نشریه مهندسی سازه و ساخت
سال هفتم شماره 3 (پیاپی 34، پاییز 1399)

Studies on damaged connections in recent earthquakes have shown that damage to some of the structures cannot be justified by the use of common failure mechanisms. This is due to the extremely low cyclic fatigue created by the critical cycle on the connections, which leads to a failure in the examined connections. In recent years, a lot of research efforts have been made to create a method for predicting a failure due to extremely low cycle fatigue to interpret the failure mechanism in steel materials and structural components. It can be said that the failure is due to the extremely low cycle fatigue state governing the steel structures in severe earthquakes. On the other hand, after the Northridge earthquake, the use of moment connection with reduced beam section, which showed good ductility in laboratory studies, expanded rapidly. But studies on this kind of connection have focused on capacity, and the extremely low cycle fatigue and failure mechanisms in this type of connection have not been considered. The present study focuses on the effect of extremely low cyclic fatigue on damaged connections with reduced beam section beams in moment steel frames. In this regard, the effects of extremely low cyclic fatigue were studied in a 6-story steel moment frame complex with reduced beam sections. Comparison of the indices of start cracking by extremely low cycle fatigue in different floor showed that this index is far from the beginning of the failure. In other words, the connections with reduced beam sections in this structure will not damage by extremely low cycle fatigue.

Output-only structural identification is conducted by output data of the structure. These data usually include structural response together with some noise. Success of output-only methods in determining the vibration parameters of a structure depends on the signal to noise ratio (SNR) of the output data. In this paper, the vibration parameters (Natural frequency and Mode shape) of a contilever beam have been obtained using output data which have different signal to noise ratios. The vibration parameters of the beam were determined using modal analysis of finite element model and considered as reference parameters. Then, appropriate input was applied to the beam and the acceleration signal was obtained. To generate noisy data, noise with different powers compared to signal powers were added to acceleration signal. The modal parameters of the beam were obtained using two output-only methods, Peak Picking (PP) and Stochastic Subspace Identification (SSI). The vibration parameters having signal-to-noise ratios greater than 25 (lower noise level) for all considered modes were identified properly. At a signal-to-noise ratio of 0.25 to 25 (higher noise level), it was not possible to identify the modal parameters of the first mode of the beam, but the parameters of the higher modes were identified with good accuracy.

Changes in modal strain energy of elements before and after damage is a robust damage detection index. However, in structures like double layer grids which have large number of elements, this method has some problems. First, In large structures this method needs more mode shapes to detect damage through modal strain energy method which in practice is difficult to determine. Second, this method introduce some healthy elements as damaged element. To overcome these problems, in this paper a two stage damage detection technique based on modal strain energy method is presented for detecting damage in double layer grids. First, the Modal Strain Energy Based Index (MSEBI) for each mode shape is determined. Then a data fusion technique based on Bayesian theory is used to combine MSEBI values obtained from each mode shape to find damaged elements. Then Charged System Search (CSS) optimization method which is a powerful optimization method is employed to optimize an objective function based on natural frequency to determine damage severity of damaged elements. To demonstrate the performance of the proposed method, a large double layer grid with 1536 elements and different single and multiple damage cases is considered. Numerical results show that the proposed method can successfully find damaged elements and their severities using only few first numbers of mode shapes and frequencies.

In this paper the compressive strength (f’c) of self-compacted concrete (SCC) containing Nanomaterials (NM) is evaluated by non-destructive ultrasonic pulse velocity method. This study involves about 13 different mixtures containing Nano oxides of Silica (NS), Aluminum (NA) and Copper (NC) with amounts ranging from 0.25 to 2 percent, as the binder in replacement of Portland cement, tested between 3 and 90 days for f’c ranging from about 20 to 54 MPa. The influence of different parameters such as type of NM, wet and dry curing conditions on the relationship between ultrasonic pulse velocity (UPV) and f’c is examined. Also, the effect of rebar on UPV is evaluated in this type of SCCs. Results show that the specimens containing NS, NA and NC with amounts of 1.5, 0.25 and 0.25% of the cement weight (optimum percentage), respectively, have the highest UPV and f’c. The specimens containing the optimum amount of NS in wet curing condition with a 22% increase in f’c compared to the control specimen has the highest f’c. In the specimen containing the optimum percentage of NM, UPV became less in the first ages and higher in the older ages more than control specimens. Exponential relationships between UPV and f’c of the specimens were determined whose coefficients are different for each added Nano. The addition of NM reduced the difference in UPV between wet and dry curing conditions so that this difference from 5% for control SCC decreased to 3.8%, 3, and 4% for SCCs containing NS, NA, and NC, respectively. The effect of steel bars on the UPV was introduced by means of correction coefficients (CC) and it was observed that the CC obtained with the proposed CC are consistent with CC recommended by BS 1881: Part 203.

