Showing papers in "Journal of Structural Engineering-asce in 1991"

RC Beams Strengthened with GFRP Plates. I: Experimental Study

Abstract: The static strength of reinforced concrete beams strengthened by gluing glassfiberreinforcedplastic GFRP plates to their tension flanges is experimentally investigated. Five rectangular beams and o...

Modal Analysis for Damage Detection in Structures

Abstract: Nondestructive inspection of structures by modal analysis of vibration response is reported. The dependence of natural frequencies and modal damping coefficients on deterioration in structures is examined. The magnitude of change in natural frequencies is a function of the severity and of the location of deterioration in structures. Ratios of changes in natural frequencies normalized with respect to the largest frequency change are independent of severity for small deterioration and can serve to indicate the location of deterioration directly. Specific deterioration events have an associated characteristic ensemble of ratios of natural frequency changes that may be compiled in advance to form a data base for later interpretation of observed modal parameter changes. The modal analysis method is demonstrated in experiments on a welded steel frame exposed to fatigue loading, and on wire ropes damaged by sawcuts. The method holds promise as a condition monitoring tool for bridges and other skeletal structures.

Establishing R (or Rw) and Cd Factors for Building Seismic Provisions

Abstract: This paper derives the basic formulas for establishing the response modification factor R and the displacement amplification factor Cd used in the National Earthquake Hazards Reduction Program (NEHRP) recommended provisions. An extension of the approach is used to derive the system modification factor Rw used in the 1988 Uniform Building Code. These terms are primarily functions of both the structural overstrength and structural ductility factors. Because of the empirical nature of the NEHRP recommended values for these factors, it is difficult to justify the relative values between R and Cd factors for most of the building systems. The structural overstrength factor, which can be determined from analytical studies, depends on structural redundancy, story drift limitations, multiple load combinations, strain hardening, participation of nonstructural elements, and other parameters. Although the ductility factor of an individual structural member can be determined experimentally, there is no general agreeme...

Fiber pullout and bond slip. I: Analytical study

Abstract: This is the first of a two‐part paper involving a fundamental study of bond in fiber‐reinforced cement composites. The concept of a relationship between the bond shear stress and the local slip at the interface between the reinforcement and the matrix for fiber‐reinforced, cement‐based composites is relatively new. Moreover, the difficulties involved in conducting strain measurements on short discontinuous fibers have made it impossible so far to develop experimental bond shear stress versus slip curves for fiber‐reinforced concrete. This paper involves a fundamental study of the bond‐stress‐slip relationship between steel fibers and concrete. The analysis consists of a primal problem, whereby a complete pullout curve is predicted from an assumed bond‐slip relationship, and a dual problem in which the bond‐slip relationship is obtained from an experimental pullout curve. The frictional behavior of the interface is also related to and explained by the shrink‐fit and fiber‐matrix misfit theory. The theory d...

Fractional‐Derivative Maxwell Model for Viscous Dampers

Abstract: A fractional-derivative Maxwell model is proposed for viscous dampers, which are used for vibration isolation of piping systems, forging hammers, and other industrial equipment, as well as for vibration and seismic isolation of building structures. The development and calibration of the model is based on experimentally observed dynamic characteristics. The proposed model is validated by dynamic testing and very good agreement between predicted and experimental results is obtained. Numerical algorithms for the solution of the constitutive relation in either the frequency or the time domain are presented. Some analytical results for a single-degree-of-freedom viscodamper system are presented. These results are useful to the design of vibration-isolation systems. Furthermore, an equivalent viscous oscillator is defined whose response is essentially the same as that of the viscodamper isolator. Finally, the model is employed in the analysis of a base-isolated model structure that has been tested on a shake table.

Simulation of dynamic load for bridges

Abstract: This paper deals with the analysis of dynamic loads in bridges. Models are developed for trucks, road surface (roughness) and the bridge. The statistical parameters are based on the results of surveys, tests and analysis. Truck parameters include the body (mass), suspensions, and tires. Road profiles are simulated using stochastic processes (power spectral density function). A bridge is treated as a prismatic beam. The Monte Carlo method is used to simulate the traffic on the bridge. Random variables include the truck type, total weight, axle distances, and speed. Calculations are carried out for steel and prestressed concrete girder bridges. The dynamic load factors for a bridge design code are calculated based on the 75 year mean maximum loads. The analysis is performed for single trucks and for two trucks (side by side). Dynamic loads are lower for heavier trucks and also lower for two trucks. The calculated static and dynamic deflections are presented in tables and graphs as a function of gross vehicle weight, and span and axle distance. Simulated deflections indicate that the dynamic component is not correlated with the static component. Therefore, the dynamic loads are lower for heavier trucks.

