Journal of Structural Engineering

Tall buildings are one of the few constructed facilities whose design relies solely upon analytical and scaled models, which, though based upon fundamental mechanics and years of research and experience, has yet to be systematically validated in full scale. In response to this need, through the combined efforts of members of academe, a design firm and a commercial wind tunnel testing laboratory, a program was initiated to monitor the full-scale response of representative tall buildings and compare this to the predicted response from wind tunnels and finite-element models used commonly in design. As part of this monitoring program, in situ periods and damping ratios over a range of response amplitudes are also being evaluated. This paper provides an overview of the monitoring program, which includes three tall buildings in the city of Chicago, details their instrumentation and modeling, and provides an example of the full-scale response data analyses being conducted.

The paper examines the seismic design force formulas that are currently prescribed in the 2003 NEHRP Provision for the design of nonstructural components in buildings, and presents new formulas to improve the these recommendations. The current code provisions are building independent and overly conservative, especially for the design of nonstructural components in tall buildings. This conservatism can be reduced by utilizing the information about the fundamental period of the building. New force formulas that depend on the building period are proposed. The proposed formulas are intended to avoid more involved analyses. They are validated by a comprehensive numerical study of several buildings of different periods analyzed for an ensemble of recorded earthquake acceleration time histories.

This paper presents two simplified methods for calculating the seismic force coefficients for flexible nonstructural components in building structures. The methods utilize the dynamic characteristics, expressed in terms of the fundamental periods and damping ratios, of the component and the supporting structure to calculate the force coefficient. The cases with the building period and component period known or unknown are considered. The formulas with less information tend to provide more conservative estimates of the force to cover the worst case situations. The new formulas now include the effect of possible resonance with higher modes. The validity of the proposed formulas is verified by a comprehensive numerical study of several buildings of different fundamental periods.

The seismic performance of steel moment-framed buildings has been of particular interest since brittle fractures were discovered at the beam–column connections in a number of buildings following the M 6.7 Northridge earthquake of January 17, 1994. A case study of the seismic behavior of an extensively instrumented 13-story steel moment frame building located in the greater Los Angeles area of California is described herein. Response studies using frequency domain, joint time–frequency, system identification, and simple damage detection analyses are performed using an extensive strong motion dataset dating from 1971 to the present, supported by engineering drawings and results of postearthquake inspections. These studies show that the building’s response is more complex than would be expected from its highly symmetrical geometry. The response is characterized by low damping in the fundamental mode, larger accelerations in the middle and lower stories than at the roof and base, extended periods of vibration after the cessation of strong input shaking, beating in the response, elliptical particle motion, and significant torsion during strong shaking at the top of the concrete piers which extend from the basement to the second floor. The analyses conducted indicate that the response of the structure was elastic in all recorded earthquakes to date, including Northridge. Also, several simple damage detection methods employed did not indicate any structural damage or connection fractures. The combination of a large, real structure and low instrumentation density precluded the application of many recently proposed advanced damage detection methods in this case study. Overall, however, the findings of this study are consistent with the limited code-compliant postearthquake intrusive inspections conducted after the Northridge earthquake, which found no connection fractures or other structural damage.

This paper examines the collapse response of Japanese low- and medium-rise steel moment-resisting frames, which may suffer stiffness degradation and strength deterioration after column buckling in severe earthquake shaking. The ground motions used were artificial accelerograms with an identical Fourier amplitude spectrum. The maximum scale for artificial accelerograms not to collapse the systems and the corresponding energy-based velocities were investigated. This study has confirmed that far-field ground motions induced smaller plastic deformations when compared to near-field ground motions, thereby allowing the systems to absorb more energy before collapse. The results from comparing the scale indicated that in moderate degradation the maximum intensities for far-field ground motions not to collapse the systems were larger than those for near-field ground motions. This trend increased with intensive degradation but reversed with slight degradation. Moderate degradation was selected to further validate the real records and damping effects. This study has provided an insight into the collapse response with respect to the type of earthquake wave and to the extent of structural degradation.

Fragility curves are constructed to assess the seismic vulnerability of a hypothetical two-story reinforced concrete frame building designed only for gravity loads. Fragility curves are also developed for the same building modestly retrofitted by means of column strengthening. A Bayesian methodology is used to construct probabilistic demand models to predict the maximum inter-story drifts, given the spectral acceleration at the fundamental period of the building. The data for the models are obtained using two-dimensional inelastic time history analyses of the building for a suite of synthetic ground motions, developed for the Memphis region. The models are developed using both equality data and lower bound data, and are developed to properly account for both aleatory and epistemic uncertainties. In the absence of probabilistic capacity models for gravity load designed structures, capacity limit states are considered based on FEMA 356 guidelines and deterministic nonlinear pushover analyses. The results quantify the vulnerability of low-rise reinforced concrete frame buildings and show the effectiveness of seismic retrofitting in reducing the probability of failure.

