Unreinforced Structures Research Articles

Seismic resistance analysis and optimization of gymnasium truss structure

The gymnasium is mainly composed of truss structure, with a large span, and requires high seismic performance. To ensure greater stability of the truss structure, modal analysis was conducted on the mechanical model, yielding the natural frequencies and modal shapes. In order to improve the seismic performance, BRB rods were added to the support points, and the frame structure was reinforced with diagonal braces. Through modal verification, it can be seen that the stiffness of the truss structure was significantly improved. The internal forces of the truss were simulated under the El Centro and Taft seismic wave conditions. The results show that the optimized structure can significantly reduce the internal forces and improve the overall seismic performance. The fundamental natural period of the reinforced structure is less than that of the unreinforced structure, and the stiffness of the reinforced steel structure increases, making it more sensitive to vertical seismic motion.

Programmable mechanical responses of a hybrid star-rhombus honeycomb based on digital design method

Hybrid honeycombs with programmable mechanical responses have received widespread attention due to their multifunctionality and superior mechanical properties. To improve the tunability of the hybrid honeycomb, a hybrid star-rhombus honeycomb (HSRH) is designed using a digital design method. Different coding strategies are obtained by programming the distribution position of the rhombic structure inside the star-shaped honeycomb. HSRH with different coding strategies are designed by coding “0″ (unreinforced structure) and “1″ (reinforced structure). The compressive stress, Poisson's ratio and energy absorption of HSRH are investigated by experiments and numerical simulations. The results show that HSRH produces ordered deformation in compression with multi-plateau characteristics. HSRH has superior energy absorption in the X direction than in the Y direction. The specific energy absorption can be regulated between 2.41 - 5.02 J/g and 1.57–4.29 J/g in the X and Y directions, respectively. HSRH with unidirectional and bidirectional filling strategies exhibit a double-plateau feature in the X and Y directions, while the HSRH with hierarchical filling strategies has a three-plateau feature in the X direction. The filling ratio plays a decisive role in the energy absorption of honeycomb and has a positive correlation with the energy absorption properties. Furthermore, diverse deformation modes can be customized by introducing pattern-coding strategies. The codable deformation mode and programmable energy absorption of the HSRH are achieved by controlling the filling ratio and coding strategy (i.e., coding the number and location of embedded rhombic structures). This study provides a digital method for the design of programmable hybrid honeycombs and facilitates their application in multi-tasking energy absorbers.

Development of structural type-based physical vulnerability curves to debris flow using numerical analysis and regression model

Urbanization has significantly increased the risk of debris flows to houses located at mountain boundaries, particularly in South Korea, where urban centers are surrounded by mountains. The degree of damage to a house strongly depends on the structural rigidity of the building, even when exposed to the same hazard intensities. However, the structural characteristics of buildings are usually disregarded in vulnerability assessments. In this study, vulnerability curves were proposed for three different structural types of buildings: brick masonry, light-weight steel frame, and wood structures, using numerical and regression analyses. A total of 22 debris flow events from 2011 to 2020 and 63 cases of damage to unreinforced structures were collected and analyzed. Back analysis of the debris flows was performed using Flow-R, and the impact pressure acting on each structure was calculated. Three different vulnerability curves were proposed using various linear and nonlinear regression analyses; the Avrami equation provided the highest statistical significance. The vulnerability curves of three different structures were similar for the slight damage range. However, over the slight damage range, the wood structure was the most vulnerable at a given hazard intensity, whereas the brick masonry structure was the least vulnerable. Validation was performed using data from recent debris flow events, revealing precise damage level predictions for 9 out of 11 buildings. Compared with previous research, the degree of damage at the same hazard intensity was found to be significantly dependent on the structural type. The developed vulnerability curve for each unreinforced structure can be used as key data for vulnerability and risk assessments.

Application of stress function and Fourier’s series for calculation of multilayer reinforced road structure

The article is devoted to the development of a method for static load calculation of a multilayer reinforced road structure. Unlike the traditional calculation practice of such structures, in which both reinforced and unreinforced layers are considered as isotropic layers, in this paper reinforced layers are presented as orthotropic layers, and unreinforced layers as isotropic. The computational model of the structure with isotropic and orthotropic layers is considered in the framework of the plane problem of elastic theory. The width of all layers in the crosswise direction is the same. The resolving equations are derived from the equations of strain compatibility involving the stress function and Fourier’s series. The unknown constants are obtained from the boundary conditions and layer coupling conditions. Formulas for determining the maximum deflections are derived. As an example, the two-layer structure calculation is given, the upper layer of which is reinforced with a geocell and the lower layer is unreinforced. A uniform loading is applied on a limited part of the upper layer surface. The paper presents a maximum deflections dependence diagram of the two-layer system on the number of terms of the series. The calculated deflections of the two-layer structure are compared with the maximum deflections of an equally thick unreinforced single-layer structure, as well as with the results of field experiments. The effect of reinforcing the upper layer of the structure is defined as the ratio of the difference between the unreinforced and reinforced maximum deflections of structures and the unreinforced structure maximum deflection. The reinforcement effect calculated according to the proposed theory was 16 %, while the effect obtained as a result of field experiment was 10 %. The difference is explained by the difference in the size and shape of the load deck, as well as the difference between the plane and volume deformation models of the theoretical model and the real structure.

