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This study presents a methodology to determine the stability and optimal profile for slopes with concave cross section under static and seismic conditions. Concave profiles are observed in some natural slopes suggesting that such geometry is a more stable configuration. In this study, the profile of a concave slope was idealized by a circular arc defined by a single variable, the mid-chord offset (MCO). The proposed concave profile formulation was incorporated into a limit equilibrium-based log spiral slope stability method. Stability charts are presented to show the stability number, MCO, and mode of failure for homogeneous slopes corresponding to the most stable configuration under static and pseudostatic conditions. It is shown that concave profiles can significantly improve the stability of slopes. Under seismic conditions, the impact of concavity is most pronounced. Good agreement was demonstrated upon comparison of the results from the proposed method against those attended from a rigorous upper bound limit analysis. The proposed methodology, along with recent advances in construction technology, can be employed to use concave profiles in trenches, open mine excavations, earth retaining systems, and naturally cemented and stabilized soil slopes. The results presented provide a useful tool for preliminary evaluation for adopting such concave profiles in practice. Key words: slope stability, limit equilibrium, concave slope, optimization, seismicity. Cette etude presente une methodologie pour determiner la stabilite et le profil optimal de pentes avec une coupe transversale concave dans des conditions statiques et sismiques. Les profils concaves sont observes dans quelques pentes naturelles, ce qui suggere que cette geometrie est une configuration plus stable. Dans cette etude, le profil d'une pente concave a ete idealise par un arc circulaire defini par une variable unique, le decalage a mi-corde (MCO-mid-chord offset). La formulation de profil concave proposee a ete incorporee dans une methode de stabilite de pente logarithmique spiroidale basee sur l'equilibre de limite. Des diagrammes de stabilite sont presentes pour montrer le nombre de stabilites, le MCO et la mode de defaillance sur les pentes homogenes correspondantes a la configuration la plus stable dans des conditions statiques et pseudostatiques. Il est demontre que les profils concaves peuvent ameliorer considerablement la stabilite des pentes. Dans des conditions sismiques, l'impact de la concavite est plus prononce. Un bon accord a ete demontre lors de la comparaison des resultats de la methode proposee contre ceux qui decoulent d'une analyse de limite superieure. La methodologie proposee, ainsi que les avances recentes en technologie de construction, peut etre employee pour utiliser les profils concaves dans les tranchees, les fouilles de mines a ciel ouvert, les systemes de retention de sol, et dans les pistes de sol naturellement cimentees et stabilisees. Les resultats presentes offrent un outil utile pour l'evaluation preliminaire pour l'adoption de tels profils concaves dans la pratique. [Traduit par la Redaction] Mots-cles : stabilite des pentes, equilibre de limite, pente concave, optimisation, sismicite., Introduction Engineered slopes and earth structures are commonly built with a planar profile in cross section. However, it has been shown that concave slopes offer higher stability (e.g., Sokolovskii 1960; [...]

The behaviour of cast-iron cross passages in the London Underground was investigated using three-dimensional finite element models. Unlike the behaviour of a full tunnel ring, the structural integrity of a tunnel cross-passage opening relies on support from adjacent linings. In clayey soils, the opening may deform further as the soil stiffness changes from undrained to drained conditions. Degradation of the circumferential bolts and trackbed may also lead to further tunnel movement. A parametric study was conducted to examine the influence of soil stiffness and structural components (e.g., bolts and lintel) on the structural integrity of a tunnel opening. Results show that a lintel effectively transfers the load above the opening to the adjacent linings, and its distortion affects tunnel deformation significantly. If a lintel is not present, both bolts and friction between tunnel segments provide shear resistance to the lining deformation at the tunnel opening against soil loading. Results are compared with field observations made at a critical cross passage in one of the London Underground tunnels. The findings contributed to identifying the critical deformation mechanisms of cast-iron tunnel cross passages, which can be useful during inspection of such structures. Key words: cast iron lining, cross-passage tunnel, bolt-spring model, lintel, bolts. Le comportement de passages souterrains structure en fonte croisant des tunnels dans le metro de Londres a ete etudie l'aide de modeles d'elements finis tridimensionnels. A la difference de l'anneau qui entoure un tunnel, l'integrite structurale de l'extremite d'un passage souterrain debouchant sur un tunnel depend du soutien assure par les parois adjacentes. Dans les sols argileux, cette extremite peut se deformer lorsque la rigidite du sol varie et que l'on passe de conditions non drainees des conditions drainees. La deterioration des boulons situes la circonference du tunnel et de la plate-forme de la voie de circulation du metro peut egalement occasionner une deformation plus importante du tunnel. On a realise une etude parametrique afin d'analyser les effets de la rigidite du sol et des elements structuraux (p. ex. les boulons et linteaux) sur l'integrite structurale d'une extremite de tunnel. Les resultats montrent qu'un linteau transfere efficacement la charge qui s'exerce dans la partie superieure de l'extremite du tunnel vers les parois adjacentes et que sa distorsion influe beaucoup sur la deformation du tunnel. En l'absence de linteau, les boulons et les frottements qui s'exercent entre les segments de tunnel contribuent la resistance au cisaillement lors de la deformation des parois au niveau de l'extremite du tunnel en presence des charges exercees par le sol. Les resultats sont compares aux observations effectuees sur le terrain, une intersection importante entre un passage souterrain et un tunnel du metro de Londres. Ces observations ont permis d'identifier les mecanismes cles de la deformation de passages souterrains structure en fonte croisant des tunnels, ce qui peut s'averer utile si l'on souhaite inspecter ce type de structure. [Traduit par la Redaction] Mots-cles: paroi en fonte, passage souterrain croisant un tunnel, modele boulon-ressort, linteau, boulons., Introduction The condition of aging underground tunnels presents a significant engineering challenge in many cities (Inokuma and Inano 1996; Mair 2008). The London Underground has a history of more than [...]

