Journal of Structural Engineering

The purpose of this paper is to study the response of the conventionally designed new Olive View Medical Center (OVMC) building at 16 km from the epicenter of the January 17, 1994 Northridge, California earthquake (M_s= 6.8). OVMC is on an alluvial deposit. The building was subjected to design level peak accelerations during the earthquake and suffered only limited structural and nonstructural damage. The recorded motions at different levels of the OVMC building as well as its associated free-field sites are analyzed using spectral analyses and system identification techniques. The new OVMC building was conservatively designed in 1976 with very high lateral load resisting capability—particularly as a reaction to the detrimental fate of the original Olive View Hospital that was heavily damaged during the 1971 San Fernando earthquake. The original hospital building was later razed. The replacement structure, the new cross-shaped OVMC building, experienced peak acceleration of 2.31g at the roof while its peak ground floor acceleration was 0.82g. The free-field peak acceleration was 0.91g. The lateral load resisting system of the OVMC building consists of concrete shear walls in the lower two stories and steel shear walls at the perimeter of the upper four stories. Spectral analysis shows that this stiff structure was not affected by the long duration pulses of the motions recorded at this site.

A new measure of the demand of earthquake ground motions on structures is presented. This measure, called the drift spectrum, is particularly useful for near-field earthquake motions that contain distinct pulses. Like the well-known response spectrum, the drift spectrum is based on a linear system model. However, the new spectrum differs from the response spectrum in that it is based on a continuous shear-beam model rather than a single-degree-of-freedom model. The continuous model more accurately predicts interstory drift than does a single-degree-of-freedom model for pulse-like excitations. The drift spectrum could be used for design in a manner analogous to the use of the response spectrum. Examples of near-field drift spectra for the Landers, Northridge, and Hyogo-ken Nanbu earthquakes records are provided. The nature of these spectra is discussed and conclusions are drawn regarding the specification of drift demand for structural design.

A practical analytical study is reported that leads to an improved definition of the correlation coefficient for seismic responses of structures. In contrast to definitions currently available, including the widely used expression developed for white-noise excitations by Der Kiureghian, the solution for the correlation coefficient reported in the present paper can efficiently account for the distribution of the frequency content of the excitation. The proposed expression for the coefficient is, in principle, applicable to excitations with narrow, intermediate, and wide frequency bandwidths. For the extreme wide-band case of a white-noise excitation, results of the proposed expression are shown to be in excellent agreement with those of that contributed by Der Kiureghian. Consequently, Der Kiureghian's correlation coefficient may be perceived basically as a special case of that introduced herein. Relevant analyses are made in both the time and frequency domains that contribute to enhance the appreciation for the factors that control the definition of response correlations. In particular, a time domain analysis is presented that leads to a simple, physically oriented interpretation of Der Kiureghian's coefficient. Furthermore, applications are reported for idealized excitations and the 1940 El Centro earthquake that elucidate the merits of the proposed correlation coefficient. The present study is of particular usefulness for estimates of peak seismic responses by using the complete quadratic combination rule, and for assessments of the potential for the pounding of structures during earthquakes.

For stiff structures with large plan dimensions, on soft soil and supported by columns on separate foundations, differential motion of the column foundations may lead to large moments and shear forces in the first-story columns, in the near-field of moderate and large earthquakes. These forces will augment the effects of the concurrently occurring dynamic response, causing larger than expected ductility, larger interstory drift, and thus larger and more dangerous participation of vertical acceleration. When the design conditions call for the connecting beams and slabs between individual column foundations, some components of motion of the first-story columns may be reduced. Then the foundation should be designed to withstand the forces created by deformation of soil. Approximate criteria for estimating the relative significance of these additional effects are presented.

A damage-controlled structure is defined as a combination of structural systems and energy transformation devices that are integrated in such a way that damage due to a major loading is restricted to a specific set of elements that can be readily repaired. This paper describes a preliminary design methodology for building structures subjected to seismic loading, and presents a comprehensive set of numerical simulations illustrating the response of typical structures designed according to this methodology. The proposed design considers the structure to consist of two independent structural systems that work together to satisfy a design requirement on the deformation. The first system functions as the principal load-carrying mechanism for vertical and lateral service loads, while the second structure's role is to dissipate and absorb the energy introduced by significant ground shaking. The key design parameters are the elastic stiffness distribution of the primary system, and the damping characteristics of the secondary system. An optimal evaluation of these parameters is presented for a specific choice of material strengths. The data show that the damage control design paradigm generates very good estimates for preliminary design in this case.

The purpose of this paper is to propose a procedure to determine in a rational but simple way equivalent static lateral forces for the seismic design of nonstructural components attached to buildings. The procedure is derived using concepts from a previously developed dynamic theory for secondary systems mounted on a primary structure and by introducing into this theory some simplifying assumptions. It takes into account the dynamic interaction between a nonstructural component and its supporting structure, the level above the base of the structure of the point or points where the nonstructural component is attached to the structure, the number of such attachment points, and the nonlinear behavior of the structure. In addition, it uses the design spectra specified by building codes for the design of the structure as the earthquake input to the nonstructural component. Presented also is a numerical example that illustrates the simplicity of the procedure and a numerical comparative study with three nonstructural components attached to a multistory frame building to verify its adequacy. Based on the simplicity and rationality of the proposed formulas and the results from this comparative study, it is concluded that the procedure is adequately simple and accurate enough for design purposes, represents an improvement over the pertinent design recommendations in current code provisions, and a suitable replacement for these recommendations.

