Journal of Structural Engineering

According to standards, the exposed steel column bases and other steel-concrete connections use anchor bolts to resist the tensile (or pullout) actions. The embedded steel sections can replace the anchor bolts in resisting pullout forces. This paper numerically studies the pullout behavior of embedded steel sections with end plates under various conditions. The numerical models are generated with a computer program. To support the numerical results, two I-sections with end plates are tested under pullout forces. Also, a number of test results on headed studs, available in the literature, are compared with the numerical results. One of the important conclusions of the study is that the shape of the embedded section does not seriously affect its pullout strength. The pullout strength is proportionally increased with the plan size of the end plate. However, the rate of increase in strength of headed I-sections is remarkably lower than that of the headed studs. Embedded sections with end plates can develop the pullout strength and stiffness capacities required by the design forces.

Composite steel-concrete beams that use high-strength (HS) steel sections are more sustainable from an environmental low-carbon perspective than those that use mild steel because less steel is needed. Quantifying the elastoplastic cross-sectional behavior of composite beams using HS steel sections is important because HS steel has significant differences in its plastic properties when compared with conventional mild steel. It is also important to establish this behavior in order to apply the principles of rigid plastic design. This paper develops a new analytical model for the cross-sectional analysis of HS steel-concrete composite beams to elucidate this elastoplastic response, so as to ascertain the validity of applying rigid plastic design. Both material nonlinearity and partial shear connection are incorporated in the analysis. The constitutive model for the steel is specified by using a multilinear stress-strain relationship, and that of the concrete is based on a model in terms of its axial stress-strain relationship that is prescribed in Eurocode 4. The slip strain between the steel and concrete components at the interface is treated as being dependent on the curvature and the degree of shear connection. The equation of horizontal equilibrium is derived, from which the curvature and the corresponding strain distribution through the depth of the cross-section can be obtained, leading to the bending moment resisted by the cross section. Comparisons with experimental results, those from rigid plastic analysis as well as with numerical solutions, demonstrate that the analytical procedure is adequate and accurate, providing a much-needed analytical solution for establishing the moment-curvature response of composite beams with HS steel.

This paper presents a novel sensor clustering-based time series approach for anomaly detection. The basic idea of this approach is that localized change in the properties of a structure may affect the relationship between the accelerations around the position where the damage occurs. Therefore, for both healthy and damaged (or unknown state) structures, autoregressive moving average models with eXogenous inputs (ARMAX) are created for different clusters using the data from the sensors in these clusters. The difference of the ARMAX model coefficients are employed as damage features (DFs) to determine the existence, location, and severity of the damage. To verify this approach, it is first applied to a 4-DOF mass spring system and then to the shear type IASC-ASCE numerical benchmark problem. It is shown that the approach performs successfully for different damage patterns. It is also demonstrated that the approach can not only accurately determine the location and severity of the damage, but can also distinguish between changes in stiffness and mass.

The hurricane hazard modeling requires a hurricane wind field model and a hurricane track model, that are generally developed based on historical wind speed and track records. Several hurricane hazard models have been proposed for engineering applications; the model used to map the hurricane wind hazard shown in several editions of a U.S. national standard is extensively documented in open literature. A term, which is expressed as the product of the hurricane translation velocity and gradient of the wind velocity relative to the moving center of the vortex in the governing equation (i.e., fluid momentum equation) to model hurricane wind field, is neglected in various publications. However, the effect of using this approximation on the calculated wind field has not been elaborated. In the research reported in this paper, the effect of this approximation on the wind field is investigated through numerical analysis. Also, a possible simplification of the track model used to estimate the extreme hurricane wind for the U.S. national standard is explored. The use of different wind field models and track models to estimate the extreme hurricane wind is carried out. Comparison of the estimated return period values of hurricane wind speeds is presented.

