Advances in Engineering Structures, Mechanics & Construction

Initial planning for the formation of the Solid Mechanics Division (SMD) at the University of Waterloo began in 1966, and it was implemented into a formal entity following the award of a National Research Council of Canada Development Grant in 1968. Upon the departure and retirement of many of its initial members over the next thirty years, SMD was reconstituted in the mid-1990s as the Structures Mechanics & Construction Division (SMCD) to reflect the changing research interests of its more recent and new members. The collective research work of both SMD and SMCD has resulted in many publications and valuable contributions to the progress of engineering structures, mechanics and construction. This paper presents a 40-year historical perspective through to 2006 of this activity at the University of Waterloo.

Two vibrating systems are said to be

isospectral

if they have the same natural frequencies. This paper reviews some recent results on isospectral conservative (i.e., undamped) discrete vibrating systems. The paper centres around two ways of creating isospectral systems: by QR factorisation with a shift, and by using the concept of isospectral flow. Both these procedures are illustrated by using FEM models.

Active cold-formed steel research started in the early 70s at the University of Waterloo with Profs. N.C. Lind, A.N. Sherbourne, J. Roorda, and R.M. Schuster, and their respective graduate students. More recently, Prof. L. Xu and his graduate students have also contributed in this area of research. Numerous publications have been documented in National and International sources over the past 35 years. Presented in this paper are the many noteworthy contributions that have been made as a result of the cold-formed steel research activities at the University of Waterloo. What is of particular interest is the number of design recommendations that have been adopted by National and International coldformed steel design Standards and Specifications. With the recently formed Canadian Cold-Formed Steel Research Group, cold-formed steel research is very much alive at the University of Waterloo.

Portland cement (PC) concrete is generally a highly durable structural material. Nevertheless, certain chemical actions and aggressive environments in a livestock building can cause deterioration and total collapses of structures have occurred long before they have reached their design life.

The sulphide and sulphate resistance of three replicates of eight different reinforced concrete mixes were investigated in a laboratory study in which one half of the 48 specimens were half submerged in a sodium sulphate solution (20,000 ppm SO

4

2t-

) and also exposed to hydrogen sulphide gas (1,000 ppm H2S). The other half of the 48 specimens was subjected to hydrogen sulphide gas only. The mixes included PC concrete with W/CM ratios of 0.4 and 0.5 and six mixes with cement replacements of slag, fly ash or silica fume, all with water/cementitious material (W/CM) ratio of 0.4.

After 23 cycles of testing over about 36 months, the electrochemical potential results and visual inspection of the reinforcing bars indicate that the PC concrete with 0.5 W/CM ratio was the least resistant against steel corrosion. Corrosion of the concrete was more critical than that of the steel. All treatments containing silica fume performed much better than PC40. Treatments that contained fly ash performed worse than plain PC concrete with the same W/CM ratio. Sulphate resistant cement concrete was more resistant than Type 10 Portland cement concrete, in both sets of tests. In general the samples that were exposed to hydrogen sulphide and sulphate corroded at a higher rate then those exposed to the H

2

S gas only.

In subsequent tests 6 of the 8 mixes were exposed to 7% sulphuric acid for about one year. Preliminary results indicate the greatest mass loss for the concrete with 0.5 W/CM ratio and very similar amount of loss for the 5 mixes with W/CM ratio of 0.4. The least amount of mass loss was experienced by the mix with sulphate resistant cement.

In order to establish a rational management program for bridge structures, it is necessary to evaluate the structural damage of existing bridges in a quantitative manner. However, it is difficult to avoid the subjectivity of inspectors when visual data are used for the evaluation of damage or deterioration. In this paper, an attempt is made to develop an optimal bridge maintenance system by using a health monitoring technique. The damage of Reinforced Concrete (RC) bridge decks is evaluated with the aid of digital photos and pattern recognition. So far, neural networks have been applied to judge the damage state of RC bridge decks. However, there are still some problems that learning data are not enough and recognition accuracy is not satisfactory. In order to solve these problems, TAM network is applied here, which is an optical system. Though the numerical examples using actual data, it is shown that the recognition rate is increased.

As the computing power of computers is constantly increasing, more accurate finite element analysis and detailed modelling of structures are sought. The critical issue of concern at hand is the characterization of complex material constitutive behaviour using numerical techniques. Finite element analysis of reinforced concrete structures under severe and reversible loadings requires a proper representation of concrete material behaviour. Abnormal loads such as impact, blast and seismic are generally reversible and cause structures to vibrate. To arrive at a reasonable approximation of damage in reinforced concrete structures under abnormal loading, the cracking of the concrete and its direction must be addressed. The inclusion of a mechanism that accounts for crack closure should be considered to include the compression strength of the cracked concrete if the load direction is reversed and the crack is closed. Thus, development of an improved material model for concrete and its implementation in a non-linear finite element code that is well suited to this class of problem is undertaken. In the work described in this paper, the methodology used in the development of this new material model for concrete is discussed. A sample case is analysed and the results of these FE analyses are discussed. The new concrete material model predicts the location and the direction of the cracks accurately and also allows for the inclusion of the compression strength of the cracked material in directions parallel to crack plane. In addition the closure of the crack and reactivation of the compression strength of the concrete orthogonal to the crack plane when the crack is closed is achieved.

In order to establish a rational impact resistant design procedure for prototype reinforced concrete (RC) structures, not only experimental study but also numerical analysis study should be conducted. However, numerical analysis method on impact response analysis for those structures has not been established yet. Here, in order to establish a rational numerical analysis method for prototype RC structures under impact loading, a falling-weight impact test was conducted for prototype RC girder with 8 m clear span. Referring to the experimental response waves, numerical accuracy was investigated varying major parameters. From this study, following results were obtained as: (1) fine mesh should be used near supporting gigues; (2) Drucker-Prager yield criterion should be applied which gives better results than von Mises one; and (3) appropriate system damping constant should be set to

h

= 0.015.

In order to assess the damage loss of reinforced masonry walls under earthquake loading, it is helpful to have a set of commonly accepted damage states. HAZUS gives detailed description for the qualitative damage states and assigns threshold drift ratios for achievement of each damage state. The HAZUS damage states are assigned based on expert opinion and judgment, and performance and experience data. Unfortunately, application of the HAZUS damage states is limited by the fact that they do not differentiate failure mode: flexure, shear and possible mixed flexure/shear. Furthermore, drift ratios defined in HAZUS have not been fully verified by experiment or experience. As a step toward addressing these deficiencies, this paper examines experimental results from three experimental programs and assesses the accuracy of the HAZUS methodology. Drift ratios at achievement of defined damage states are compared with HAZUS definitions. Results show that the HAZUS methodology tends to overestimate the drift ratio achieved by a wall at a given level of damage. In this paper, only experimental results for concrete masonry unit (CMU) walls are considered.