Nonlinear Dynamic Analysis of 3‐D‐Base‐Isolated Structures

Abstract: In base‐isolated structures specially designed isolation systems provide the needed flexibility and energy dissipation capacity. The isolation systems, which can be either elastomeric or sliding systems, exhibit highly nonlinear behavior. The existing algorithms cannot analyze base‐isolated structures with sliding isolation systems accurately. This paper presents an analytical model and a solution algorithm developed for nonlinear dynamic analysis of three‐dimensional‐base‐isolated structures with elastomeric and/or sliding isolation systems. The novelty of the analytical model and solution algorithm is its capability to capture the highly nonlinear frictional behavior of sliding isolation systems in plane motion. Nonlinear behavior is restricted to the base and the superstructure is considered to be elastic at all times. Biaxial and uniaxial models, which can represent both elastomeric and sliding isolation bearings, are presented. The solution algorithm consisting of the pseudoforce method with iteratio...

RC Beams Strengthened with FRP Plates. II: Analysis and Parametric Study

Abstract: Analytical models based on the compatibility of deformations and equilibrium of forces are presented to predict the stresses and deformations in concrete beams strengthened with fiber composite plates epoxy-bonded to the tension face of the beams. The models are given for beams having rectangular and T cross sections. A parametric study is conducted to investigate the effects of design variables such as plate area, plate stiffness and strength, concrete compressive strength, and steel reinforcement ratio. The moment versus curvature diagrams for various combinations of these variables are plotted and compared. The results indicate that bonding composite plate to a concrete beam can increase the stiffness, yield moment, and flexural strength of the beam. The method is particularly effective for beams with a relatively low steel reinforcement ratio.

Experimental Study of Friction‐Pendulum Isolation System

Abstract: A shake‐table study of the friction‐pendulum isolation system, installed in a six‐story, quarter‐scale, 52‐kip model structure, is presented. Two bearing materials are studied, one with a peak friction coefficient of 0.075 and another of 0.095. In both cases, the isolation system has a rigid‐body mode period of 1 sec. The isolated structure is found to be capable of withstanding strong earthquake forces of different frequency content. In tests with the El Centra motion, the isolated structure sustains, without any damage, a peak ground acceleration six times greater than what it could under fixed‐based conditions. It is found that the bearing displacements are low and that the permanent bearing displacements at the end of free vibration are very small, in general, not exceeding 6% of the bearing design displacement. The system is shown to have quantifiable properties, and analytical techniques are presented that provide accurate prediction of the response.

Closure of "Numerical Solution for Response of Beams With Moving Mass"

Abstract: An analytical-numerical method is presented that can be used to determine the dynamic behavior of beams, with different boundary conditions, carrying a moving mass. This article demonstrates the transformation of a familiar governing equation into a new, solvable series of ordinary differential equations. The correctness of the results has been ascertained by a comparison, using finite element models, and very good agreement has been obtained. Furthermore, the article shows that the response of structures due to moving mass, which has often been neglected in the past, must be properly taken into account because it often differs significantly from the moving force model.

Fiber Pullout and Bond Slip. II: Experimental Validation

Abstract: This is the second of a two‐part paper involving a fundamental study of the bond‐stress‐slip relationship between steel fibers and cement‐based matrices It presents a validation of the analytical model developed in the first part by providing comparisons between experimentally and analytically predicted pullout load curves of steel fibers embedded in cementitious matrices Very good agreement is observed The model is also used to derive, for the first time, bond shear stress versus slip curves typical of fiber‐reinforced concrete and considered a constitutive property of the fiber‐concrete interface This has good practical significance since, while pullout tests of fibers are easy to conduct, there is currently no experimental method that directly yields their bond shear stress versus slip relationship

Bridge live-load models

Abstract: Live load is an important load component for highway bridges. The paper summarizes the available data base and formulates the approach to calculating maximum moments and shears for various time per...

Optimal Steel Frame Design by Simulated Annealing

Abstract: A simulated annealing strategy is developed for use in the discrete optimization of three-dimensional steel frames. This strategy randomly perturbs the current design to create a candidate design. A probabilistic acceptance criterion is then employed to determine whether the candidate design should replace the current design or be rejected. This acceptance criterion allows worse designs to be accepted in the initial stages of the strategy. The likelihood of accepting worse designs is small in the final stages of the strategy. The strategy is presented and illustrated on a three-dimensional, six-story, unsymmetrical frame. The frame is realistically loaded with gravity and seismic loads. Members in the frame must be selected from among discrete standardized shapes. The strategy is able to treat multiple section properties per member without having to curve-fit dependent properties as functions of a single independent property. Performance of the strategy is compared to that of the branch-and-bound method. Approximation techniques aimed at reducing computation time are investigated.

