Iranian Journal of science and Technology (B: Engineering)
Volume:35 Issue: 4, August 2011

In this paper, a new hybrid method is developed for optimal design of truss structures. This method is based on a modified multi-objective particle swarm optimization, tournament decision making process, and a local search algorithm. In structural design practice, different objectives are usually considered in the selection of the final design in which most of these objectives contradict each other. The use of a multi-objective optimization method guides the decision makers to find the most suitable design. Incorporating a decision making process with this optimization, it becomes possible to find a solution which covers most of the requirements. The developed hybrid algorithm is applied to three truss structures to illustrate its ability in finding the optimal solution.

Shear wall with coupling beam is a very effective means of providing lateral bracing for buildings subjected to earthquakes. The strength, stiffness and ductility of coupling beams considerably influence the behavior of coupled wall systems, since even the local failure of each coupling beam could lead to the global failure of the structure. In this paper, two common efficient strengthening methods for improving the behavior of RC coupling beams have been investigated. The nominal shear strength of an FRP-strengthened concrete member can be determined by adding the contribution of the FRP external shear reinforcement to the contributions from the reinforcing steel and the concrete. Numerical studies were conducted to analyze three groups of RC coupling beams, of which one was strengthened by externally bonded steel plates and the other two strengthened with FRP sheets. Fiber Reinforced Polymer (FRP) composites have found increasingly wide applications in structural engineering due to their high strength to weight ratio, high corrosion resistance and ease of installation. The FRP-strengthened groups differ on how the FRP is attached to the coupling beam. The material properties used for modeling the concrete, reinforcement and FRP were adopted due to experimental data. Based on the numerical results, strengthening using steel plates upgraded the lateral load bearing capacity up to 66%, while the improvement in deformation capacity was about 50%. Also, the FRP sheets could considerably enhance the strength and deformation capacity of coupling beams; as in the most effective procedure (wrapping) increased the shear force by 84% and chord rotation capacity by 77%.

The traditional pushover analysis procedure does not represent adequately, the effects of varying dynamic characteristics caused by structural yielding and the contributions of higher modes to the structural responses. The recently developed modal pushover analysis (MPA) procedure significantly improves the traditional pushover analysis by including the effects of a sufficient number of modes in the analysis.The present study evaluates the accuracy of the modal pushover analysis in estimating the seismic demands of vertically irregular planar moment-resisting frames in comparison with the exact results from nonlinear time-history analysis (NLTHA). In this research, seven irregular 12-story frames representing three types of height-wise irregularities: mass irregularity, vertical geometric irregularity (i.e., setback) and difference in floor levels, and also one regular 12-story frame as a reference are analyzed due to an ensemble of seven ground acceleration records. The earthquake records are chosen such that a wide range of frequency contents, as well as the effects of both near and far fault distances, be taken into account in the analysis. Findings from this investigation indicate that in most of the irregular models considered, the method of MPA can reasonably estimate the values of critical parameters such as peak floor displacement and story-drift ratios. Furthermore, it is observed that the inclusion of the first two modes improves the accuracy of predictions relative to using only the fundamental mode in the analysis. Such degree of improvement in accuracy is not reached by including the third mode effects in the MPA procedure.

The ultimate bearing capacity of strip foundations situated on reinforced soils has been analyzed in this paper using the stress characteristics method. A computer code has been written to analyze the slip line net and to calculate the ultimate load distribution beneath the foundation. The ultimate bearing capacity is expressed in terms of bearing capacity factors. Increase in the ultimate bearing capacity due to reinforcement is expressed by introducing another bearing capacity factor, Nt. Earthquake effect has been considered using horizontal and vertical pseudo-static seismic coefficients, Kh and Kv. Design charts have been provided giving the bearing capacity factors for the seismic case. These charts can be used for design purposes for reinforced soils. However, the obtained results should be further compared with future experimental results in order to attain greater confidence in design. Effects of reinforcement and horizontal earthquake coefficient on the failure pattern have also been investigated.

Laboratory data that relates the liquefaction resistance of sand and silt mixtures to shear wave velocity are presented and compared to the liquefaction criteria derived from seismic field measurements based on the deterministic and probabilistic methods. In the deterministic method, cyclic triaxial and resonant column tests were conducted on specimens of Firoozkooh clean sand and sand-silt mixtures with a silt content up to 60% at different densities (Dr =15, 30, 60 and 75%). Cyclic undrained strength (CRR) and small strain shear wave velocity were determined for identical specimens formed using the undercompaction method and laboratory results were converted to field and compared to liquefaction criteria derived from seismic field measurements in previous research. In the probabilistic method, the three-sigma rule was used to analyze the uncertainty factors in liquefaction hazard analysis based on laboratory data. Probabilistic diagrams were also compared with that of other researchers. Results show that the use of the existing field-based correlations to assess CRR is overestimated in comparison to field cyclic resistance evaluated by laboratory testing for the Firoozkooh sand-silt mixtures containing 60% fines. For clean sand and specimens containing up to 30% fines, the results of this study on cyclic resistance were fairly consistent with previously published results.