The results of a set of experimental tests concerning the cyclic behaviour of prefabricated column-tofoundation connections is presented. The tests allow to compare the response of cast-in-place connections against pocket foundation and grouted sleeve solutions. The results demonstrate that grouted sleeves ensure a ductility similar to the one of cast in situ column-foundation connections and of pocket foundations, although a slightly smaller dissipation capacity is observed. It is found that in grouted sleeves connections the damage is localized at the column base, in the thin grout layer existing between the prefabricated column and the foundation. As a result, very little damage may be observed in the column, allowing an easier post-seismic column repair.

This paper presents the results of experimental study on air-void stability in fresh self-consolidating concretes. Two series of self-consolidating concrete were undertaken for conducting laboratory tests. Each series of concrete included three different fresh mixtures. The air-void stability in fresh concretes was investigated with respect to post-mixing and agitation. The air content of fresh concretes was determined at various test stages and adjusted considering aggregate correction factors. The flowing ability of the fresh concretes was also examined with regard to slump and slump flow. The entire testing period involved four stages extended to 60 and 90 minutes for series 1 and 2, respectively. Test results reveal that the slump and slump flow of the concrete mixtures were consistent in all test stages, and the loss of air content was minimal. The maximum loss of air content over the period of 60 and 90 minutes was less than 1.0%. Rice husk ash did not affect the air-void stability in fresh concretes. However, it increased the demand for high-range water reducer and air-entraining admixture. The overall test results indicate that the air-void stability in all fresh self-consolidating concretes was satisfactory.

This paper describes one phase of an extensive experimental program that has recently been completed at the University of Toronto. In this phase, eighteen lightly-reinforced shear-critical reinforced concrete beams were loaded to failure. The abilities of the ACI-318 shear design method and a simplified design method based on the Modified Compression Field Theory to predict the failure loads are compared. It is found that the ACI design method is dangerously unconservative when applied to large beams and oneway slabs constructed without stirrups, while the simplified MCFT design method is both safe and accurate. Studies of the mechanism of shear transfer indicate that approximately one quarter of the shear in a reinforced concrete beam constructed without stirrups is transferred in the compression zone, with the rest carried primarily by aggregate interlock. The development of theoretically-sound shear design methods must therefore be based on the fact that aggregate interlock plays a critical role in the shear behaviour of reinforced concrete structures.

Corrosion of steel reinforcement is one of the main durability problems facing reinforced concrete infrastructures worldwide. This paper gives an overview on a seven year research program conducted at the University of Waterloo, sponsored by ISIS (Intelligent Sensing for Innovative Structures) Canada, to examine the viability of using fibre reinforced polymer (FRP) composites as a repair and strengthening method for corroded reinforced concrete structures. The majority of the research was carried out in the laboratory utilizing large-scale members. The results revealed that FRP repair successfully confined the corrosion cracking and improved the structural performance of corroded beams. Analytical models were developed to validate the experimental data. The FRP repair system was implemented in Fall 2005 to address corrosion damage in a bridge in the Region of Waterloo.

Straw-bale construction is an emerging building method and many builders choose to plaster the straw bales with earthen plaster to reduce the embodied energy of the structure. A better understanding of the parameters affecting earthen plaster strength is essential for safe and effective use of this building technique. This study investigated the importance of initial plaster moisture content, drying time, clay content and, moisture content at the time of testing. Clayey silt soil, bagged ball clay and lime-cement are compared as plaster binders for straw-bale applications. Compressive testing was conducted on 50-mm plaster cubes and 100-mm by 200-mm plaster cylinders. It was found that as initial moisture content increased, strength and modulus of elasticity was unaffected for the earthen plaster. As the drying time increased between 10 days and 18 days, strength was unaffected but modulus of elasticity increased proportionally. As clay content increased, strength increased proportionally and stiffness was unaffected. As moisture content at the time of testing increased, both the strength and the stiffness decreased proportionally. Plaster made with soil was found to have greater strength than the plaster made with bagged clay or lime-cement plaster.

This paper provides an overview of the Generalised Beam Theory (GBT) fundamentals and reports on the novel formulations and applications recently developed at the TU Lisbon: the use of conventional GBT to derive analytical distortional buckling formulae and extensions to cover (i) the buckling behaviour of members with (i

1

) branched, closed and closed/branched cross-sections and (i

2

) made of orthotropic and elastic-plastic materials, and (ii) the vibration and post-buckling behaviours of elastic isotropic/orthotropic members. In order to illustrate the usefulness and potential of the new GBT formulations, a few numerical results are presented and briefly discussed. Finally, some (near) future developments are briefly mentioned.

Having many attractive advantages, genetic algorithms (GAs) have been applied to many design optimization problems. However, the practical application of GAs to realistic tall building design is still rather limited, since GAs require a large number of structural reanalyses and perform poorly in local searching. While the Optimality Criteria (OC) method can be applied effectively to the element sizing optimization of tall buildings, there is no guarantee that the OC method can always lead to the global optimum. In this paper, the so-called hybrid OC-GA method is presented to fully exploit the merits of both OC and GA for topology and element sizing optimization of braced tall steel frameworks. While the GA is particularly useful in the global exploration for optimal topologies, the OC technique serves as an efficient local optimizer for resizing elements of selected topologies. The effect of population size and the importance of the local OC search operator have been investigated. The applicability and efficiency of the hybrid OC-GA method were tested with two braced steel building examples. Results indicate that the incorporation of the OC operator into the GA has remarkably improved the efficiency and robustness of the evolutionary algorithm and thus make the hybrid method particularly useful for topology optimization of practical tall building structures involving a large number of structural elements and the use of numerous structural forms.

Due to corrosion and the continuous demand to increase traffic loads, there is a great need for an effective, cost-efficient system which can be used for the repair and strengthening of steel highway bridge girders. Recently, research has been conducted to investigate the use of carbon fiber reinforced polymer (CFRP) materials to address this need. This paper describes the details of an experimental program which was conducted to investigate the fundamental behavior of steel-concrete composite bridge girders strengthened with new high modulus CFRP (HM CFRP) materials. The behavior of the beams under overloading conditions and fatigue loading conditions was studied as well as the possible presence of a shear-lag effect between the steel beam and the CFRP strengthening. A series of proposed flexural design guidelines are presented which can be used to establish the allowable liveload increase for a strengthened beam and to design the required HM CFRP strengthening.

