Passive isolator, 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 considered. 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 the top floor 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 and with absence of vibration control systems under the N00W component of El Centro 1940 and N90W component of the Mexico City earthquake excitation records. The results show that the integrated passive/active vibration control system is most effective in suppressing the peak structural acceleration for the El Centro 1940 earthquake when compared with the passive or active vibration absorber alone. The active vibration absorber, however, is the only system that suppresses the peak acceleration of the structure for the Mexico City 1985 earthquake.
This paper investigates the in-plane nonlinear elastic and inelastic buckling behaviour and the strength of fixed circular steel arches by using a rational finite element model. It is found that the elastic and inelastic buckling behaviour of a fixed arch is quite different from that of a pin-ended arch. The design equation for pin-ended steel arches in uniform compression cannot be used directly for the design of fixed steel arches, nor can the design interaction equation for pin-ended steel arches be used for the design of fixed steel arches that are subjected to combined axial compressive and bending actions produced by general in-plane loading. A design equation for the strength of fixed steel arches that are subjected to uniform compression is proposed. The finite element investigations show that this proposed design equation provides good predictions for the strengths of fixed steel arches in uniform compression. An interaction equation for strength design of fixed steel arches that are subjected to combined bending and axial compressive actions against in-plane failure is also proposed. The finite element investigations show that the proposed design equation provides good lower bound predictions for the strengths of fixed steel arches in combined compressive and bending actions.
This paper outlines the design economics, cost function and modelling of optimal design of reinforced concrete beams for different design conditions. It is shown that there can be large variations in the cost of a beam depending on the unit cost of the materials and shuttering, the beam dimensions and the reinforcement ratio. As there are an infinite number of alternative beam dimensions and reinforcement ratios that yield the same moment of resistance, it becomes difficult to achieve the least-cost design by the conventional methods. This paper presents a geometric programming model which gives the unique least-cost design of a beam, considering the cost of materials and shuttering and the structural requirements. Application of this new design technique is illustrated with example problems.
An experimental study of block shear of coped beams with a welded clip angle connection is presented in this paper. Twelve full-scale coped steel I-beam tests were conducted. The test parameters included the web block aspect ratio (height to width ratio of the web block connected to the clip angle) and the connection rotational stiffness. Out of the 12 test specimens, eight test specimens failed in block shear of the connection, namely, tensile fracture of the block width (the web underneath the clip angle) and shear yielding of the block height (the web along the vertical side of the clip angle). Two test specimens failed by local web buckling at the cope, one test specimen failed in the welds and the remaining one did not fail due to the limited capacity of the loading jack.The test results showed that in general the block shear capacity of the test specimens increased with increasing web block aspect ratio and increasing connection rotational stiffness. The current design specifications (AISC-LRFD, CAN/CSA-S16-09, CAN/CSA-S16-01, BS EN 1993-1-8-2005, BS5950-1:2000, and AIJ-1990) provide conservative estimates of the block shear capacity of the test specimens except for the specimen that had the smallest connection rotational stiffness. It should be noted that none of the design equations evaluated in this programme consider the influence of the web block aspect ratio and the connection rotational stiffness.
An efficient computational method for lowering the cost of the free vibration, stress, and buckling analyses of multilayered composite cylinders is presented. The analytical methodology is based on the linear three-dimensional theory of elasticity where the cylinders are assumed to have simply supported curved edges, and the fibers of the different layers are either in the circumferential or in the longitudinal direction. The full equations of the finite element model are solved for a single pair of Fourier harmonics, and the response that corresponds to the other Fourier harmonics is generated utilizing a reduced system with considerably fewer degrees of freedom.
: The critical design decisions in bridge design are made at the preliminary design stage. This stage depends on the expertise of the designer, built up from extensive experience. Experience is difficult to acquire, and may be entirely lacking when new technology is introduced. As a result, there is little shareable and transferable collective design knowledge within the profession. This paper explores how preliminary design knowledge may be generated, updated and used, utilizing techniques of machine learning from the field of artificial intelligence. A model of the preliminary design process is first presented as a sequence of five tasks and then specialized to the design of cable-stayed bridges. A computer tool serving as a design support system is described whose design follows the model of the preliminary design process, and a design example using the tool is presented. The key property of the system is its adaptive nature: it acquires knowledge from information on existing bridge...
