This thesis studies the uni-axial behaviour of circular double-skinned concrete-filled-steel-tubular (CFST) columns with external confinement in form of external steel rings. Particular attention is paid to the experimental behaviour of double-skinned CFST columns and theoretical model for evaluating the loadcarrying capacity of un- and ring-confined double-skinned CFST columns. Experimental studies on circular double-skinned CFST columns with various spacing of confinement, concrete strength and hollow ratio were conducted and discussed comprehensively. The mechanical properties of double-skinned CFST columns such as elastic stiffness, elastic strength, load-carrying capacity and ductility are presented. From the result, it is found that the elastic stiffness, elastic strength, load-carrying capacity and ductility are enhanced by installing the external steel rings to the outer tube as external confinement. To verify the effectiveness of external steel rings, the Poisson’s ratios of the double-skinned CFST columns are listed and found to be similar to that of concrete so that a perfect bonding is maintained. To emphasis the excellent performance of double-skinned CFST columns with external rings under uni-axial compression, the load-carrying capacity, elastic strength and elastic stiffness are compared to those of single-skinned CFST columns and reinforced concrete columns. To fill up the gap that no design model is provided in Eurocode 4 (EC4) for confined double-skinned CFST columns, a theoretical model based on the force equilibrium condition is proposed for evaluating the load-carrying capacity of both un- and ring-confined double-skinned CFST columns. The model takes into account the composite action between the steel tubes and core concrete. To verify the proposed model, numerous test results obtained by the author and other researchers are used for comparing the theoretical results. According to the above theoretical model above, a parametric study is carried out to investigate the effect of various geometry and material properties on the load-carrying capacity of double-skinned CFST columns. The confining pressure is expressed in terms of geometry and material factors. A simplified design formula is proposed to facilitate the preliminary design of double-skinned CFST columns with and without external confinement. / published_or_final_version / Civil Engineering / Master / Master of Philosophy

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This thesis presents the development and validation of a high ductility seismic resistant steel brace connector called the Yielding Brace System (YBS) that improves the earthquake performance of steel braced frame buildings. The connector is comprised of two steel castings which dissipate seismic energy through flexural yielding of specially designed triangular yielding fingers. In this body of work, the need for such a system is presented along with a summary of previously developed steel castings for enhanced earthquake performance of building structures. The development of the YBS concept is then discussed in detail and equations are developed to predict the elastic and plastic response of a YBS connector based on the geometry of the yielding fingers. The low-cycle fatigue life of the cast steel material used for the yielding elements of the YBS is characterized based on the results of several cyclic, small-scale yielding fingers tests and a low-cycle fatigue life prediction model is derived. Following this, the design of a prototype connector for the second storey brace of a fictitious six storey sample building located in Los Angeles is presented. This design is conducted using the low-cycle fatigue prediction model, the response prediction equations and non-linear finite element analysis. Results of four full-scale prototype tests are then presented. Two of the tests are axial tests of the device alone, while the other two are full-scale braced frame tests. Finally, the design of a 12-storey sample building is presented. This building design is then evaluated via non-linear time-history analysis using the FEMA P-695 methodology. The results from these analyses are then discussed and compared to a similar study conducted on the same building designed with buckling restrained braces. This work shows that the Yielding Brace System is a highly ductile, seismic resistant brace that can be used as an alternative to the buckling restrained brace with the potential to provide a stiffer structure with increased ductility.

Elliptical hollow sections (EHS) are the newest steel shape to emerge in the industry, but appropriate design guidance is lacking, being completely absent from Canadian codes and guidelines. Geometric property and compressive resistance tables were established to be potentially added to the Canadian guides. The equivalent RHS method, originally proposed by Zhao and Packer in 2009, was simplified and modified to validate its use for the design of EHS columns and beams. An experimental programme was developed to investigate the behaviour of EHS-to-EHS welded connections. Twelve T and X connection tests were performed to study the effect of connection angle, orientation type and loading. Two methods were developed to predict connection capacities and failure modes: the equivalent CHS and the equivalent RHS approaches. Both methods proved to be conservative on average, but the equivalent RHS approach proved to be more successful at capturing the actual failure mode of EHS-to-EHS connections.