This paper describes an analytical model developed to predict the behaviour of axially loaded slender members composed of steel hollow structural sections (HSS), retrofitted with carbon-fibre reinforced polymer (CFRP) composite sheets. A previous experimental study by the authors showed that gain in strength due to CFRP retrofitting was highly sensitive to the specimen's imperfection. As such, developing an analytical model was necessary to uncouple the effects of imperfection and number of CFRP layers. The model predicts the load versus axial and lateral displacements, and accounts for initial imperfection, and the contribution of CFRP sheets. The model was verified against results of the experimental program and showed reasonable agreement. The model was then used in a steel plasticity, the built-in through-thickness residual stresses, geometric non-linearity, including increased both the axial strength and stiffness substantially. parametric study. The study demonstrated that retrofitting slender HSS columns using CFRP sheets CFRP layers, it is believed that geometric imperfections have also varied among the specimens. As such, no specific correlation could be established between the amount of strength gain and the amount of CFRP.

This paper is meant to present themechanical characteristics of austenitic stainless steel rebars under monotonic and cyclic loadings. Furthermore, some results concerning the experimental tests on column prototypes subjected to cyclic loadings are presented, with the intent of comparing with results obtained on analogous columns reinforced with standard high ductile carbon steel rebars.

This paper presents a new plastic-hinge method for inelastic analysis of steel frames. The proposed plastic-hinge model employed two parameters in the modeling. The first parameter involves mimicking the spread of plasticity through a section depth, while the second incorporates the spread of plasticity along a member length. Procedures to determine the key parameters are developed using moment-curvature-thrust relationship for steel beam-columns. The proposed analysis method is especially advantageous when modeling the spread of plasticity along a member length using various discretization schemes. Two numerical examples are performed to demonstrate the accuracy and simplicity of the method.

This paper discusses the results of extensive design experiments in which memetic algorithms were applied to optimize topologies of steel structural systems in tall buildings. In these experiments, evolutionary algorithms were employed to determine optimal configurations of structural members (topology optimization) while the optimal cross-sections of members (sizing optimization) were found using continuous/discrete optimization algorithm implemented in SODA. The impact of all major evolutionary computation parameters on the performance of memetic algorithms was investigated. Two classes of complex structural design problems were considered: design of a wind bracing system in a tall building and design of the entire steel structural system in a tall building. The total weight of the structural system was assumed as the optimality criterion with respect to which the designs were optimized while satisfying all design requirements specified by appropriate design codes.

In the conducted experiments various key EC parameters and their values were considered having the largest impact on the performance of memetic algorithms. It was discovered that the type of EA, the rate of mutation operator, and the size of parent population were critical for the success of structural optimization processes. Specifically, evolution strategies produced on average significantly better results than genetic algorithms for the design problems considered in the paper. Also, low mutation rates, i.e. 0.025, resulted in best performance of memetic algorithms. Furthermore, small parent population sizes were generally preferred to large populations. For the simpler problem of conceptual design of a wind bracing system, optimal results were produced even when the population with a single member was used. In the case of the second and more complex design problem slightly larger population sizes were required consisting of 5 members.

Results of a large number of design experiments allowed formulating initial recommendations regarding optimal parameter settings for memetic algorithms for structural design applications. The experiments also produced a body of structural design knowledge, both quantitative and qualitative in nature. They identified regions of the design spaces in which high-performance solutions can be found. They also defined the ranges of the total weight of structural systems associated with highperformance solutions for both classes of design problems. Furthermore, significant qualitative differences between high-performance solutions have been identified. The structural shaping patterns exhibited by high-performing designs ranged from crossed macrodiagonal patterns composed of X bracings to irregular patterns consisting of various types of bracings.

In order to precisely investigate an interaction between column flange and top angle's vertical leg and the effects including prying action of bolts on

M - θ

r

characteristics of top-and seat-angle connections, nonlinear finite element (FE)analyses for top-and seat-angle connections as one of the angle type connections were performed. In those analysis, contact model with small sliding option was applied between contacting pair surfaces of all connecting elements. Bolt pretension force was introduced in the initial step of analysis. Numerical results together with those estimated by using Kishi-Chen power model were compared with the experimental ones to examine those applicabilities.Parametric study was also performed by varying connection parameters, material properties of connection assemblages,and magnitude of bolt pretension.From this study, the following results are obtained:(1)proposed numerical analysis method can be applicable to estimate elasto-plastic nonlinear behavior of angle type connections; (2)pretension force of bolts has no effect on prying action at the initial and ultimate level of loading,but gives some effects a little at the intermediate loading level; (3)a prying action can be increased with decreasing the thickness of flange angle or increasing of gage distance from the angle heel to the centerline of bolt hole; and (4)the initial connection stiffness and ultimate moment capacity of the connections can be increased due to increasing of angle thickness, beam depth and bolt diameter,and decreasing of gage distance.

This paper presents an efficient and practical procedure for the optimal design of added damping in framed structures. The total added damping is minimized while inter-story performance indices for linear and nonlinear structures are chosen and restricted to allowable values under the excitation of an ensemble of realistic ground motion records. Optimality criteria are formulated based on fully stressed characteristics of the optimal solution and a simple analysis/redesign procedure is proposed for attaining optimal designs. Results of three examples presented compare well to those obtained using formal gradient based optimization.

This article presents an efficient method for inelastic analysis of semirigid planar steel frameworks. A compound element comprised of a plastic-hinge element and a semirigid connection element is located at member ends that may potentially undergo inelastic deformation. Nonlinear inelastic flexural behaviour is modeled by an empirical relation between moment and rotation for which the parameters are available from experimental results. A four-parameter model is employed to simulate the nonlinear moment-rotation behaviour of semirigid connections. The member stiffness matrix involving the compound element is expressed explicitly in terms of stiffness degradation factors that vary depending on the loading level. This permits direct account for the combined influence of inelastic and nonlinear connection behaviour on structure stiffness. A semirigid steel portal frame is analyzed to illustrate the proposed analysis method, and the results are compared with those obtained from experiments involving the same frame.

Semirigid connections show nonlinear behavior even due to small loadings. Therefore linear analysis is not a proper solution algorithm for structures that have such connections; rather a nonlinear analysis should be done. The conventional methods of nonlinear analysis of frames are inherently iterative, and their final results include some small order of approximation. They usually are done through modification of the stiffness matrix of structure and/or load vector. In this paper, a new method of nonlinear analysis has been presented that contrary to iterative methods, it is non-iterative. It does the analysis in one step without change in the initial model and stiffness matrix of the structure or its load vector. Theoretically it does not include approximation and gives exact results. In this method to force internal moments follow their nonlinear moment-rotation curves, some virtual moments (that are primarily unknown) are imposed to the structure at semirigid connections. To find the unknown virtual moments, a quadratic programming problem is formulated and solved. After finding the values of virtual moments, employing superposition principle, exact nonlinear response of structure is obtained and internal forces and moments of members are calculated.