Real fires start from localized burning and will not develop to flashover in an open space or in large open-plan compartments. Temperatures of gas and exposed steel columns in localized fires are not uniform. Current structural fire design methods are based on uniform heating and, therefore, do not account for temperature gradients in real fires. This paper presents a numerical investigation of the behavior of bare steel columns subjected to a localized fire. The buckling behaviors of steel columns surrounded by and adjacent to a localized fire are investigated. Simple approaches are provided to calculate the temperatures of steel columns surrounded by and adjacent to a localized fire. Sequentially coupled thermal-mechanical simulations were conducted. Unrestrained, axially restrained, and rotationally restrained steel columns of various load ratios and dimensions subjected to different heating conditions were considered. The study found that the behavior of steel columns subjected to a localized fire may be completely different from that of columns subjected to a standard International Organization for Standardization (ISO) 834 fire. The steel columns surrounded by a localized fire fail by local buckling, while the same columns subjected to a standard ISO 834 fire fail by global buckling. The temperatures at which steel columns buckled (buckling temperature) when surrounded by a localized fire are higher than those of columns subjected to both a standard ISO 834 fire and an assumed uniform steel temperature condition. However, the buckling temperature of steel columns adjacent to a localized fire may be much lower than that of the columns subjected to both a standard ISO 834 fire and an assumed uniform steel temperature condition. The location of the localized fire relative to the column has a significant effect on the temperature and buckling behavior of steel columns. Structural fire design for steel columns based on a standard ISO 834 fire or a uniform steel temperature may be conservative if the potential real fires are localized fires surrounding the columns and may not be conservative if the potential real fires are localized fires adjacent to the columns.

Novel statically indeterminate structures generally contain both self-stress modes and internal mechanisms, and have few or no load-bearing capacities before being prestressed. These structures could not maintain stable equilibrium states until appropriate initial prestresses are assigned. The initial prestress design is the basic and key step for novel kinematically indeterminate structures. In this study, an efficient symmetry method is proposed for determining integral prestress modes for various cable-strut structures with multiple independent self-stress modes. Group theory and its matrix representations are introduced to calculate the linear independent self-stress modes retaining full symmetry. Subsequently, integral prestress modes are directly extracted from the null space of the first block matrix of the symmetry-adapted equilibrium matrix. Manual classifications of member types and double singular value decompositions on large-sized matrices are avoided in the calculation process. Therefore, the symmetry method shows great advantage in terms of computation efficiency, especially for the structures with high-order symmetry, complex geometric configurations, or many independent self-stress modes. Illustrative examples are presented to verify the robustness and efficiency in calculating integral prestress modes for different types of symmetric cable-strut structures. The proposed method can play an important role in the force-finding analysis and optimum prestress design of various cable-strut structures.

The axial compressive strength of flange-damaged steel columns is studied using an equilibrium-based analytical approach. Compared to a related investigation that employed a variational technique, the presented model shows similar trends regarding severely reduced axial compressive strength associated with local flange damage. An empirical equation based on a parametric study performed using the developed technique is proposed as a tool that can be used by practicing engineers to determine a quick and reasonable approximation of the capacity of locally flange-damaged steel columns. The developed equation is validated using detailed finite-element analysis.

Database-enabled design (DED) frameworks offer convenient meshing of wind-tunnel driven databases with state-of-the-art analysis methodologies for the design of low-rise to high-rise buildings. These procedures can serve as an alternative or supplementary tool to codes and standards. Despite their obvious advantages over simplified code-specified procedures, the response estimates obtained from these data-driven frameworks are by nature susceptible to the accuracy of both the analysis schemes employed within the procedure as well as the aerodynamic databases that are driven by different wind tunnels, instrumentations, and experimental techniques. To examine and quantify these potential sources of discrepancy, this paper attempts to evaluate the performance of various cyber-based DED frameworks that are currently used in the preliminary design of tall buildings. To this end, a systematic comparison is carried out among along-wind and across-wind response estimates of a typical building example using some of the most recently proposed DED frameworks.