A procedure is developed where stresses from a finite element analysis are incorporated into a limit equilibrium framework to evaluate the stability of three-dimensional slopes. An independent stress-deformation analysis is performed to calculate the internal stress state for the slope. The stress distribution is imported into the three-dimensional slope stability analysis in the form of a regular grid. The slip surfaces considered in the limit equilibrium analysis are ellipsoidal and discretized using a series of triangular planes. The normal and shear force acting at the centroid of individual triangular planes can be computed from the internal stress distribution. Subsequently, the factor of safety of a selected slip surface can be calculated directly without using an iterative procedure. A series of verification examples are presented to confirm that the proposed method provides the required accuracy and flexibility to assess the stability of slopes typically encountered in practice. Sensitivity analyses are presented to show how the procedure used to compute the forces acting on each triangular plane, the number of planes used to discretize the slip surface, and Poisson's ratio influence the computed factors of safety, but do not limit the successful application of the methodology. Key words: three-dimensional slope stability, stress-based slope stability, Poisson's ratio. Cet article presente une procedure dans laquelle les contraintes d'une analyse par elements finis sont incluses dans un systeme d'equilibre limite pour evaluer la stabilite de pentes en trois dimensions. Une analyse independante en contrainte-deformation est realisee afin de calculer l'etat des contraintes internes de la pente. La distribution des contraintes est importee dans l'analyse de stabilite de pente en trois dimensions sous la forme d'une grille rectangulaire. Les surfaces de glissement considerees dans l'analyse d'equilibre limite sont ellipsoidales et discretisees a l'aide d'une serie de plans triangulaires. Les forces normales et de cisaillement exercees au centroide des plans triangulaires individuels peuvent etre calculees a partir de la distribution des contraintes internes. Ensuite, le facteur de securite d'une surface de glissement selectionnee peut etre calcule directement sans avoir recours a une procedure iterative. Une serie d' exemples de verification sont presentes pour confirmer que la methode proposee fournit la precision et la flexibilite necessaire pour evaluer la stabilite de pentes typiquement rencontrees sur le terrain. Des analyses de sensibilite sont presentees pour demontrer comment la procedure utilisee pour calculer les forces qui agissent sur chaque plan triangulaire, le nombre de plans utilises pour discretiser la surface de glissement, et le ratio de Poisson influencent les facteurs de securite calcules, mais sans limiter l'application adequate de la methodologie. Mots-cles : stabilite de pente en trois dimensions, stabilite de pente basee sur les contraintes, ratio de Poisson. [Traduit par la Redaction], Introduction A variety of procedures are available to evaluate the stability of slopes ranging from simple expressions such as the use of a stability number to rigorous numerical methods. The [...]

Interstory drift, the relative translational displacement between two consecutive floors, is an important engineering demand parameter and indicator of structural performance. The structural engineering community would benefit well from accurate measurements of interstory drift, especially where structures undergo inelastic deformation. Unfortunately, the most common method for obtaining interstory drift, double integration of measured acceleration, is problematic. Several issues associated with this method (e.g., signal processing steps and sparse instrumentation) are illustrated using data from shake table studies and two extensively instrumented buildings. Some alternative contact and noncontact methods for obtaining interstory drift are then presented. DOI: 10.1061/(ASCE)ST.1943-541X.0000255 CE Database subject headings: Data processing; Displacement; Probe instruments; Buildings; Instrumentation; Drift; Measurement; Floods. Author keywords: Data processing; Displacement; Acceleration; Sensors; Buildings; Instrumentation; Drift.

Since the early 20th century combined RC-masonry buildings have become more common in European, Mediterranean, and Southern America countries. Despite the diffusion of this combined building typology, the international guidelines have not followed building transformation evolutions and, in particular, for combined RC-masonry buildings, nowadays, international guidelines are not exhaustive to deal with specific issues of this building typology. Although there is a well-established background focused on the nonlinear analysis of masonry structures and RC frames, the knowledge of numerical and experimental criteria for the study of interaction effects in combined RC-masonry buildings is limited. In this paper, nonlinear static analyses (pushover analyses) on three- dimensional combined RC-masonry buildings have been performed to obtain capacity curves of single-resistant systems and of the whole building. The results confirm the code guidelines for the design of new combined RC-masonry buildings and provide interesting insights about the seismic behavior of combined RC-masonry buildings obtained from the rehabilitation of original masonry structures. DOI: 10.1061/(ASCE)ST.1943-541X.0000249 CE Database subject headings: Reinforced concrete; Masonry; Buildings; Seismic effects; Comparative studies; Rehabilitation. Author keywords: Combined RC-masonry; Existing and new buildings; Seismic load repartition; Pushover analysis.

The paper focuses on the development of a general method for extreme response estimation of dynamical systems subjected to random excitations. One of the most important elements in structural safety is an assessment of the exceedance or failure probability. The frequently used Poisson assumption tends to overestimate the exceedance level if the response spectrum has narrow- banded features since the so-called clustering effects occur. The latter inaccuracy may be corrected by using an alternative approach studied in this paper, which enables accurate prediction of extremes irrespective of clustering effects. DOI: 10.1061/(ASCE)0733-9399(2010)136:3(290) CE Database subject headings: Engineering mechanics; Dynamics response; Structural reliability; Nonlinear analysis; Random processes; Predictions; Statistics. Author keywords: Extreme response; Narrow banded process; Clustering; Mean up-crossing rate; Nonlinear dynamic systems; Monte Carlo simulation.

