UBC Theses and Dissertations

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UBC Theses and Dissertations

Through bolt connections for composite columns McLellan, Andrew Bruce


The use of concrete filled hollow structural sections as columns in buildings and bridges has many advantages and is steadily becoming more popular. The coimection of steel beams to such columns has been a controversial issue, mainly because the transfer of the beam shear to an axial load in the concrete core is not well understood. Friction is often relied upon to transfer loads although some design codes require direct bearing on the concrete. The development of an inexpensive connection which bears directly against the concrete was embarked upon to improve the versatility and cost-effectiveness of composite columns in buildings. Connections typically used in steel construction were studied. From this study, the concept of "through bolt connections" evolved, resulting in a system with great versatility that can be used for many different configurations of connections and structural types. An experimental study was undertaken to examine the shear load transfer to the concrete core. Square hollow sections, 305x305x12 mm and 1500 mm long, were filled with 30 MPa concrete, to which W460x61 beams were attached with 25 mm high strength steel bolts using a standard end plate connection. All tests were conducted in a cruciform configuration with monotonically applied load, varying the moment-to-shear ratio, the bolt tensioning and the bolt embedment conditions. From the experimental results two types of transfer mechanism were identified: bearing of the bolt on the concrete and friction between the concrete core and the steel shell. The friction capacity, without post-tensioning of the bolts, was found to be substantial for the particular specimens tested. This was further increased proportionally as bolt tensioning was applied. Yielding of the bolts during advanced load stages, however, caused relaxation of the applied bolt tension, thus reducing the benefits of enhanced friction. On the other hand, relatively small beam end moments applied to the connection were found to increase the friction capacity by a substantial amount. The major load transfer, however, occurred through direct bearing of the bolts on the concrete. This bearing capacity was found to be much higher than anticipated with the result that bolt shear at the beam-to-column interface became the governing failure mode. Based on the observed behaviour of the connections, several design philosophies are proposed. Several quantitative parameters have been identified to require further research. This study indicates that the through bolt connection provides a practical and reliable load transfer mechanism, while also being adaptable and easy to fabricate. From the results presented here, it will be possible to focus further research on the development of simplified code formulations which represent a realistic estimate of the connection capacity.

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