The softened membrane model for torsion (SMMT) was the first rational theory to incorporate the precracking and postcracking torsional behavior of solid RC members. A series of theoretical and experimental investigations were conducted in Taiwan to extend the SMMT to hollow RC members. This paper proposes a generalized SMMT based on the results of these investigations, determines the constitutive relationships of concrete for thin- and thick-walled hollow RC members, and develops a unified SMMT theory for hollow and solid RC members. The proposed SMMT for hollow RC members (SMMT-H) is also employed to derive a rational $T_{cr}$ and ${\theta }_{cr}$ formula for such members, completing a unified set of noniterative rational $T_{cr}\text{-}{\theta }_{cr}$ formulas for both hollow and solid RC members. In the experiment, the proposed SMMT-H analyzed the torsional responses of the specimens reasonably well, and predicted the cracking points and precracking torque-twist curves almost perfectly. The proposed rational formula for hollow RC members, which accurately predicted the $T_{cr}$ and ${\theta }_{cr}$ of the specimens, is an almost-perfect simplification of the SMMT-H in terms of $T_{cr}$ and ${\theta }_{cr}$. A numerical example illustrating the simple application of this formula is presented.

This paper introduces a framework for the performance-based design (PBD) of high-rise buildings with respect to the occupant comfort performance objective. The performance is expressed in terms of the probability—conditional on the wind event intensity—that the fraction of the building occupants on one floor that perceive the motion is greater than a chosen threshold value. The framework is fully probabilistic, and numerous uncertainties affecting both the structural response and the human perception threshold are considered in the assessment of the exceedance probability. In addition, a new database-enabled online analysis module based on the proposed PBD framework [Performance-Based Data-Enabled Design module for Occupant Comfort (PBDED-OC)] is presented. The PBDED-OC allows the assessment of the occupant comfort performance of a tall building by taking advantage of databases of high-frequency base balance measurements or by adopting specific user-supplied wind tunnel data and is envisaged to be particularly useful during the preliminary design phase.

This paper investigates the rocking response and stability analysis of an array of slender columns caped with a rigid beam which are vertically restrained with elastic prestressed tendons that pass through the centerline of the columns while anchored at the foundation and the cap-beam. Following a variational formulation, the nonlinear equation of motion is derived in which the stiffness and the prestressing force of the tendons are treated separately. In this way, the postuplift stiffness of the vertically restrained rocking frame can be anywhere from negative to positive depending on the axial stiffness of the vertical tendons. The paper shows that the tendons are effective in suppressing the response of rocking frames with small columns subjected to long-period excitations. As the size of the columns, the frequency of the excitations, or the weight of the cap-beam increases, the vertical tendons become immaterial given that most of the seismic resistance of tall rocking frames originates primarily from the mobilization of the rotational inertia of their columns. The paper concludes with the presentation and validation of an equivalent rigid-linear system so that the rocking response of vertically restrained rocking frames can be computed with popular open-source or commercially available software simply by employing existing elastic-mutilinear elements.

This paper assesses the performance of a newly developed blind bolt, intended for use in constructing bolted moment-resisting connections to concrete-filled tubular steel profiles. A total of ten connection tests are reported, with each configuration having been subjected to a predominantly tensile force in a representation of the tension region of a typical moment connection. The test variables included type of fastener, addition of concrete to the tube, strength of the concrete, spacing among bolts, and bolt class. On the basis of deformability response, the benefits of filling the tubular member with concrete are highlighted. The favorable performance that results from using a relatively, high-grade concrete infill is also highlighted. The addition of a concrete infill to the tube stiffens and strengthens the otherwise relatively flexible tube walls, enhancing overall connection behavior in terms of stiffness, strength, and ductility. The performance of connections to concrete-filled tubular steel profiles using blind bolts with headed anchors is shown to be suitable for moment-resisting construction.

