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CONTENTS
Volume 1, Number 4, December 2001
 

Abstract
The response of multi-story building structures to lateral loads, mainly due to earthquake andrnwind, is investigated for preliminary design purposes. Emphasis is placed on structural systems consisting ofrnrigid and braced steel frames. An attempt to gain a qualitative understanding of the influence of bending andrnshear stiffness distribution on the deformations of such structures is made. This is achieved by modeling thernstructure with a stiffness equivalent Timoshenko beam. It is observed that the conventional stiffnessrndistribution, dictated by strength constraints, may not be the best to satisfy deflection criteria. This isrnparticularly the case for slender structural systems with prevailing bending deformations, such as flexiblernbraced frames. This suggests that a new approach to the design of such frames may be appropriate whenrnserviceability governs. A pertinent strategy for preliminary design purposes is proposed.

Key Words
rigid frame; braced frame; performance-based design; preliminary design; serviceability; stiffness equivalence; Timoshenko beam; stiffness optimization.

Address
C.J. Gantes, I. Vayas

Abstract
In an experimental and analytical study on the structural behavior of slender circular steel-concreterncomposite columns, eleven specimens were tested to investigate the effects of three ways to apply arnload to a column. The load was applied eccentrically to the concrete section, to the steel section or to the entirernsection. Three-dimensional nonlinear finite element models were established and verified with the experimentalrnresults. The analytical models were also used to study how the behavior of the column was influenced by thernbond strength between the steel tube and the concrete core and the by confinement of the concrete core offeredrnby the steel tube. The results obtained from the tests and the finite element analyses showed that the behaviorrnof the column was greatly influenced by the method used to apply a load to the column section. When relyingrnon just the natural bond, full composite action was achieved only when the load was applied to the entirernsection of the column. Furthermore, because of the slenderness effects the columns did not exhibit thernbeneficial effects of composite behavior in terms of increased concrete strength due to the confinement.

Key Words
composite column; confined concrete; bond; load application; hollow steel section; non-linear finite element analyses; experiments.

Address
Mathias Johansson and Kent Gylltoft, Department of Structural Engineering, Concrete Structures, Chalmers University of Technology, SE-412 96 Goteborg, Sweden

Abstract
This paper treats the failure analysis of prestressing steel wires with different kinds ofrnlocalised damage in the form of a surface defect (crack or notch) or as a mechanical action (transversernloads). From the microscopical point of view, the micromechanisms of fracture are shear dimplesrn(associated with localised plasticity) in the case of the transverse loads and cleavage-like (related to arnweakest-link fracture micromechanism) in the case of cracked wires. In the notched geometries thernmicroscopic modes of fracture range from the ductile micro-void coalescence to the brittle cleavage,rndepending on the stress triaxiality in the vicinity of the notch tip. From the macroscopical point of view,rnfracture criteria are proposed as design criteria in damage tolerance analyses. The transverse load situationrnis solved by using an upper bound theorem of limit analysis in plasticity. The case of the cracked wire mayrnbe treated using fracture criteria in the framework of linear elastic fracture mechanics on the basis of arnprevious finite element computation of the stress intensity factor in the cracked cylinder. Notchedrngeometries require the use of elastic-plastic fracture mechanics and numerical analysis of the stress-strainrnstate at the failure situation. A fracture criterion is formulated on the basis of the critical value of therneffective or equivalent stress in the Von Mises sense.

Key Words
prestressing steel wires; fracture criteria; damage tolerance; structural integrity; engineering safe design

Address
J. Toribio, Department of Materials Engineering, University of Salamanca, E.P.S., Campus Viriato, Avda. Requejo 33, 49022 Zamora, SpainrnA. Valiente, Department of Materials Science, Polytechnic University of Madrid, ETSI Caminos, Ciudad Universitaria, 28040 Madrid, Spain

Abstract
The growing use of unprotected or partially protected steelwork in buildings has caused a livelyrndebate regarding the safety of this form of construction. A good deal of recent research has indicated that steelrnmembers have a substantial inherent ability to resist fire so that additional fire protection can be either reducedrnor eliminated completely. A performance based philosophy also extends the study into the effect of structuralrncontinuity and the performance of the whole structural totality. As part of the structural system, thermalrnexpansion during the heating phase or contraction during the cooling phase in most beams is likely to bernrestrained by adjacent parts of the whole system or sub-frame assembly due to compartmentation. This has notrnbeen properly addressed before. This paper describes an experimental programme in which unprotected steelrnbeams were tested under load while it is restrained between two columns and additional horizontal restraintsrnwith particular concern on the effect of catenary action in the beams when subjected to large deflection at veryrnhigh temperature. This paper also presents a three-dimensional mathematical modelling, based on the finiternelement method, of the series of fire tests on the part-frame. The complete analysis starts with an evaluation ofrntemperature distribution in the structure at various time levels. It is followed by a detail 3-D finite elementrnanalysis on its structural response as a result of the changing temperature distribution. The principal part of thernanalysis makes use of an existing finite element package FEAST. The effect of columns being fire-protectedrnand the beam being axially restrained has been modelled adequately in terms of their thermal and structuralrnresponses. The consequence of the beam being restrained is that the axial force in the restrained beam starts asrna compression, which increases gradually up to a point when the material has deteriorated to such a level thatrnthe beam deflects excessively. The axial compression force drops rapidly and changes into a tension forcernleading to a catenary action, which slows down the beam deflection from running away. Design engineers willrnbe benefited with the consideration of the catenary action.