The method is capable to model semirigid connections with multilinear moment-curvature relations. The formulation of the problem for bilinear and trilinear moment-curvature relations has been presented here. Two examples are presented to demonstrate the robustness, capability and validity of the method.

Estimation of ductility demand distribution through the height of the structure is a very hard task for seismic design engineers working on performance based design of buildings. In this paper a modified direct displacement based design procedure has been proposed. In this method the design force distribution among the height of the structure is obtained based on various ductility demand distributions derived from modal characteristics of the structure and mathematical formulations. The method has been applied to the moment steel frames in low, medium and high rise buildings and the results of various ductility distributions have been compared. The plastic mechanism has also been modeled and the efficiencies and deficiencies of each have been discussed through various numerical examples. The effect of yield mechanisms and ductility demand patterns for various building types on the equivalent SDOF parameters have been investigated compared to the time history analysis results to find the sensitive parameters.

The code ”Plastique“, suitable for the inelastic dynamic analysis of R/C structures and its theoretical background are presented. Every structural entity is represented by a single nonlinear element through the implementation of a macro modeling approach. Three different types of 2D-macro elements are formulated namely; beams, columns and shear walls. The structural behavior of each element is evaluated using a flexibility formulation based on both element edge regions that follow a distributed plasticity law. A fiber model is used to define the monotonic strength envelope at each section. The hysteretic behavior of the structural elements is monitored by a smooth hysteretic model of Bouc-Wen type. This model is capable to express the stiffness degradation, strength deterioration and pinching phenomena which are observed in R/C elements under cyclic loading. Plane frames consisting of combinations of plane elements are linked at the levels of floors via diaphragms to assemble the 3D mathematical model of the structure. Solutions are obtained by direct integration of the equations of motion, while an iterative procedure is implemented to satisfy equilibrium at every time step. Finally, a damage analysis is performed using an appropriate damage model. The numerical examples presented herein the reveal the features of the proposed analysis scheme.

It is anticipated that the construction of buildings that incorporate light gauge steel frame/wood panel shear walls as primary lateral load resisting elements will increase across Canada in coming years. At present, a codified method for the prediction of shear wall strength and stiffness is not available in Canada. For this reason an investigation of various analytical prediction methods was completed. The racking strength and stiffness of steel frame/wood panel shear walls have been shown to be highly dependent on the behaviour of the sheathing connections. An experimental program involving over 200 small-scale tests was first carried out to establish the performance of steel stud to wood sheathing connections. This information was then utilized in a comparison of five existing analytical/mechanics based methods to predict the strength and deflection of wood framed shear walls. These existing analytical methods were adapted for use with the steel framed walls. A comparison of the predicted strength and deflection values was then made with the results of full-scale shear wall tests. Based on the comparison between test and predicted shear wall response the elastic models presented by Küllsner & Lam were recommended for use to predict the lateral resistance and deflection of light gauge steel frame/wood panel shear walls under monotonic and cyclic loading. At the same time, the shear capacity and initial stiffness as measured from tests of single sheathing connections with an edge distance of 25 mm, and which were evaluated using an equivalent energy approach, were recommended as the input connection parameters for both the strength and deflection models.

In current construction practice, lateral strengths of shear wall panels with cold formed steel framing are primarily determined by tests owing to the lack of analytical methods. Meanwhile, the use of numerical methods such as the finite element method has been limited to researchers investigating the behaviour of SWP. Moreover, the finite element method has rarely been employed in design practice to determine the lateral strength of shear wall panels because the modelling is cumbersome. Presented in this paper is an analytical method to determine the ultimate lateral strength of shear wall panels. The method accounts for the factors that affect the behaviour and the strength of shear wall panels, such as material properties and thickness of sheathing, sizes of the C-shape steel studs, spacing of fasteners, and so on. Lateral strengths obtained from the proposed method for sheathing wall panels were compared with those of recent experimental investigations. The results of the comparison demonstrate that the predicted lateral strengths are in good agreement with those of the tests. Therefore, the proposed method is recommended for engineering practice.

This paper demonstrates how to decompose general stability solutions into useful subclasses of buckling modes through formal definition of the mechanical assumptions that underlie a class of buckling modes. For example, a thin-walled lipped channel column as typically used in cold-formed steel can have its buckling mode response decomposed into local, distortional, global, and other (transverse shear and extension) modes. The solution is performed by writing a series of constraint equations that are consistent with the mechanical assumptions of a given buckling class. The mechanical assumptions that defined the buckling classes were determined so as to be consistent with those used in Generalized Beam Theory (see e.g., Silvestre and Camotim 2002a,b) The resulting constraint equations may be used to constrain the solution before analysis, and thereby provide the opportunity to perform significant model reduction, or may be employed after the analysis to identify the buckling classes that participate in a given buckling mode. This paper shows the framework for this process in the context of the finite strip method (building off of Ádány and Schafer 2004, 2005a,b) and discusses some of the interesting outcomes that result from the application of this approach. Of particular interest, and discussed here, is the definition of global buckling modes, and the treatment of members with rounded corners — each of which provide certain challenges with respect to traditional definitions of the buckling classes. Examples are provided to illustrate the technique and challenges. The long-term goal of the work is to implement the procedures in general purpose finite element codes and thus enable modal decomposition to become a widely available tool for analyzing thin-walled member cross-section stability.

In this paper, tests and finite element analysis are used to study the shear resistance of cold-formed steel stud walls in low-rise residential structures. Firstly, the shear resistance of cold-formed steel stud walls under monotonic loading is tested. The test models, including walls with single-sided gypsum sheathing, walls with single-sided oriented strand board sheathing, and walls with gypsum sheathing on the back and oriented strand board on the face are made in full scale of engineering project. The test apparatus and test method and the failure process of specimens are introduced in detail. Then, the finite element analysis model of cold-formed steel stud walls considering geometric large deformation and materials nonlinear is presented to study their shear resistance. Walls were simulated as shell elements. The studs and tracks are simply connected. The screws connecting the sheathings to the frame are modeled by coupling methods. The solution method of equations is selected by ANSYS program automatically. Finite element analysis results in this paper are close to that of experiment. The results of test and finite element analysis show that sheathing materials influences the wall's shear resistance more greatly. The strength of steel has a less influence on the shear resistance of walls. As the decrease of stud spacing, height of wall and screw spacing at the perimeter, the walls' load ability increases obviously.

