Journal of Structural Engineering

A novel approach to nonlinear finite-element analysis of concrete structures exposed to fire is presented. The proposed formulation refers to frame systems, but it can be extended to other types of structures. The main novelty of this formulation is the use of a special class of evolutionary algorithms, known as cellular automata, to describe the heat transfer process induced by fire and to create an effective link between the simulation of the thermal process and the structural analysis. The heat transfer process is reproduced by considering heat conduction, heat convection, and thermal radiation. The temperature effects on the structural performance are taken into account by means of temperature-dependent thermal and mechanical properties of concrete and steel. In this way, the general criteria for nonlinear finite-element analysis of concrete structures are applied to formulate a cellular reinforced concrete beam element with temperature-dependent characteristics. The effectiveness and applicability in engineering practice of the proposed formulation is demonstrated through applications. The results prove the accuracy of the proposed procedure and show that, for statically indeterminate structures, fire safety needs to be evaluated at the global level by taking into account the actual role played by the structural scheme.

In this paper, design expressions are derived that extend the American Iron and Steel Institute (AISI) direct strength method (DSM) to cold-formed steel columns with holes. For elastic buckling-controlled failures, column capacity is accurately predicted by using existing DSM design equations and the cross-section and global elastic buckling properties calculated including the influence of holes. For column failures in the inelastic regime, in which strength approaches the squash load, limits are imposed to restrict column capacity to that of the net cross section at a hole. The proposed design expressions are validated with a database of existing experiments on cold-formed steel columns with holes, and more than 200 nonlinear finite-element simulations that evaluate the strength prediction equations across a wide range of hole sizes, hole spacings, hole shapes, and column dimensions. The recommended DSM approach is demonstrated to provide a broad improvement in prediction accuracy and generality when compared to the AISI main specification, and, with the recent introduction of simplified methods for calculating elastic buckling properties including the influence of holes, it is ready for implementation in practice.

Ground movement along a buried pipeline often places the line in net compression. The local buckling resistance at cold bends is less than at straight sections, and hence, local buckling tends to occur at bends. Correspondingly, this full-scale testing program was conducted to investigate the local and global load-deformation relationships for seven cold bend pipes under applied loads. The specimens were loaded under bending, axial load, and internal pressure until local buckling occurred, and their curvatures were subsequently increased to establish their postbuckling behaviors. It was demonstrated though these tests that the buckling strain of the cold bend pipes was considerably lower than a straight pipe and that the presence of internal pressure exhibited a predominant role on the specimens load-deformation behaviors. It was also established that the behavior of the specimens was relatively unaffected by the bending methods that were employed during cold bending.

The design and behavior of a ductile structural system called a knee-braced moment frame (KBMF) are presented in this paper. The design of this structural system is based on a capacity-design concept that results in ductile behavior. For this system, the frames are designed so that the knee braces will yield and buckle under seismic loads; this is followed by plastic hinging of beams at the ends of the beam segments outside the knee portions. Through this concept, inelastic activities are confined to the designated elements. The knee braces also provide much less obstruction than the braces of conventional systems, making this structural system architecturally attractive. The design concept of this new structural system is addressed first. Results from an experimental study into the seismic behavior of the proposed system are then presented. Two approximately half-scale KBMF specimens were tested. The load-deformation characteristics obtained from the test indicate that the newly developed system can be a viable alternative to conventional structural systems. Finally, results from dynamic analyses of an example KBMF structure designed by the proposed method are provided.

A recent study has shown that residual drifts after earthquakes that are greater than 0.5% in buildings may represent a complete loss of the structure from an economic perspective. To study the comparative residual drift response of special moment-resisting frames (SMRFs) and buckling-restrained braced (BRB) frames, buildings between 2 and 12 stories in height are designed according to ASCE 7-05 and investigated numerically. This investigation includes pushover analyses as well as two-dimensional nonlinear time-history analyses for two ground motion hazard levels. The two systems show similar peak drifts and drift concentration factors. The BRB frames experience larger residual drifts than the SMRFs; however, the scatter in the residual drift results is large. Expressions are proposed to estimate the residual drifts of these systems as a function of the expected peak drifts, the initial recoverable elastic drift, and the drift concentration factor of each system. When subjected to a second identical earthquake, both framing systems experienced larger-than-expected drifts when an initial drift greater than 0.5% was present.

The maximum inelastic response of bilinear single-degree-of-freedom (SDOF) systems when subjected to ground motions without distinguishable pulses is revisited with dimensional analysis by identifying timescales and length scales in the time histories of recorded ground motions. The characteristic length scale is used to normalize the peak inelastic displacement of the bilinear system. The paper adopts the mean period of the Fourier transform of the ground motion as an appropriate timescale and examines two different length scales that result from the peak ground acceleration and the peak ground velocity. When the normalized peak inelastic displacement is presented as a function of the normalized strength and normalized yield displacement, the response becomes self-similar, and a clear pattern emerges. Accordingly, the paper proposes two alternative predictive master curves for the response that solely involve the strength and yield displacement of the bilinear SDOF system in association with either the peak ground acceleration or the peak ground velocity, together with the mean period of the Fourier transform of the ground motion. The regression coefficients that control the shape of the predictive master curves are based on 484 ground motions recorded at rock and stiff soil sites and are applicable to bilinear SDOF systems with a postyield stiffness ratio equal to 2% and an inherent viscous damping ratio equal to 5%.

This paper demonstrates the size effect on the shear strength of reinforced concrete (RC) beams with stirrups and does so in two separate and independent ways: (1) by fracture mechanics, based on finite-element analysis calibrated by a large beam test; and (2) by purely statistical analysis in which a newly assembled database of 234 tests is filtered to eliminate spurious size effects caused by nonuniformity of secondary influencing parameters. Both ways show that stirrups, whether minimum or heavier, cannot suppress the size effect completely, although they can mitigate it significantly for beam depth $d<1\mathrm{m}$ (39.4 in.). The effect of stirrups is to push the size effect curve in logarithmic scale into sizes larger by about one order of magnitude. For beam depths $d<0.5\mathrm{m}$, 1, 2, and 6 m (19.7, 39.4, 78.7, and 236.2 in.), the percentages of beams whose shear strength is below the code limit are calculated as 3.5, 6.5, 15.7, and 55.1%, respectively. The corresponding failure probabilities are ${10}^{-6}$, ${10}^{-5}$, ${10}^{-4}$, and ${10}^{-3}$, whereas ${10}^{-6}$ is the generally accepted standard for a tolerable maximum in risk analysis. It follows that, for beams with stirrups having depth $>1\mathrm{m}$ (39.4 in.), the size effect cannot be neglected.

Timber-concrete composite structures are often used as upgrade solutions for existent timber floors. The timber cross section is maintained and a concrete layer is poured over the floorboard surface. Floorboards help with loading distribution on the beams. The study presented is based on laboratory tests that used specimens made with a lightweight aggregate concrete (LWAC). The use of this type of concrete, compared with normal weight concrete (NWC), has the advantage of lower dead weights, and might be very important in many cases, namely, when the supporting structure has limited strength. Many design works of timber-concrete composite connections do not take into account the timber interlayer, which is known to reduce the load-carrying capacity and the stiffness of the connection. Previous investigations of NWC found that the inclusion of the interlayer leads to average reductions of 30% and 50% for the load capacity and slip modulus, respectively. The current investigation with LWAC found that such reductions are up to 10 and 30%, respectively.