Journal of Structural Engineering

Reinforced concrete (RC) structures in a chloride environment are exposed to a particularly aggressive form of deterioration. Managing the maintenance and repair of structures subjected to these conditions is an important issue facing asset owners and operators. In the present paper, the criterion for repair is the extent of corrosion induced cracking. Common maintenance criteria and maintenance techniques are described and these are incorporated into a two-dimensional spatial time-dependent reliability model. This allows the number, timing, and extent of maintenance actions to be predicted over the service life of RC structures. The analysis includes the effect of inspection interval, maintenance technique, repair efficiency, and repair threshold. Results are presented for a typical RC bridge deck subject to a corrosive marine environment. It was found that repair efficiency affecting time to corrosion initiation of repaired concrete is more critical to the performance of the repair than the repair efficiency that affected corrosion rate.

To date, attention of researchers concentrated mainly on the serviceability failure of corroded reinforced concrete (RC) beams caused by excessive cracking. However, cracking of the concrete cover and a reduction in the bond strength (which may result in slip between the corroding reinforcement and the concrete) also decrease the stiffness of RC beams. As a result of it, displacements of the RC beams increase and may exceed a limit value specified in a code, i.e., it has been noted that corrosion may also cause the serviceability failure due to excessive displacements. In this paper the effect of corrosion on deflections of RC beams and, subsequently, on the probability of serviceability failure due to excessive deflection is considered. A nonlinear finite element model is used to describe the structural behavior of RC beams with corroding reinforcement. Uncertainties associated with the model and available data are taken into account. Results show that an increase in deflections due to corrosion has a lesser effect on serviceability of RC beams than corrosion-induced cracking.

In the conventional seismic design methods, heightwise distribution of equivalent seismic loads seems to be related implicitly on the elastic vibration modes. Therefore, the employment of such a load pattern does not guarantee the optimum use of materials in the nonlinear range of behavior. Here a method based on the concept of uniform distribution of deformation is implemented in optimization of the dynamic response of structures subjected to seismic excitation. In this approach, the structural properties are modified so that inefficient material is gradually shifted from strong to weak areas of a structure. It is shown that the seismic performance of such a structure is better than those designed conventionally. By conducting this algorithm on shear-building models with various dynamic characteristics, the effects of fundamental period, target ductility demand, number of stories, damping ratio, postyield behavior, and seismic excitations on optimum distribution pattern are investigated. Based on the results, a more adequate load pattern is proposed for seismic design of building structures that is a function of fundamental period of the structure and the target ductility demand.

High-rise concrete shear walls are often supported near or below grade by stiff floor diaphragms connected to perimeter foundation walls. When a large portion of the overturning moment in the wall is transferred to the foundation walls by force couples in two or more stiff floor diaphragms, the maximum bending moment (flexural plastic hinge) occurs above the diaphragms and the shear force reverses below the flexural hinge. Depending on the stiffness of floor diaphragms, and on the shear rigidity and flexural rigidity of the high-rise concrete walls, the reverse shear force below the flexural hinge may be much larger than the base shear above the flexural hinge. Nonlinear dynamic analyses indicate the maximum reverse shear force is proportional to the bending moment capacity of the wall and inversely proportional to the accompanying base shear force. An upper-bound estimate of bending moment capacity of the high-rise wall combined with an assumed zero base shear force can be used in a simple nonlinear static analysis to estimate the maximum shear force below the flexural plastic hinge. A nonlinear shear model can be used to determine whether diagonal cracking of the wall and yielding of horizontal wall reinforcement will reduce the reverse shear force without causing a shear failure. Increasing the quantity of horizontal reinforcement in the wall above a certain limit may not prevent a shear failure and thus a different design solution will need to be found. An upper-bound estimate of floor diaphragm stiffness should be used in order to not underestimate the shear strain demand on high-rise walls.

Strengthening of masonry-infilled reinforced concrete (RC) frame buildings with an open first story is needed because of the inherent irregularity in such buildings resulting in their poor performance as observed during several past earthquakes. In the present study, several strengthening schemes were evaluated for their effectiveness in improving the performance of such buildings, based on nonlinear analyses of typical RC frames. Instead of using predetermined multiplying factors for designing the first-story members for higher forces as recommended in some national codes, a rational method was developed for the calculation of the required increase in strength of open first-story columns. Other strengthening schemes studied were providing additional columns, diagonal bracings, and lateral buttresses in the open first story. Code methods were found to increase only lateral strength (and not the ductility) of such frames; whereas, some of the alternate schemes studied were found to improve both lateral strength and ductility for improved seismic performance.

This paper examines the impact responses of reinforced concrete (RC) beams through an experimental study and presents an analytical model developed to predict the maximum midspan deflection and maximum impact load, which aids as an important performance index to evaluate the damage levels of RC beams when subjected to impact loadings. The experimental study involves a drop hammer impact test and investigates the influence of drop height and the effect of the amount of longitudinal steel reinforcement contributes to the response of RC beams. The RC beam specimens used in the experiment comprised of under-reinforced sections provided with sufficient amount of transverse reinforcements to allow for an overall flexural failure. The experimental impact responses of the RC beams were simulated with two-degree-of-freedom mass-spring-damper system model, in which the loading rate effects were duly considered. The analytical results are in good agreement with the experimental results for the RC beams that exhibited overall flexural failure.