Mortar is suitable for utilizing in compressed parts such as columns and arches due to its considerable compressive strength. However, despite this advantage, the relatively low tensile strength and brittleness of the mortar limit its use for components that are under tensile loads. Using fibers and nano-particles can be a way to reduce these problems. In this research work, various fiber reinforced and nano-engineered cement mortars have been successfully prepared through the addition of nanosilica into plain and fiber reinforced mortars. In this way, effects of nano-SiO2 on the mechanical properties of cement mortars with polypropylene fibers and also without it have been evaluated. Polypropylene fibers were used in lengths of 6-18 mm and aspect ratios of 300-900. The effect of fibers in two different percentages of 0.1% and 0.2%, and the influence of nano-silica in various percentages of 1, 2, 3, 4 and 5% on mortars with a water-to-binder weight ratio of 0.485 were evaluated and compared. A total of 108 cubic mortar samples with a dimension of 5×5×5 cm3 and 108 rectangular cubic samples with a dimension of 16×4×4 cm3 were made according to ASTM standards, and compressive and flexural strength tests were carried out on samples at the ages of 7 and 28 days. The results of the experiments indicated a significant enhancement in the mechanical properties of the prepared mortars, as the values of 7-day and 28-day compressive strength of the sample containing 0.1% fiber and 3% nano-SiO2 were increased by 51% and 61%, respectively, compared to the control sample. Besides, the values of 7-day and 28-day flexural strength of the sample containing 0.2% fiber and 3% nano-SiO2 were increased by 48% and 55%, respectively, compared to the control sample. A noticeable increase in mechanical properties indicated the suitable performance of this type of mortar.

Concrete is a porous material. Water and other fluids can penetrate it and effect its durability. So permeability is one of the most effective parameters of concrete structures durability. Design and construction of concrete, which has a less permeability in addition of providing the desirable compressive strength is of great importance. In this paper, the permeability of 7-day concrete samples containing different pozzolans was investigated using cylindrical chamber method. The mass percentages of silica fume, zeolite and fly ash equal to 5, 10, 15 and 20 percentage that replace type II Portland cement and the existing method of British standard (BS EN 12390-8:2009) were used to compare the obtained results. The percentage of the permeable voids volume was also measured based on ASTM C642-06 standard and used as an index to evaluate permeability. Samples strength was also investigated using the results obtained from the twist-off method. The results of the permeability tests show that all of the pozzolans used in this investigation decrease the percentage of the permeable voids volume and consequently decrease the samples permeability, while silica fume increases the samples compressive strength and zeolite and fly ash decrease samples compressive strength. A very good correlation was also observed between the cylindrical chamber and British standard method results.

In a knee steel braced frame the main diagonal braces are connected to short knee elements. The knee elements act sacrificially and reducing the demands on the main structure during an earthquake, and can be replaced after the earthquake. However, careful design of the knee elements is required to ensure that they are able to absorb energy through repeated large plastic deformation without suffering collapse or instability. A program of laboratory testing has been undertaken in order to optimize the design and useful results from experimental tests are presented in this paper. For testing, the knee elements were mounted horizontally in a reaction frame and loaded vertically by a hydraulic actuator. In this research one kind of new weakening symmetrical perforated knee element are studied. Extensive experimental program of experimental testing by applied cyclic loading behavior on perforated knee element fuse and compare that’s result with a solid element fuse has been undertaken. ANSYS software was used for simulation and the numerical analyses in order to verified experimental tests for specimens. Results show that applied cyclic loading on both knee specimens produce sustain stable hysteresis loops and it is shown that excellent performance to absorb energy can be achieved using perforated knee elements.