This paper presents a full-scale active tensegrity structure at EPFL and demonstrates how it can learn as well as carry out self-diagnosis and self-compensation. Tensegrities are generally flexible structures: small loads may lead to large displacements. We thus control slope by actively modifying the self-stress state between cables and struts. The structure benefits from past experience through case-based reasoning. It memorizes past control commands and adapts them in order to react to new applied loads up to forty times more rapidly than without this previous control information. Redundancy of this structure provides opportunities for ”fault tolerant“ behavior. The active control system can also be used to perform self-diagnosis and then to self-compensate local damage. For many cases of local damage, the structure remains capable of satisfying control goals. This paper also summarizes a multi-objective optimization method for control according to four criteria. In contrast with other applications involving multiple objectives, such as design where users prefer choices, this is a control task, thereby requiring identification of a single solution only. Also, the single dominant objective usually generates hundreds of possible solutions. Four objectives are evaluated firstly using Pareto optimality and then a unique solution is chosen through successive filtering of candidate solutions using a hierarchy of objectives. The combination of advanced computing techniques with structural control of serviceability criteria is providing many new possibilities for structural engineers. These results are expected lead toward more autonomous and self-adaptive structures that are able to evolve as their environment changes.

Two different approaches for modelling the mechanical behaviour of polyethylene materials are presented. In the first one, the emphasis is on the relationships between molecular features and mechanical properties. In the proposed model, the material is analyzed from a microscopic viewpoint and considered as an aggregate of crystals. The constitutive equation is expressed in a viscoplastic framework considering degradation at large deformations. For the second approach, the material response is considered to be nonlinear viscoelastic. A phenomenological approach is adopted, and attention is given on the formulation of a model that can be implemented for structural analysis of components such as pipes. In this part of the study, numerical and experimental data of creep for a medium-density polyethylene pipe material are presented. The efficacy of the micro- and macro-mechanical approaches is confirmed by experimental results.

A finite element-based simulation of neurulation, a critical developmental event common to all vertebrates, is presented for an amphibian embryo. During this process, a sheet of tissue rolls up to form a tube, the precursor of the spinal cord and brain. Material property data for the simulation are based on the cellular fabric of the tissues and on tensile test data, and geometric data are obtained from three-dimensional reconstructions. A spatio-temporal correlation system is used to organize and correlate the data and to construct the finite element model. The simulations predict morphogenetic movements similar to those which occur in real embryos.

Stochastic structural mechanics deals with the analysis of random phenomena occurring in structural systems or components. There are two major categories of structural uncertainties which involve spatial correlation and which consequently require the treatment as random fields. These are:

Material properties such as modulus of elasticity or strength

Geometrical properties such as shape or thickness of structural components. The outcome of the stochastic structural analyses is significantly affected by the appropriate treatment of the random properties in the context of the Finite Element method.

The paperwill provide an overviewof random field representation as appropriate for the stochastic finite element method (cf. Matthies and Bucher, 1999). This includes integral representation models as well a point representation models. In addition, conditional random fields as required in the presence of pointwise deterministic information (e.g. from measurements) are introduced.

Example applications illustrate these concepts and discuss the numerical implications of random field modeling. These applications involve static and dynamic problems which arise in system identification (Macke and Bucher, 2000; Bucher et al., 2003) as well as dynamic stability issues due to geometrical imperfections of shells (Most et al., 2004).

The network of channels called Navigli was built in the surrounding country and downtown Milan, Italy. Wave propagation measurements were taken along the channel for a total length of 80 m. Each test consisted of the simultaneous measurement of the response of the wall with 15 transducers to an impulse load. The relative condition of the wall is evaluated by considering three main wave characteristics: group velocity, phase velocity, and attenuation coefficient. A fuzzy logic model is developed to make a relative evaluation of the condition of the sidewall.

This paper presents a solution for stress and deformation fields induced by a central crack in an elasticplastic plate subject to tensile load. The solution is controlled by a crack opening parameter related to material modulus and far-field stress.

The micro/meso-mechanical approach for composites is commonly based on the analysis of a representative volume element or a so-called repeated unit cell (RUC). Through analysis of the RUC model one can predict not only macroscopic mechanical properties but also microscopic damage initiation and its propagation in composites. In this paper, we present an overview of our contributions in the following three essential areas in the micro/meso-mechanical analyses: (1) a unified form of periodic boundary conditions for the RUC modelling; (2) a nonlinear viscoelastic constitutive model for polymer matrix materials; and (3) a post-damage constitutive model based on the concept of smeared crack. Application examples combining the above three topics are presented, in which three types of glass/epoxy laminates are analyzed using finite element method. The predicted results are compared with experimental data and they are in good agreement.

A new perspective is presented on the Galerkin solution for linear stochastic algebraic equations, that is, linear algebraic equations with random coefficients. It is shown that (1) a stochastic algebraic equation has an optimal Galerkin solution, that is, a Galerkin solution that is best in the mean square sense, and (2) the optimal Galerkin solution is equal to the conditional expectation of the exact solution with respect to a σ-field coarser than the σ-field relative to which this solution is measurable. Galerkin solutions that are not optimal are called sub-optimal. Both optimal and suboptimal Galerkin solutions are defined and constructed. Optimal and sub-optimal Galerkin solutions are used to calculate statistics of the displacement of a simply supported plate sitting on a random elastic foundation. The accuracy of these Galerkin solutions is assessed by Monte Carlo simulation.

A new sampling technique referred to as the hypercube point concentration sampling technique is proposed. This sampling technique is based on the concepts of the Latin hypercube sampling technique and the point concentration method. In the proposed technique, first, the probability density function of the random variables is replaced by a sufficiently large number of probability concentrations with magnitudes and locations determined from the moments of the random variables. In other words, the probability density function is replaced by the probability mass function determined based on the point estimate method. The probability mass function is then used with the Latin hypercube sampling technique to obtain samples. For evaluating statistics of a complicated performance function of an engineering system, the proposed technique could be more efficient than the Latin hypercube sampling technique since for a given simulation cycle the required number of evaluations of the performance function in the former is less than that in the latter. The proposed sampling technique is illustrated through numerical examples.

An application of a comprehensive and compact methodology to obtain the asymptotically-correct stiffness matrix of anisotropic, thin-walled, closed cross-section, and rotating slender beams is presented. The Variational Asymptotic Method (VAM), which utilizes small geometrical parameters inherent to thin-walled slender beams, is used to obtain the displacement and strain fields, and the cross-sectional stiffness matrix without any ad hoc assumptions. The advantage of this approach is that the asymptotically-correct and populated 4 × 4 cross-sectional stiffness matrix provides all the necessary information about the elastic behavior of the rotating beam, thereby nullifying the need for refined beam theories that incorporate higher order deformation modes, like the Vlasov's mode. The implementation of the theory usingMATLAB was validated against the Vartiational Asymptotic Beam Sectional Analysis (VABS) computer software, a two-dimensional finite element program that utilizes a more general approach to the VAM that is applicable to thick/thin-walled anisotropic crosssections with arbitrary geometry. Sample applications of the theory to rotor blades are presented. The paper concludes with a discussion of how the presented material would be used directly in the dynamic modelling of rotating helicopter blades.

