This paper proposes an energy-based method to theoretically determine the collapse resistance of reinforced-concrete (RC) beam-slab subassemblies subject to a middle column loss at large deflection. It considers the contributions to internal energy dissipation due to extension of rebar in the slab and beams, the additional resultant bending moment from membrane forces in the slab and catenary tensile forces in beams, and the sectional bending moment at plastic hinges of beams and along yield lines of slabs. Furthermore, the effect of tensile forces on the sectional bending moment of the slab and beams is also accounted for. The effectiveness and accuracy of the proposed method are validated against available test results. The proposed method is found to produce accurate predictions on the collapse resistance of the subassemblies, as well as capture the main feature of mechanism of the subassembly at large deflections. It is concluded that the energy dissipation due to the extension of rebar and additional resultant bending moment significantly contribute to the resistance of subassemblies at large deflections. The interaction between membrane forces and bending moment of the beam and slab sections has a detrimental effect on the collapse resistance. Neglecting this effect will overestimate the collapse resistance for the subassembly.
© 2018 ICE Publishing: All rights reserved.

This article experimentally studies the behaviour of high-strength bolted connections with slot bolt holes under cyclic load to evaluate its seismic performance. A total of six specimens varying in the bolt diameters and pretension are designed and tested. The results show the connections with slot holes have good energy dissipation capacity. It is found that connections with M30 bolts, of which the hysteretic loops are fuller, have better energy dissipation capacity than that of M20 bolts connections. With the increase of number of loading cycles, the sliding force of the connections decreases. On the other hand, the ultimate bearing capacity of the connections does not decrease much both for M30 and M20 bolts. As the experiments proceed, the friction coefficients and the areas of the hysteresis curves decrease in a similar trend because of the smoothing of surfaces. The experimental results indicate the potential application of the connections with slot bolt holes as dual-function parts in structures to simultaneously provide stiffness and energy dissipation capacity.
© The Author(s) 2018.

This paper investigates disproportionate collapse resistance of braced steel frames exposed to fire. The influence of type, number and location of bracing systems on the global collapse is studied. The results show that using braces can enhance the fire-induced collapse resistance by 1 hour. For a single-compartment fire, the collapse can be prevented by using either horizontal or vertical braces. The presence of vertical braces at interior bays is essential to prevent collapse. It is necessary to use a combined horizontal and vertical bracing to prevent collapse for interior multi-compartment fires. For multi-compartment fires at corner, fire protections are required for the perimeter columns to prevent global collapse. Slabs have beneficial influence on the collapse resistance which can be resisted by a tensile ring around the perimeter of the heated slab, and also by tensile yield lines extended to the frame edge. It is suggested to ensure the fire partition at corner regi on to avoid fire spread to adjacent compartments since spreading of corner fires are more dangerous than that of interior fires.
© 2019 by The Hong Kong Institute of Steel Construction. All rights reserved.

This paper experimentally studies the seismic behavior of high strength bolted connections with slot bolt holes at ambient and elevated temperatures. A total of 6 specimens varying in bolt diameters, pretensions, temperatures are designed and tested. The results show that the connections with slot holes at both ambient and elevated temperatures have good energy dissipation capacity. It is found that connections with M30 bolts have better energy dissipation capacity and load-bearing capacity than those with M20 bolts, due to the larger friction force and sliding distance in M30 bolts since they have a larger pretention and wider slot hole. The sliding forces of the connections decrease with the increase of the number of loops. The sliding force decrease by about 50% after 60 loops at elevated temperatures, compared with 35% at ambient temperature. While the ultimate bearing capacity of the connections does not decrease much. The friction coefficient and area of hysteresis curves decrease in a similar trend by about 40% after the first 30 loops. After 60 loops, they are reduced to 40% and 50% of their initial values for the connections with M20 bolts and M30 bolts, respectively. The experimental results indicate the potential application of the connections with slot bolt holes for a dual-function component in a structure to simultaneously provide stiffness and energy dissipation capacity.
© 2018, Hong Kong Institute of Steel Construction. All rights reserved.

This paper numerically investigates the mechanism and influencing factors of the tensile membrane action (TMA) developed in reinforced concrete slabs at large displacements and elevated temperatures. Explicit dynamic analyses are performed using LS-DYNA. The numerical model is first validated against ambient and fire tests on simply supported slabs. It shows that the slab responses are not sensitive to the mesh size. It is found that the 30min of heating in a standard fire can be scaled down to 1 s of computing time. Parametric studies are followed to demonstrate the influence of load ratio, boundary condition, slab thickness, reinforcement layout and aspect ratio on the occurrence and development of TMA. Five failure modes of slabs, initiated by the rupture of reinforcement, are found depending on the boundary condition, reinforcement layout and aspect ratio. As the aspect ratio increases, the location of the reinforcement rupture moves from the slab center to the intersections of longitudinal and diagonal yield lines, and further extends to the corners. For the presence of horizontal restraint at perimeter, the rupture of reinforcement occurs at the longer edges of slabs. Slabs with more reinforcement placed along the long span may fail by the rupture of reinforcement along the short span at the center of the slab. The reinforcement along the short span plays a key role in the load-bearing capacity of rectangular slabs. It is found that reinforced concrete slabs may have a fire resistance of 2 h at least due to the enhancement of TMA. The critical reinforcement temperature of 600 °C is necessary to ensure the efficiency of TMA. The deflection limit of span/20 reasonably predict the failure of slabs although a slab can resist loads at a deflection up to span/12 without collapse by means of TMA. It is recommended to increase the reinforcement ratio to enhance the effect of TMA.
© 2017 Elsevier Ltd