Quantification of building system performance and response parameters such as response modification coefficient ($R$), system overstrength factor $({\mathrm{\Omega }}_0)$, and deflection amplification factor ($C_d$) is of great importance to reliably assess the performance of RC walls. Values of these parameters used in current design are mainly based on judgment and qualitative comparisons of the known response capabilities of relatively few lateral-force resisting systems in use. In order to assess the validity of these parameters, 20 special and 20 ordinary RC walls (archetypes), with varying seismic design conditons and physical parameters, were designed and modeled in a software framework. Static pushover and incremental dynamic analyses using 44 ground motions were conducted on each archetype using standard methodology, and the obtained adjusted collapse margin ratios are compared to established limits. Results indicate that the parameters of the archetypes designed based on current code provisions are within reasonable limits with the exception of archetypes having height-to-length aspect ratios of 3 or greater, where the use of a larger $R$ value is suggested.

In this paper, novel geometrically nonlinear analytical solutions (NASs) and linear analytical solutions (LASs) for the postelastic response analysis of biconvex and biconcave prestressed suspension cable trusses subjected to a uniformly distributed load are presented. The derived analytical method provides a straightforward determination of the intensity of vertical uniformly distributed loads applied over the entire span of a structure, as well as the accompanying forces and vertical deflections corresponding to the postelastic behavior of cable trusses. In this solution, the direct use of experimental data, such as the actual stress-strain properties of high-strength steel cables, is implemented. Resultant forms of the deflection and cable equations are modified because of the effects of geometrically nonlinear truss behavior in the postelastic range that need to be incorporated. The application of the described approaches and derived equations is illustrated by numerical examples. Resultant responses (forces and deflections) of the biconcave cable truss in the postelastic range obtained by the present geometrically nonlinear and linear analytical solution are compared with those obtained by the nonlinear FEM.

Steel arches are often restrained by elastic lateral braces to improve their out-of-plane stability. Because in real structures, the braces are not infinitely elastic and the strength consideration can have an effect on the stiffness requirement of the braces, it is important for the lateral braces to have sufficient stiffness and to be configured properly. This paper presents an investigation of the threshold stiffness and arrangement of discrete lateral braces required for preventing steel arches from buckling out of their plane. An energy approach is used to derive the elastic threshold stiffness for discrete lateral braces, and the effect of out-of-plane geometric imperfections on the elastic threshold stiffness of the lateral bracing is also studied. The number of lateral braces and the length of the arch segment between adjacent braces are also derived analytically by comparing the in-plane buckling loads of the entire arch and the out-of-plane buckling loads of the arch segment between adjacent braces. The threshold stiffnesses derived in this study agree well with the finite-element results. The agreement between the analytical solutions for the minimum number of lateral braces and the maximum length of the arch segment between the braces and the finite-element results is also very good.

Earthquake-induced damage to structural components and spray-applied fire-resistive material (SFRM) in steel gravity frame beam-column connection regions can impact connection performance during an ensuing fire. Bolted connections are particularly susceptible to failure at elevated temperatures due to a rapid loss in bolt capacity with increasing temperature, which occurs at an accelerated rate compared to the thermal degradation of structural steel. In order to characterize structural and SFRM damage over a range of seismic drift demands, two large-scale SFRM insulated beam-column connection assemblies utilizing single-plate and unstiffened seated connection designs, with composite floor slabs, were tested under combined gravity and lateral loading. SFRM cracking, debonding, and spalling were observed during both tests, exposing critical connection elements. In addition, the single-plate connection assembly, which was developed in accordance with U.S. national guidelines but contained design features based on empirical requirements that may lead to deficient performance during a seismic event, experienced multiple bolt shear failures at an interstory drift demand less than 3%. The tests demonstrate that structural and SFRM damage are credible threats during large-magnitude ground shaking, and suggest that demands on gravity connections during a postearthquake fire could be significantly greater than those assumed in a conventional design for fire safety.