This paper summarizes the results of a comprehensive statistical study of inelastic displacement ratios that allow the estimation of maximum lateral inelastic displacement demands from maximum elastic displacement demands for structures built on soft soil sites. These ratios were computed for single-degree-of-freedom systems undergoing six levels of inelastic deformation when subjected to 116
earthquake ground motions recorded on bay-mud sites of the San Francisco Bay Area and on sites in the former lake-bed zone of Mexico City. These soft soil deposits are characterized by low shear wave velocities, high water contents, and high plasticity indices. The influence of period of vibration normalized by the predominant period of the ground motion, the level of inelastic deformation, earthquake magnitude, and epicentral distance are evaluated and discussed. Mean inelastic displacement ratios and their corresponding dispersion are presented. The effect of stiffness degradation on inelastic displacement ratios is also considered. For this purpose, mean ratios of
maximum inelastic displacement demands of stiffness degrading systems to maximum inelastic displacement demands of nondegrading systems are presented. Finally, a simplified equation to estimate mean inelastic displacement ratios obtained through nonlinear regression analyses is provided to aid designers estimate inelastic displacement demands of structures built on soft soil sites.
A new connection system for a concrete filled steel tube composite column and reinforced concrete beams is proposed. In this connection, the steel tube is interrupted while the reinforced concrete beams are continuous in the joint zone. Multiple lateral hoops that constitute the stiffening ring are used to confine the core concrete in the connection zone. The transfer of moment at the beam ends can be ensured by continuous rebars; the weakening of the axial load bearing capacity due to the interruption of the steel tube can be compensated by the confinement of the stiffening ring. Using these configurations, concrete casting and tube lifting can be made more convenient since welding and hole drilling in situ can be avoided. Axial compression experiments on six specimens and reversed cyclic loading tests on three interior column specimens and three corner column specimens were conducted to evaluate this new beam-column system; load-deflection performance, typical failure modes, stress and strain distributions, and the energy dissipation capacity were obtained. The experimental results showed that the effective confinement can be achieved by the stiffening ring, and an excellent axial bearing capacity can be obtained, as well as a superior ductility and energy dissipation capacity. As a new connection system for the concrete filled steel tube composite column with reinforced concrete beams, it can also be applied to other types of confined concrete columns.
A near full-scale 3D jointed precast prestressed concrete beam to column connection designed and constructed in accordance with an emerging Damage Avoidance Design (DAD) philosophy is tested under displacement controlled quasi-static reverse cyclic loading. The performance of the subassembly is assessed under unidirectional loading along both orthogonal directions as well as under concurrent bi-directional loading. The specimen is shown to perform well up to 4% column drift with only some minor flexural cracking in the precast beams, while the precast column remains uncracked and damage-free. This superior performance is attributed to steel armoring of the beam-ends to mitigate the potential for concrete crushing. Under bi-directional loading a tapered shear-key layout is used to effectively protect the beams against adverse torsional movements. A three-phase force-displacement relationship is proposed which gives due consideration to: the prerocking flexural deformation of the beam; the rigid body kinematics during the rocking phase; and the yielding of the external dissipaters and post-tensioning tendons. Good agreement between the proposed theoretical model and experimental observation is demonstrated. An equivalent viscous damping model is also proposed to represent both change in the prestress force in the subassembly and yielding of the supplemental energy dissipaters in the rocking connection.
This paper discusses the optimum design of tuned mass damper (TMD) for seismically excited building structures. In the design process the multi degree of freedom structures are considered so that it makes improvement to the available design procedures so far, where usually only single mode model is considered. The H-2 norm of the transfer function from the external disturbance to a certain regulated output is taken as a performance measure of the optimization criterion. The genetic algorithm, which has been successfully applied in many applications, is used to find the optimum value of TMD parameters. The numerical examples for optimum parameters of TMD for multi degree of freedom structures are presented to show the effectiveness of this design procedure. II is shown that by using the proposed procedure, the optimum value of the mass damper can be determined without specifying the modes to be controlled. A comparison is also made to the Den Hartog and Warburton approaches.
A tensegrity is a lightweight space structure consisting of compression members surrounded by a network of tension members. They can be easily dismantled and therefore provide innovative possibilities for reusable and modular structures. Tensegrities can adapt their shape by changing their self stress, and when equipped with sensors and actuators, they can adapt to changing environments. A full-scale prototype of an adjustable tensegrity has been built and tested at Swiss Federal Institute of Technology (EPFL). This paper begins with a description of important aspects of the design, assembly, and static testing. Tests show that the structure behaves linearly when subjected to vertical loads applied to a single joint. Nonlinearities are detected for small displacements, for loads applied to several joints and for adjusting combinations of telescoping compression members. To predict behavior, dynamic relaxation - a nonlinear method - has been found to be reliable. Appropriate strut adjustments found by a stochastic search algorithm are identified for the control goal of constant roof slope and for the load conditions studied. When adjusting struts, an excessive number of adjustable members does not necessarily lead to improved performance.
