duration of wood joints under high stress level: experimentation and modelling, COST 508-Wood mechanics; Workshop, May 1994, p.128-135

Curtu, I. & Ghelmeziu, N. (1984). Mecanica lemnului si materialelor pe baza de lemn (Mechanics of wood and wood products), Editura Tehnica, Bucuresti

Curtu, I. et al. (1988). Imbinari in lemn--structura, tehnologie, fiabilitate ( Wood joints--structure, technology, reliability), Editura Tehnica, Bucuresti

Madsen, B. (1992). Structural Behaviour of timber. Timber engineering Ltd, ISBN 0-9696462-0-1, North Vancouver, British Columbia Canada

DATES, R[odica] N[icoleta]; TERCIU, O[vidiu]; BABA, M[arius]; STAN, G[ianina] I[leana] & CURTU, I[oan] *