Review of effects of loading rate on reinforced concrete

Abstract: A survey of the behavior of reinforced concrete subjected to dynamic loading is presented. Provisions for seismic design in current design codes have been developed on the basis of results obtained from static tests. Realistic methods of design should take into consideration the strain‐rate‐dependent properties of reinforced concrete in order to accurately predict the behavior of a reinforced concrete structure subjected to dynamic loads. The response of reinforced concrete materials and elements to different strain rates is reviewed and discussed. As rate of loading increases, concrete compressive strength, steel yield strength, and flexural capacity of reinforced concrete member also increase. The increase in flexural capacity of individual members as a result of high strain rates, if significant, may shift the failure mode of a structure from a preferred ductile manner to a less desirable brittle mode.

Full‐Scale Implementation of Active Control. I: Design and Simulation

Abstract: An active bracing system has been designed and fabricated for implementation to a full‐scale dedicated test structure for structural response control under seismic loads. Presented in this paper, the first part of a two‐part series, are the design of the active system, control algorithm development, and simulated control results under design earthquakes. Detailed design and analysis of the active system are carried out with respect to hardware development, control force constraints, and power and energy requirements. It is shown that a full‐scale efficient active structural control system can be developed within limits of current technology. Simulation results provide information on performance bounds that can be expected of active systems in structural control under seismic loads and under constraints imposed by practical considerations. The installation and observed performance of the system under actual ground motions will be the subject of the second paper in this series.

Reliability of highway girder bridges

Abstract: Reliability procedures are developed for girder bridges. Load models are based on the available statistical data. The derivation of load models is described in other papers. Resistance models are developed by simulations using the available test results for materials and components. The girder behavior is described by a moment curvature relationships. Bridge capacity is determined in terms of the maximum truck load before failure. The analysis is performed for a single unit truck, a semitrailer, and combinations of these trucks. For a given truck position, the load is gradually increased until the deformation (maximum deflection) exceeds the critical level. The developed resistance models are used in the reliability analysis. Reliability indices are calculated for noncomposite and composite steel girders, reinforced concrete T-beams, and prestressed concrete girders. The calculations are performed for girders and structural systems. Reliability indices for the system are higher than for girders. The effect of correlation between girder strengths is analyzed. Girder bridges behave as parallel systems. Sensitivity functions are developed for various parameters related to the considered girders. The developed approach is demonstrated on typical girder bridges.

Review of Effects of Loading Rate on Concrete in Compression

Abstract: A survey of the behavior of concrete subjected to dynamic loading is presented Since the mechanical properties of materials are functions of the rate of loading, they need to be determined with re

Identification of Linear Structures Using Discrete‐Time Filters

Abstract: Most of the previous studies considered structural system identification in the continuous‐time domain. The discrete‐time approach to the problem is more natural since all the recordings are in the discrete‐time form. This study presents a discrete‐time method for system identification by using discrete‐time linear filters. The method itself is well known in electrical and systems engineering fields, and therefore is not new. The objective in the paper is to present the method by emphasising its relation to the more familiar continuous‐domain modal analysis approach that is widely used in structural engineering. In addition to the method, some practical but important problems are also discussed in the paper, such as the processing of data, the selection and validation of models in the identification, and the detection of soil‐structure interaction. As an example, a 12‐story building was identified by using recordings from the magnitude 6.4, San Fernando, California earthquake of February 9, 1971.

Flexure and Shear Behavior of Concrete Beams during Fires

Abstract: This paper describes the results of a research program to study the behavior of reinforced concrete beams exposed to severe fires. Data are presented from fire tests of six full-scale beams continuous over one support that were fabricated using normal-weight concrete. Four beams were exposed to the standard ASTM E119 fire, and two to a short-duration high-intensity (SDHI) fire developed using realistic fire-load and compartment-ventilation parameters. All six beams developed significant shear cracks near the continuous support rather early in the fire, but eventually failed from excessive flexural cracking and deformation. Mathematical models for predicting thermal and structural response of concrete beams exposed to natural fires, as well as the ASTM standard exposure, were also developed. These models predicted the experimental behavior with sufficient accuracy for purposes of limit-states design.

