Search

The remanent magnetism method allows the identification of potentially unsafe conditions in pretensioned and posttensioned concrete structures by locating fractures in the prestressing steel. This nondestructive method identifies fractures of single wires, even when they are bundled with intact wires. The magnetic field of tendons is measured at the concrete surface, once they have been premagnetized with an electromagnet. Fractures produce characteristic magnetic leakage fields, which can be measured with appropriate sensors at the concrete surface. The parameters associated with fractured wires have been quantitatively identified in the laboratory and have been confirmed in the field. The knowledge of these parameters allows us to draw conclusions about the reduction of a cross-sectional area or the number of fractured wires in a tendon. The method has been successfully applied outside of the laboratory on full size bonded and unbonded posttensioned structures. CE Database subject headings: Nondestructive tests; Concrete, prestressed; Steel; Fractures; Bridges; Corrosion.

The technique of adhesively bonding steel or fiber-reinforced plastic plates to the surfaces of reinforced concrete (RC) structural elements is being adopted worldwide to strengthen or repair reinforced concrete buildings and bridges. However, the prevention of premature debonding or peeling of externally bonded plates is a most critical aspect of design. One of the major modes of debonding in plated RC beams is shear peeling induced by the formation of critical diagonal cracks caused by the applied vertical shear forces. In this paper, mathematical models are developed for quantifying the shear peeling resistance of RC beams bonded with plates of different configurations. These models are based on the simplified theory of plasticity concepts for shear in reinforced concrete beams and also on procedures to compute the bond strength between an adhesively bonded plate and a concrete element. The models are validated against 32 test results from reinforced concrete beams adhesively bonded with either steel plates or steel angles attached to the tension faces, or tension face plated beams additionally bonded with side plates to improve their shear peeling resistance.

Stability of a newly proposed structural system, namely, the prebuckled or prestressed dome with ring stiffener, is investigated. The prestressed dome is different from traditional domes in that it is made by first elastically buckling shallow, straight radial members and then attaching them to their supports. A special-purpose algorithm devised to consider a staged construction method of the system using nonlinear finite-element models is implemented into the program named DOME3D. Stability of the prestressed dome with two different rise-to-span ratios is studied with a varying number of ring stiffeners. The study shows that the stiffened system is not vulnerable to loading imperfections leading to side-sway buckling. The stiffening methods increased the limit load by more than 100% and changed the buckling mode of the system. For some stiffener configurations, the behavior of the stiffened domes was similar to shell-like domes. The results showed that the stiffening system leads to significant increase in the strength-to-weight ratio of the novel type structure, and improvement in the overall stability behavior of the dome.

In recent years, increasing interest has been shown throughout Europe in developing and designing slim floor systems in steel-framed buildings. This paper presents the fire resistance behavior of the composite asymmetric slim floor beam as an isolated member and as a part of the frame using numerical analysis methods. Three schemes were investigated, including isolated beams, a plane subframe with semirigid beam-to-column connections, and a 3D slim floor frame system. The first scheme aimed to explore the fire resistance of the beams according to standard fire-testing methodology. The objective of the second scheme was to reveal the effect of frame continuity on the fire resistance of the slim floor beam and the mechanical interaction between the frame elements. The third scheme was to preliminarily identify the influence of the composite slab on the beam behavior in fire. The investigations show that the isolated slim floor beam has a 60-min standard fire resistance without any additional fire protection, if the load ratio is

This paper presents a performance-based optimization method for optimal topology design of bracing systems for multistory steel building frameworks with overall stiffness constraint under multiple lateral loading conditions. Material removal criteria are derived by undertaking a sensitivity analysis on the mean compliance of a structure with respect to element removal. A performance index is proposed to evaluate the performance of resulting bracing systems in the optimization process. In the proposed method, unbraced frameworks arc initially designed under strength constraints using commercial standard steel sections from databases. The optimal topology of a bracing system for the multistory steel building framework is then generated by gradually removing inefficient materials from a continuum design domain that is used to stiffen the framework until the performance of the bracing system is maximized. Two design examples are provided to illustrate the effectiveness of the performance-based design optimization method proposed for the conceptual layout design of lateral bracing systems for multistory steel building frameworks.

The concept which states that the effective length is equal to the distance between points of contraflexure is taken from an understanding of the Euler buckling behavior of columns. This concept of effective length is erroneous but it can still be applied if the evaluation is based on beam-column behavior. Approximate formulas for effective length of steel columns are provided as an alternative to the nomogram technique.

Circular steel silos and tanks are often constructed from rolled-steel plates with welded or bolted lap splices. Where the lap runs horizontally around the silo circumference, the lap connection causes a local eccentricity in the line of vertical thrust in the shell wall. This eccentricity induces high local bending stresses and circumferential membrane stresses, which are detrimental to the strength of the structure. Although elastic buckling of the shell wall may be the critical failure mode for thin lap-jointed cylinders under axial load with no or small internal pressure, axisymmetric plastic collapse is the likely failure mode when the internal pressure is high, especially for relatively thicker cylinders. This paper presents a detailed study of the axisymmetric collapse behavior and strength of lap-jointed cylinders under combined axial compression and internal pressure and establishes easy-to-use design equations for such shells.

Analytical and experimental approaches are presented to monitor the tension levels in posttensioned high-strength steel rods of stressed-timber systems. The, dynamic response of these rods under an impact load is correlated with the rod tension. The dominant frequencies of the rod stressed to different tension levels are consistently distinguishable. An increase in frequency is consistently observed with increases in the rod tension levels. An analytical relation is developed to correlate the dominant frequencies to the tension level in the rod. The proposed technique can predict the tension levels with a 85-90% accuracy of the experimental results. The nondestructive approach proposed herein is intended for future use in structural inspection of stressed timber bridges.

Bicycle wheels achieve their structural efficiency by making use of prestressing in three ways. Tests show that the bottom spokes carry virtually all the load by compressive forces, which reduce the tensile prestress set up in the spokes when the wheel was made. The test results are compared with an analysis that considers the spokes as a disk that can carry force in one direction only. This is shown to give good agreement, as does an analysis that considers the rim as a straight beam on an elastic foundation. The behavior of the wheel with an inflated tire is also considered, and it is shown that good comparisons with theory are obtained if the reaction from the road is assumed to be distributed over a specific length of the rim. Prestressing is shown to be important also in the mechanism by which the various forces are transmitted through the tire from the road to the rim.

An alternative direction technique to a five-step analysis techniques for obtaining final stresses, strain and rotation in a timedependent analysis of composite steel-concrete sections is presented. The original method presented in R. Ian Gilbert's 1989 article is supplemented by astep-by-step presentation of the alternative method. Application of the proposed technique using an example from Gilbert shows that both methods yield the same results.

Results of two separate investigations on out-of-plane stability of steel arches complement one another, particularly in terms of postbuckling behavior, slenderness, ultimate load and rolling residual stresses. One investigation showed that the column buckling curves can be used for stocky arches, while the other demonstrated that these can also be used for slender arches. An alternative definition for the ultimate load, or the load belonging to the point in the load-displacement diagram that is defined as min/d(load)/d(displacement)/, is put forth.