Shake table test on a five-story residential masonry building strengthened by engineered cementitious composites based on minimum disturbance

Multi-storey masonry structures are popular structural systems for residential buildings in China in the 1970 s, which need strengthening urgently due to poor seismic performance. To save construction time and achieve minimum disturbance, a single-sided engineered cementitious composites (ECC) strengthened method for masonry structures was proposed. To investigate the effectiveness of the strengthening method, two 1/4-scaled 5-storey masonry structures, including a reference unreinforced structure (US) model and a corresponding retrofitted structure (RS) model, were designed and tested by shake table tests. The comparisons between the US model and the RS model were analyzed in terms of the natural frequency, acceleration amplification factor, acceleration response, and storey drift response. The results show that the strengthening method can improve the seismic performance of the masonry structure. The connection method can ensure that the existing masonry walls and ECC layers work together. The initial natural frequency of the RS model is 1.3 times that of the US model. The maximum storey drift of the US model is 3.0 times that of the RS model at PGA = 0.550 g. To ensure the strengthening efficiency, the acceptable structural aspect ratio range for the strengthening method was suggested, and an improved strengthening method was proposed.

Investigation of energy absorption performances of a 3D printed fiber-reinforced bio-inspired cellular structure under in-plane compression loading

This article proposes glass-fiber-reinforced bone-inspired cellular structures to enhance energy absorption capability. The elastic modulus of the bone-inspired unit cell is obtained analytically based on the energy method and then employed in Particle Swarm Optimization algorithm to get optimized cellular structures. In the optimized cellular structure, the stiffness is optimized and the energy absorption capacity is investigated. A Fused Filament Fabrication 3D printing process is used to fabricate the cellular structures with continuous glass fiber-reinforced polylactic acid (PLA). In-plane compression tests are performed to investigate the mechanical performance of cellular structures. Finite Element Modeling (FEM) is conducted to analyzed the mechanical performance of the structures. In FEM, the failure criterion is determined using the maximum stress and VUSDFLD subroutine, and the damage growth is modeled by decreasing the mechanical properties. A good agreement between numerical and experimental results was observed. Results demonstrated that the energy absorption in glass-fiber-reinforced PLA is ∼250% higher than in the un-reinforced structure. The optimized cellular structure exhibits a stable prolonged plateau stress region and very high specific energy absorption parameters.

Finite element analysis of bond behavior in corroded reinforced concrete beams: State-of-the-art

The article conducts a comprehensive examination of various aspects related to rebar corrosion, encompassing the corrosion mechanism, its implications on design criteria, the modeling of bond interfaces under both corroded and non-corroded conditions, and the modeling of reinforced concrete (RC) beams affected by corrosion, employing both empirical and analytical methodologies. The initial stages of corrosion instigate a gradual transformation of rebar into rust. One notable consequence of reinforcement corrosion is the generation of expansive pressure, leading to concrete cracking, spalling, and detachment of the concrete cover. Additionally, it diminishes the effective cross sectional area of the rebar, ultimately resulting in a decline in the concrete's bond strength and gradual structural deterioration. Ultimately, continuous corrosion can lead to a complete loss of bond between the concrete and rebar, representing the most severe form of damage attributable to corrosion. This poses a critical threat, particularly in cases where the beam functions as an unreinforced structure, potentially culminating in sudden structural failure. This paper primarily underscores the utilization of the Finite Element Method (FEM) for evaluating the impact of bond deterioration between concrete and reinforcement caused by corrosion. The paper effectively employs this technique to predict and analyze the structural damage in corroded RC beam specimens.

Validation of a practice-oriented floor spectra formulation through actual data from the 2016/2017 Central Italy earthquake

Analytical expressions for the floor spectra evaluation play a key role in the correct definition of the seismic input induced to non-structural elements or local mechanisms in existing buildings. They have to be able properly assessing the possible amplification phenomena, but also correctly describing the effects of nonlinearities due to structural damage. Due to the complexity of such phenomena, data on existing structures hit by earthquakes constitute a precious source for a better understanding of the topic and the validation of analytical expressions. In this framework, the paper aim is twofold. On one hand, it evaluates the entity of seismic amplification through experimental evidences from in-situ measurements on two existing monitored unreinforced structures. On the other hand, it presents the application on them of an analytical expression for the floor spectra already developed by the Authors. The case-studies are the former courthouse of Fabriano (Ancona, Italy) and the Pizzoli’s town hall (L’Aquila, Italy). They were both hit by the 2016/2017 earthquake in Central Italy and are permanently monitored by the Italian seismic monitoring system of the Italian Department of Civil Protection. With the aim of validating the above-mentioned expression, the paper shows the comparison between experimental and analytical floor spectra for various minor events and mainshocks of the Central Italy earthquake. Since the two case-studies exhibited different damage levels (from slight to moderate, respectively), the comparison allowed us to verify the reliability of the expression both in the pseudo-elastic and moderate nonlinear fields.

