Wang, Zhiyu
(2012)
Hysteretic response of an innovative blind bolted endplate connection to concrete filled tubular columns.
PhD thesis, University of Nottingham.
Abstract
Concrete filled steel tubular (CFT) columns can employ the advantages of both materials: steel and concrete. Connection to such columns, however, is problematic. This is especially so if the required connection is both bolted and moment-resisting.
To address this issue, a novel blind-bolted and moment-resisting connection to CFT column has been developed. This connection uses an innovative blind-bolt, introduced in previous research at the University of Nottingham and is termed the Extended Hollobolt (EHB). The EHB has been developed to provide sufficient tensile resistance and stiffness for the connection to develop resistance to moment. Previous research work has studied the performance of this connection under monotonic loading. The performance of such connection under cyclic loading, however, is not yet investigated. The work presented in this thesis addresses this gap in knowledge.
This thesis reports on a series of full scale testing of joints consisting of beam endplates connected to CFT columns using this blind bolt. The test connections were constructed with relatively thick endplate so as to isolate the CFT column and the blind bolt as the relatively weak elements in the connection system. This study focuses on the behaviour of the connections with principal failure modes attributed by the blind bolt and the CFT column.
The experiments were conducted to obtain insights into the hysteretic moment-rotation relationship, available ductility & energy dissipation capacity, observe typical failure modes, and develop relevant understandings of the Extended Hollobolt-endplate connection subjected to cyclic loading. The selected connection details were chosen to examine the influential parameters of the joint hysteretic moment-rotation behaviour.
From the experimental results, two representative failure modes, bolt fracture and column face bending failure, were observed and categorized in relation to the connection configuration. The connection behaviour are described and compared with respect to the influences of bolt grade, cyclic loading procedure, tube wall thickness and concrete grade.
Based on the experimental hysteretic moment-rotation relationships, an evaluation of the cyclic characteristics and an analysis' of the cumulative damage were carried out for the two representative connection categories. The joint hysteretic moment-rotation response was assessed in term of degradations of strength, stiffness, ductility, and energy dissipation. The use of damage levels and stages in the interpretation of damage evolutions for the connection behaviour is also described in this thesis. The findings of cumulative damage analysis suggest that the energy based cumulative damage index outweighs the other indices in characterizing the progressive damage process of the connections in this study. This is especially the case for those related to hysteresis loops at repeated cycles of each loading amplitude.
Following the experimental study, 3D nonlinear finite element models of the connections were developed to analyse the mechanical response of the connection. The comparison of the numerical and experimental moment-rotation envelope curve is discussed with respect to related geometric and material parameters. The connection failure modes and displacement distributions were further examined as a supplement to the experimental findings that were necessarily limited by instrumentations.
This work also presents mathematical models for the hysteretic moment-rotation relationships simulating the loading, unloading and reloading segments of the hysteresis loops. Characterizing parameters were introduced to, allow for the softening slope, linear segment slope, and degradations of strength and energy dissipation.
This study concluded that the proposed finite element model simulates well the behaviour of the connection with good prediction of the moment-rotation envelope curves and of the failure mode. It is also concluded that the proposed mathematical models define well the non-linear loading and unloading paths with reasonable accuracy. It is finally claimed that the Extended-Hollobolt endplate connection provides a stable improvement in strength and stiffness under cyclic loading compared with other similar connections.
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