A compendium of mechanical properties of carbon and low alloy steels following hydrogen exposure has been assembled from literature sources. The property sets include yield strength, ultimate tensile strength, uniform elongation, reduction in area, threshold stress intensity factor, fracture toughness, and fatigue crack growth. These properties are from literature sources under a variety of test methods and conditions. The collection of literature data is by no means complete, but the diversity of data and dependency of results oil test method are sufficient to warrant a design and implementation of a standardized test program. The program would be needed to enable a defensible demonstration of structural integrity of a pressurized hydrogen system. It is essential that the environmental variables be well-defined (e.g., the applicable hydrogen gas pressure range and the test strain rate) and the specimen preparation be realistically consistent (such as the techniques to charge hydrogen and to maintain the hydrogen concentration in the specimens). [DOI: 10.1115/1.3141435]

Many probabilistic methodologies have been proposed that address the reliability and risk factors unique to composite structures, incorporating micromechanics, laminate theory, manufacturing defects, operating environment, and impact damage, but few have addressed the importance of inspection intervals and damage-detection capabilities. The present study is based on a probabilistic damage-tolerance analysis with consideration of the following parameters: inspection intervals; statistical data on damages; loads; temperatures; damage-detection capability; and residual strength of the new, damaged, and repaired structures. The inspection intervals are formulated based on the damaged structure's probability of failure and the quality of its repair. This approach enables accidental, random damage events to be assessed quantitatively, allowing aircraft manufacturers, operators, and flight certification authorities to better evaluate and predict the damage tolerance and safety of an aircraft structure. Engineers can use this methodology to incorporate structural risk and maintenance costs into their design and inspection criteria. Its validity is demonstrated on several existing structural components, and special attention has been paid to the availability and cost of the probabilistic input data.

The failure probability of a series structural system theoretically involves multidimensional integration and is usually difficult to calculate. The search for efficient computational procedures for estimating system reliability has resulted in several approaches, including bounding techniques and efficient Monte Carlo simulations. For the narrow bound method, the joint failure probability of every pair of failure modes needs to be calculated. In the present paper, in order to improve the accuracy of the narrow bound estimation method, a computationally effective point estimation method for calculating the joint probability of every pair of failure modes is proposed and examined for series systems. Based on the computational results of several illustrative examples, it can be seen that the results by the present method are in good agreement with those obtained through integration. DOI: 10.1061/(ASCE)0733-9399(2007)133:5(588) CE Database subject headings: Structural reliability; System reliability; Joints; Failures; Integrated systems.

Size effect is studied here with reference to the bonding of short, deformed bars, embedded in normal-strength concrete (NSC) and high-performance concrete (HPC). Tests on 48 cylindrical specimens reinforced with a single bar and subjected to a pull-out or push-in force show that bond exhibits a strong size effect, which is well described by Bazant's general-type size-effect law. Four diameters are considered ([d.sub.b]=5, 12, 18, and 26 mm), with bonded length-to-bar diameter ratios equal to 3.5 (HPC) and 5 (NSC). All specimens are highly confined by means of a steel jacket to prevent or control cover splitting and to investigate bond behavior in highly confined conditions. Test results on short, anchored bars were instrumental in working out the local bond stress-slip law, taking into account size effect, which appears in the formulation of the maximum bond stress through the bar diameter. Short embedments also prevented bar yielding. The proposed local bond stress-slip law (1) is formulated as an extension of the law suggested in European Model Code MC90, also including some later proposals; (2) fits quite well with the available test data on short, well-confined anchored bars; (3) introduces the favorable effects that the confining reinforcement (generally consisting of stirrups and longitudinal bars) has on bond strength; and (4) may be easily introduced into a design code. CE Database subject headings: Anchorages; Bonding strength; Confinement; Cracking; Fracture mechanics; High strength concretes; Slip; Size effect.

Accurate and efficient response gradient computations for nonlinear geometry are required in structural reliability, optimization, system identification, and response sensitivity analysis of frame structures that undergo large displacements. In this paper, the exact response gradient of beam-column finite elements under large displacements is derived considering uncertain material and geometric parameters. The element response formulation takes place in a corotational reference frame that displaces and rotates with the element, thus permitting the separation of nonlinear material from nonlinear geometric effects in the computation of the response, as well as of the gradient of the response. Relative to the corotational reference frame, small deformation theory suffices for all structural engineering applications. Thus, the proposed response gradient computations are applicable to most beam-column element formulations available in the literature, including force-based and mixed formulations. CE Database subject headings: Beam columns; Finite elements; Frame structures; Nonlinear analysis; Geometric nonlinearity; Sensitivity analysis; Structural reliability.

This paper presents the test results on 12 two-span continuous T-beams strengthened using different external tendon types and profiles and subjected to third-point loadings. Test results indicated that an increase in ultimate strength with sufficient ductility could be achieved using short tendons located over the critical sections. Such a strengthening scheme led to a more ductile beam behavior compared to those with continuous tendons over both spans or with draped tendons within each span. Parabolic tendons anchored beyond the interior support however effectively strengthened the negative moment region with improved ductility at ultimate limit state. Furthermore, beams strengthened with carbon fiber-reinforced polymers tendons showed similar response to those with steel tendons, while beams subjected to unsymmetrical loading suffered from larger deflections and lower ultimate load compared to those subjected to symmetrical loading. Theoretical predictions based on the concept of bond reduction coefficients were found to agree with the test results. Finally, a parametric study was carried out to establish design charts that can be used for the strengthening of continuous beams. A design example is presented to illustrate the use of the charts. CE Database subject headings: Continuous beams; Fiber reinforced polymers; Structural strength.