Behavior of rc frame strengthened using structural steel bracing

Abstract: A retrofit strengthening technique for improving the seismic behavior of a nonductile reinforced concrete (RC) frame was investigated experimentally. The original frame contained deep, stiff spandrel beams and short, flexible columns that were susceptible to shear failure under lateral loads. The strengthening technique used a steel X‐bracing system attached to the exterior of the frame using epoxy‐grouted dowels. A two‐thirds‐scale frame model consisting of two bays and three levels was subjected to statically applied cyclic lateral load. Test results indicated substantial increases in both lateral stiffness and strength. The lateral capacity of the strengthened frame was governed by brace buckling and eventual connection failures and column shear failures. Measured brace buckling loads agreed well with predicted capacities assuming fixed (K=0.5) connections, and dowel performance was excellent. Bracing‐system elements attached to the side faces of the concrete columns also substantially increased column...

Role of Stirrups and Residual Tensile Strength of Cracked Concrete on Bond

Abstract: The local bond‐slip law of an anchored ribbed bar after the splitting of its surrounding concrete is studied. Particular attention is devoted to understanding the reason why several experimental results show a different variability of this law along the bar. Actually, the bond is governed by the local confining action, which can vary; and its variability depends on both concrete mechanical properties and geometrical characteristics of the specimen. A significant contribution to this action is due to the transverse reinforcement and the residual tensile strength of splitted concrete. Another contribution is due to the structural characteristics, such as the transverse support restraints and possible lateral loads. A refined analytical model involving the local mechanical properties of concrete and the interaction phenomena between the principal and transverse reinforcement is developed. Experimental confirmations of the theoretical results are also presented. Finally, diagrams that are significant for desi...

Validation of design recommendations for integral abutment piles

Abstract: Because integral‐abutment bridges decrease the initial and maintenance costs of bridges, they provide an attractive alternative for bridge designers. The objective of this project is to develop rational and experimentally verified design recommendations for piles in these bridges. Field testing consists of instrumenting two bridges in Iowa to monitor air and bridge temperatures, bridge displacements, and pile strains. Core samples are also collected to determine coefficients of thermal expansion for the two bridges. Design values for the coefficient of thermal expansion of concrete and revised temperature ranges for the deck and girders of steel and concrete bridges are recommended. A girder extension model is developed to predict the longitudinal bridge displacements caused by changing bridge temperatures. The pile is idealized as an equivalent cantilever. The frame model better predicts both the longitudinal displacement and weak‐axis pile strains. A lateral frame model is presented to predict the later...

Evaluation of Torsional Provisions in Seismic Codes

Abstract: The effects of plan asymmetry on the earthquake response of code-designed, one-story systems are identified with the objective of evaluating how well these effects are represented by torsional provisions in building codes. The earthquake-induced deformations and ductility demands on resisting elements of asymmetric-plan systems, designed according to several different codes, are compared with their values if the system plan were symmetric. The presented results demonstrate that the design eccentricity in building codes should be modified in order to achieve the desirable goal of similar ductility demands on asymmetric-plan and symmetric-plan systems. The design eccentricity should be defined differently for elastic and inelastic systems; in the latter case, it should vary with the design force level or anticipated degree of inelastic action. However, it does not appear possible to reduce the additional element deformations due to plan asymmetry by modifying the design eccentricity; these deformations should be provided for in building design.

Load-time response of colliding concrete bodies

Abstract: Concrete‐to‐concrete collisions may occur between breakwater armor elements In assessing the resistance of the individual armor elements, next to concrete technology aspects and hydraulic aspects, information should be obtained regarding the load‐time history of concrete‐to‐concrete impact The latter aspect is studied in a series of dynamic experiments on different contact surface geometries Variables in the experiments are the impact velocity, the concrete quality, the mass of the striker element, and the contact surface geometry, ie, spherical/planar, conical/planar, truncated conical/planar, and spherical/corrugated geometries of the striker/target surface A simple elastoplastic contact model is developed, based on four parameters that can be determined from static tests on the same contact surface geometry

3‐D Nonlinear Seismic Behavior of Cable‐Stayed Bridges

Abstract: The dynamic behavior of three-dimensional (3-D) long-span cable-stayed bridges under seismic loadings is studied. The cases of synchronous and nonsynchronous support motions due to seismic excitations of these flexible structures are considered; furthermore, effects of the nondispersive traveling seismic wave on the bridge response are studied. Different sources of nonlinearity for such bridges are included in the analysis. Nonlinearities can be due to: (1) Changes of geometry of the whole bridge due to its large deformations including changes in the geometry of the cables due to tension changes (known as the sag effect); and (2) axial force and bending moment interaction in the bridge tower as well as the girder elements. A tangent stiffness iterative procedure is used in the analysis to capture the nonlinear seismic response. Numerical examples are presented in which a comparison is made between a linear earthquake-response analysis (based on the utilization of the tangent stiffness matrix of the bridge at the dead-load deformed state) and a nonlinear earthquake-response analysis using the step-by-step integration procedure. In these examples, two models having center (or effective) spans of 1,100 ft (335.5 m) and of 2,200 ft (671 m) are studied; this range covers both present and future designs. The study sheds some light on the salient features of the seismic analysis and design of these long contemporary bridges.