Observations obtained in the course of previous earthquakes and studies performed by different researchers indicate the influence of cladding (façade) on seismic performance of structure. there is a dearth of technical knowledge in this field and no proper understanding exists of the actual response of different facade systems to earthquakes, and few studies, especially in Iran, have conducted about how building facade is damaged and about the performance analysis. As a matter of fact, even considering the cladding as non-structural members, those will interact with the encompassing frame during strong earthquakes, and these interactions can alter seismic performance of the structure. This is even more important when it comes to concrete frames wherein material properties and cladding arrangement along the structure height impose significant impacts on the failure mode and failure mechanism of the encompassing frame. In the present paper, three-, five-, and nine-story frames with conventional moment-resisting systems were analyzed using OpenSees Software by considering two typical façades in Iran, namely brick-finished and granite-finished structures. Results of the analysis show that, materials and cladding arrangement impose significant impact on the wall stiffness, distribution of seismic forces by elements, and lateral displacement of the structure. Moreover, it is observed that, with increasing the number of stories, effects of cladding on the structure behavior and performance decrease gradually.

Seismic isolation systems represent one of the most effective solutions to reduce near-fault damages; the systems can be used at either foundation or story levels. The main advantage of seismic isolation system is that it increases fundamental period of structure to longer periods. Among other advantages of seismic isolators, one can refer to the dissipation of input energy into the structure which lowers the transmitted acceleration to the above structure. In the present research, performance of combined concrete-steel structures with or without seismic isolation systems at story level, are investigated under near-field earthquakes. For this purpose, three structures of 4, 7, and 10 stories with/without seismic Lead Rubber Bearing (LRB) isolation systems with different damping ratios and periods were modelled. The analysis results indicate that, the isolation systems of longer period and smaller damping ratio were associated with lower relative displacement, acceleration and base shear, improving the structure performance. Results of analysis show that, with increasing the structure height (compare the results of the 10 story building with those of the 4 and 7 story counterparts), variations in base shear is reduced in the models with different isolation systems; i.e. the taller the structure, the more faded the role of the isolator. Furthermore, a study of the variations of damping ratio and period of isolator makes it obvious that, with increasing the period of isolators, shear force of the stories in the isolated structures decreases, while it slightly increases with increasing the damping percentage of the isolator.

Exact seismic evaluation of underground structures such as tunnels requires dynamic analysis, due to the complex dynamic behavior of soil and the interaction of soil and structure. Dynamic analysis in order to simulate the actual seismic response of a structure is possible only with the appropriate acceleration time history (compatible with the local site soil conditions). Considering the vertical component of the earthquake on the site is one of the important factors for achieving the real responses of the structure (such as near fault earthquakes). In this paper, using ABAQUS finite element software, the soil and tunnel system was modeled using linear time histories analysis under four near field recordings of the earthquake, taking into account the various proportions of horizontal and vertical components in the twelve loading combinations, and the correlation effect Seismic components have been investigated by combining different loads on acceleration, stress and strain, axial force, bending moment and shear force maximum in tunnel lining. The studies showed that increasing the vertical to horizontal ratio has a very small effect on the maximum horizontal acceleration on the tunnel Liner. By examining the axial forces, bending moment and shear force, increasing the ratio of vertical components to horizontal has the greatest effect on the axial force response. Also, by comparing the maximum stress and strain under the twelve loading combinations for the selected records, the maximum strain in the tunnel liner is less than the allowable strain rate for the concrete and in the case of loading, the stress in the tunnel liner is greater than the permitted stress Concrete and in the opposite direction are confident.

