Kang Seok Lee

The harmony search heuristic algorithm for discrete structural optimization

Many methods have been developed and are in use for structural size optimization problems, in which the cross-sectional areas or sizing variables are usually assumed to be continuous. In most practical structural engineering design problems, however, the design variables are discrete. This paper proposes an efficient optimization method for structures with discrete-sized variables based on the harmony search (HS) heuristic algorithm. The recently developed HS algorithm was conceptualized using the musical process of searching for a perfect state of harmony. It uses a stochastic random search instead of a gradient search so that derivative information is unnecessary. In this article, a discrete search strategy using the HS algorithm is presented in detail and its effectiveness and robustness, as compared to current discrete optimization methods, are demonstrated through several standard truss examples. The numerical results reveal that the proposed method is a powerful search and design optimization tool for structures with discrete-sized members, and may yield better solutions than those obtained using current methods.

Flow and Compressive Strength of Alkali-Activated Mortars

The authors present results of testing performed on 18 fly ash (FA)-based mortars and 36 slag-based mortars activated by sodium hydroxide and/or sodium silicate powders. Activator sodium oxide mixing ratio to source materials, fine aggregate-binder ratio (s/b), and water-binder ratio (w/b) are the main variables investigated. That much higher compressive strength and slightly less flow were exhibited by slag-based alkali-activated (AA) mortars when compared to FA-based AA mortars for the same mixing condition was shown by test results. For AA mortar initial flow and 28-day compressive strength evaluation, nonlinear multiple regression analysis developed feed-forward neural networks and simplified equations were developed. A comprehensive database of the results of 82 tests of sodium silicate activated mortars was used for neural network training and testing and simplified equation calibration. Estimation of slag-based AA mortar compressive strength development was also done using the ACI 209R specified formula calibrated against the collected database. Test results were in good agreement with predictions obtained from developed simplified equations and trained neural network, although there was slight overestimation of slag-based AA mortar early strength by the proposed simplified equations.

Harmony search for structural design

Various algorithms have been developed and applied to structural optimization, in which cross-sectional areas of structure members are assumed to be continuous. In most cases of practical structure designs, however, decision variables (cross-sectional areas) are discrete. This paper proposes a combinatorial optimization model for structural design using a new nature-inspired algorithm, harmony search (HS). HS is also compared to genetic algorithms through a standard truss example. Numerical results reveal that the proposed HS is a powerful search algorithm for combinatorial structure optimization.

Tests on Cementless Alkali-Activated Slag Concrete Using Lightweight Aggregates

Five all-lightweight alkali-activated (AA) slag concrete mixes were tested according to the variation of water content to examine the significance and limitation on the development of cementless structural concrete using lightweight aggregates. The compressive strength development rate and shrinkage strain measured from the concrete specimens were compared with empirical models proposed by ACI 209 and EC 2 for portland cement normal weight concrete. Splitting tensile strength, and moduli of elasticity and rupture were recorded and compared with design equations specified in ACI 318-08 or EC 2, and a database compiled from the present study for ordinary portland cement (OPC) lightweight concrete, wherever possible. Test results showed that the slump loss of lightweight AA slag concrete decreased with the increase of water content. In addition, the compressive strength development and different mechanical properties of lightweight AA slag concrete were comparable with those of OPC lightweight concrete and conservative comparing with predictions obtained from code provisions. Therefore, it can be proposed that the lightweight AA slag concrete is practically applicable as an environmental-friendly structural concrete.

Structural Performance of Reinforced Concrete Beams Strengthened with Sprayed Fiber Reinforced Polymers

The main purpose of this study is to develop a sprayed FRP repair and strengthening method, which is a new technique for strengthening the existing concrete structures by mixing one of the carbon or glass chopped fibers and one of the epoxy or vinyl ester resins with high-speed compressed air in open air and randomly spraying the mixture onto the concrete surface. At present, the sprayed FRP repair and strengthening method using the epoxy resin has not been fully discussed. In order to investigate the material property of the sprayed FRP, this study carried out tensile tests of the material specimens, which were changed with the combinations of various variables including the length of chopped fiber and the mixture ratio of chopped fiber and resin. These variables were set to have the equal material strength, compared with that of one layer of the FRP sheet. As a result, the optimal length of glass and carbon chopped fibers was fumed out to be 38 mm, and the optimal mixture ratio between chopped fiber and resin was also turned out to be 1 : 2 from each variable. And also, the thickness of the sprayed FRP to have the equal strength to one layer of the FRP sheet was finally calculated. In is study, a series of experiments were carried out to evaluate the strengthening effects of flexural beams, shear beams and damaged beams strengthened with the sprayed FRP method, respectively. The results revealed that the strengthening effects of the flexural and shear specimens were reasonably similar to those of the FRP sheet, and the developed Sprayed FRP technique is able to be used as a strengthening scheme of existing RC building.