Wind pressure differences were measured across the rain screen and across the air barrier assembly of precast open rain screen wall panels. Twelve panels were instrumented mostly on the west, north and east walls of the 24th floor of a 27 storey office building in Montreal, U.S.A. The maximum load measured on the rain screen in one year of continuous monitoring was 285 Pa, duration of one second. The largest pressure difference across an entire panel of the building envelope never coincided with peak pressure differences on the rain screen. However, maxima due to wind only, across windward panels, ranged from 400 Pa to 475 Pa. Pressure differences across the air barrier assembly of wall panels included stack and heating, ventilating and air conditioning effects of up to 150 Pa when outside temperatures dropped to -20DEGREESC.
DOI: 10.1061/(ASCE)0733-9445(2003)129:10(1312) This paper delineates the development of a prototype hybrid knowledge-based system for the optimum design of liquid retaining structures by coupling the blackboard architecture, an expert system shell VISUAL RULE STUDIO and genetic algorithm (GA). Through custom-built interactive graphical user interfaces under a user-friendly environment, the user is directed throughout the design process, which includes preliminary design, load specification, model generation, finite element analysis, code compliance checking, and member sizing optimization. For structural optimization, GA is applied to the minimum cost design of structural systems with discrete reinforced concrete sections. The design of a typical example of the liquid retaining structure is illustrated. The results demonstrate extraordinarily converging speed as near-optimal solutions are acquired after merely exploration of a small portion of the search space. This system can act as a consultant to assist novice designers in the design of liquid retaining structures.
Keywords: Algorithms; Knowledge-based systems; Liquids; Structural design.
A truck passing over a bridge induces loads which vary with time. To test the ability of available analytical techniques to estimate fatigue life under such loading, crack growth rate tests are conducted with eleven compact‐type specimens using constant and variable amplitude load‐time histories. One variable amplitude load‐time history used in these tests is recorded from an in‐service bridge; other histories used are constructed to isolate minor cycle amplitude and minor cycle mean relative to major cycle mean as test variables. The data show that the mean level of minor cycles within a complex cycle significantly affects the damage caused by the complex cycle, causing the standard rainflow counting‐Miner's rule fatigue life estimating technique to be unconservative. Furthermore, cycles below the constant amplitude fatigue stress range threshold are shown to be damaging when applied as part of a variable amplitude load‐time history. It is proposed that complex load‐time histories be converted to single equivalent cycles through a damage index.
This paper investigates the minimum and maximum crack spacings in concrete pavements from the energy viewpoint and explores mechanisms that control spacing. An analytical model, which is composed of two cohesive cracks and an elastic bar restrained by distributed elastic springs, is proposed as an idealization of the cracking pattern in the concrete. By varying the length of the elastic bar of the analytical model, the tensile forces acting on the cohesive cracks and the energy profiles are investigated. It is demonstrated that the cracking pattern varies with the length of the elastic bar (i.e., the spacing between the two possible cracks), from which the minimum and maximum crack spacings are obtained. Numerical analyses are made of a model pavement and the results indicate that it is the energy minimization principle that governs the cracking pattern. The practical spacings evaluated by numerical analyses fall within the minimum and maximum crack spacings given by the practical observation.
Structural analyses of tensegrity structures must account for geometrical nonlinearity. The dynamic relaxation method correctly models static behavior in most situations. However, the requirements for precision increase when these structures are actively controlled. This paper describes the use of neural networks to improve the accuracy of the dynamic relaxation method in order to correspond more closely to data measured from a full-scale laboratory structure. An additional investigation evaluates training the network during the service life for further increases in accuracy. Tests showed that artificial neural networks increased model accuracy when used with the dynamic relaxation method. Replacing the dynamic relaxation method completely by a neural network did not provide satisfactory results. First tests involving training the neural net work online showed potential to adapt the model to changes during the service life of the structure.
Recent earthquakes such as Loma Prieta, Northridge, and Kobe have demonstrated a need for a new design philosophy of bridge piers that avoids damage in order to ensure post-earthquake serviceability and reduce financial loss. Damage Avoidance Design (DAD) is one such emerging philosophy that meets these objectives. DAD details require armoring of the joints; this eliminates the formation of plastic hinges. Seismic input energy is dissipated by rocking coupled with supplemental energy dissipation devices. In this paper the theoretical performance of a DAD bridge pier is validated through bi-directional quasi-static and pseudodynamic tests performed on a 30% scale specimen. The DAD pier is designed to rock on steel-steel armored interfaces. Tension-only energy dissipaters are used to increase tie down forces and further reduce dynamic response. The seismic performance of the DAD pier is compared to that of a conventional ductile pier. Results show that one can have 90 percent confidence that the DAD pier will survive a design basis earthquake without sustaining any damage, whereas for the conventional design substantial damage is sustained.