A systematic study is made of the effects of base uplifting on the seismic response of cylindrical liquid-storage tanks that are supported directly on flexible soil foundations. First, a detailed investigation is made of the effects of system parameters on the uplifting resistance and energy dissipation capacity of the partially uplifted base plate. It is shown that: (1) the hydrodynamic base pressures reduce the uplifting resistance as well as the energy dissipation capacity of the base plate; (2) the uplifting resistance increases with increase in the thicknesses of the base plate and the tank wall, and the stiffness of the foundation soil; and (3) the energy dissipation capacity increases with an increase in the base-plate thickness and the yield level of plate material, and reduces with increase in foundation stiffness and the thickness of the tank wall. Next, an efficient method is presented for the dynamic response analysis of flexibly supported unanchored tanks. It is shown that the flexibility of the foundation reduces the overturning base moment, and reduces significantly the axial compressive stresses in the tank wall, but these reductions are accompanied by increased values of plastic rotations and (in some cases) base uplifting and hoop compressive stresses.

Cyclic, transverse loads were applied to the bents of a three-span reinforced concrete bridge. At a load equal to 45% of the bridge's weight, the bent drift ratio was 0.5%. The bridge's high stiffness was attributed to its continuous superstructure and stiff abutments, which resisted approximately 80% of the applied load. After the soil surrounding the abutments had been excavated, the bridge's stiffness was 15% of the initial stiffness. After the researchers had isolated the bridge superstructure from the abutments, the stiffness was 9% of the initial stiffness. Despite the bents' poor details, damage was limited to yielding of the wingwalls and column cracking; similar bridges should resist likely transverse seismic motions with little damage. The tests provided estimates of the abutment and bent resistances that can be used to evaluate modeling procedures. Following existing modeling procedures, the researchers assembled a model that reflected the measured nonlinear properties of the concrete, steel, soil, bearing pads and polystyrene. The model reproduced the measured response well.

Two reinforced concrete bridge bents were subjected to large, transverse displacements. The bents contained detailing deficiencies typical of the 1960s, including minimal transverse reinforcement, short reinforcing splices, and a lack of top reinforcement in the footings. Spalling of the concrete cover at the column tops began at a drift ratio of 1.5. At this drift ratio, the column displacement ductility was approximately four, the curvature ductility was eight, and nominal curvature in the plastic hinges corresponded to a nominal maximum concrete strain of 0.01. In spite of their deficiencies, the bents resisted transverse loads equal to nearly 40% of the bridge's weight at a drift ratio of 3%. The ductile response was attributed to a low shear demand and to the influence of the soil in redistributing the rotational demands away from the column splices. The measured response was reproduced well by an analytical model that considered the nonlinear force-deformation relationships of the columns and soil. The results of a parametric study indicated that the soil surrounding the column bases increased the column shear demand by 25%.

A reinforced concrete, box-girder bridge will collapse during an earthquake if the relative displacement between adjacent frames at an in-span expansion joint exceeds the available seat width. To identify factors that most affect these relative displacements, 216 nonlinear response-history analyses were conducted for various frame, abutment, and restrainer properties, as well as for four ground motions. The researchers considered only longitudinal motion of straight bridges without skew, and the ground motions were assumed to be coherent. Maximum relative joint displacements were sensitive to the stiffnesses of adjacent frames, the frames' effective periods, and the restrainer properties. The relative displacement between the frames and abutment seats was sensitive to the overall stiffness and weight of the bridge. Based on the results of the parametric study, new design procedures are proposed for designing in-span seismic restrainers and abutment seats. Examples illustrate both procedures. Compared with existing design methods, the proposed methods result in restrainer designs that are more consistent with the results of nonlinear analysis.

Random vibrations of a bridge structure under propagating, seismic excitations are considered and joint effects of pseudostatic and dynamic vibrations are studied. Respective matrix equations of motion are derived. Detailed numerical analysis of a four-span bridge modeled as a three-dimensional (3D) frame under kinematic wave excitations with an oblique angle of propagation is carried out. Key parameters of seismic loads are identified and their effect on both displacement and force response is analyzed. The root-mean-square response is normalized with respect to uniform excitations. The normalized displacement response stays between 0 and 1 while the normalized force response may exceed 1, indicating additional spatial effects. These effects were identified to result from pseudostatic response, mostly for low apparent wave velocity with respect to support distances. A sensitivity, first passage, analysis is carried out to study the joint effect of propagation velocity and angle on the response.