A high Reynolds number wind-tunnel test was conducted to assess Reynolds number effects on the stability and control characteristics of a realistic, second-generation supersonic transport concept. The testing included longitudinal and lateral/directional studies at transonic and low-speed, high-lift landing conditions across a range of Reynolds numbers from that available in conventional wind tunnels to near-flight conditions. Results presented focus on Reynolds number sensitivities of the stability and control characteristics at Mach 0.30 and 0.95 for a configuration including empennage. The angle of attack where the pitching-moment departure occurred increased with higher Reynolds numbers for both the landing and transonic configurations. Stabilizer effectiveness and directional stability increased with the Reynolds number for both configurations. The landing configuration without forebody chines exhibited a large yawing-moment departure at high angles of attack and zero sideslip that varied with increasing Reynolds numbers. This departure characteristic nearly disappeared when forebody chines were added. The landing configuration's rudder effectiveness also exhibited sensitivities to changes in Reynolds number. This study extends the existing Reynolds number database for supersonic transports operating at subsonic conditions.

Wave induced pore pressures in bottom soils have an influence on the geotechnical stability of structures and may also have an effect on the transport of sand. Wave induced pore pressures in a seabed of sand or clay has been an item for extensive research. One of the major difficulties in analyzing and interpreting both laboratory and field data has been related to the question of air/gas content in the pores of fine soil. Only a small fraction of air/gas significantly changes the bulk modulus of the water/gas mixture and also the attenuation of the wave induced pressures in the soil with depth. The reason for the difficulty of analyzing data has been the lack of good instrumentation for measuring the gas content in the seabed soil. DOI: 10.1061/(ASCE)0733-950X(2007)133:1(83) CE Database subject headings: Coastal engineering; Wave pressure; Measurement; Saturation; Gas.

The stability and second-order analyses of three-dimensional (3D) multicolumn systems including the effects of shear deformations along the span of each column are presented in a condensed manner. This formulation is an extension to an algorithm presented recently by the writer in 2002 and 2003 by which the critical load of each column, the total critical load, and the second-order response of a 3D multicolumn system with semirigid connections can be determined directly. The proposed solution includes not only the combined effects of flexural deformations and shear distortions along the columns in their two principal transverse axes, but also the effect of the shear forces along each member induced by the applied end axial force as the columns deform and deflect (as suggested by Haringx in 1947 and explained by Timoshenko and Gere in 1961) in their two principal transverse axes. The extended characteristic transcendental equations (corresponding to multicolumn systems with sidesway and twist uninhibited, partially inhibited, and totally inhibited) that are derived and discussed in this publication find great applications in the stability and second-order analyses of 3D multicolumn systems made of materials with relatively low shear stiffness such as orthotropic composite materials (fiber reinforced plastic) and multilayer elastomeric bearings used for seismic isolation of buildings. The phenomenon of buckling under axial tension in members with relatively low shear stiffness (observed by Kelly in 2003 in multilayer elastomeric bearings, and recently discussed by the writer in 2005) is captured by the proposed method. Tension buckling must not be ignored in the stability analysis of multicolumn systems made of columns in which the shear stiffness GAs is of the same order of magnitude as [[pi].sup.2]EI/[h.sup.2]. DOI: 10.1061/(ASCE)0733-9399(2007)133:1(106) CE Database subject headings: Buckling; Building codes; Columns; Computer applications; Frames; Shear deformation; Compression.

Gravity loads play an important role in the linear and nonlinear behavior of buildings during earthquakes. They can also be the cause of ultimate collapse of the structures. In this study, the governing equation of continuous uniform shear buildings under the effects gravity loads is derived and eigenfrequencies, displacement, and drift mode shapes are obtained by eigenanalysis. It is shown that how the geometric properties of the structure affect the fundamental oscillation period and response of the building. Inclusion of the effects of the gravity loads makes the solution of the governing differential equation dependent on the Bessel functions of the first and second kind. The modal load and mass equations are solved using the orthogonality relations of Bessel functions. Effects of gravity on displacement and drift behavior of shear buildings on soft soils and rock subjected to limited near-fault earthquake excitations are shown. DOI: 10.1061/(ASCE)0733-9399(2007)133:1(48) CE Database subject headings: Shear walls; Drift; Lateral displacement; Structural stability; Frequency analysis; Eigenvalues; Eigenvectors; Gravity loads.

In this section, we consider the stability of the scattering problem with respect to a small perturbation of the potential. We will study two cases. If the potential in the [...]

Two general methods are proposed to calculate the aircraft equivalent singly vulnerable area. One is called the Monte Carlo simulation based method, and the other is called the aircraft multiple hit vulnerability based method. The Monte Carlo simulation method compares all the aircraft's unique existent states to a 'model of filling boxes with balls,' and by randomly and uniformly sampling the threat hit locations, the expected number of hits on the vulnerable area required to kill an aircraft [E.sub.AV] can be attained. In the second method, [E.sub.AV] is obtained using the cumulated kill probability of an aircraft subjected to multiple threat hits. After getting the [E.sub.AV], the aircraft equivalent singly vulnerable area can be calculated. Four examples demonstrate the correctness and feasibility of the two general methods. Analysis shows that the developed methods overcome the shortcomings of currently used methods in that these methods can deal with the calculation of an aircraft equivalent singly vulnerable area for the case in which 1) the aircraft may have several sets of multiply vulnerable components, and the multiply vulnerable components of each set do not always have the same vulnerable area, and 2) the aircraft vulnerable components can overlap in any arbitrary manner.