This paper deals with experimental investigations into flexural-torsional ultimate resistance of in-plane-pinned circular steel arches. Three I-section arches with the same span but different rise-to-span ratios were tested under different loading conditions. A test rig comprising loading frames and lever beams was used to apply symmetrical three-point loads over the full span and unsymmetrical two-point loads over half the span to the arch rib. These concentrated loads are directed toward fixed points during flexural-torsional buckling of the arches. The arches are supported in such a way that their ends in plane are pinned and out of plane are semirigidly restrained. Geometric imperfections are measured before loading. The test results show that when the test arches reach their load-carrying capacity, plastic zones form in the arch rib, and the out-of-plane flexural-torsional deformation is significant. It is found that the pinned arches buckle in an asymmetric double-wave S-shaped flexural-torsional failure mode as a result of the semirigid restraints for bending out of plane at arch ends, rather than in a one-wave C-shaped buckling mode for rigid restraints. Comparisons between the experimental and numerical results indicate that the finite-element method developed in the paper can predict the inelastic flexural-torsional response of steel arches very well.

Design requirements for development length of lap-spliced reinforcing bars in concrete refer to the associated rules for anchorage, practically treating the two problems as one. This was based on the empirical observation that development capacity of reinforcement obtained from both lap-splice and anchorage tests converge to the same experimental values. Nevertheless, from a theoretical viewpoint the similarity between the states of stress of these two problems is not well addressed. In order to trace the underlying relationship between these two bond conditions, the field equations governing lap-splice behavior are established and solved from first principles using simplified constitutive relationships for steel and bond between reinforcement and concrete; the state of stress thus estimated is subsequently compared with the state of stress that occurs along an anchorage of the same length. Because lap splices occur in the clear span of members in the presence of flexural moment, an essential point in the solution of the differential equations of bond is the onset of crack stabilization in the splice region. Both longitudinal and transverse concrete cover cracking is considered in solving for the associated bond stress distribution, whereas debonding that initiates at the lap ends and spreads inward to cross inner flexural cracks is reproduced in the solution. Model verification is based on comparisons with indicatively selected published data from elastic lap-splice tests. The main conclusion derived from the model is the confirmation of the empirical rule embedded in the Code Design procedures, namely that bond in lap-splice regions behaves exactly the same as in the anchorage.

This study proposes a modal analysis for a simplified model of a linked building system, i.e., a system consisting of two adjacent tall buildings connected through links such as skybridges or skygardens. The simplified model was developed by modeling each building as a cantilever beam and allowing for the effects of varying link mass, axial stiffness, bending stiffness, and link location. The analytical solutions for the modal properties of the simplified model were then derived by solving the characteristic equations in the related boundary value problem. After validating the accuracy of the analytical solution by comparing it with the associated finite-element model (FEM), the effects of the link parameters on the modal properties, and the wind-induced responses of the system were examined by using these analytical solutions. Meaningful results were obtained and discussed. The analytical solution and results can be used for rapidly evaluating the modal properties of the linked building systems and for optimally designing the link during the preliminary design stage.

A field trial involving the surface detonation of the equivalent of 5,000 kg of trinitrotoluene (TNT) was carried out in Woomera, South Australia. The overpressures, impulses, and related information obtained from this trial were analyzed and compared against available predictive procedures and previously published data sets. A predictive procedure that is adopted in the widely used Unified Facilities Criteria (UFC) 3-340-02 is based on equations that are established through a small data set, in which issues such as afterburning, increased pressure as a result of the chemical reaction zone, and blast wave asymmetricality were not addressed. The data set contains data from mid and far-field explosions, in which the pressure data in the data set were not directly measured, but were inferred through the arrival-time data. This leads to a significant level of uncertainty in the UFC 3-340-02 procedure, especially for closed-in explosions. Differences between the field measurements and the predictions made when using the blast-parameter charts in UFC 3-340-02 and a computational fluid dynamics code are identified and analyzed in this paper. The findings of the analysis imply that the current predictive approach will neither capture the variabilities of blast parameters in an actual event, as it does not account for the shape of the charge, nor capture the uncertain behavior of the blast wave at a close standoff distance. This may lead to an overly conservative prediction of the impulse or an underestimation of the peak overpressure. The findings of the analysis show that in an uncontrolled blast event, the reliability of the empirical charts provided in the UFC 3-340-02 is questionable.

