Journal of Structural Engineering

A nonlinear analysis that focused on geometric effects and also included the long-term effects of creep and shrinkage performed on the Tucker High School roof revealed an unstable shell with little capacity for increased loads. The hipped hyperbolic paraboloid roof collapsed under dead loads alone in 1970 after seven years of use. The nonlinear model included a realistic description of the geometry of the roof by using shell elements and solid elements instead of shell elements and beam elements. The finite-element analysis used the large rotation geometric nonlinearity and therefore accounted for the deformed geometry of the shell. The study, based on an uncracked material, also assumed that the incremental creep strains were proportional to the stresses and the incremental creep strain was exponentially decaying in time. The magnitudes of the creep strains after one year were close to the average for lightweight concrete based on the ACI guide.

This paper presents a numerical method, based on a simple transformation concept, for analyzing the failure loads of slender reinforced concrete columns under uniaxial bending. Both material and geometric nonlinearities are taken into account; this method is applicable to material failure and buckling failure. The concrete and steel of the critical section are transformed into an equivalent material, which is homogeneous and linearly elastic, by using the secant modulus ratio; the cross-sectional width parallel to the neutral axis accordingly expands or contracts. The modulus of elasticity of concrete as proposed by Carrasquillo and colleagues is used as the basis for transformation. A numerical example is presented to illustrate the method. For 82 columns in six experimental investigations carried out in the past by other researchers the predicted failure loads are as close as the predictions of the P-Delta method and generally closer to the experimental values, compared with the predictions of ACI 318 and EC 2. The proposed method allows for rapid and accurate analysis of the failure load of slender columns.

This paper presents results from a continuing research program that aims to study confinement of concrete by lateral reinforcement. The current work deals with the experimental behavior of high-strength concrete (HSC) and ultrahigh-strength concrete (UHSC) column behavior. Realistically sized columns (305 × 305 × 1,473 mm) with heavy stubs (508 × 762 × 813 mm) were tested under moderate to high axial load levels and reversed cyclic displacement excursions. The behavior of UHSC and HSC columns were examined and compared to normal strength concrete column behavior. The variables studied in this research program are the concrete strength, axial load level, steel configuration, amount of lateral steel, and the presence of a heavy stub. A performance-based design procedure for the design of confinement reinforcement in HSC and UHSC columns is presented and compared with current American Construction Institute (United States) and New Zealand Standards (New Zealand) codes.

The ductility of concrete structures is an issue that is continually being addressed as higher strength concretes are produced, better numerical models have become available, and seismic mechanics become better understood. For high strength concrete columns in axial compression, there are some questions as to the ability of the ties to provide similar levels of confinement as currently exist in normal strength concrete columns having similar tie arrangements. The ratio of the confining stress to the concrete strength is lower for high strength concrete. In many of the empirically based models used for the design of high strength concrete columns, it is taken that the ties are at yield at the peak load. In this paper, a finite-element model is developed to investigate the response of concentrically loaded columns with concrete strengths up to 100 MPa. Numerical examples are presented where it is shown that: (1) for confined high strength concrete columns, the tie steel is not at yield at the peak load; and (2) tension strains at the cover-core interface of high strength concrete columns are large enough to account for the early cover spalling that has been observed experimentally.

A constitutive model for tension in cracked reinforced concrete members is presented. Special emphasis is placed on the bond between concrete and reinforcement as the main factor influencing tension stiffening in cracked reinforced concrete. The model was developed for members subjected to bending and axial loads. The constitutive formulation was implemented into a cross-sectional analysis program and was used to analyze 14 specimens, which were tested in bending and in-plane loading. The comparison between the experiments and the analyses is presented and shows the same special characteristics for both the experimental results and the analyses.

Fiber-wrapping or encasement of concrete in fiber-reinforced plastic (FRP) shells significantly enhances strength and ductility of concrete columns. However, design of such hybrid systems requires an accurate estimate of the performance enhancement due to the confinement mechanism. Current design procedures are simple extensions of the models developed for conventional reinforced concrete columns. Previous studies have demonstrated that such models may not be conservative for FRP-encased concrete. A simple model is presented to predict the complete bilinear stress-strain response of FRP-confined concrete in both axial and lateral directions. The model is based on correlation between the dilation (expansion) rate of concrete and the hoop stiffness of the restraining member. The parameters of the model are directly related to the material properties of the FRP shell and the concrete core. The predicted stress-strain curves compare favorably with the results of the present study, as well as tests by others on both fiber-wrapped and FRP-encased concrete columns.

