Journal of Structural Engineering

Clause 10.15.3 of the 1995 ACI code (ACI 318-95) defines the effective strength of a high-strength concrete interior column with an intervening normal strength concrete slab. This provision is based on laboratory tests in which no load was applied to the slab. Such tests may overestimate the strength of the column because they do not properly model the confinement conditions of the column-slab joint. This paper reports experimental results of tests with properly modeled confinement conditions. The test results show that the intensity of slab load, the aspect ratio of the joint, and the rectangularity of the column affect the strength of an interior column-slab joint. A new design equation replacing the provision in Clause 10.15.3 is proposed.

In this study, 16 reinforced concrete (RC) simple-span beams were tested statically up to failure to quantitatively investigate the arch action. Variables included four shear-span ratios (2, 2.5, 3, and 4), two longitudinal steel ratios (1 and 2%), and the presence or absence of stirrups. The measured longitudinal reinforcement tensions in the shear span were significantly higher than the calculated values by beam theory. This may be attributed to the reduction of the internal moment arm length by the development of the arch action. The measured steel tension in a shear span was resolved into two components—one for beam action and one for arch action. After reaching the cracking load, the ratio of steel tension from arch action to that from beam action was constant, ranging from 0.2 to 1.2 at the center of the shear span. At ultimate load, over the entire length, the reinforcement force in the beams without web reinforcement and with shear span-to-depth ratio smaller than 3 was almost constant because the beams acted as tied arches. On the basis of the experimental findings, an equation is proposed to predict the internal moment arm length, which then leads to a new method to calculate the steel tension.

Instability of reinforcement in adequately detailed reinforced concrete (RC) structural elements is a symptom of excessive deformation in compression, and it typically marks the limit of usable ductility of the member. Because the supply of ductility (deformability) is a measure of the effectiveness of the confining reinforcement, detailing necessary for reinforcement stability is intimately related to that required for confinement. Existing design models consider the two problems independently, and associate the initiation of bar buckling with the occurrence of a critical milestone of material behavior under compression. The resulting requirements for lateral support of the reinforcement do not correlate well with those obtained from studies for confinement and rarely control in cases of earthquake design. The objective of this paper is to develop alternative requirements for reinforcement stability that recognize the interaction between displacement ductility demand in the critical section, tie effectiveness, limiting concrete strain, bar size, and tie spacing. To quantify this relationship an extensive database consisting of over 300 column tests was compiled from international literature. Tests considered in the database were conducted under repeated axial and flexural load reversals, simulating earthquake effects on specimens with a variety of confining and longitudinal steel arrangements. The database was systematically analyzed to identify from experimental evidence the critical condition for reinforcement stability and its relationship to the limiting average compressive strain and/or ductility demand in the plastic hinge zone, and to the confinement geometry and effectiveness. Recognizing the interdependence between critical buckling conditions and confinement presents an opportunity to relate detailing requirements for the stability of reinforcement to detailing for ductility, and to distinguish between designs intended to satisfy different levels of deformation demand required for construction in various zones of seismic risk.

In conventional design of shear-dominated reinforced concrete elements shear resistance is assumed to comprise reinforcement and concrete contributions. Because the design code assumes these contributions to be uncoupled and independent of the load history, the estimated nominal shear strength of reinforced concrete is insensitive to the number and intensity of imposed load cycles. However, in accordance with recent experimental evidence, the concrete contribution to shear resistance degrades with increasing ductility demand, thereby rendering the estimated nominal shear strength conservative for low levels of deformation, and increasingly unconservative for larger imposed displacements. This paper attempts to establish the parametric relationship between shear strength and deformation demand through a nonlinear analytical model of cyclic plane-stress states in reinforced concrete. The formulation establishes tensorial, constitutive, and equilibrium relations, all expressed in terms of average stresses and strains. The model was used to conduct a parametric investigation of several design variables. The rate of strength degradation and ductility capacity were used to gauge the parametric sensitivity of the problem. It was found that the calculated concrete shear contribution degraded with increasing deformation demand and was enhanced by an increase in the level of axial load and the amount of transverse reinforcement. A simplified design equation was developed from the analytical results to express the dependence of the concrete contribution term on the intensity of imposed deformation demand.

