Chun Man Chan

Experimental determination of the tension-softening relations for cementitious composites

Abstract A novel experimental technique based on the J-integral is employed to experimentally determine the tension-softening (σ-δ) relations in cementitious composites. The σ-δ relation provides information on fracture resistance and could be used for numerical simulations of crack formation and propagation in structures constructed from materials which exhibit “tension-softening” behavior. With this method, no complicated modifications to testing machines are required. In the experiment, two pre-cracked specimens with slightly different notch lengths are used. The corresponding values of load, load point displacement and crack tip separation are simultaneously recorded. From this experimental data, the J-integral as a function of crack tip separation as well as the tension-softening curve can be deduced. These curves also provide a measure of the critical energy release rate. The experimental technique has been applied to mortar. It is suggested that the relatively simple technique can provide reliable material parameters for characterization of fracture resistance in plain and fiber reinforced cementitious composites independent of specimen geometry, size and loading configurations.

An optimality criteria algorithm for tall steel building design using commercial standard sections

Grierson and Chan (1991a, b) recently presented an efficient optimality criteria (OC) technique for the optimum design of large-scale tall steel building frameworks. In this paper, further developments of the technique are described concerning a detailed procedure to assign optimum discrete section sizes to structural members. A 60-storey, 7-bay framework example is presented to illustrate the practical applicability and efficiency of the proposed design optimization algorithm.

Integrated Reliability-Based Seismic Drift Design Optimization of Base-Isolated Concrete Buildings

This paper presents an effective numerical reliability-based optimization technique for the design of base-isolated concrete building structures under spectrum loading. Attempts have been made to automate the integrated spectrum analysis, reliability analysis, and design optimization procedure and to minimize the total cost of the base-isolated building subjected to multiple design performance criteria in terms of the story drift of the superstructure and lateral displacement of the isolation system or corresponding reliability constraints. In the optimal design formulation, the cost of the superstructure can be expressed in terms of concrete member sizes while assuming all these members to be linear elastic under a specified design earthquake. However, the base isolation is assumed to behave nonlinearly and its cost can be related to the effective horizontal stiffness of each isolator. Based on the principle of virtual work, the drift responses and corresponding reliability indexes can be explicitly formulated and the integrated optimization problem can be solved by an optimality criteria method. The technique is capable of achieving the optimal balance between the costs of the superstructure and isolation systems while the seismic drift performance or corresponding reliability of a building can be simultaneously considered. An illustrative example shows that conventional deterministic design optimization cannot ensure designs with satisfactory reliability levels, whereas the reliability-based design optimization can achieve the objective when uncertainties are considered. It is believed that such an optimization technique provides an effective tool for seismic design of building structures.

Stiffness Optimization for Wind-Induced Dynamic Serviceability Design of Tall Buildings

Contemporary tall buildings with increasing height and slenderness are highly sensitive to the actions of wind. The structural design of modern tall buildings is generally governed by the need to provide adequate strength and stiffness against dynamic movement induced by strong wind. In addition to the strength-based safety design considerations, the major design effort of a tall building is related to the assessment of the wind-induced serviceability design requirements in terms of lateral drift and motion perception criteria. With tall buildings of today continuing to increase in height, the mitigation of wind-induced vibrations in tall buildings becomes a more critical challenge in the design synthesis process. This paper presents an integrated dynamic analysis and computer-based design optimization method for minimizing the structural cost of tall buildings subject to wind-induced serviceability acceleration design criteria. Once the optimal dynamic serviceability design problem is explicitly formulat...

Lateral stiffness characteristics of tall reinforced concrete buildings under service loads

This paper presents an analytical method for quantitatively predicting the effects of cracking on the lateral deflection and stiffness characteristics of tall reinforced concrete buildings under service loads. The effects of cracking in tall reinforced concrete buildings can be considered using an element stiffness reduction model. This model determines the probability of cracking occurrence by dividing the area of the moment diagram, Scr, where the working moment exceeds the cracking moment by the total area of the moment diagram, S. A practical cracking analysis method can be established by integrating the proposed stiffness reduction model with an iterative algorithm and commercial linear finite element analysis package. The proposed method has been validated by good agreement of results between the numerical computation and experimental testing of large-scale rigid-frame and wall-frame structural sub-assemblages. The effectiveness of the numerical analysis method is also illustrated through a practical 40-storey reinforced concrete building example. The cracking effects on the lateral deflection and stiffness characteristics of this building were analysed both explicitly and quantitatively. Copyright

An optimality criteria design method for tall steel buildings

Abstract An efficient computer-based method is developed for the optimum design of tall steel building frameworks. Specifically, an optimality criteria method is applied to minimize the weight of a lateral load-resisting structural system of fixed topology subject to constraints on overall and interstorey drift. By exploiting the fact for building frameworks that member forces are relatively insensitive to changes in member sizes, rigorously-derived optimality criteria are shown to be readily satisfied through an iterative redesign procedure that converges in but a few cycles. A steel framework example is presented from a variety of viewpoints to illustrate the features of the design optimization method.

INTEGRATED STRUCTURAL OPTIMIZATION AND VIBRATION CONTROL FOR IMPROVING WIND-INDUCED DYNAMIC PERFORMANCE OF TALL BUILDINGS

Structural optimization and vibration control have long been recognized as effective approaches to obtain the optimal structural design and to mitigate excessive responses of tall building structures. However, the combined effects of both techniques in the structural design of wind-sensitive tall buildings with excessive responses have not been revealed. Therefore, this paper develops an integrated design technique making use of both the advantages of structural optimization and vibration control with an empirical cost model of the control devices. While the structural optimization is based on a very efficient optimality criteria (OC) method, a smart tuned mass damper (STMD) is used for the structural control purposes. Utilizing data obtained from synchronous pressure measurements in the wind tunnel, a 60-story building of mixed steel and concrete construction with three-dimensional (3D) mode shapes was employed as an illustrative example to demonstrate the effectiveness of the proposed optimal performance-based design framework integrating with structural vibration control.

Stiffness optimization technique for 3D tall steel building frameworks under multiple lateral loadings

Abstract Although todays engineering computer technology allows for precise analysis of the structural response of a building, it does not readily provide insight for economical design. Due to the complex nature of a modern tall building consisting of thousands of structural members, the traditional design method is generally highly iterative and time consuming. This paper describes an efficient computer-based technique for least-weight design of three-dimensional (3D) tall steel building frameworks under multiple lateral loading conditions. Stiffness constraints in terms of interstorey drifts are considered and optimum discrete member sizes are automatically selected from databases of commercial standard steel sections. The technique is remarkably efficient and the optimum design generally converges in a few cycles. The designs of two 3D lateral-load resisting building frameworks are presented as illustrations. The effectiveness and suitability of the technique for the design of large-scale tall steel building frameworks are discussed.

Alternative methods for the optimal design of slender steel frameworks

Abstract A mathematical programming (MP) method and an optimality criteria (OC) method are alternatively applied for the design of tall slender steel frameworks. For both cases, the design objective is to minimize the weight of the lateral load-resisting structural system of given topology subject to constraints on interstorey drift. The MP and OC solution algorithms are first described, and then both methods are applied for the minimum-weight design of a range of steel frameworks having differing numbers of storeys.

Design Optimization of Tall Steel Building Frameworks

This lecture presents an efficient computer-based method for the optimum design of tall steel building frameworks. Specifically, an Optimality Criteria method is applied to minimize the weight of a lateral load-resisting structural system of fixed topology subject to constraints on interstorey drift. A range of steel framework examples is presented to illustrate the features of the design optimization method.