We derive here equivalent self-adjoint systems for conservative systems of the second kind. Existence of the symmetrized systems confirms that certain conservative systems of the second kind behave as a true conservative system. In this way, study of stability can be carried out on the symmetrized system. In general, it is easier to study a self-adjoint system than a nonself-adjoint system. For the conservative system of the second kind, including the Pflüger column, we also presented a lower bound self-adjoint system. For a linear conservative gyroscopic system, we gave a zero parameter sufficient condition for instability and one for stability. The criteria depend only on the characteristics of the system. For a simple 2-DOF system, the present criteria yield the exact solutions.

During conceptual structural design the engineer proposes initial structural solutions to early architectural designs. At this stage, the decisions made by the engineer are based mostly on knowledge about structural behaviours and experience on the applicability of available construction technologies and materials to different design situations. This research proposes a knowledge-based computer approach to assist the engineer in proposing feasible structural solutions to the architect interactively. With this approach a structural solution is developed by the engineer from an overall description to a specific one through the progressive use of knowledge. A first prototype has been implemented and is being enhanced with a knowledge-base for design exploration. Therefore, an example of envisioned computer support is used to illustrate the capabilities of the proposed approach.

In this paper, applications of crystal plasticity theory to the numerical modelling of large strain plasticity phenomena are considered. In particular, instabilities and localized deformation phenomena for face-centred cubic (FCC) and body-centred cubic (BCC) polycrystals subjected to various deformation modes are investigated. In-house finite element analyses based on a rate-dependent crystal plasticity model have been developed to simulate the large strain behaviour for sheet specimens subjected to plane strain and plane stress deformation modes. In the formulation, the plastic deformation of an individual crystal is assumed to be due to crystallographic slip and simulations are performed using two approaches. In the first approach, each material point in the finite element analysis is considered to be a polycrystalline aggregate having a large number of FCC or BCC grains, and the Taylor theory of crystal plasticity is adopted to model the behaviour of the polycrystal. In the second approach, each grain is represented individually using one or more finite elements, and the constitutive response within each element is given by the single crystal constitutive model. Both approaches account for initial textures, as well as texture evolution during large plastic deformations. The numerical analyses incorporate parallel computing features. The results of simulations for the above-mentioned deformation modes are discussed, and in certain cases comparisons are made with experimental results for rolled aluminum sheet alloys and for draw quality steels.

At the conceptual stage of the design process where only a partial specification for a design is available and due to fuzzy nature of information at this stage it is difficult to program every design requirements. Experience has shown that evolutionary computation EC, (particularly the genetic algorithm) to be an effective decision support tool for conceptual design. To make EC useful in this stage of the design it needs strong human interaction and guidance to lead the search in discrete regions of the search space to explore and discover more appropriate design concepts. Humans are extremely good at perceptual evaluation of designs according to criteria that are extremely hard to program (Eckert et al., 1999). As a result, they can provide useful fitness evaluation for interactive evolutionary systems. They can also include personal preferences to lead the search and exploration to a preferred direction. This kind of interaction is extremely important to satisfy design/client requirements, particularly at the conceptual stage of the design process. This paper introduces a novel approach which demonstrates that interactive use of evolutionary computation, assisted by visualisation tools, leads to a human-led search. A system which support human-led search and it is based on an interactive visualisation clustered genetic algorithm, developed by Packham and coworkers (Packham, 2003; Packham and Denham, 2003; Packham et al., 2004; Rafiq et al., 2004), is introduced and its application on an example of a multi-disciplinary decision making process is demonstrated.

Any construction project that is completed on-time at the lowest total cost requires the consideration of logistics processes and economics. This study investigates the cost implications of moving and transforming materials in various materials network configurations associated with popular prefabrication construction methods. Efforts focus on the trade-offs that exist among contract-to-completion times, transportation costs, and assembly costs for the alternative construction methods. The findings suggest that the competitive advantage of prefabrication methods can be enhanced through an optimal combination of reduced construction times as well as the number, distance, and configurations of materials and sub-assembly shipments.

Delamination of sandwich columns is studied using a relatively simple cohesive layer model. The model is described in some detail and is incorporated as a user supplied element (UEL) in a finite element package. The model is shown to predict accurately the test results of delamination of a facing sheet of a sandwich member. The accuracy of the model is seen to be superior to a model previously proposed by the authors, which predicts an earlier termination of crack growth. The UEL model is applied to a sandwich column investigated by earlier investigators — a column that is relatively stout (ratio of length (L) to depth (d) ≈ 7.3) and has stiff facing sheets (ratio of depth d to the thickness (h) of facings ≈ 15). The model is able to capture the onset of delamination buckling, sudden delamination growth at nearly constant compression, stable delamination growth and reaching of a limit point of the load carrying capacity. A slender sandwich column with relatively thin facings (L/d ≈ 15, d/h ≈ 40) is next considered. It indicates that overall bending tends to inhibit delamination growth under quasistatic loading as it tends to keep the delaminated surfaces in contact.

The moment Lyapunov exponents are important characteristic numbers for determining the dynamical stability of stochastic systems. Monte Carlo simulations are complement to the approximate analytical methods in the determination of the moment Lyapunov exponents. They also provide criteria on assessing how accurate the approximate analytical methods are. For stochastic dynamical systems described by Itô stochastic differential equations, the solutions are diffusion processes and their variances may increase with time of simulation. Due to the large variances of the solutions and round-off errors, bias errors in the simulation of momemt Lyapunov exponents are significant in the cases of improper numerical approaches. The improved estimation for some systems is presented in this paper.

A computer-automated design and optimization process for pile foundations with rigid concrete slabs is presented. Optimality Criteria methodology is used to provide optimal pile designs. A threedimensional optimization computer program has been developed that designs a foundation system with an optimal number of piles, geometric layout, pile orientation, batter, and size for a given structure subjected to multiple load cases. The optimization procedure controls displacements while reducing the overall weight of the pile foundation design. A new method for optimizing weightless variables, such as batter, was also created. Thus, the challenges of optimizing variables that indirectly affect the weight of the pile foundation can still be designed to create weight savings. In one example, the total volume of the steel piles is reduced from 61,920 in3 to 49,570 in3 by optimizing only the pile sizes. Furthermore, the weight is reduced again by simultaneously optimizing each pile group's size coupled with the weightless variable, batter.