Key Words
fire engineering; fire resistance; steel beams; fire tests; with resiseants; fire element analysis; catenary action.

Address
T. C. H. L i u and J.M. Davies, Manchester School of Engineering, Oxford Road, Manchester, M13 9PL, UK

Abstract
We analyzed the experimental and theoretical behavior of a particular type of steel joint designed tornconnect beam to beam and able to transfer both shear forces and bending moments. This joint is characterizedrnby the use of steel plates and bolts enclosed in the width of the beams. The experimental investigation wasrncarried out characterizing the constituent materials and testing in flexure beams constituted by two portions ofrnbeams connected in the middle with the joint proposed. Connections having different characteristics in termsrnof thickness of plates, number and type of bolts were utilized. Flexure tests allow one to determine the load-deflectionrncurves of the beam tested and the moment-rotation diagrams of the connections, highlighting thernstrength and the strain capacity of the joints. The proposed analytical model allows one to determine thernmoment-rotation relationship of the connections, pointing out the influence of the principal geometrical andrnmechanic characteristics of single constituents on the full properties of the joint.

Key Words
experimental testing; semirigid connections; monotonic behavior; failure mode; beam-beam joints; load-deflection curves; moment-rotation curves; analytical model.

Address
G. Campione and N. Scibilia, Dipartimento di Ingegneria Strutturale e Geotecnica, Universita di Palermo, Viale delle Scienze-I-90128, Italy

Abstract
The damage suffered by steel structures during the Northridge (1994) and Kobe (1995) earthquakesrnindicates that the fully restrained (FR) connections in steel frames did not behave as expected. Consequently,rnresearchers began studying other possibilities, including making the connections more flexible, to reduce thernrisk of damage from seismic loading. Recent experimental and analytical investigations pointed out that thernseismic response of steel frames with partially restrained (PR) connections might be superior to that of similarrnframes with FR connections since the energy dissipation at PR connections could be significant. Thisrnbeneficial effect has not yet been fully quantified analytically. Thus, the dissipation of energy at PRrnconnections needs to be considered in analytical evaluations, in addition to the dissipation of energy due tornviscous damping and at plastic hinges (if they form). An algorithm is developed and verified by the authors tornestimate the nonlinear time-domain dynamic response of steel frames with PR connections. The verifiedrnalgorithm is then used to quantify the major sources of energy dissipation and their effect on the overallrnstructural response in terms of the maximum base shear and the maximum top displacement. The resultsrnindicate that the dissipation of energy at PR connections is comparable to that dissipated by viscous dampingrnand at plastic hinges. In general, the maximum total base shear significantly increases with an increase in thernconnection stiffness. On the other hand, the maximum top lateral displacement Umax does not always increasernas the connection stiffness decreases. Energy dissipation is considerably influenced by the stiffness of arnconnection, defined in terms of the T ratio, i.e., the ratio of the moment the connection would have to carryrnaccording to beam line theory (Disque 1964) and the fixed end moment of the girder. A connection with a Trnratio of at least 0.9 is considered to be fully restrained. The energy dissipation behavior may be quite differentrnfor a frame with FR connections with a T ratio of 1.0 compared to when the T ratio is 0.9. Thus, for nonlinearrnseismic analysis, a T ratio of at least 0.9 should not be considered to be an FR connection. The studyrnquantitatively confirms the general observations made in experimental results for frames with PR connections.rnProper consideration of the PR connection stiffness and other dynamic properties are essential to predictrndynamic behavior, no matter how difficult the analysis procedure becomes. Any simplified approach mayrnneed to be calibrated using this type of detailed analytical study.

Key Words
seismic loading; energy dissipation; steel frames; partially restrained; fully restrained; inelastic analysis; connection stiffness; time history analysis; connection parameters.

Address
Alfredo Reyes-Salazar, Facultad de Ingenieria, Universidad Autonoma de Sinaloa, Culiacan, Sinaloa, MexicornAchintya Haldar, Department of Civil Engineering and Engineering Mechanics, University of Arizona, Tucson, Arizona, USA


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