Advances in Structural Engineering

This article presents an experimental study on two innovative types of cast steel connectors, which would be used as a replaceable energy-dissipating component in the beam-to-column joint. The main aim was to use the connectors as the main source of deformation and energy dissipation, and to realize rapid repair of the joints by simple replacement of these connectors after earthquakes. A total of eight cyclic tests under two loading procedures were conducted on three types of connectors, namely, traditional welded T-stubs W, C1 type cast steel connectors, and C2 type cast steel connectors. The new cast steel connector specimens were observed to have good energy dissipation capacity with stable and plump hysteretic curves, and the equivalent viscous damping could achieve up to 43.7%. The ductility, which was governed by fracture of energy-dissipating elements, was shown to be satisfactory. The L/t ratio (the length of energy-dissipating element to its thickness ratio) was found to be an important factor influencing the cyclic performances of C2 connectors. The accumulated plastic damage of C2 connector caused by bidirectional reversed tension and compression actions did not significantly weaken its energy dissipation capacity, which demonstrated the feasibility of employing C2 connector only in the beam bottom flange of a beam-to-column joint.

Modern fatigue design in civil constructions is mainly limited to the use of S-N curves and the hypothesis of Palmgren-Miner, as described in design standards and Eurocode. While using the latter, the fatigue evaluation may be conservative, since the outdated S-N curves are compared to current construction technology and weld properties. This shortcoming has a direct influence on the current design of orthotropic steel decks. To increase the understanding of the fatigue behaviour, an improved analysing tool using linear elastic fracture mechanics and extended finite element model is proposed. As a result, thickness effects are evaluated for both the longitudinal stiffener and the deck plate. These calculations indicated that increasing the thickness of the deck plate and the longitudinal stiffener increases the fatigue life of the structure. However, the thickness should be limited to maintain the advantage of a light-weighted construction.

It is always a challenge to efficiently and accurately estimate the force on structures from falling objects. This study aims to predict the maximum impact force on reinforced concrete beams subjected to drop-weight impact using artificial neural network. A new empirical model including a comprehensive version and a simplified version is proposed to estimate the maximum impact force. The model was verified against a database collected from the literature including 67 reinforced concrete beams tested under drop-weight impacts. The database covers the concrete strengths ranging from 23 to 47 MPa, the projectile mass from 150 to 500 kg, and the impact velocity up to 9.3 m/s. The prediction of the comprehensive version of the proposed model fits the experimental results very well with an average absolute error of 11.6%. The simplified version of the proposed model is established for easy estimation, with the average error of 23.2% in prediction of the maximum impact force.

Progressive collapse is a form of disproportionate damage, which is induced by an initial local failure that propagates throughout a large portion or the entirety of structural systems. The initial local failure may occur anywhere in a structural system, which consequently exhibits different collapse behaviors. However, the progressive collapse resistance is an inherent property of the entire structural system regardless of any specific initial local failure. This article proposes a collapse probability index to assess the progressive collapse resistance of reinforced concrete frame structures. Such an index is capable of measuring all typical initial local failures instead of any specific ones. The collapse fragility curve is further established based on the collapse probability index, which varies with an increase in the nominal gravity. The structural redundant resistance to progressive collapse can be assessed by the fragility curve. Furthermore, a simplified assessment method is proposed to measure the collapse probability indices feasibly of reinforced concrete frames under different nominal gravity levels. The proposed method based on the collapse probability index can comprehensively evaluate the progressive collapse resistance of reinforced concrete frames which is validated through analysis of a single four-story and a group of eight-story reinforced concrete frame structures.

Cable–cylinder bracing (cable bracing system with central cylinder) is one of the modern bracing systems in which a pair of cables passes through a cylinder at their point of intersection. This research focuses on the seismic behaviour of moment-resisting frames which have been strengthened using cable–cylinder bracing. For this purpose, steel moment-resisting frames, with different numbers of storeys, were strengthened with two different types of cable bracing: cable cross-bracing and cable–cylinder bracing, and their seismic performances were compared. After performing verification, the original moment-resisting frames and those strengthened with cable–cylinder bracings and cable cross-bracings were subjected to the scaled records of six well-known earthquakes. The hysteresis, drift distribution and column axial force variation curves were plotted for the storeys at the height of the strengthened frames and compared to those of moment-resisting frames. Moreover, the energy dissipation values were also derived from the mentioned records. Based on the obtained results, the moment-resisting frames strengthened with cable–cylinder bracings performed much better compared to those strengthened by cable cross-bracing.