NASA's effort to develop a large payload, high altitude, long-duration balloon, the ultralong duration balloon, focuses on a pumpkin shape superpressure design. It has been observed that a pumpkin balloon may be unable to pressurize into the desired cyclically symmetric equilibrium configuration, settling into a distorted, undesired state instead. Hoop stress considerations in the pumpkin design lead to choosing the lowest possible bulge radius, whereas robust deployment is favored by a large bulge radius. Mechanical locking may be a contributing factor in the formation of undesired equilibria. Long term success of the pumpkin balloon for NASA requires a thorough understanding of the phenomenon of multiple stable equilibria. This paper uses the notion of stability to classify balloon designs. When we applied our finite element model to a balloon based on the NASA Phase IV-A pumpkin design, we found the fully inflated/fully deployed strained equilibrium float configuration was unstable. To demonstrate our approach for exploring the stability of constant bulge radius designs and their sensitivity to parameter changes we carry out a number of parametric studies. We focus on analytical studies, but we also compare our results with flight data whenever possible.

The static equilibrium position and its associated dynamic stability of a cylinder situated in the wake of an upstream cylinder is investigated in this paper. Both the upstream and downstream cylinders are elastically mounted on springs to allow for streamwise and transverse displacements. Due to the wake effect the downstream cylinder is subject to a lift force as well as a drag. It is shown that under certain flow conditions there exist multiple stable and unstable equilibria for the downstream cylinder. There also exist a critical flow velocity and once this velocity is exceeded no equilibrium positions of the downstream cylinder can be found, which suggests a likely occurrence of clashing between the two cylinders. [DOI: 10.1115/1.1555111]

In this paper, the stability of a thin cylindrical shell subjected to axial leakage flow is discussed. In this paper, the second part of a study of the axial leakage flow-induced vibration of a thin cylindrical shell, we focus on lateral vibration, that is, the beamlike vibration of a shell. The coupled equations between a shell and a fluid are obtained by using the Donnell's shell theory and the Navier-Stokes equation as same as the former paper. The influence of the axial velocity on the unstable vibration phenomena is clarified concerning the beamlike vibration mode of a shell. The numerical results on shell theory are compared with the ones on beam theory which have been already reported by the authors; and the numerical parameter studies are done for various dimensions of a shell and a fluid. [DOI: 10.1115/1.1565077]

This paper presents stability analysis and design recommendations for the falsework of wood and metal post shores. Based on the setup of shoring systems used on actual construction sites, analysis models of the shoring system have been derived. The experimental test results indicate that the base stiffness to the ground of shores is 50 t-cm/rad for (490 kN-cm/rad) wood post shores and 70 t-cm/rad (686 kN-cm/rad) for metal post shores of which the joint stiffness between members is 750 t-cm/rad (7,355 kN-cm/rad). With these stiffnesses, factors of 0.75 for wood post shores and 0.85 for metal post shores are used to modify the critical loads of shoring systems calculated from individual shores. The critical loads of a shoring system increase with the number of fixed strong shores, but are not affected by the number of leaning columns. In simplifying shore design, the LeMessurier formula is used for the strength computation of shoring systems composed of wood post shores. The critical loads of shoring systems increase linearly with the number of strong shores, but they are invariant with the positions of strong shores. If the required number of strong shores is defined, the critical loads of shoring systems can be found by interpolation. CE Database keywords: Falsework; Stability; Shoring; Critical load.

This paper describes an investigation into the effective length of columns subjected to axial loads, for the various conditions encountered during the construction of buildings using the lift-slab method. Many designers of lift-slab construction have assumed an effective length factor, [KAPPA], for all conditions, equal to 2.0, and have believed that this assumption is reasonable or conservative. The investigation shows that for many conditions a [KAPPA] value equal to 2.0 is reasonable or conservative; however, the investigation also shows conditions where effective length factors greater than 2.0 should be used, and where effective lengths much lower than 2.0 could be used. DOI: 10.1061/(ASCE)0887-3828(2002)16:4(141) CE Database keywords: Columns; Buckling; Slabs; Soil-structure interaction; Stability; Effective length.

The failure of two reinforced concrete structures is recounted, one involving a warehouse structure and the other an offshore platform base structure. Design details and factors leading to the collapses are identified and discussed. The structures were subsequently analyzed using nonlinear finite-element analysis procedures, taking into account relevant second-order behavior models. The analyses provided an accurate assessment of the load capacities and failure modes observed, as well as meaningful insights into the underlying behavior mechanisms and factors leading to the failures. This paper supports the view that nonlinear analysis techniques have become useful everyday tools for design office applications, particularly in forensic work, and also gives evidence suggesting that errors in the design of modern structures can be potentially more catastrophic than in the past, and that advanced assessment techniques will assume increased importance as a result. CE Database keywords: Structural failure; Concrete structures; Finite element method; Shear.

Two manufactured metal building warehouses loaded with agricultural products failed in service. Inspection revealed considerable damage to the structure and the foundation. The building owner filed suit against the building supplier, the building erector, and the soils testing laboratory whose engineer had designed the foundation. The agricultural product imposed substantial outward lateral pressures on the walls of the structures. Review of the available design documents indicated that these loads had not been accounted for in design. A structural analysis revealed that elements of the structure were underdesigned for the agricultural product loads. In addition, the foundation did not have any slab reinforcement to resist the loads. To prevent similar failures, these loads must be accounted for in design. The steel structure design and foundation design were both deficient. A contributing factor was the lack of communication between the designer of the structure and the designer of the foundation, due to the lack of a single engineer of record to take responsibility for the buildings. CE Database keywords: Structural failure; Litigation; Loads; Building design.