The paper presents some results of a recently completed research project funded by the German Science Foundation (DFG) dealing with the behavior of seismically excited granular material silos. A numerical model consisting of up to five components for describing the dynamic behavior of the silo is presented. The intergranular strain approach for hypoplasticity theory has been used for the granular material and soil–structure–interaction effects have also been taken into account. Results of numerical simulations obtained with this model are shown and compared to measurements carried out at a shaking table in Saclay, France. A squat and a slender silo have been simulated numerically in order to check the design loads specified by the European Code for silos subject to seismic loads. It follows that the Eurocode provisions for slender silos are quite good while for squat silos a reduction of the active mass assumed would be in order.

This paper revisits a fundamental assumption used in most classical failure theories for postbuckled web plates under shear, namely that the compressive stresses that develop in the direction perpendicular to the tension diagonal do not increase any further once elastic buckling has taken place. This assumption naturally led to a well-known theory that tension field action in plate girders with transverse stiffeners must be anchored by flanges and stiffeners in order for the webs to develop their full postbuckling strength. However, a careful examination of the results of the nonlinear finite-element analyses carried out for this study reveals that the diagonal compression continuously increases in close proximity to the edges after buckling, thereby producing in the web panel a self-equilibrating force system that does not depend on the flanges and stiffeners. These findings provide a fuller understanding of the actual mechanics of tension field action.

The moment magnifier method was introduced into design practice to eliminate the need for extensive calculations, based on the solution to a differential equation, for the second-order moments in slender columns. This method is strongly influenced by the effective flexural stiffness $\left(EI\right)$ , which varies due to the nonlinearity of the concrete stress–strain curve and the cracking along the column length among other factors. The $EI$ equations given in the ACI building code are approximate when compared to the EI values computed from the axial load–bending moment–curvature relationships. This study was undertaken to determine the influence of a full range of variables on $EI$ used for the design of slender, tied, composite columns in which the bending moment was applied about the minor axis of the steel shape encased in concrete, and also to examine the existing ACI $EI$ equations. Approximately 12,000 isolated square composite columns, each with a different combination of specified properties of variables, were simulated and used to generate the stiffness data. The columns studied were subjected to short-term ultimate loads and equal and opposite end moments causing symmetrical single curvature bending. A new nonlinear equation for $EI$ was then developed for use in design of slender composite columns subjected to minor axis bending and is proposed as an alternative to the existing ACI $EI$ equations.

Residual stresses play an important role in the behavior of welded structures subjected to cyclic loading. Two different details were evaluated, including three cast shoe base socket connection to extruded tube and two through plate socket connection joints using the hole drilling technique. Hole drilling rosette strain gauges were installed adjacent to the fillet weld toes, and strains measured as holes were milled in $0.254\mathrm{mm}$ $\left(0.01\mathrm{in.}\right)$ increments to a maximum depth of $2.286\mathrm{mm}$ $\left(0.09\mathrm{in.}\right)$ . Measured strains were converted to stresses using both the power series and integral methods. Stresses determined from the two methods were compared, and based on sensitivity to errors; the power series methodology was taken to provide more reliable results.

A lateral bracing system is installed at the top flange level of a steel tub girder to form a quasi-closed box, thereby increasing torsional resistance and retaining the original box shape. A single diagonal (SD) type or crossed diagonal type is typically adopted along with intermediate internal bracing systems (also called internal cross frames) that are placed at either even or odd numbered spacing. A significant coupling action between SD bracing members and internal cross-frame members has been found to occur in the case of a SD lateral bracing system and internal cross frames placed at odd numbered panel spacing. The member forces developed in lateral bracing are 25–30% higher than those in structures with internal cross frames installed at even numbered panel spacing. Matrix equations were formulated to compute the brace forces developed in both SD lateral bracing and K-shaped internal cross-frame members subjected to bending action. Member forces computed from the new equations were compared with those from three-dimensional finite-element analysis, and an excellent agreement between the two solutions was found.

