Testing and analysis of connections with small-diameter fasteners in natural bamboo

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

In bamboo construction screws are rarely seen, even though they are commonly used in timber connections. This is mostly due to the perceived risk of splitting, which is commonly observed in nailed bamboo connections due to the increase in tangential stress arising upon the nail insertion. However, screws, unlike nails, do not displace the material but cut it out with the thread while being drilled-in. This fundamental difference creates a promising hypothesis that screwed connections, in fact, may not be as brittle as widely considered. Therefore, the aim of the thesis was to investigate the properties and applicability of the use of screws in natural, round bamboo, and provided these are acceptable, to propose a design guidance for the studied connection.

The experimental part of the thesis comprised 330 tests. This included 17 embedment tests, 65 tests on nailed connection loaded parallel to fibre and 248 tests on screwed connection loaded parallel to fibre. In addition, 31 tests on a bolted connection loaded perpendicular to fibre originating from an external source were used. The experimental data was used in analytical modelling to predict the connection capacity and to estimate stiffness and ductility. The capacity of bolted connection loaded perpendicular to fibre was also modelled using the Finite Element Method.

The analytical models for the connection capacity were based on existing models for timber. Since bamboo density varies across the wall thickness (it is more fibrous towards the outer surface), the timber yield model, used for connections loaded parallel to fibre, was adjusted to include the density gradient. It was found that the gradient does not affect yield capacity. The splitting capacity of connections loaded perpendicular to fibre was derived based on a timber fracture model by considering the round and hollow bamboo cross-section.

The thesis findings confirmed that the behaviour of the proposed connection resembles the behaviour in timber. The observed ductile failure modes corresponded to the failure modes predicted by the timber dowelled connection theory. Similar to timber connections, the group effect in screws arranged in line along fibre was observed (reduction of capacity per fastener). For connections loaded parallel to fibre, the predicted ductile and brittle capacities were found to match the observed results well. For connections loaded perpendicular to fibre, a timber fracture model, which was adapted to suit bamboo, was found to result in relatively accurate predictions of the observed capacity, which was supported by the Finite Element results. Based on the study findings the proposed connection with screws and metal plates appears to be an attractive alternative to the common mortar-infilled bolted joint.
Date of Award3 Oct 2023
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorJames A P Norman (Supervisor), Eleni Toumpanaki (Supervisor) & David Trujillo (Supervisor)

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