Compression-web members in metal-plate-connected wood trusses may buckle elastically as long slender columns. One common construction technique is to nail a side member on the compression-web member (main member) so that the cross section is 'T' shaped, thus increasing the cross-sectional dimensions and the critical buckling load. The bracing efficiency of the T-brace is not known. The objective of this study was to assess the influence of the main-member length, side-member length, and connection method on T-brace efficiency by measuring the first buckling-mode critical load. Finite-element models were developed with simply supported ends and side members that varied from full length to one-quarter length. The models had side members attached with either 16d common nails or phenol-resorcinol adhesive. The results of a finite-element investigation were used to determine which T-brace geometries to test. The testing program included full-length, half-length, and quarter-length side members nailed or glued to 1,830 and 3,050-mm main members. The testing results paralleled the finite-element results. The T-brace assemblies exhibited a linear minor-axis bending deflection from the onset of loading until an instability load was reached; the deflection then increased without bound. Some slipping between the main and side members was observed in the nailed assemblies, and twisting was evident in the deformed shape of the adhesive-attached assemblies. T-bracing was most efficient for very long members; its effectiveness increased with increasing side-member length. Adhesive-attached side members were more effective than nail attached. The effects of end geometry and out-of-plane metal plate bending were not investigated but could have a substantial effect. DOI: 10.1061/(ASCE)0733-9445(2002)128:3(374) CE Database keywords: Bracing; Trusses; Webs; Finite-element method; Wood.

Research is presented describing the study of isoguanosine and isocytidine related RNA duplexes to measure structure and stability.

Elastomeric seismic isolation bearings are subjected to large axial loads and lateral displacements during strong earthquakes. The existing Koh-Kelly model for elastomeric bearings accounts for axial load effects on horizontal stiffness. This linear model is based on small displacements and rotations and predicts stable postcritical behavior or increasing critical load with increasing horizontal displacement; however, unstable postcritical behavior is observed in the bearing test results presented in this study. The analytical model developed in this study, based on the Koh-Kelly model, includes large displacements, large rotations, and nonlinearity of rubber, and it predicts unstable postcritical behavior. The formulation of the analytical model, calibration, and verification using experimental results are presented. It is shown that: (1) the critical load reduces with increasing horizontal displacement; and (2) the horizontal stiffness reduces with increasing horizontal displacement and axial load. It is also shown that the critical load capacity at a horizontal displacement equal to the width of the bearing is not equal to zero, as predicted by the approximate procedure used in design, but higher.

Design and operational issues with respect to the use of inflatable-deployable foam-rigidized components in aerospace structures are investigated in this paper. The sample structures used in this study are fabricated from flexible Kapton film formed into a cylindrical shell by bonding a flat sheet along a longitudinal seam. This tubular shell is injected with a hardening urethane foam to form a composite strut coupon. As with all structures touted for aerospace use, the survivability of foam-rigidized structures when subjected to a micrometeor flux is of interest. This issue is investigated in this paper by performing two controlled experiments: (1) Foam-rigidized test coupons were evaluated in their pristine state to determine structural properties, severely damaged in a controlled fashion, and then evaluated again to compare the undamaged to damaged behavior; and (2) a single specimen was repeatedly tested and then slightly damaged to examine how structural behavior evolves as damage accumulates. The results of these experiments are then used to draw conclusions about the utility of foam-rigidized structures in space applications, including an evaluation of appropriate structural health monitoring strategies.

The ductility of an elastic structure with a growing crack may be defined as the ratio of the additional load-point displacement that is caused by the crack at the moment of loss of stability under displacement control to the elastic displacement at no crack at the moment of peak load. The stability loss at displacement control is known to occur when the load-deflection curve of the whole structural system with the loading device (characterized by a spring) reaches a snapback point. based on the known stress intensity factor as a function of crack length, the well-known method of linear elastic fracture mechanics is used to calculate the load-deflection curve and determine the states of snapback and maximum loads. An example of a notched three-point bend beam with a growing crack is analyzed numerically. The ductility is determined and its dependence of the structure size, slenderness, and stiffness of the loading device is clarified. The family of the curves of ductility versus structure size at various loading device stiffnesses is found to exhibit at a certain critical stiffness a transition from bounded single-valued functions of D to unbounded two-valued functions of D. The method of solution is general and is applicable to cracked structures of any shape. The flexibility (force) method can be adapted to extend the ductility analysis to structural assemblages provided that the stress intensity factor of the cracked structural part considered alone is known. This study leads to an improved understanding of ductility, which should be useful mainly for design against dynamic loads.

A new [C.sub.0] finite element for accurate analysis of laminated composite beam structures is developed. The element formulation is based on a quadratic zig-zag layerwise theory developed previously by the writers. The theory assumes a zig-zag distribution of the in-plane displacement field through the thickness and satisfies the interlaminar shear stress continuity across the layer interfaces. In developing the finite-element formulation, the shear strain fields are made field consistent, and thus the shear locking phenomenon is eliminated. A new transverse normal strain is derived by assuming the transverse normal stress to be constant through the thickness of the laminate. This assumption is shown to remove Poisson's ratio stiffening. The results obtained from the present finite element are found to be in good agreement with exact elasticity solutions available for simply supported beams. A multisublaminate approach that is simple to implement with the present element is shown to improve the predictions of the present model for complex laminated structures.