The durability of reinforced concrete (RC) structures depends mainly on concrete permeability that allows the aggressive agents to reach the reinforcement and to start corrosion. In addition to the permeability connected to porosity and microcracking, another important and well-known cause of degradation is concrete cracking that directly exposes the reinforcement to aggressive agents. Besides flexural cracks, other kinds of cracks can occur in RC beams, of which splitting cracks have particular relevance. They develop longitudinally along the anchored bar and expose a large area of the bar to the environment so that the corrosion resistance to splitting cracks should be lower than the resistance to flexural cracks. To study the structure durability related to splitting cracks, two aspects should be considered: (1) The resistance to corrosion for a given splitting crack opening; and (2) the evaluation of the splitting crack width. To shed some new light on the second aspect, in this paper a relationship between the maximum splitting crack opening and the flexural crack opening is proposed and contrasted with experimental results. Furthermore, an approximated relationship, for practical use, is determined.

In reinforced concrete beams, the sectional definition of the moment-curvature relationship is not straightforward owing to the presence of cracks. In general, this problem has been solved by considering a representative portion of the beam and by defining the curvature 1/r as the ratio between the rotation of the portion considered and its length. Large portions of the beam with many cracks lead to “average” moment-curvature relationship, while small portions delimited by two consecutive flexural cracks lead to “local” moment-curvature relationship. The difference between these two definitions is studied in this paper introducing a “general model.” With this model the entire beam is modeled through a succession of blocks divided by flexural cracks taking into account the bond between steel and concrete. Due to the practical impossibility of univocal definition of the crack pattern evolution, it is proposed to use a “range model.” This “range,” delimited by the curves of maximum and minimum deformability, includes the moment-curvature relationship (locals and average) obtained from all possible crack patterns.

Ten large-scale tests on floors comprising hollow-core slabs supported on beams were carried out. All tests ended in web shear failure of slabs close to the supports of the beam. In one test the shear resistance was not reduced, but in others was only 40–77% of the reference value measured on nonflexible supports. In one test yielding of the reinforcement in the supporting beam preceded failure. In other tests the beams did not yield and their deflection at slab failure was very small, typically L_b/1,000–L_b/300. Reduction of shear resistance is due to transverse deformation of the slab ends resulting from deflection of the beam. The reduction is so great that it must be considered in design, even though collapses due to such reduction, thanks to the high safety factors applied, have neither occurred nor are likely to occur in conventionally designed floors.

A simple calculation model is proposed for prediction of the shear resistance of prestressed hollow-core (PHC) slabs supported on beams. A simplified principal stress criterion for the web shear failure is introduced. The criterion is based on three stress components. The vertical shear stress τ_zy and the axial stress σ_y are calculated as for slabs supported on nonflexible bearings. The transverse shear stress τ_zx is calculated applying beam theory to a composite beam comprising the beam, joint concrete, a section of the slabs, and the optional topping. The width of the section is an experimental parameter that takes into account the degree of composite interaction between the slabs and the beam. It is assumed to be proportional to the span of the beam. Also presented are the modifications needed when the beams are continuous, the floor is covered with reinforced concrete topping, or the ends of the hollow cores are filled with concrete.

Rings or ringbeams are often used to stiffen cylindrical tanks, silos, towers, and pressure vessels. A ring acts together with the axisymmetric shell to form a composite structure. The structural analysis of such a composite structure under general loadings is usually not simple, and often has to be numerical. Although the membrane stress in a symmetrical ring stiffener under axisymmetric loadings may be calculated from a simple effective ring analysis, no existing simple method is able to predict the bending stresses caused by either eccentric loading (bending moment) or the use of an asymmetric section as a ring stiffener. In this paper, the deformations of a general unsymmetrical ring stiffener are analyzed using linear shell bending theory. A new rational method of determining both the precise membrane and the bending stresses throughout the discrete ring stiffener using effective ring analysis is devised. The new method is suitable for use in a shell design standard.

This note describes a simplified model of damage for reinforced concrete (RC) frame members. The model is different from those proposed in the literature because it combines the concept of inelastic hinge and the methods of fracture and continuum damage mechanics. The model allows the characterization of the following effects: (1) unsymmetrical cross section with different yield capabilities at positive and negative bending; (2) influence of the axial force on the flexural member behavior, stiffness, and strength degradation due to cracking of the concrete; (3) plastic deformations due to the yield of the reinforcement; and (4) different stiffness under positive and negative flexure due to unsymmetrical cracking even in symmetrical sections and low cycle fatigue. The modeling of the low cycle fatigue effects, which is the main object of this note, is made using concepts from fracture mechanics. Numerical simulations of tests reported in the literature validate the model.