Volume 5 Issue 7 - September 12, 2008
Rotational performance of traditional butted timber joints with gap
Min-Fu Hsu

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This paper was awarded as "Emachu Fund"

Timber connections play a crucial role on structural behavior of timber constructions, as they usually affect the internal force distribution in the whole structure. Recent years have seen increased attention being given to structural behavior of timber joints in literatures. Fig. 1 shows three various types of timber joints used in residential houses, which are the continuous, butted and dovetail types, found in field survey. This study attempts to explore the rotational performance of butted timber joints by comparing the theoretical model established in this study with the experimental results and discuss the differences of these two various types Taiwanese timber joints in behavior when subjected to moment action.
Fig. 1. Three various types of timber joints found in field survey

A total of 21 full-scaled butted timber joints were tested. The experimental setup is illustrated in Fig. 2. The material used in the specimens is Chinese Fir(Cunninghamia lanceolata). A theoretical model was proposed to predict the behavior of timber connections subjected to moment action. Fig. 3 compares one of the results obtained from experiments and theoretical models in 21 tests. The accepted agreement can be found in all 21 tests.
Fig. 3. Comparison of experimental and predicted results of specimen BN-G-3
Fig. 2. Experimental setup

So far we have proposed theoretical models to estimate butted and continuous timber joints. Hence this paper compares the mechanical behavior of butted and continuous timber joints by using the models we developed. Fig. 4 illustrates the comparison of the butted and continuous timber joints with identical geometries. The timber joints compared have beam section of 180×60 mm, and column section of 180×180 mm, and a 2 mm gap. The MOE of beam perpendicular to the grain was assumed as 0.4 Gpa. From Fig. 4 we found the initial stiffness and moment resistance decrease dramatically once the beam becomes discontinuous and just butted together. In the example taken in this study, the initial stiffness of timber joints drop to 21.3% once the carpenters adopt the butted timber joints. This is not very preferable.
Fig. 4. Comparison of moment-resistance curves of butted and continuous timber joints

Similar failure modes occur at the 21 specimens, which include slight friction and embedment failures on the beam. No mechanical failure was observed on the column. The end of the beam positioned inside the column was polished due to the friction. This might result in changing of the friction coefficient at the contact surfaces. Comparing the observed failures with those found in continuous timber joint, the damage of butted joint is much slighter and no shear failure perpendicular to grain occurs.

Another noteworthy phenomenon is that the butted timber connection starts to show moment resistance at the rotation approximately double of that in the continuous case. The gap between beam and slot on column will result in rigid body rotation of beam, thus it is reasonable for butted timber joints to have larger rotation without any moment resistance, and these connections are more sensitive to the gaps between beams and slots on columns.

A theoretical model was proposed to explore the mechanical behavior of butted timber joints and verified by a total of 21 full-scale tests. Observation from this article shows that the butted timber connections show considerable lower initial stiffness and moment resistance than the continuous timber joints. Also we found the butted timber connections are more sensitive to the gaps, which means larger initial rotation. This paper not only support the superiority of mechanical performance of continuous timber joints, but also reveals the necessity of research effort to be devoted to the reinforcement of butted timber joints in the future.
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