Buckling of Variable Cross‐Section Columns: Integral‐Equation Approach

Abstract: A semianalytical procedure is presented for the axial buckling of elastic columns with step-varying profiles. Profiles with continuous variations can be approximated, to any desired degree of accuracy, by a series of step variations. The formulation leads to a general procedure that can be applied to any continuous or discontinuous profile regardless of the number of steps. Since the step changes of the profile are represented by distributions, the differential equation cannot be solved in the ordinary sense. The differential equation is therefore converted to an integral equation. The solution of the integral equation is obtained by polynomial functions. The formulation involves a significant amount of algebraic manipulations. This problem is alleviated by using a symbolic manipulation system to carry out the algebraic manipulations. The integral kernels are obtained for the different boundary conditions. Formulas for buckling loads for members with variable profiles an different boundary conditions can be obtained in terms of the section and profile parameters. The example problems present the solution of some common columns with variable cross sections. The procedure can be used to derive the elastic and geometric stiffness matrices for beam columns with variable cross sections as well as for other elements.

Earthquake-Induced Base Sliding of Concrete Gravity Dams

Abstract: Analytical procedures are developed considering hydrodynamic effects to determine the response history of earthquake-induced sliding of a gravity dam monolith supported without bond on a horizontal, planar surface of rock. The results presented indicate that the permanent sliding displacements of dams induced by ground motions with peak acceleration of 0.5g may range from a few inches to a couple of feet. Also examined in this work is the possibility of estimating the sliding displacement of a flexible dam due to ground acceleration by analyzing the sliding response of a rigid dam due to an average (over height) acceleration computed from linear analysis of the flexible dam without sliding at the base. The results presented indicate that this approximate procedure, which has been widely used in estimating the deformations of embankment dams, can provide an order-of-magnitude estimate of the concrete dam sliding displacement, which is conservative for most cases when this displacement is practically significant.

Reliability of Short Composite Beam‐Column Strength Interaction

Abstract: The strength of a short composite steel-concrete beam column is affected by variations in the strengths of concrete and steel, the cross-sectional dimensions of concrete and steel section, the placement of steel section and reinforcing bars, and the strength model itself. The effects of these factors on the variability of the ultimate strength of square short composite beam columns in which steel shapes are encased in cast-in-place concrete are studied. The results indicate that the specified concrete strength, the structural steel ratio, and the end eccentricity ratio influence the probability distribution properties of short composite beam-column strength. The end eccentricity ratios of 0.5 or less are critical since they have a very significant effect on the beam-column strength. As expected, the beam columns with slenderness ratios near the ACI Building Code upper limit for short columns produce lower strengths than the corresponding cross sections.

T-Joints in Rectangular Hollow Sections Subject to Combined Actions

Abstract: An extensive test program on a range of cold‐formed rectangular and square hollow sections subjected to combined bending and concentrated force is described. The concentrated force was applied by welding a square hollow section (called the branch member) to the center of the member under test (called the chord member) and perpendicular to it. The chord member was simply supported with the concentrated force at the center of the span. The span was varied in order to determine the interaction relationship between bending moment and concentrated force. Tests under pure moment and pure concentrated force were performed for control purposes. The ratio of the width of the branch member to the chord member (β) used varied from 0.5 to 1.0 so that failure modes ranging from local bending and buckling of the face of the chord member to web crippling were observed. In addition, the thickness of the chord‐member sections was varied so as to produce a range of section slenderness and failure modes from buckling to yie...

Effect of Base Uplift on Dynamic Response of SDOF Structures

Abstract: In this study, a parametric investigation for the case of a single-degree-of-freedom (SDOF) oscillator and a harmonic excitation is performed. The simplest case of a rigid base on a rigid foundation is examined first. Based on the results of this analysis, an empirical foumula is established, which permits the computation of the maximum of deformation. Using this formula, an approximate method for the estimation of earthquake-response spectra for uplifting structures utilizing linear-repsonse spectra is proposed. This method gives reasonably good results for all periods. The analysis is then extended to the case of flexible foundations. The behavior of the system described here is nonlinear overall, although the equations of motion can be linearized in each of the two regimes of the response, during full contact and after uplift. The details of the study are described, and the results are discussed.