This article experimentally studied the sliding behaviour of high-strength bolted connections. A total of 14 specimens varying in types of hole and bolt diameters were designed and tested under ambient temperature and target temperature of 130°C and 200°C, respectively. The experimental results show that the pretension in the bolts increases with the increase in temperature below 200°C. After 200°C, the pretension reverses and starts to decrease. It was found that the reverse temperature depended on the relative thermal expansion of the steel plates and bolts. The model of temperature-dependent thermal expansion coefficients given by Australian standard AS 4100 is in good agreement with the test results. The friction coefficient increases with the increase in temperature. The friction coefficients of connections with slot holes increase by 12% at 200°C compared with that at ambient temperature. At the same temperature, the friction coefficients of connections with slot holes are found to be 12% lower than that of connections with standard holes.
© 2017, © The Author(s) 2017.

This paper numerically investigates disproportionate collapse resistance of three-dimensional steel-framed structures exposed to compartment fires. The effect of fire protections (low, medium, high) as well as fire locations (corner, edge and interior) on collapse modes and load redistribution schemes is studied. The results show that the frames do not collapse immediately after this local failure but experience a relatively long withstanding period of at least 60 min. This is attributed to the increasing deflection of heated slabs, resulting in increased lateral displacements of adjacent cool columns which governs their buckling. This indicates that the "fire rating" of a structure against global collapse is somewhat 1-hour longer than that of individual members. It is found that the fire protection of steel members has significant effect on the resistance of structures against fire-induced disproportionate collapse. The frames with a medium level of fire protection (2-hour fire rating for columns) withstand the fire. A comparison between 2D and 3D models shows that the 2D model produces conservative results by underestimating the collapse resistance of structures. It cannot capture the load redistribution in a 3D model where more loads are distributed along the short span than those along the long span. The presence of slabs for delaying the global collapse cannot also be simulated by a 2D model. It is recommended that the fire protection of perimeter columns should be enhanced to 2-hour fire rating and slabs should be protected to delay and prevent the collapse of structures.
© 2017

This paper presents experimental investigations on the thermal and mechanical behavior of composite floors subjected to ISO standard fire. Four 5.2 m×3.7 m composite slabs are tested with different combinations of the presence of one unprotected secondary beam, direction of ribs, and location of the reinforcement. The experimental results show that the highest temperature in the reinforcements occurs during the cooling phase (30–50 °C increment after 10-min cooling). The temperature at the unexposed side of the slabs is below 100 °C up to 100-min heating, compared to the predicted fire resistance close to 90 mins from EC4. For the slabs without secondary beams, the cracks first occur around the boundaries of the slab, while for the slabs supported by one unprotected secondary beam, concrete cracks first occur on the top of the slab above the beam due to the negative bending moment, and later on develop around boundaries. Debonding is observed between the steel deck and concrete slab. The secondary beam significantly impacts the deformation shape of tested slabs. Although a large deflection, 1/20 of the span length, is reached in the tests, the composite slabs can still provide sufficient load-bearing capacity due to membrane action. The occurrence of tensile membrane action is confirmed by the measured tensile stress in the reinforcement and compressive stress in the concrete. A comparison between measured and predicted fire resistance of the slabs indicates that EC4 calculations might be used for the composite slabs beyond the specified geometry limit, and the prediction is conservative.
© 2017 Elsevier Ltd

This paper proposes two reduced-order modeling approaches (Equivalent spring model and Single degree of freedom model) to simplify the traditional dynamic analysis of a whole structure by two steps: (1) local damage analysis of columns under blast on a member level, and (2) global analysis with an equivalent spring model considering the failure process of damaged columns. This avoids the global structural analysis in Step 1 and also avoids the simulation of blast loads in Step 2. The failure time and residual load-bearing capacity of damaged columns under blast are considered and their effect on the global collapse resistance is studied. The results show that they have significant effect on the collapse resistance. The longer the failure time, the larger the residual resistance, the smaller the structural displacement, and the greater the collapse resistance. It is found that the dynamic behavior of structures is sensitive to the ratio of column failure time to the structural vibration period t₀/T. The larger the t₀/T, the smaller the dynamic amplification effect. It is suggested to consider a smaller dynamic amplification factor rather than 2 for t₀/T >0.3. For the selection of analysis methods for blast-induced collapse of structures, it is recommended to adopt the alternate path method for t₀/T<0.2, and static analyses for t₀/T>3. While for 0.2< t₀/T<3, dynamic analyses considering the effect of failure time and residual resistance of damaged columns should be conducted.
© 2018, Hong Kong Institute of Steel Construction. All rights reserved.