The postbuckling behavior of an elastic fixed-end column with an elastic brace at the center is investigated. Attention is focused on those of brace stiffness near its threshold value at which, under axial load, the column becomes critical with respect to two buckling modes simultaneously. We show that, for the brace stiffness greater than the threshold value, there are precisely two secondary bifurcation points on each primary postbuckling path bifurcating from one of the least two classical buckling loads, and the corresponding secondary postbuckling paths connect all of these secondary bifurcation points in a loop. For the brace stiffness less than the threshold value, no secondary bifurcation occurs. The asymptotic expansions of the primary and secondary postbuckling paths are constructed. The stability analysis indicates that, when the brace stiffness goes beyond its threshold value, the primary postbuckling path with a node in the center becomes unstable from stable by means of the secondary bifurcation (i.e., secondary buckling occurs).

The relationship between axial load and end-rotational stiffness is derived. Reliable postelastic performance of concentrically braced frames requires that braces be designed for predictable buckling behavior. For cross-braced frames, restraint provided by the complementary brace (including both translational and rotational stiffness) can have a significant effect on buckling strength; this effect is complicated by the tension in the complementary brace, which increases its stiffness. Differential equations of stability are employed to determine upper and lower bounds of buckling strength, as well as to formulate the relationship between axial load and rotational stiffness. Given current detailing practices, it is desirable to control the plane of buckling so that the ductility of end-gussets is utilized; values of the effective length factor are provided to allow the design of braces for predictable buckling behavior. Out-of-plane buckling of systems with crossed torsionally stiff braces may place severe torsional demands on the end-gussets of the brace in tension; in-plane buckling is a viable alternative if it can be shown that it occurs at a lower load than out-of-plane buckling.

Four evaluations of a study on the stability of uniformly reinforced slopes are presented. One raises questions about the definition of pore water pressure used in deriving the analytical expression for computing the reinforcement needed to prevent the collapse of slopes. Another argues that the modeling of the soil as a 3D continuum can be applied to various reinforcement patterns regardless of the distribution of inclusions in the soil masss.

This paper is intended to verify the concern expressed by researchers that stiff asphalt-mineral filler mastics may lead to the cracking of flexible pavements in the brittle-ductile region. Test results obtained from the direct tensile test show that both tensile stress and tensile strain increase with increasing mineral fillers. The failure energy of asphalt-mineral filler mastics also increases with filler concentration because of increases in strength and strain. With increasing tensile strength, it is implied that there is a good adhesion between asphalt binders and mineral fillers. The tensile strength of asphalt-mineral filler mastics decreases as the size of filler particles increases. However, at very low temperatures plain asphalts and mastics exhibit low strains, indicating that mineral fillers may not be the cause of cracking. Interaction mechanisms between mineral fillers and asphalt binders are proposed to explain the increase of stress and strain. After reviewing various composite models, a particulate-filled composite model is selected to accurately predict the tensile strength of asphalt-mineral filler mastics.

The stability of an elastic wheelset coupled with torsional spring and damper is studied in this paper. With flexible elements between two wheels, the advantages of both rigid and independently rotating wheel systems may be obtained. Previous investigations indicated that axle flexibility will affect the vehicle dynamic behavior and an optimal design may improve the system performance. Those studies were limited to constant wheel/rail geometry as the wheelset rolls along the track. In this paper, it is intended to determine the critical speed regions for both constant and time-varying models. The variation in conicity is assumed to be periodic thus the Floquet stability concept may be employed. The computation of the state-transition matrix is based on a Runge-Kutta algorithm.

The dynamical behaviors of a flexible rotor supported by two plain cylindrical bearings surrounded by squeeze film damper are investigated. In this study, the Hopf bifurcation theory is used to investigate the stability of the equilibrium position of the system, then the unbalanced rotor response is determined by the shooting method and the stability of these solution is examined using the Floquet transition matrix method. It is shown that the quasi-periodic motion and period-doubling motion may be excited by the rotor unbalance.

This paper investigates the vibration of an annular disk that is subjected to rotation and in-plane frictional traction distributed over a sector of the disk's two faces. Technical applications include noise, vibration, and harshness in automotive and aircraft disk brakes, clutches, transmissions, and other rotating machine components. To the degree that the rotor-to-stator friction in such cases is directed along the disk's deformable surface, it is treated here as a nonconservative follower-type load. The vibration model incorporates membrane stiffness which derives both from rotation, and from the stresses established as a result of friction. The plane stress state is determined in closed form as a Fourier series, and that solution is compared with the companion, but computationally intensive, results from finite element analysis. For the cases of sector-shaped and full annular loading the vibration model predicts the critical mode, which is defined as the one that becomes dynamically unstable at the lowest friction level. Vibration modes and propagating waves that fall into opposite symmetry classes are shown to have opposite stability characteristics in the presence of frictional loading.

In this article we consider the influence of the prismatic kinematic pair lubrication film on the planar vibrations of a constant cross-sectional straight link that is attached to the joint. The main objective is to develop an analytical model that incorporates the effect of the lubricant film on the vibration of elastic links in mechanisms. It has been assumed that the beam on which the prismatic kinematic pair translates is a linear elastic body. Equations for the translational and rotational motions of the link are developed by applying Hamilton's principle. Kinetic energy that is required for the application of this principle has been derived by utilizing a generalized velocity field theory for elastic solids. This approach provides means to include the inertia terms directly in the equations of motion. The pressure field exerted through the viscous, incompressible, lubricant film is obtained from the solution of the Reynolds equation of lubrication. We introduce a scheme to solve the resulting two sets of equations for the vibrations of the link and the motion of the fluid. The pressure field is used to compute the external force exerted by the fluid on the link. The utility of the method is demonstrated by considering a planar mechanism that includes an elastic element with a prismatic joint.