A new ant algorithm, namely Colony-Mutated Ant System (CMAS), circumventing the premature convergence phenomenon is proposed in this paper and applied to pipe network optimization problems. The method uses a simple but effective mechanism, namely Pheromone Replacement Mechanism (PRM), to make sure that the global-best solution path always has the maximum trail intensity. This mechanism introduces enough exploitation into the method and, more importantly, enables one to exactly predict the number of global-best solutions at each iteration of the algorithm without the necessity of calculating the cost of the solutions created. This number is used as a measure for premature convergence of the method at each iteration. The colony is then mutated such that a predefined number of global-best solutions survive the mutation process. Two different mutation mechanisms, namely one-bit and uniform mutation are introduced and used. The probability of mutation is adjusted at each iteration so that the required number of global-best solutions survive the mutation. The method is shown to produce results comparable to Max-Min ant system (MMAS) algorithm, while requiring less free parameter tuning. The application of the method to a benchmark example in the pipe network optimization discipline is presented and the results are compared. The results indicate that the proposed CMAS method shows improved performance with improved convergence characteristics. Furthermore, the method requires less computational effort for tuning purposes due to the fewer number of free parameters compared to the MMAS method.

The Christiansen's friction factor, Fc, is usually used to compute head loss due to friction in laterals of pressurized irrigation systems. In this research, this factor was evaluated considering variable discharge for outlets located along the laterals. For this purpose, two geometrical progressions were established between the outlets’ discharges. The first progression was appointed between the discharge of the first outlet and the outlet which is assumed to possess the average pressure. The next progression was developed between the discharge of this outlet and the most downstream outlet which is assumed to operate at least discharge. The proposed F-factor is a function of number of outlets, the permitted variation of pressure between the first and the last outlet, and also the formula being used to calculate friction head loss along the lateral. The results showed that Christiansen’s method accurately estimates total friction head loss along the laterals such that maximum relative error caused by assuming equal discharges in outlets does not exceed 3%.

A numerical model called MIKE21 was evaluated for simulation of dam and dike breaks. In recent years in Iran and other parts of the world some dike-breaches have been reported. Application of two-dimensional models in simulation of dike-breaches is inevitable, because of the nature of the flood propagation after the break. However, in Iran, there are very few two-dimensional models used in river and floodplain issues. Hence, the performance of MIKE21 as a numerical model for simulation of dike break was investigated in this research. MIKE21 was originally developed for flow simulation in coastal areas, estuaries and seas. MIKE21’s performance in simulation of dam and dike-break was studied via comparison with analytical solutions, other numerical approaches and available experimental data. Fair agreement has been observed, but care should be taken when modeling shock waves with MIKE21. The study was finally extended to a real case study. Some dike-break scenarios were considered in the Helleh River, in Iran and the results are discussed and presented.

For many geotechnical engineering structures, the bearing capacity and load-deformation response depend on the stress-relative displacement behavior of interfaces in the contact area with surrounding soil. This issue may become of paramount importance considering that many of the existing interface constitutive models reveal remarkable limitations in the simulation of the intense contraction observed in the initial stage of shearing. In this research note, a proper state dependent constitutive equation is introduced to improve this deficiency. Direct comparisons against experimental data and some of the recent interface models are presented to show the improvements achieved.

Collapsible soils have considerable strength and stiffness in their dry natural state but settle dramatically when they become wet. This paper documents a low-cost, qualitative evaluation scheme using fuzzy set analysis to determine regional collapsibility based on subjective knowledge of the geological and geotechnical conditions and their uncertainty. The geological and geotechnical factors and their category were defined based on relevant literature. Each factor and category was then weighted or rated using linguistic terms developed from expert assessment. The linguistic data or information obtained from the assessments was represented and processed using fuzzy sets. To assess the criteria, 87 collapse potential tests were performed on undisturbed soil samples gathered from 27 different locations throughout Iran. It is shown that the geotechnical criteria predict soil collapsibility much better than the geological criteria.

This research has accomplished a comparative rheological test on the unmodified and nanoclay modified bitumen. The results show that compared to unmodified bitumen, the nanoclay modification leads to changes in rheological parameter by increasing stiffness and decreasing the phase angle hence; it can also reduce aging effect on bitumen. Further, the comparison of the rutting parameter (G*/sinδ) shows that the nanoclay modification could increase the rutting resistance of bitumen where the improvement is dependent upon the type and amount of nanoclay. The fatigue resistance parameter (G*sinδ) at the low to medium temperature shows that the nanoclay modification reduces the fatigue life, however, it exhibits the same fatigue life as that of the unmodified bitumen after a particular ageing condition.