This thesis presents the development and validation of a high ductility seismic resistant steel brace connector called the Yielding Brace System (YBS) that improves the earthquake performance of steel braced frame buildings. The connector is comprised of two steel castings which dissipate seismic energy through flexural yielding of specially designed triangular yielding fingers. In this body of work, the need for such a system is presented along with a summary of previously developed steel castings for enhanced earthquake performance of building structures. The development of the YBS concept is then discussed in detail and equations are developed to predict the elastic and plastic response of a YBS connector based on the geometry of the yielding fingers. The low-cycle fatigue life of the cast steel material used for the yielding elements of the YBS is characterized based on the results of several cyclic, small-scale yielding fingers tests and a low-cycle fatigue life prediction model is derived. Following this, the design of a prototype connector for the second storey brace of a fictitious six storey sample building located in Los Angeles is presented. This design is conducted using the low-cycle fatigue prediction model, the response prediction equations and non-linear finite element analysis. Results of four full-scale prototype tests are then presented. Two of the tests are axial tests of the device alone, while the other two are full-scale braced frame tests. Finally, the design of a 12-storey sample building is presented. This building design is then evaluated via non-linear time-history analysis using the FEMA P-695 methodology. The results from these analyses are then discussed and compared to a similar study conducted on the same building designed with buckling restrained braces. This work shows that the Yielding Brace System is a highly ductile, seismic resistant brace that can be used as an alternative to the buckling restrained brace with the potential to provide a stiffer structure with increased ductility.

Elliptical hollow sections (EHS) are the newest steel shape to emerge in the industry, but appropriate design guidance is lacking, being completely absent from Canadian codes and guidelines. Geometric property and compressive resistance tables were established to be potentially added to the Canadian guides. The equivalent RHS method, originally proposed by Zhao and Packer in 2009, was simplified and modified to validate its use for the design of EHS columns and beams. An experimental programme was developed to investigate the behaviour of EHS-to-EHS welded connections. Twelve T and X connection tests were performed to study the effect of connection angle, orientation type and loading. Two methods were developed to predict connection capacities and failure modes: the equivalent CHS and the equivalent RHS approaches. Both methods proved to be conservative on average, but the equivalent RHS approach proved to be more successful at capturing the actual failure mode of EHS-to-EHS connections.

The use of cold-formed steel structures is increasing rapidly around the world due to the many advances in construction and manufacturing technologies and relevant standards. However, the structural behaviour of these thin-walled steel structures is characterised by a range of buckling modes such as local buckling, distortional buckling or flexural torsional buckling. These buckling problems generally lead to severe reduction and complicated calculations of their member strengths. Therefore it is important to eliminate or delay these buckling problems and simplify the strength calculations of cold-formed steel members. The Hollow Flange Beam with two triangular hollow flanges, developed by Palmer Tube Mills Pty Ltd in the mid-1990s, has an innovative section that can delay the above buckling problems efficiently. This structural member is considered to combine the advantages of hot-rolled I-sections and conventional cold-formed sections such as C- and Z-sections (Dempsey, 1990). However, this structural product was discontinued in 1997 due to the complicated manufacturing process and the expensive electric resistance welding method associated with severe residual stresses (Doan and Mahendran, 1996). In this thesis, new fastening methods using spot-weld, screw fastener and self-pierced rivet were considered for the triangular Hollow Flange Beams (HFBs) and the new rectangular hollow flange beams (RHFBs). The structural behaviour of these types of members in axial compression was focused in this research project. The objective of this research was to develop suitable design models for the members with triangular and rectangular hollow flanges using new fastening methods so that their behaviour and ultimate strength can be predicted accurately under axial compression. In the first stage of this research a large number of finite element analyses (FEA) was conducted to study the behaviour of the electric resistance welded, triangular HFBs (ERW-HFBs) under axial compression. Experimental results from previous researchers were used to verify the finite element model and its results. Appropriate design rules based on the current design codes were recommended. Further, a series of finite element models was developed to simulate the corresponding HFBs fastened using lap-welds (called LW-HFBs) and screw fasteners or spot-welds or self-piercing rivets (called S-HFBs). Since the test specimens of LW-HFBs and S-HFBs were unavailable, the finite element results were verified by comparison with the experimental results of ERW-HFB with reasonable agreement. In the second stage of this research, a total of 51 members with rectangular hollow flanges including the RHFBs made from a single plate and 3PRHFBs made from three plates fastened with spot-welds and screws was tested under axial compression. The finite element models based on the tests were then developed that included the new fasteners, contact simulations, geometric imperfections and residual stresses. The improved finite element models were able to simulate local buckling, yielding, global buckling and local/global buckling interaction failure associated with gap opening as agreed well with the corresponding full-scale experimental results. Extensive parametric studies for the RHFBs made from a single plate and the 3PRHFBs made from three plates were undertaken using finite element analyses. The analytical results were compared with the predictions using the current design rules based on AS 4100, AS/NZS 4600 and the new direct strength method. Appropriate design formulae based on the direct strength method for RHFBs and 3PRHFBs were developed. This thesis has thus enabled the accurate prediction of the behaviour and strength of the new compression members with hollow flanges and paved the way for economical and efficient use of these members in the industry.

Thesis (Ph. D.)--Lehigh University, 2004. / Includes vita. In two parts. Includes bibliographical references (leaves 323-327).