Reactions, moments, displacements, and rotations due to axle loading in a two‐span continuous, composite‐steel girder test bridge were analyzed and compared with those calculated by finite element, American Association of State Highway and Transportation Officials (AASHTO), and the National Cooperative Highway Research Program (NCHRP) analysis methods. The simulated truck axle loads were applied on the test bridge on one, two, and three lanes to maximize positive moment at 0.4 L and negative moment at 1.0 L. The results of the study show that finite element analysis most accurately predicted the bridge behavior under the truck axle loading. The AASHTO and NCHRP analysis methods gave inaccurate results, since the loads were applied away from the supports.
Experimental and theoretical studies have been performed to predict the fire resistance of circular hollow steel columns filled with bar-reinforced concrete. A mathematical model to calculate the temperatures, deformations, and fire resistance of the columns is presented, Calculated results are compared with those measured. The results indicate that the model is capable of predicting the fire resistance of circular hollow steel columns, filled with bar-reinforced concrete, with an accuracy that is adequate for practical purposes. The model enables the expansion of data on the fire resistance of circular concrete-filled steel columns, which at present consists predominantly of data for columns filled with plain concrete, with that for columns filled with bar-reinforced concrete. Using the model, the fire resistance of circular concrete-filled steel columns can be evaluated for any value of the significant parameters, such as load, column-section dimensions, column length, and percentage of reinforcing steel without the necessity of testing.
Experimental and theoretical studies have been carried out to predict the fire resistance of rectangular hollow steel columns filled with bar-reinforced concrete. A mathematical model to calculate the temperatures, deformations, and fire resistance of the columns is presented in this paper. Calculated results are compared with those measured. The results indicate that the model is capable of predicting the fire resistance of square hollow steel columns, filled with bar-reinforced concrete, with an accuracy that is adequate for practical purposes. The model enables the expansion of data on the fire resistance of square concrete-filled steel columns, which at present consists predominantly of data for columns filled with plain concrete with data for columns filled with bar-reinforced concrete. Using the model, the fire resistance of square concrete-filled steel columns can be evaluated for any value of the significant parameters such as load, column-section dimensions, column length, and the percentage of reinforcing steel, without the necessity of testing.
This paper is primarily concerned with the formation of barrel-vault space trusses derived from a flat configuration by posttensioning. To obtain a fairly general picture of the shape-formation process by means of posttensioning, both gently and sharply curved barrel-vault models are studied. The structures are devised to be near mechanisms during the posttensioning operation. They are considered to be near mechanisms because only the flexural stiffness of the top chords provides any appreciable resistance to deformation apart from friction and self weight. Therefore, no appreciable axial force is induced in the members of the structures during the shape-formation process. The result of experimental and theoretical work on the shape formation of barrel-vault space trusses by means of posttensioning is presented here. The shape-formation process, referred to as self-erection, can lead to significant economies in the construction of large-span lightweight structures by eliminating or minimizing the need for scaffolding and heavy cranes.
A series of shallowly embedded steel column base consisting of an exposed column base and a covering reinforced concrete floor slab were tested under horizontal cyclic loading to very large deformation. By adjustments to the floor slab thickness, shape, and reinforcing bars in the slab, the initial stiffness, maximum strength, and dissipated energy of the shallowly embedded column base increase significantly with respect to those of the exposed column base. It is found to be practical to strengthen the shallowly embedded column base so that it would behave like a fully embedded column base. Punching shear failure in the floor slab around the column due to the uplift of the base plate occurs when the shallowly embedded column base fails. Based on the plastic theory, a mechanical model that considers the contributions of the anchor bolts and the bearing and punching shear of the floor slab is proposed to evaluate the maximum strength. The evaluated results have good agreement with the test results, with errors not greater than 20%.
When crest-fixed thin steel roof and wall cladding systems are subjected to wind uplift or suction loading, local pull-through or pull-out failures occur prematurely at their screwed connections. During high wind events such as storms and hurricanes these localised failures then lead to severe damage to buildings and their contents. In recent times, the use of thin steel battens/purlins has increased considerably. This has made the pull-out failures more critical in the design of steel cladding systems. Recent research has developed a design formula for the static pull-out strength of screwed connections in steel cladding systems. However, the effects of fluctuating wind uplift or suction loading that occurs during high wind events are not known. Therefore a series of cyclic wind uplift/suction tests has been undertaken on connections between thin steel battens made of different thicknesses and steel grades, and screw fasteners with varying diameter and pitch. Tests revealed a significant reduction to pull-out strength caused by fluctuating wind loading. Simple design equations and suitable recommendations were developed to take into account this strength reduction. This paper presents the details of the cyclic tests and the results.