Seismic isolation bearings consisting of lubricated sliding bearings and steel dampers exhibit nearly elastoplastic behavior. Accordingly, they can transfer force to bridge substructures which is independent of the level and frequency content of seismic excitation. While this is a desirable property, it is achieved at the expense of potential for development of significant permanent displacements. This paper reports on a study of one such system that consists of E-shaped steel dampers and which found a number of applications in bridge seismic isolation. The paper demonstrates that indeed significant permanent displacement develop, particularly in earthquakes with shock-loading characteristics. Moreover, the paper demonstrates the significance of restoring force in isolation systems and concludes that the current American Association of State Highway and Transportation Officials (AASHTO) design guidelines for these systems are appropriate.

In this paper, a passive isolator, an active vibration absorber, and an integrated passive/active (hybrid) control are studied for their effectiveness in reducing structural vibration under seismic excitations. For the passive isolator, a laminated rubber bearing base isolator, which has been studied and used extensively by researchers and seismic designers, is studied. An active vibration absorber concept, which can provide guaranteed closed-loop stability with minimum knowledge of the controlled system, is used to reduce the passive isolator displacement and to suppress vibration. A three-story building model is used for the numerical simulation. The performance of an active vibration absorber and a hybrid vibration controller in reducing peak structural responses is compared with the passively isolated structural response under the N00W component of the El Centro 1940 and N90W component of the Mexico City 1985 earthquake excitation records.

Because of the stochastic nature of earthquakes and limited capacities of actuators, it is conceivable that the actuators used for the control of civil engineering structures may be saturated during strong earthquakes. In this paper, the stability of actively controlled linear structures under a variety of popular control methods during actuator saturation is investigated. Sufficient conditions to guarantee the asymptotic stability of the structure during actuator saturation are examined and discussed. These conditions involve the solution of a system of simultaneous linear matrix inequalities (LMI). A simple and efficient computer code to solve a system of LMI, based on the MATLAB LMI toolbox, is presented for use by the readers. Based on sufficient conditions, a method for designing general controllers, that guarantee asymptotic stability during actuator saturation, is presented. It is shown analytically that Lyapunov controllers, H_∞-type controllers, and sliding-mode controllers are asymptotically stable during actuator saturation. For multi–degrees-of-freedom (MDOF) systems using pole assignment and linear quadratic regulator (LQR) controllers, it has been shown through extensive simulations results that: (1) asymptotic stability of structures is more likely to be guaranteed for low-gain controllers than for high-gain controllers; (2) adding active damping to the structure is much more beneficial than adding active stiffness, in terms of asymptotic stability; and (3) asymptotic stability can be guaranteed for quite high levels of added active damping to various modes of the structure. Further extensive simulation results demonstrate that static output feedback controllers using only velocity feedback are always asymptotically stable in the range of practical applications. Likewise, for static output controllers using collocated sensors, the region of asymptotic stability is quite large.

Periodic interior buckling of regular multistory and multibay rectangular elastic frames with elastic bracing is analyzed. It is shown that there exists a certain critical bracing stiffness for which the critical loads for the nonsway (symmetric) and sway (antisymmetric) buckling modes coincide. Simple formulae for the critical stiffness are given. For the critical and softer bracing, the type of postcritical buckling behavior is the unstable symmetric bifurcation, exhibiting imperfection sensitivity according to Koiter's 2/3-power law. For stiffer bracing, there is no imperfection sensitivity. The critical bracing, however, represents a naive optimal design which should be avoided because the imperfection sensitivity is the strongest. It is recommended that the truly optimal bracing should be significantly stiffer (perhaps 1.1 to 2 times as stiff). The buckling behavior, including the postcritical imperfection sensitivity, is similar to that of a portal frame analyzed before. The solution also provides a demonstration of a simple method for the initial postcritical analysis of frames recently proposed by Bažant and Cedolin, which is based on energy minimization. In this method, the distribution of cross section rotations is assumed to be the same as in the classical linearized theory. The curvatures and deflections are obtained from the rotations by integration with at least a second-order accuracy (in terms of the rotations), and the axial shortening with at least a fourth-order accuracy.

The pseudodynamic test is increasingly being used for testing structures under seismic loads. Due to the limited capacity of available equipment and also to economic reasons, testing is often required to be carried out on scaled models rather than full-scale structures. Although the similitude laws are well established, the selection of a suitable set of scale factors is often complicated due to the infinite number of possible sets. The various aspects to be considered in selecting a suitable set of scale factors are clarified. The question of possible differences in test results due to a difference in testing procedure is answered by means of tests on identical specimens using three typical procedures. It was found that the results obtained can be considered to be identical for practical purposes.

In the most simple case of plastic collapse, it is said that a hinge forms in a member when the moment capacity is reached. In the more complex case of plastic collapse, when the effect of axial load on the moment capacity is considered, it is argued here that an “offset hinge” is formed since as you move along an interaction or limit surface, the incremental moment is proportional to the incremental axial load. In this note the stiffness of a plane, elastic, straight member with offset hinges at either or both ends is derived. An example of plastic collapse taken from the European literature is included.