A total of 67 web crippling tests were conducted on carbon fibre–reinforced polymer strengthened cold-formed stainless steel square and rectangular hollow sections in this article. Several carbon fibre–reinforced polymer strengthening schemes were adopted, such as strengthened with carbon fibre–reinforced polymer sheets, strengthened with carbon fibre–reinforced polymer plates and strengthened with anchored carbon fibre–reinforced polymer plates. Two loading conditions of end-two-flange and interior-two-flange have been considered. The tests were performed on five different tubular section sizes which covered a slightly wide range of measured web slenderness ratios from 18.70 to 68.41. The effects of carbon fibre–reinforced polymer length, number of carbon fibre–reinforced polymer layers and carbon fibre–reinforced polymer strengthening schemes on the strength enhancement have been discussed based on the test results obtained from this study. It was found that the web crippling capacity of stainless steel tubular sections can be obviously increased by carbon fibre–reinforced polymer strengthening, especially for those sections with large values of web slenderness under end-two-flange loading condition. However, the strengthening enhancement of carbon fibre–reinforced polymer sheets and carbon fibre–reinforced polymer plates is quite limited due to the debonding between stainless steel tube and carbon fibre–reinforced polymer. The anchored carbon fibre–reinforced polymer plates can effectively delay or totally prevent the debonding failure. The web crippling behaviour of cold-formed stainless steel square and rectangular hollow sections strengthened with anchored carbon fibre–reinforced polymer plates has been significantly improved. Finally, the web crippling test results obtained from this study are compared with the current design strengths.

The primary objective of this article is to study the effect of beam depth on the performance of shear-deficient beams externally strengthened with carbon fiber–reinforced polymer composites. The investigated parameters include overall behavior up to failure, the onset of the cracking, crack development, and ductility. The experimental results showed that externally bonded carbon fiber–reinforced polymer increased the shear capacity of the strengthened reinforced concrete beams significantly depending on the variables investigated. The use of carbon fiber–reinforced polymer composites is an effective technique to enhance the shear capacity of reinforced concrete beams. For the beams tested, as the depth of the reinforced concrete beams was increased from 225 to 450 mm which is equivalent to a/d ratio of 2.7–1.2, respectively, there was corresponding 15%–19% increase in contribution of the carbon fiber–reinforced polymer strips in terms of ultimate load. The impact of the beam depth is more pronounced on the ultimate load than the corresponding deflection of the control and strengthened beams. The results indicated that the beam depth has an influence on the angle at which primary cracking angle varied from 33, 44, 50, and 54 for beam of a/d of 2.7, 1.9, 1.5, and 1.2, respectively. If the shear crack crossed carbon fiber–reinforced polymer strips above its development length, the carbon fiber–reinforced polymer strips could not be expected to reach its ultimate strength and was thus only partially effective as observed in beams with different depths.

Fire is a common secondary disaster following blast events. Because the strength of steel is sensitive to temperature, the steel structure that survives an explosion threat may collapse in the post-explosion fire. Thus, it is significant to investigate the progressive collapse mechanism of steel structures subjected to explosion and post-explosion fire. This article presents an accurate analytical model for evaluating the progressive collapse potential of a steel frame subjected to a confined explosion and post-explosion fire. The macro-model of the shear tab connection is established, and a material model that can reproduce the nonlinear dynamic behaviour of connections under combined hazard is compiled as subroutines. The finite element package AUTODYN is adopted to calculate the internal blast load to accurately analyse the effect of blast-induced damage on the fire resistance of the steel structure. The analytical results indicate that the gravity columns and columns in the interior moment resisting frames are key components for structural stability under combined hazards. The blast not only results in geometrical damage to the structural members, improving the load–displacement effect, but also leads to failure of the shear tab connections which increases the effective length of the column. Therefore, the failure time of the column under the combined hazard of blast and fire is earlier than the fire only case. Blast damage to the connection may also result in connection failure before column buckling in a fire, leading to local collapse prior to progressive collapse. Moreover, the collapse time–charge weight interaction diagram is generated to determine the weakest area of the steel frame. The membrane action of the floor is the only mode to redistribute load after the failure of the damaged column in a fire. Thus, increasing the reinforcement ratio in the floor may help mitigate the progressive collapse potential of the steel frame.

In the existing fragility assessment, most bridges are short-span bridges and a few bridges have long spans such as cable-stayed bridges. In this study, a procedure is proposed to conduct the component and system seismic fragility analysis of long-span cable-stayed bridges. Three critical issues are addressed in the procedure: (1) the optimal intensity measure of cable-stayed bridges, (2) limit state models of various components, and (3) contribution of individual components to the entire system failure of bridges. This study chooses a long-span cable-stayed bridge with the most common configuration in China and builds the numerical model of its multiple components using OpenSEES that can account for their nonlinear response and uncertainties in the ground motion and material properties. Four typical intensity measures are compared with respect to four characteristic properties including efficiency, practicality, sufficiency, and proficiency. Peak ground velocity turns out to be the optimal intensity measure. Limit states of pylon sections are derived by a numerical simulation based on pushover analysis and China’s guidelines. The pushover results indicate that the limit state of their section curvature depends highly on the section type and axial compression coefficient. A joint probabilistic seismic demand model and Monte Carlo simulation are employed to obtain an accurate system fragility estimate of cable-stayed bridges by accounting for the contribution of each component to the overall bridge system. The system fragility curves based on Monte Carlo simulation lie much closer to the upper bound fragilities given small correlation coefficients, implying that seismic demands of various components conditioned on the peak ground velocity are not correlated.