George Vasdravellis

Large-Scale Experimental Validation of Steel Posttensioned Connections with Web Hourglass Pins

A new self-centering beam-to-column connection is proposed. The connection uses posttensioned high-strength steel bars to provide self-centering capability and carefully designed energy-dissipation (ED) elements that consist of steel cylindrical pins with an hourglass shape. The proposed ED elements have superior ED and fracture capacity, and are placed between the upper and the bottom flanges of the beam such that they do not interfere with the composite slab. A simplified performance-based procedure was used to design the proposed connection. The connection performance was experimentally validated under quasi-static cyclic loading. The specimens were imposed to drift levels beyond the expected design ones to identify all possible failure modes. The experimental results show that the proposed connection eliminates residual drifts and beam damage for drifts lower than or equal to 6%. A simplified analytical procedure using plastic analysis and simple mechanics was found to accurately predict the connection behavior. Repeated tests on a connection specimen were conducted, along with replacing damaged ED elements. These tests showed that the proposed ED elements can be easily replaced without welding or bolting, and hence the proposed connection can be repaired with minimal disturbance to building use or occupation in the aftermath of a major earthquake.

Seismic design, modelling and assessment of self-centering steel frames using post-tensioned connections with web hourglass shape pins

A new self-centering steel post-tensioned connection using web hourglass shape pins (WHPs) has been recently developed and experimentally validated. The connection isolates inelastic deformations in WHPs, avoids damage in other connection parts as well as in beams and columns, and eliminates residual drifts. WHPs do not interfere with the composite slab and can be very easily replaced without bolting or welding, and so, the connection enables non-disruptive repair and rapid return to building occupancy in the aftermath of a strong earthquake. This paper presents a simplified nonlinear model for the connection and the associated beams and columns that consists of nonlinear beam-column elements, and hysteretic and contact zero-length spring elements appropriately placed in the beam-column interface. The model was calibrated against experimental results and found to accurately simulate the connection behaviour. A prototype building was selected and designed as a conventional steel moment-resisting frame (MRF) according to Eurocode 8 or as a self-centering steel MRF (SC-MRF) using the connection with WHPs. Seismic analyses results show that the conventional MRF and the SC-MRF have comparable peak storey drifts, and highlight the inherent potential of the SC-MRF to eliminate damage in beams and residual drifts. The paper also shows that repair of damage in the conventional MRF will be costly and disruptive after the design basis earthquake, and, not financially viable after the maximum considered earthquake due to large residual drifts.

Design rules, experimental evaluation, and fracture models for high-strength and stainless steel hourglass shape energy dissipation devices

Steel yielding hysteretic devices provide a reliable way to increase the energy dissipation capacity of structures under seismic loading. Steel cylindrical pins with hourglass shape bending parts (called web hourglass shape pins—WHPs) have been recently used as the energy dissipation system of posttensioned connections for self-centering steel moment-resisting frames. This work evaluates the cyclic behavior of WHPs made of high-strength steel and two grades of stainless steel, i.e., austenitic grade 304 and duplex. Design rules for WHPs are established using principles of mechanics. Twenty-six tests using different cyclic loading protocols and different WHP geometries were conducted. The tests showed that the WHPs have stable hysteretic behavior and high fracture capacity. WHPs made of duplex stainless steel have the most favorable and predictable performance for seismic applications. Two micromechanics-based fracture models, i.e., the void growth model and the stress-modified critical strain model, were calibrated and their parameters are provided for high-strength steel and the two types of stainless steel. The ability of the cyclic void growth model to predict fracture in WHPs under cyclic loading is also evaluated.

Behaviour and design of composite beams subjected to sagging bending and axial compression

Abstract This paper presents an experimental and numerical study on the ultimate strength of steel–concrete composite beams subjected to the combined effects of sagging (or positive) bending and axial compression. Six full-scale composite beams were tested experimentally under sagging bending and increasing levels of axial compression. A nonlinear finite element model was also developed and found to be capable of accurately predicting the nonlinear response and the combined strength of the tested composite beams. The numerical model was then used to carry out a series of parametric analyses on a range of composite sections commonly used in practice. It was found that the sagging moment resistance of a composite beam is not reduced under low-to-moderate axial compression, while it significantly deteriorates under high axial compression. Sectional rigid plastic analyses confirmed the experimental results. The moment–axial force interaction does not change significantly between full and partial shear connection. Based on the experimental and numerical results, a sagging moment–axial compression interaction law is proposed which will allow for a more efficient design of composite beams.

Shear Strength and Moment-Shear Interaction in Steel-Concrete Composite Beams

Steel-concrete composite beams are currently designed against shear by neglecting the contributions of the concrete slab and composite action, while the moment-shear interaction is not addressed in current structural codes of practice. This paper presents an experimental and numerical study on the shear strength and moment-shear interaction in simply-supported steel-concrete composite beams. Fourteen composite beams and one steel beam were tested under combined bending and shear. The effects of partial shear connection and shear reinforcement in the slab were also studied. A nonlinear finite element model was developed and found capable of accurately predicting the behavior of the composite beams. Extensive parametric studies were then conducted using the validated numerical model. The results allowed for the derivation of a moment-shear interaction law for composite beams and highlighted the high degree of conservatism in current structural specifications. It is shown that both the concrete slab and the composite action contribute significantly to the shear strength of a composite section and that the main factors that influence the shear capacity of a composite beam are the slab thickness and the degree of shear connection. Based on the experimental and numerical results, a design model is proposed for a more efficient design of compact composite beams in regions where the acting shear is high.

Minimal-damage steel frames using post-tensioned connections with web hourglass shape pins: design and assessment

A new self-centering steel post-tensioned connection using web hourglass shape steel pins (WHPs) has been recently developed and experimentally validated. The connection isolates inelastic deformations in WHPs, avoids damage in other connection parts as well as in beams and columns, and eliminates residual drifts. WHPs do not interfere with the composite slab and can be very easily replaced without bolting or welding, and so, the connection enables non-disruptive repair and rapid return to building occupancy in the aftermath of a strong earthquake. This paper presents a simplified nonlinear model for the connection and the associated beams and columns that consists of nonlinear beam-column elements, and hysteretic and contact zero-length spring elements appropriately placed in the beam-column interface. The model was calibrated against experimental results and found to accurately simulate the connection behaviour. A prototype building was selected and designed as a conventional steel moment-resisting frame (MRF) according to Eurocode 8 or as a self-centering steel MRF (SC-MRF) using the connection with WHPs. Seismic analyses results show that the conventional MRF and the SC-MRF have comparable peak storey drifts, and highlight the inherent potential of the SC-MRF to eliminate damage in beams and residual drifts. The paper also shows that repair of damage in the conventional MRF will be costly and disruptive after the design basis earthquake, and, not financially viable after the maximum considered earthquake due to large residual drifts.