A cyclic bidirectional loading experiment is carried out to examine the ultimate seismic behavior of thin-walled circular steel columns by using an accurate three-dimensional (3D) experimental system recently developed by the writers. This experimental system is characterized by its unique 3D hinge and 3D transducer that are employed to apply accurate 3D force components to the top of columns by considering geometrical nonlinearity precisely. Cyclic circular loads are adopted as virtually the severest bidirectional horizontal loads for circular columns. From the experimental results, it is observed that the strength and ductility of the columns decrease considerably under the cyclic circular loads, compared with those under the conventional cyclic unidirectional loads. The experimental results are also used to confirm the validity of the nonlinear finite element method (FEM) shell analysis where a three-surface cyclic plasticity constitutive model is implemented after modifying the original three-surface model to consider the behavior under large plastic strain. With the extensive numerical results obtained from the nonlinear FEM analysis, empirical prediction equations are derived to evaluate the strength and ductility of thin-walled circular steel columns under the cyclic circular load with one loading cycle.

This paper presents a study on the strength and behavior of cold-formed steel $Z$ -sections with simple and complex stiffeners subjected to major axis bending. The study involved both experimental and numerical investigations, in which a series of specimens were examined. All specimens were failed by the combined mode of shear and moment. The results from the investigations were analyzed and compared with the combined moment and shear strengths predicted by international design rules. The suitability of the design rules in predicting the combined capacity of $Z$ -section with complex stiffeners was assessed through a reliability analysis.

This paper addresses, both theoretically and experimentally, the buckling of shallow concrete arches that may occur due to the effects of creep and shrinkage deformations of the concrete. Under sustained loading, these viscoelastic effects cause time-dependent changes of the deflections of a shallow arch and can lead to the establishment of a geometric configuration at which the arch is no longer stable, and which heralds the onset of creep buckling. By using a virtual work formulation, the equilibrium and buckling equations of an arch with horizontal spring supports are derived in closed form, and using the algebraically tractable age-adjusted effective modulus method to model the time-dependent behavior of the concrete allows the buckling load and the prebuckled life of the arch to be determined using prescriptive equations. Long-term tests on three $4.25\mathrm{m}$ span arches under sustained loading are described. Each arch failed because of creep buckling several months after first loading. These tests are used to validate the theory developed in the paper and to provide benchmark data for calibration. The study shows that creep and shrinkage of the concrete have a profound influence on the structural behavior of shallow concrete arches.

This paper addresses the usage of a simplified homogenization technique for the analysis of masonry subjected to out-of-plane loading. The anisotropic failure surface, based on the definition of a polynomial representation of the stress tensor components in a finite number of subdomains, is combined with finite element triangular elements employed for the upper and lower bound limit analyses. Several comparisons between the proposed model and experimental data available in the literature are presented, for wallettes subjected to bending at different orientations and for different panels loaded out of plane. The limit analysis results allow us to identify the distribution of internal forces at critical sections and to obtain the collapse modes, as well as the failure loads. Excellent results are found in all cases, indicating that the proposed simple tool is adequate for the safety assessment of out-of-plane loaded masonry panels. The combined usage of upper and lower bound approaches, and their respective simplifications, allow us to define a narrow interval for the real collapse load.

This paper presents a parametric study aimed at uncovering general design principles that govern the structural performance of honeycomb-type rigidified inflatable structures (RIS) as load-bearing wall systems for use in residential housing. This study involves the use of finite element modeling and optimization. A series of honeycomb-type RIS wall systems, each comprising different honeycomb cell sizes, are examined. The problem at hand is stated in the form of minimizing material volume subject to: permissible stress, maximum allowable deflection, and membrane thickness. The optimization results help identify optimal design configurations for given sets of loading conditions and material properties. The effects of various design parameters, such as cell size, material properties, and membrane thicknesses, are discussed. The performance of honeycomb-type RIS wall systems is compared with that of rectilinear-type RIS wall systems, which were studied previously. The work presented makes a significant step in establishing the feasibility of RIS for housing applications.

In the equivalent-static-force procedure, the heightwise distribution of seismic load is implicitly related to the linear dynamic response. However, this contradicts the fact that structures undergo large nonlinear deformation during severe earthquakes. Hence, the question arises, how should the lateral loading pattern in seismic design be modified to ensure that the ductility demands are the minimum? This technical note addresses this question in detail. The shear and frame building models of multistory structures have been studied. Employing a new assessment criterion, the adequacy of various distribution patterns is examined, and a more efficient pattern is introduced. It has been shown that for any given earthquake, there can be found a unique strength distribution pattern that leads to the most uniform distribution and the least amount of ductility demand.