This paper presents a consistent approach for the optimal seismic design of added viscous damping in framed structures. The approach presented is appropriate for use in elastic as well as yielding frames. The sum of added damping is chosen as the objective function and the performance of the structure, under the excitation of an ensemble of deterministic ground motion records, is constrained. The performance of the structure is measured by the maximal inter-story drifts in both the linear and nonlinear cases. The nonlinear case however, uses an additional performance measure of the normalized hysteretic energy of the plastic hinges

Gradients of the performance measures are first derived to enable the use of an appropriate first order optimization scheme. Moreover, an efficient selection scheme enables the consideration of only a few records rather than the whole ensemble, hence making the optimization process efficient in terms of the computational effort.

A methodology of enhanced computational efficiency is presented for continuum topology optimization of sparse structural systems. Such systems are characterized by the structural material occupying only a small fraction of the structure's envelope volume. When modeled within a continuum mechanics and topology optimization framework such structures require models of very high refinement which is computationally very expensive. The methodology presented herein to deal with this issue is based on the idea of starting with a relatively coarse mesh of low refinement and employing a sequence of meshes featuring progressively greater degrees of uniform refinement. One starts by solving for an initial approximation to the final material layout on the coarse mesh. This design is then projected onto the next finer mesh in the sequence, and the material layout optimization process is continued. The material layout design from the second mesh can then be projected onto the third mesh for additional refinement, and so forth. The process terminates when an optimal design of sufficient sparsity, and sufficient mesh resolution is achieved. Within the proposed methodology, additional computational efficiency is realized by using a design-dependent analysis problem reduction technique. As one proceeds toward sparse optimal designs, very large regions of the structural model will be devoid of any structural material and hence can be excluded from the structural analysis problem resulting in great computational efficiency. The validity and performance characteristics of the proposed methodology are demonstrated on three different problems, two involving design of sparse structures for buckling stability, and the third involving design of a hinge-free gripper compliant mechanism.

Reliability can be considered as a rational evaluation criterion in assessment of bridge structures. The traditional element-based approach to bridge design and evaluation does not allow for consideration of interaction between the components that form a structural system and, therefore, it can be conservative. Safety of the structural system also depends on the degree of redundancy (load sharing) and ductility. As a result, it has been observed that the load carrying capacity of the whole structure can be much larger than what is determined by the design of individual components. Therefore, this paper is focused on the system behavior. The objective is to formulate a limit state function for the whole bridge, identify the load and resistance parameters, and develop an analysis procedure to assess the reliability of the bridge as a structural system. The major steps of the procedure include selection of representative structures, formulation of limit state functions, development of load and resistance models, development of the reliability analysis method, reliability analysis of selected bridges, and formulation of recommendations for practical bridge assessment. The live load is considered in form of a design truck. The analysis is performed for different values of span length, truck position (transverse and longitudinal), number of vehicles on the bridge (multiple presence), girder spacing, and stiffness of structural members (slab and girders). For each combination of these parameters, the bridge resistance is determined in terms of the weight of a truck (or trucks) causing an unacceptable deflection or instability of the considered bridge. The reliabilities are also calculated for individual components (girders) and compared to system reliabilities of the bridge. The resulting system reliability can serve as a tool in the development of a rational bridge design and evaluation procedure.

Probabilistic assessment of the ductility demand and reliability analysis were carried out for bilinear hysteretic SDOF systems. The assessment considered two sets of strong ground motion records, and was focused on the evaluation of the mean and the coefficient of variation of the ductility demand for a given value of the normalized yield strength. The results indicate that the ductility demand could be modeled as a Frechet (Extreme value type II) variate. Based on the obtained results, empirical equations were provided to predict the mean of the ductility demand for bilinear SDOF systems of different natural vibration periods, damping ratios, and ratios of the post yield stiffness to the initial stiffness. The numerical results show that the coefficient of variation (cov) of the ductility demand can go as high as to about 1.0 depending on the characteristics of the structure. Also, a simple approach was given to estimate the probability of incipient damage and the probability of incipient collapse using the developed probabilistic characterization of the ductility demand. The approach, which could be suitable for carrying out design code calibration analysis, is illustrated numerically.

This paper presents a practical approach for maintenance optimization of a network of aging highway bridge decks that integrates a stochastic deterioration model based on Bogdanoff's cumulative damage theory with an effective multi-objective optimization approach. The multi-objective maintenance optimization takes into account all relevant objectives, such as improving bridge deck condition, minimizing maintenance costs, and minimizing traffic disruption and associated user costs. The consideration of these three objectives enables to take full advantage of the available bridge inspection data and implicitly lead towards the minimization of the risk of failure due to bridge deck deterioration and maintenance activities. A multi-objective optimality index is proposed as an optimality criterion for priority ranking of the deficient bridge decks for maintenance. The obtained optimal maintenance project prioritization strategy achieves a satisfactory trade-off or compromise between the selected relevant and competing optimization objectives. The proposed approach is illustrated on a small network of ten bridge deck projects that are optimized for maintenance.

The probabilistic modelling of deterioration in the time-dependent reliability analysis is a necessary step for developing a risk-based approach to the life cycle management of infrastructure systems. The decisions regarding the time and frequency of inspection, maintenance and replacement are confounded by sampling and temporal uncertainties associated with the deterioration of structural resistance. To account for these uncertainties, probabilistic models of deterioration have been developed under two broad categories, namely the random variable model and stochastic process model. The paper presents a conceptual exposition of these two models and highlights their profound implications to the age-based and condition-based preventive maintenances policies. The proposed stochastic gamma process model of deterioration is more versatile than the random rate model commonly used in the structural reliability literature.

Underground utility services play an essential role in sustaining urban life. The majority of these utility services are delivered through pipeline networks, which are mostly buried underground and are interconnected through other urban systems to distribute or collect basic sustainable needs such as treated water, waste water, gas, communication, and power. Deterioration of underground infrastructure systems occurs due to ineffective maintenance management practices. Because new installation can be very costly and disruptive, the best course of action is to maintain the present infrastructure in a more effective way to maximize life span and prevent catastrophic failures. The accurate evaluation of current underground infrastructure must be done before any crucial decisions including lifecycle, rehabilitation and replacement intervals, and appropriate remedial methods can be made. Unfortunately, traditional technologies and management approaches have been limited by the use of insufficient data in the evaluation of the structural integrity of an aged infrastructure. This paper describes the testing, development, and application of a novel assessment technology, which combines in-pipe Ground Penetrating Radar (GPR) with Digital Scanning and Evaluation Technology (DSET) robotics to collect accurate information about the condition of the inside wall of concrete sewer pipes. A case study applying this innovative technology to sections of large diameter PVC-lined concrete pipe in the City of Phoenix is presented. The study and adoption of innovative pipeline assessment methods provide better information to improve the decision-making process, thereby making economical decisions to optimize resources in more efficient ways.