In recent decades, concurrent with expansion of cities, the demand for new buildings and structures has increased. On the other hand, aggregates constitute the biggest portion of concrete. As such, providing natural aggregates for producing concrete has become a challenge due to shortage of resources. Also, treatment of construction waste materials resulted from demolition of old structures has been another concern. One way to handle these problems is reusing concrete waste as a new material in new construction. Nowadays, recycled concrete aggregate (RCA) is used in concrete as a partial or complete replacement for natural aggregate (NA). Performance of recycled aggregate concrete (RAC) is influenced by the characteristics of the parent concrete from which RCA is obtained. In this research, the mechanical properties and the durability of three types of RCAs from different sources were evaluated. For this purpose, the aggregates' Los Angeles abrasion value and freeze/thaw soundness value were determined. Moreover, eight concrete mixes incorporating different amounts of the RCAs as coarse aggregate were cast and their water absorption coefficient, compressive strength and deicer-salt scaling resistance were assessed. The results showed that the mechanical properties of RCAs had a direct relationship with the mechanical properties and durability of RACs. Also, the type of RCAs had no significant impact on the performance of RACs. However, increasing the replacement amount of RCA decreased the strength and deicer-salt scaling resistance of RACs.

This study investigates method of producing non-uniform support acceleration in actual site condition. Factors affecting the non-uniform excitation of topographic site include time delay between the arrival time of incident wave at the canyon floor and any particular point at higher elevation, causing phase difference. Another important factor in generating non-uniform accelerations is the topography amplification. Therefore, in the present study, two dimensional models of a canyon site subjected to Ricker wave with different predominant frequencies, shear wave velocities, shape ratios have been developed and the results of boundary element analyses in time domain (HYBRID code) were obtained in different elevations on the canyon surface. Then, by conducting a series of statistical analyses (Symbolic Regression and Genetic Programming) on the results from numerous presumed cases, the relationships for calculating the time delay between support responses of V-shaped canyons was obtained. To verify the accuracy of the proposed relationships, the results were compared with the records from Pacoima dam site during two different earthquakes and good agreement of the results was observed. Eventually, by implementing the presented relationships into random vibration method, non-uniform acceleration of the site was simulated. The results indicate that the proposed relationships have an appropriate level of accuracy to calculate time delay on the V-shaped canyons.

Replacing of conventional concrete by High Performance Fiber Reinforced Cementitious Composite (HPFRCC) could improve tensile strength, bending strength, shear strength and other capability of behaviour such as ductility, energy absorption and toughness. Strain hardening behaviour in (HPFRCC) could result in restrict of crack and prevent from crack widening. Also, because of the high strain capacity of (HPFRCC), using this material in the beam-column connection is more attention paid by researchers. After verification of numerical models for frame using OPENSEES software and comparison with experimental results, two 2D frames with 3 and 6 stories has been created to study. These frames have been designed based on ACI 318. Each frame is considered in 4 formats containing conventional concrete in all elements (RC), containing of (HPFRCC) in beam-column connection (RCH1), containing of (HPFRCC) in beam-column connection and first story column base (RCH2) and containing of (HPFRCC) in all elements. Results from nonlinear static analysis (pushover analysis) show increase of lateral strength, ductility and ultimate displacement of frames containing (HPFRCC) with respect to (RC) frames. Also, using of (HPFRCC) in frames has been decreased maximum story drift ratio, maximum roof displacement and has been increased maximum base shear and story shear in nonlinear time history analysis.

Compressive strength test of concrete is one of the most important tests for controlling the quality of concrete in construction structures and, moulds with different shapes and dimensions (cubic and cylindrical) are used to provide experimental specimens based on the workshop conditions and the regulations of the relevant codes and standards. The purpose of this research is to investigate and determine changes in the conversion factors for the strength of specimens with different shapes and dimensions for C35 and C45 concretes made with different types of cement. In this research, 425-1, 2, and 5 cements were used for the preparation of specimens. A total of 162 cubic and cylindrical specimens with dimensions (200 × 200 × 200) mm, (150 × 150 × 150) mm and (150x300) mm at the age of 3, 7 and 28 days have been tested for compressive strength. The results of this study show that in addition to the shape and size of the samples, the type of cement used also affects the strength and conversion factors for the strength of the specimens with various shapes and sizes. The results also show that the difference in conversion factor for strength obtained with the corresponding conversion factors provided by the Iranian National Building Code, Part 9 at the age of 3 and 7 days of the specimens is maximum and is minimum at the age of 28 days.