A pilot study was conducted to evaluate existing rock-sizing techniques for stabilizing transition toes of embankments. The U.S. Bureau of Reclamation and the U.S. Army Corps of Engineers (Campbell) procedures were applied and determined to be conservative in sizing riprap. Embankment-overtopping tests were conducted placing 8.9, 13.0, and 19.8-cm-diameter stones at the slope transition. An alternative method was developed for sizing toe rock based upon the unit discharge, embankment slope, and flow concentration. The results indicate that an embankment toe can be stabilized with a smaller median stone size than previously anticipated. These results were verified for unit discharges of 0.54 [m.sup.3]/s/m or less.

The stability condition of cantilevered masonry piers subjected to their own weight and to a concentrated compressive top load is investigated, considering no-tension material with nonlinear stress-strain law in compression, of an experimental nature, and including softening behavior. The analysis is carried out by improving a numerical approach adopted in previous works where stability problems were solved assuming an infinitely elastic linear constitutive law in compression. Before the geometrical nonlinearity is considered, the limit equilibrium of a typical pier rectangular cross section is detected assuming unlimited available compressive strain. This preliminary analysis allows one to determine analytically the limit value that has to be imposed on the eccentricity of the resultant compressive force and to derive the moment-curvature relationships on which the second-order effects depend. Then the stability domains are derived in dimensionless form and their boundaries are modeled by analytical approximate expressions of practical use. Some numerical examples show that, depending on the average normal stress level, the assumption of an approximate linear constitutive law in compression, affected by the same elasticity modulus as that at the origin of the actual stress-strain law, can provide an unacceptable overestimate of the critical load.

The main objective of this study is to assess the structural integrity of the double-deck through-truss Brent-Spence bridge when subjected to a seismic event from the New Madrid, Wabash Valley, or Anna seismic zones. To achieve this, the scope of work was divided into several tasks as follows: field testing of the main bridge, analytical modeling, determination of the site-specific ground motion, and seismic response analysis. This paper deals with the first two tasks. The ambient vibration properties of the main bridge were determined through field testing under traffic induced excitation. The purpose of measuring the ambient vibration properties was to determine the mode shapes and the associated natural frequencies. These vibration properties were subsequently used as the basis for calibrating the finite-element model for seismic response analysis. A three-dimensional finite-element model of the main bridge was developed for the free vibration analysis. The seismic response analysis using this calibrated finite-element model is presented in the second part of this two-part paper.

This is the second part of a two-part paper on field testing and analytical studies related to seismic behavior of the Brent-Spence bridge. This part of the paper deals with seismic evaluation of the bridge, while the first part dealt with free and ambient vibration studies of the bridge by field testing and finite-element analysis. Site-specific ground motion scenarios are developed for the bridge to represent probable earthquakes that may occur in these seismic zones. The time histories of these events are then used in the seismic analysis of the main bridge and response spectra are used for analyzing the approach spans. A three-dimensional finite-element model of the main bridge was subjected to the time histories to determine maximum responses (displacements, forces, and stresses). The seismic analysis of approach spans dealt only with the potential for loss of span due to excessive longitudinal displacement along the highway main line. For the maximum credible earthquake of magnitude 8.5 [M.sub.s] (7.3 [m.sub.b,Lg]) on the Richter scale at New Madrid, the seismic evaluation indicates that the main bridge will survive the earthquake in the elastic range without significant damage and no loss of span. The approach spans are found to be vulnerable to loss of span failure in the 8.5 [M.sub.s] event.

The strength and ductility of steel frames depend strongly on the behavior of their beam-to-column connections. The fracture of beam-to-column connections of steel buildings in the Northridge Earthquake generated concern about the reliability of current design and construction technology on steel connections. There is an urgent need to find a reliable method to retrofit these connections and to use it for the design of new buildings. A simple method is proposed here to enhance the ductility of beam-to-column connections. On trimming the beam flanges slightly, the ductility of the connection can be improved greatly. Experimental study of the newly designed connections under cyclic load shows that the ultimate strengths are almost unaltered, whereas the stiffness is decreased only slightly. However, the plastic rotational capacity can be increased several times. Moreover, the fracture proneness of connections due to welding sensitivity and stress concentration from abrupt geometry alternation can be minimized and the energy dissipation capacity can be improved. A shaking table test of a steel frame designed according to the proposed connection method also behaves better than that of a conventional moment-resisting frame.

On Oct. 17, 1989, a tall, steel-framed, San Francisco office building with 47 above-grade stories was excited by the Loma Prieta earthquake. The building response was recorded by accelerometers maintained by the California Division of Mines and Geology Strong Motion Instrumentation Program. From the records, important characteristics of the building response can be identified. In this paper, a computationally efficient, approximate dynamic analysis of the building using a reduced-order continuum model is conducted. By making use of the repetitive nature of the lattice framing system, finite-element models generated using continuum methodology have significantly fewer degrees of freedom than models generated using classical discrete finite-element techniques. These reduced-order models can therefore lead to significant savings in the computational effort and cost typically associated with dynamic analyses of large structures. The vibrational characteristics of the continuum model, as well as the results of the dynamic analysis, are compared to the building's recorded response. The continuum model is accurate in capturing the dominant periods of vibration as well as the time-history response and shows promise as a tool for preliminary design of large lattice structures.