A novel yield criterion that includes different strengths along each material axis is presented. The criterion includes two different fracture energies in tension and two different fracture energies in compression. The ability of the model to represent the inelastic behavior of orthotropic materials is shown, and a set of experimental tests to characterize the constitutive behavior of masonry is proposed. The capability of the model to reproduce the strength behavior of different masonry types is demonstrated through a comparison with available experimental data in masonry panels subjected to uniform biaxial loading conditions. Finally, the model is validated against experimental results on masonry shear walls and good agreement is found. A clear understanding of the behavior of masonry shear walls, and the nonlinear phenomena involved in its collapse, is presented with the help of a detailed comparison between numerical and experimental results.

A numerical model for the study of spatial structures consisting of curved, three-dimensional members with variable cross sections is presented, together with its application to the nonlinear geometric and material analysis of skeletal masonry constructions. Nonlinear material behavior is included in the model by means of elastoplastic constitutive equations under shear and compressive stresses, while a linear-elastic perfectly brittle behavior is assumed in tension. The dependency of shear strength upon the applied compression is taken into account by means of the Mohr-Coulomb failure criterion. Nonlinear geometric effects caused by the imposition of the equilibrium condition upon the deformed configuration of the structure are considered, but it is assumed that the increments of both displacements and sectional rotations are moderately small. Three examples are presented. The first is a circular helicoid formerly studied by Young and Scordelis (1958). A very good agreement and accuracy level were obtained, even though a very small number of elements, two or three, were used. The second example deals with the analysis of a masonry arch up to failure including nonlinear material and geometric effects. Finally, the advantage of the presented formulation in the analysis of large structures is shown through the study of a Gothic vault.

Fire resistance of floor structures using steel sections has been modeled using bending-moment resistance calculations based on temperature data measured in fire tests. Three types of floors are investigated: (1) conventional composite floor using protected I-beam; (2) noncomposite SLIMFLOR; and (3) composite SLIMDEK asymmetric beam slim floor systems. The effect on construction using fire-resistant steels is investigated; the steels under consideration having a strength reduction factor of around 0.5 at 700°C, an increase of some 100°C compared with conventional structural steels. It has been found that the improvement in fire resistance is limited, especially in the case of slim floor systems where there is already efficiently built-in fire resistance. The fire resistance is increased by 22–24 min by using the fire-resistant steels for the I-beam floor, whereas such improvement is only 11–16 min in the case of slim floor constructions. To achieve a 30-min increase in fire resistance in the I-beam floor structure, the strength reduction factor curve of the steel needs to shift by 150°C to higher temperatures.

Computer programs were developed and used to investigate the influence of various factors on the fire resistance of steel hollow structural section columns filled with steel fiber-reinforced concrete. Results from the parametric studies were used to develop simple equations for calculating the fire resistance of steel fiber-reinforced concrete (SFRC)-filled hollow structural section (HSS) columns with circular and square cross sections. The validity of the calculation procedure was established by comparing the calculated fire resistances with those obtained from fire tests on columns. These equations provide a rational and easy-to-use design method for evaluating the fire resistance of hollow steel columns filled with steel fiber-reinforced concrete for any value of the significant parameters that determine it, such as load, cross-sectional dimensions, effective length, and concrete strength. The proposed equations give reasonable estimate of fire resistance for SFRC-filled circular HSS columns; however, the predicted fire resistances can be overconservative for SFRC-filled square HSS columns. Hence, these equations can be used in preliminary design and, where available, the designer should rely on furnace testing when not satisfied with the estimated fire resistance values.