This paper presents a review of the design provisions for screw-fastened cold-formed steel connections in the United States, including criteria for testing and evaluating the strength of screw fasteners. In addition, the paper examines the relationship between the connected elements/components computed connection strength and the strength of the connector (screws). Fastener strength test data from three independent manufacturers is presented and it is shown that as the thickness or yield strength of the connected elements/components is increased, the ratio of the fastener strength to the bearing/tilting strength decreases to values less than one-half in some cases. Analysis of the test data showed that even when the 2001 North American Specification (NASPEC) for the Design of Cold-Formed Steel Structural Members statistical requirements for distribution of the test data are met, the computed safety and resistance factors can exceed the recommended limits in Section E4 of the 2001 NASPEC. On this basis, it is recommended that the current language in the 2001 NASPEC be revised to specify, if necessary, limits on manufacturer computed safety and resistance factors. Finally, it is recommended that fastener tilt during screw shear strength tests be eliminated to facilitate an accurate evaluation of the shear strength of a fastener.

Screws are the only fasteners approved in the 2006 International Building Code for attaching wood structural sheathing to cold-formed steel framing for shear walls. Steel pins also provide an effective and efficient means of attaching wood structural sheathing. However, the proprietary nature of steel pins, coupled with the need to demonstrate equivalent performance to code-approved elements, necessarily requires manufacturers to consider some form of reversed cyclic testing to justify shear wall values for seismic design. This paper presents the results of an experimental program that evaluated the performance of wood panel sheathed cold-formed steel frame shear walls in which the wood panels were attached using one type of pneumatically driven steel pins. For the assemblies tested and the specific pin type used, several useful performance characteristics and characteristic values were identified to facilitate overall performance comparisons with other types of shear wall configurations. It is shown that the average ratio of the peak strength to the derived allowable strength exceeded 2.8, the average maximum usable displacement exceeded 3.5% of the wall height, the average ratio of the peak strength displacement to the allowable strength displacement exceeded 9.3, and the average maximum usable displacement was 35% greater than the peak strength displacement.

Testing showed that the hysteretic behavior of nailed wood joints governs the response of many wood systems when subjected to lateral loadings. Unfortunately, commercially available software does not have the appropriate hysteretic element for a nailed wood joint, and the accuracy and versatility of the previously developed nail joint elements are not satisfactory. A general hysteretic model, Bouc-Wen-Barer-Wen, was modified to represent the hysteretic behavior of a nailed joint, in which the hysteretic constitutive law is characterized by a series of ordinary differential equations. It can produce versatile and smoothly varying hysteresis curves, is nonlinear, history dependent, and includes stiffness and strength degradation, and pinching. Based on the test data, the suitable parameters for different joint configurations can be estimated through genetic algorithms. This model was embedded in commercially available software, ABAQUS/Standard (Version 6.5), as a user-defined element which accounted for the coupling property of the nail joint action. A detailed shear wall was modeled and analyzed, and the results agreed well with the test data.

After modifications, the Bouc-Wen-Barber-Wen (BWBN) model was used in simulating the hysteretic behavior of wood light-frame shear walls. A complete shear wall model is composed of boundary framing members and two-diagonal modified BWBN hysteretic springs. The parameters of the springs can be estimated based on the shear wall test data or detailed shear wall simulation results. To verify the accuracy and efficiency of this shear wall model, a two-story residential building was simulated and a three-dimensional dynamic analysis was conducted on it. The resultant structural responses agreed with the test results well.

Contemporary tall buildings with increasing height and slenderness are highly sensitive to the actions of wind. The structural design of modern tall buildings is generally governed by the need to provide adequate strength and stiffness against dynamic movement induced by strong wind. In addition to the strength-based safety design considerations, the major design effort of a tall building is related to the assessment of the wind-induced serviceability design requirements in terms of lateral drift and motion perception criteria. With tall buildings of today continuing to increase in height, the mitigation of wind-induced vibrations in tall buildings becomes a more critical challenge in the design synthesis process. This paper presents an integrated dynamic analysis and computer-based design optimization method for minimizing the structural cost of tall buildings subject to wind-induced serviceability acceleration design criteria. Once the optimal dynamic serviceability design problem is explicitly formulated, a rigorously derived optimality criteria method is developed for seeking the optimal distribution of element stiffness for a tall building system, satisfying the peak acceleration design constraints. Two full-scale 60-story building examples of mixed steel and concrete construction, with and without complex three-dimensional (3D) mode shapes, are used to illustrate the effectiveness and practical application of the optimal design technique. The results showed that the computer-based optimal design technique provides a powerful tool for wind-induced dynamic serviceability design of tall general buildings with complex 3D mode shapes.

This paper describes an alternative perspective in evaluating of computational models for predicting deflection of cracked reinforced concrete (RC) structures. The concept of robustness to uncertainty in modeling input parameters is introduced in addition to the classical perspective of modeling accuracy. A new method to quantify robustness to uncertainty is proposed and applied to the problem of modeling deflection of RC structures. Monte Carlo simulation is used for uncertainty propagation in serviceability calculations. The propagation of random uncertainties in deflection of a continuous one-way RC slab is examined. We introduce a method for the practical use of the proposed robustness metric by considering the reliability analysis. A cracked plane frame model to simulate deflection of the RC slab was adopted. We show that by considering a threshold of uncertainty in predicted deflection, the robustness to uncertainty in model inputs for different deflection prediction models can be evaluated. Our investigation proves that a tradeoff between model robustness and model accuracy exists in modeling deflection of RC structures.