A New Structural Optimization Method for Structures with Discrete Sizing Variables

Many methods have been developed and are in use for structural sizing optimization, in which the cross-sectional areas, i.e., sizing variables are usually assumed to be continuous. In most practical design problems in structural engineering, however, the design variables are discrete. This paper proposes an efficient optimization method for structures with discrete sizing variables based on the harmony search (HS) heuristic algorithm. The recently developed HS algorithm was conceptualized using the musical process of searching for a perfect state of harmony. It uses a stochastic random search instead of a gradient search, so derivative information is unnecessary. A discrete search strategy using the HS algorithm is presented in detail and its effectiveness and robustness, compared to current discrete optimization methods are demonstrated through a standard truss example. Numerical results reveal that the proposed method is a powerful search and design optimization method for structures with discrete sizing variables and may yield better solutions than those obtained using the current methods.

Shear Behavior of Reinforced Concrete Beams Strengthened by Epoxy Mortar Panel with Steel Fibers

The purpose of this study was to develop a new strengthening material that increases the shear strength and improves the deformation capacity of reinforced concrete (RC) beam members. A new composite material named epoxy mortar with steel fiber (EMSF) was prepared by inserting steel fibers into an epoxy mortar to provide the new strengthening material with high strength-toughness characteristics. In this study, materials and structural tests were employed. The materials tests assessed the influence of the volume percentage and the length of steel fiber on the tensile strength and toughness capacity of the composite material, EMSF. The structural tests help determine the influence of the EMSF panel type and thickness on the shear capacity and shear failure mode of strengthened RC beam members. The results of the two test groups are summarized as follows: 1) the results from the material tests indicated that the EMSF panel with 35 mm steel fibers exhibited high tensile strength, and the EMSF panel with both 35 mm and 50 mm steel fibers showed good toughness capacity; 2) the RC beam strengthened by an EMSF panel with short (35 mm) fibers possessed the highest shear strength, 3) the RC beam strengthened by an EMSF panel with both short (35 mm) and long (50 mm) fibers exhibited improved deformation capacity; and 4) a shear strength equation was derived from the equilibrium of the cracked RC beam members and is in good agreement with the test results.

Discrete-Continuous Configuration Optimization Methods for Structures Using the Harmony Search Algorithm

This paper proposes an efficient structural optimization methods based on the harmony search (HS) heuristic algorithm that treat integrated discrete sizing and continuous geometric variables. The recently developed HS algorithm was conceptualized using the musical process of searching for a perfect state of harmony. It uses a stochastic random search instead of a gradient search so derivative information is unnecessary. A benchmark truss example is presented to demonstrate the effectiveness and robustness of the new method, as compared to current optimization methods. The numerical results reveal that the proposed method is a powerful search and design optimization technique for structures with discrete member sizes, and may yield better solutions than those obtained using current methods.

Flexural behaviour prediction for RC beams in consideration of compressive stress distribution of concrete with electric arc furnace oxidising slag aggregates

Abstract Research on the efficient utilisation of electric arc furnace oxidising slag, a by-product of the steel industry, is necessary as it is similar to natural aggregates in terms of chemical composition. The slag is also being produced in increasing amounts every year. This study performed flexural experiments on simply support beams with aggregate type and examined their concrete compressive strength, tension reinforcement ratio and compressive reinforcement ratio as variables to assess the effects of compressive stress distribution of concrete with electric arc furnace oxidising slag aggregates on the flexural behaviour of reinforced concrete beams. Compared to specimens with natural aggregates, electric arc furnace oxidising slag aggregates showed a smaller deflection before the yielding of tension reinforcement and ductile behaviour after yielding due to differences in compressive stress distribution. In addition, the proposed equations for the equivalent rectangular stress block parameter were found to provide more accurate predictions than the criterion used by other countries.

Size effect on shear strength of reinforced concrete beams with tension reinforcement ratio

It is well known that shear stress at peak of reinforced concrete beams decreases with increasing effective depth. Thus, several existing design codes and models have included various forms of formulas considering the size effect on shear strength of reinforced concrete beams; however, past experimental researches show that tension reinforcement ratio is also associated with the shear strength of reinforced concrete beams. To examine the effect of tension reinforcement ratio and effective depth on shear strength of reinforced concrete beams, this study has conducted experiments in which the effective depth (150, 300, 500, and 780u2009mm) and tension reinforcement ratio (1%, 2%, and 3%) are employed as variables. Besides, a formula for the shear strength considering both variables is proposed based on data samples collected from the present experiment and previous research. The proposed formula gives predictions comparable to the results of existing shear tests. Furthermore, rational predictions are made for effective depth of beams, compressive strength of concrete, shear span-to-depth ratio, and even tension reinforcement ratio exceeding 3%.