The system reliability evaluation of ductile frame structures presents two difficulties. One is the identification of significant failure modes; the other is the computation of overall failure probabilities contributed from these significant modes. In this paper, a practical procedure is developed, in which a failure mode independent performance function is defined using load factor obtained by limit analysis, and a response surface approach is used to approximate the performance function. With the performance function of explicit second-degree polynomial, the failure probability can be readily obtained using First and Second Order Reliability Method (FORM/SORM). Several examples are investigated, and it is found that the obtained response surface is a good approximation of the inner connotative surface of the limit state surfaces of the structural system. The proposed procedure has good efficiency and enough accuracy for system reliability evaluation of ductile frame structures. The difficulty in both failure mode identification and failure probability computation can be avoided by using the proposed procedure.

Static and dynamic tests were conducted on wood frame shear walls to (1) determine the wall resistance to lateral loading; (2) examine the wall performance under fully reversed cycles of dynamic loading; and (3) compare the static and dynamic performance as measured using the same test facility. Tests were conducted on standard 2.44 by 2.44 m (8 by 8 ft) walls, constructed of plywood or oriented strand board sheathing. Four specimens were tested statically using the traditional ASTM E-564 test procedure: three half-cycles, loading monotonically to failure. Eight specimens were tested dynamically, using a proposed test standard that was recently developed by the Structural Engineers Association of Southern California. A comparison of the static and dynamic test results demonstrates several key differences. The ultimate loads measured in the dynamic tests are slightly less than those measured in the static tests; however, the ductility of the wall, when measured dynamically, is between 34 and 42% less than the corresponding static ductility. Furthermore, damage in the static tests is characterized by the pulling away of the sheathing from the frame, extraction of the sheathing nails, and splitting of the bottom plate. Damage in the dynamic tests is generally confined to pullout and fatiguing of the sheathing nails, which eventually break off. The dynamic tests show a reduction in load and stiffness and pinched hysteresis with continued cycling at a given displacement. The results of this study suggest that the actual load factors for a shear wall subjected to an earthquake will be significantly lower than the intended design.

This paper presents the results of an ultimate load test and analysis of a posttensioned and shaped hexagonal grid dome. The hexagonal grid dome is one type of the recently proposed posttensioned and shaped single chorded space trusses which are constructed by means of posttensioning. The dome is first assembled as a planar layout on the ground, and then is deformed into its final space shape by a posttensioning operation. After shape formation, the dome is loaded to failure in order to determine the ultimate load. Finite-element analyses that correlate experimental forces in individual members, and prestress forces caused by posttensioning, have been made in an attempt to predict the ultimate load of the posttensioned and shaped dome. Comparison between theory and experiment shows that the test dome did not reach the theoretical ultimate load, and that the posttensioning had some effect on the ultimate load capacity. The nonlinear finite-element method based on the experimental results can be helpful to predict the ultimate load of posttensioned and shaped domes.

The paper presents a new element for geometrically nonlinear analysis of frame structures. The proposed formulation is flexibility based and uses force interpolation functions for the bending moment variation that depend on the transverse displacements and strictly satisfy equilibrium in the deformed configuration. The derivation of the governing equations and their consistent linearization are substantially more involved than for stiffness-based elements. Nonetheless, the element offers significant advantages over existing stiffness-based approaches, since no discretization error occurs and all governing equations are satisfied exactly. Consequently, fewer elements are needed to yield results of comparable accuracy. This is demonstrated with the analysis of several simple example structures by comparing the results from flexibility and stiffness-based elements.

In the current paper, the effect of plastic anisotropy on the elastoplastic buckling behavior of individual outstand plate elements is investigated. The elastoplastic plate-buckling problem is very sensitive to modeling parameters, and there is still some discrepancy between tested and simulated results. Stress-strain behavior and R-values have been measured by performing uniaxial tension tests in different directions in extruded plates of aluminum alloy 7108, temper T7. Several anisotropic yield criteria, found in the literature, have been calibrated in different ways, using both initial yield-stress values and R-values. An explicit finite element code, LS-Dyna3D, has been used to study the effect of plastic anisotropy on plate buckling. The yield criteria are evaluated for this particular problem. A special rig for testing of individual outstand elements has been designed, and plates of different width-thickness ratio have been tested. The tested plates have been compared with the results from the numerical analysis.