Nonstationary extreme winds are responsible for many structural damages. However, empirical models are not available for these winds, especially for fluctuations, mainly due to the difficulties in mathematical treatments. In this study, nonstationary wind characteristics are studied and analytic models are also proposed based on field measurements of wind speeds. Firstly, the discrete wavelet transform and kernel regression method are used to infer the time-varying mean and variance of the nonstationary extreme wind, respectively. Then, based on the estimated evolutionary power spectral density (EPSD), transient features of nonstationary winds are examined. Results show that spectral variations in nonstationary wind fluctuations including concerned downbursts and typhoons are relatively weak. This means that these nonstationary fluctuations can be modeled as uniformly modulated processes. Also, the validity of nonstationary wind spectrum models directly extended from current stationary wind spectra is evaluated. The study demonstrates that this extension is not appropriate. Furthermore, two analytical models are suggested to characterize nonstationary wind fluctuations, including a fully nonstationary process model and a simplified uniformly modulated process model. Both models have physical meaning and provide satisfactory fitting for the estimated EPSD. They will be helpful in the Monte Carlo simulation and structural dynamic analysis. An evaluation of these models is conducted based on the structural dynamic analysis using a series of tall buildings. Results show that these two models have good performance in structural response prediction.

Recycled aggregate concrete–filled steel tube (RACFSTs) are attracting increasing attention from academics as a solution to use recycled aggregate concrete (RAC) as main load bearing members in multilevel or even high-rise buildings. Despite their promising future for structural use, limited attention has been devoted to investigate the time-dependent behavior of such elements. This paper reports on an investigation of how the incorporation of recycled aggregates (RAs) affects the long-term responses of these composite specimens. To achieve this, 21 RACFST specimens with different aggregate replacement ratios have been prepared for the long-term tests. The accuracy of currently available RAC concrete models in predicting the creep behavior of the RACFST specimens has been investigated based on the related experimental results. Replacing natural coarse aggregates with recycled aggregates can increase the long-term deformation of the composite columns by 22% but barely affects the rate of creep development of the specimens. The observations from the research reported in this paper indicate that it is reasonable to account for such effects by multiplying an amplification factor to the available concrete models for normal composite specimens. Benchmarking analyses have shown that the available amplification factors proposed for RAC members can be used to predict the long-term responses of RACFST specimens by considering a nil exposed perimeter. Parametric analysis has been carried out to evaluate the time effects on the static responses of RACFST specimens.

In this paper, three-dimensional (3D) continuum-based finite-element (FE) simulations are implemented for estimating intermediate damage limit states in flexure-dominated ductile reinforced concrete (RC) bridge columns. Results from the 3D FE simulations were compared and validated against the experimental data from four large-scale tests. Statistical error measures and test analysis correlation metrics were utilized to quantitatively evaluate the accuracy of the models. The validated models were then applied to determine the intermediate damage limit states based on the simulation results. Onset of yielding was associated with the tensile strains on the extreme reinforcement, and compressive strains on the outer concrete surface elements were used to identify the initiation and significant growth of spalling of the cover concrete. The results show that the 3D FE simulations were efficient in predicting intermediate damage limit states in a consistent manner with the experimental observations extracted from the actual tested columns.