The level of automation technology and processes control, within the construction sector, faces unique challenges if it is to catch up with automotive and aerospace applications. The construction industry has problems relating to health and safety, environmental legislation and traditional methods of procurement. These are compounded by diminishing skills in the labour force. One way to address these issues is by increased automation and integration of design, modelling and process control. Digital Fabrication has demonstrated the feasibility of the integration of design and component production on a large scale. Freeform Construction builds on Digital Fabrication by integrating the control of final material deposition. This paper reports on recent meetings held with industrialists to gauge their perceptions of the technology and encourage discourse to identify both applications and opportunities for the wider research community. Examples of digital fabrication in construction are discussed. Freeform Construction is defined and potential applications are presented. An example of physical model generation from construction CAD software is described.

Risk management, as it relates to construction, is vital to the successful undertaking and completion of any construction project. One way to effectively manage project's risks is to develop more reliable means of identifying the most critical risks and the associated effective response methods. Research studies have extensively addressed this aspect of risk management. However, a small fraction of this research focused on identification of the critical risks encountering contractors working in the construction industry of developing countries, and few tackled identifying the risk mitigation measures employed in such an industry by domestic, international, and multinational contractors.

This paper presents a comprehensive methodology that addresses the risk identification and response methods for developing countries represented by Egypt. The paper is based mainly on the approaches used by large contractors either domestic or international.

The investigation, via a comprehensive questionnaire survey, tries to identify the most critical and significant risks that face the contractors working in the Egyptian construction industry and their associated effectively employed risk mitigation/elimination measures. Twenty-nine (29) construction project risks are classified into six (6) main categories according to their type and hundred and forty (140 risk) mitigation/elimination measures are introduced to overcome the impact of risks under each of these risk categories.

According to the collected data and the results of the statistical analysis procedures employed, the most critical risk encountered by the contractors working in the Egyptian construction industry are: 1)the financial inability of the client; 2)the improper management of construction projects; 3)inflation and interest rates; 4)in-house cash shortage; and 5)Foreign exchange and convertibility. 101 risk response methods were found to be effective from the 140 methods introduced. The most commonly used risk response method was the risk reduction technique.

This paper introduces amodel for optimizing and visualizing infrastructuremaintenance programs of multiple-distributed sites. Two unique aspects of the model are discussed in the paper: the underlying Geographic Information System (GIS); and the powerful scheduling engine that optimizes execution plans. The GIS system stores and represents two main levels of information about the scattered sites involved in a construction/maintenance program. The first level pertains to pre-planning data such as resources, locations, optional estimates, and work constraints. This information is then used by the scheduling engine to generate an optimum schedule, and accordingly, a second layer of GIS information is generated containing activities' start and finish dates and the assigned crews. This layer of information is then used by the GIS system to visualize the crews' work assignments in a legiblemanner. An implementation program BAL is presented on an example application to illustrate the benefits of using GIS to supportmunicipalities and owner/contractor organizations administering large number of infrastructure assets, such as buildings, highways, and bridges, etc.

Transparent glass constructions are being used more and more often in the construction industry using up-to-date carrying systems. The progress in this field is determined by the methods to couple glass systems and transparent substructures in a durable manner while keeping the dimensions of the substructures as small as possible.

Experimental investigations have shown that carrying glass-hybrid beams are possible and that there are appropriate adhesive and surface pre-treatment substances available on the market to connect glass and plastic in an orderly manner.

This paper presents an experimental approach for the development of new bridge restrainer system using laminated fiber reinforced rubber (LFRR). After Kobe earthquake on January 1995, the design concept for the bridge restrainer has been revised so that the bridge should install the shock absorber which may be prevented from falling down due to earthquake shock. However, the shock absorbing system for the bridge restrainer has been required to satisfy the two performance requirements of high energy absorption and reduction of impact load. To this end, the laminated fiber reinforced rubber was developed to apply to the new bridge restrainer system as a shock absorber. In this study, the three kinds of tests of static compression, rapid speed loading and weight dropping impact for the LFRR specimen were first performed in order to investigate the efficiency of LFRR as a shock absorber. Then, the rubber-rolled pin was also developed as a new bridge restrainer system from the viewpoints of impact load reduction and high energy absorption.

The comparative merits of hand and automated upper and lower bound techniques for the collapse load estimation of reinforced concrete slabs are examined. Examples, drawn from both theoretical and practical design work, are used to show that both hand and automated upper bound yield line techniques can produce significant, unsafe errors. Automated lower bound solutions, however, are shown to consistently provide safe estimates that are not unduly conservative, provided appropriate formulations are adopted. As long as the engineer is willing to dispense with the crutch of a yield line pattern, it is therefore contended that, whilst Heraclitus may be correct in that both the upper and the lower bound roads can lead to one and the same collapse load, the lower bound road gets you there, certainly more safely, and usually quicker, as the Traditional Song suggests.

Centering on worldwidly present urban areas, there have been many high-rise landmark buildings constructed in recent years. It is recognized that reinforced concrete has merit over steel frame construction in high-rise buildings, such as less sway in high winds, better human life protection in case of accidental heavy damage, better noise resistance. The use of high-strength concrete is rising, not only for pillars, in high-rise buildings. The paper points out on the need of classifying the HP-HSC for the different requested characteristic that materials have to exhibit on different structural elements of a complex structure. Among types of concrete, which binds together characteristics of High Strength Concrete (HSC) and High Performance Concrete (HPC), particular reference is made to Limestone Concrete (LSC).

Existing literature provides data on self-levelling, high performance, rapid hardening concrete, able to reach in few days the standard of HPC (Kelham, 1998; Montgomery et al., 1998; Nehdi et al., 1998. In particular the technology here referred for limestone concrete is not the usual one, but it makes reference to a mix design, characterized by an industrially produced limestone aggregates, with total absence of Silica Fume or any other addition of pozzolanic material or accelerating admixture (Cangiano, 2005; Cangiano et al., 2004.

The paper points out the significance of Limestone Concrete, as High Performance Concrete, application, starting from the following key construction requirements: in large public works with characteristic of very high durability, the choice of a technical solution it is not at all dependent on the construction cost only. In fact in this work, life service and safety performances, that slightly increase the construction costs, are of paramount importance. Starting from this key assumption, new materials, and in particular new concretes, may be able to notably cut life service and safety costs, considerably improving the performance/cost ratio of the selected solution, due to the large cut of maintenance costs. The paper wants to briefly explain the state of the art and the today frontier which lead to the material basic choices in structural design of high-rise buildings. In particular the paper refers to a comprehensive campaign of tests, in a starting-up phase, shared among different university and private laboratories in Italy, which aims to draw Guide Lines for different specific uses of Limestone Concrete, as HPC, in different structures typologies and environmental conditions.