Timber-concrete composite beams (TCCs) are made from a concrete topping fastened to a timber beam. Different methods can be used to construct the TCCs. The concrete topping can be either poured on the timber beam with preinstalled connectors or prefabricated off-site and then connected to the timber beam. The timber beam can be shored during assembling with the concrete topping or left unshored. The use of the shored construction is usually recommended as a way to reduce long-term deflection; however, it is costly. This paper investigates the influence of the construction method on the long-term behavior. A previously developed rigorous uniaxial finite element (FE) software has been first validated on some long-term tests performed in Sweden on TCCs with prefabricated concrete slabs and different connection systems. The FE model has then been used to carry out some analyses where the deflection in the long term has been compared with the same TCC constructed in different ways. Parameters investigated include the mode of construction of the concrete slab (prefabricated off-site or cast in situ), the assembly of the composite beam (with shored or unshored timber beam), the storage time of the concrete slab, and the time between the end of construction and the application of the live load. The outcomes of this numerical study indicate that by prefabricating the concrete slab it is possible to reduce the long-term deflection, particularly when the slab is stored for at least 28–56 days before it is connected to the timber beam and when the timber beam is shored for at least 1 day. In these cases, the effect of concrete shrinkage is reduced, as well as the instantaneous and delayed deflection due to the self-weight of the beam. Conversely, systems with concrete poured on the timber beam should be shored for a longer time (at least 7 days). Shoring the timber beam does not markedly change the increase in long-term deflection due to concrete shrinkage; however, it does raise the tensile stresses in the concrete slab during the period the shores are left in place, leading to the potential for cracking when stiff shear connectors are used and for long shoring times.

This paper presents the seismic performance of two types of cold-formed steel wall systems as characterized during a full-scale five-story building shake table test program: (1) an exterior architectural façade, which spanned the lower three levels of the building, and (2) interior partition wall systems distributed at all levels of the building. The test building was subjected to a suite of earthquake input motions of increasing intensity, first while the building was isolated at its base, and subsequently while it was fixed to the shake table platen. This paper presents the observed physical and measured responses for the two cold-formed steel wall systems and in particular associates physical damage states with measured drift demands in the building. The tests illustrate damage mechanisms for cold-formed steel wall systems when used in architectural façades and highlights areas that should be given particular consideration during seismic design.

This paper presents a mathematical programming–based approach for optimal retrofitting of steel structures with braces, subject to some given system performance criteria. The aim is to ensure the safety of the post-retrofitted structures under applied forces and limited displacement conditions. The proposed scheme uses a simple form of the classical ground structure–type concept to accommodate possible brace locations. Three rehabilitation cases are studied, each of which is formulated as an instance of a nonconvex and nonsmooth optimization problem generally referred to as a mathematical program with equilibrium constraints or MPEC. In spite of the fact that this type of problem is known to be challenging to solve in the mathematical programming literature, a simple, efficient, and robust approach to process is proposed. The system performance of all retrofitted structures is validated using exact nonholonomic evolutive analyses.

This paper investigates the axial compressive behavior of a through-beam connection between concrete-filled steel tubular (CFST) columns and reinforced concrete (RC) beams. In this connection system, the steel tube is completely curtailed at the corresponding beam location, with the longitudinal steel reinforcing bars for the beam remaining continuous through the connection region. A strengthening ring beam is used to enlarge the connection zone in order to compensate for the possible decrease of the axial load-carrying capacity as a result of the discontinuity of the steel tube that encases the column. Two series of axial compressive tests on 32 beam-column specimens are reported. The first series of tests, including 5 full-scale connection specimens with CFST columns, show that with proper design, the axial load-carrying capacity of the connection can be higher than that of the CFST column. The feasibility of this type of connection is verified. The second series of tests, including 27 specimens with only the connection zone, evaluate the ultimate strength of the connection. It is shown that the height and the area ratio of the reinforcement in the ring beam have significant influence on the axial load-carrying capacity of the connection zone. Taking into consideration the confinement produced by multiple layers of ring bars and the effect of local compression, a formula for predicting the ultimate axial compressive strength of this connection is proposed. The accuracy of the proposed formula is validated through comparisons of the theoretical and experimental results.

The basic concept of a deployable pyramidal shelter structure based on origami is introduced. Various radially foldable systems are composed by leaf units in leaf-in and leaf-out patterns. Then the geometry and motion of the system are studied. The geometry in the fully deployed configuration for the leaf-out pattern is identical to that for the leaf-in pattern. During motion, the leaf-in pattern reveals actually folding incompatibility. In other words, there are two strain free states; this indicates that a structure is self-locking, i.e., a strain will need to be induced in order to move the structure from one configuration to the other. The strain of the system is irrelevant to the number of leaf elements. The structure with the leaf-out pattern has rigid mobility.