Design and fatigue analysis of diagonally reinforced structures with a negative Poisson’s ratio lattice material

In this paper, a typical negative Poisson’s ratio lattice structure was studied and several diagonally reinforced 3D lattice structures were designed and analysed using the finite element method. On this basis, the fatigue life of the unreinforced and reinforced structures was calculated using SIMULIA Fe-safe software. Meanwhile, the fatigue failure process of the lattice structure was systematically simulated and quantitatively analysed by combining the multiaxial fatigue damage model. Results show that the enhancement design from the cell structure can provide an anti-diagonal shear enhancement effect. The structure A, B, and C can decrease the maximum von Mises stress by 95.8, 97.1, and 94.66%, and increase the compressibility by 80%, 56%, and 127%, respectively. The structure A enhancement yields the best overall performance in terms of structural stress, compressibility, and negative Poisson’s ratio properties enhancement. Compared with the unreinforced structure, the lifetime distribution of the structure A reinforced structure changes in both position and level. The overall lifetime has been improved from 102.813 of the unreinforced structure to 107 of the reinforced structure. Quantitative calculation of the fatigue damage is consistent with the fatigue life prediction results, which further validate the effectiveness of the diagonal enhancement method and the enhancement structure of the negative Poisson’s ratio lattice structure.

Parametric Comparison of Retrofitting Methods in Masonry using Finite Element Analysis

Seismic strengthening of buildings is necessary to control the extent of damage in case of earthquakes. The concept of an equivalent diagonal strut, provision of bands and intrusion wall are studied as structural efforts to improve the seismic stability of buildings. Static analysis and free vibration analysis are performed on various configurations. ANSYS software is used for modelling and analysis, where the models are subjected to equivalent lateral loadings. The analysis is performed using the Finite Element Method (FEM). Stress contours, lateral deformations by static analysis, natural frequency by free vibration analysis and their corresponding mode shapes for each of the models are studied. The obtained results are compared with the unreinforced masonry model. Results of the analysis showed that there is the highest increase in stiffness value to 43.38 % and a decrease in the deformation to 30.25% in the model with intrusion walls when compared to the basic unreinforced model. It is found that parameters like lateral deformations and stiffness showed positive improvements by the provisions of the configurations when compared to the unreinforced structures. This paper can be further extended to implement the dynamic analysis.

Laboratory and numerical analysis of failure of stone masonry arches with and without reinforcement

Nowadays, there are several existing masonry arch structures which are usually protected. Therefore, the maintenance and determination of their load-bearing capacity is a very important issue. Consequently, it is necessary to understand the behaviour of these structures, but the investigation of an old structure can cause difficulties. Nevertheless, a lot of tests can be performed on a smaller model under laboratory conditions. It is easier to understand their structural behaviour and determine the parameters that significantly influence the load-bearing capacity. It also gives the chance to build and verify a numerical model that can be used to model existing structures. In this research, destructive tests of small scale stone masonry arches were performed under laboratory conditions. The analyses were carried out with and without reinforcement to determine the differences in structural behaviour, load-bearing capacity, and failure type. For the tests, carbon fibre reinforced polymer (CFRP) plates were used to strengthen the arch. The properties of the building materials of the stone masonry arches were tested. The paper presents the results and experiences of the construction and the loading of small scale masonry arches. Furthermore, it gives a comparison between the behaviour of reinforced and unreinforced structures.

Experiment and Numerical Simulation of Wooden Door Frame

To prevent the wooden door frame of traditional rural houses from being stuck due to diamond deformation under earthquake and improve the seismic capacity of rural houses, an innovative method of reinforcing the angular displacement of the wooden door frame with channel steel and the diagonal brace is proposed. The rationality of the finite element simulation is demonstrated by comparing the results of finite element simulation and quasistatic test based on reinforced and unreinforced wooden door frame specimens. On the basis of the finite element model of wooden door frame, the seismic performance of channel type and diagonal brace thickness of reinforced wooden door frame and the seismic performance of friction coefficient of unreinforced wooden door frame are studied, respectively. The results show that the lateral stiffness and the lateral bearing capacity of the reinforced wooden door frame increase with the increase of channel steel type and the diagonal brace thickness. The height of the channel steel section of the seismic reinforcement structure should be half of the unreinforced structure. With the increase of the friction coefficient, the lateral bearing capacity of the unreinforced wooden frame increases, while the ductility of the unreinforced wooden frame decreases.

Numerical study on masonry vaults reinforced by composite

The article describes numerical study of masonry vaults, based on steel beams, reinforced with carbon fiber composites. Two amplification systems were investigated: a solid mortar with an embedded grid and a discrete one in the form of tapes (stripes). Numerical simulation based on nonlinear analysis revealed the amplification efficiency under different arrangements, cracking and destructive load, the stress-strain state of the vaults. Many models are considered: not strengthening; not strengthening with damage; reinforced without damage, reinforced with damage. The simulation results in ANSYS PC were compared with each other to assess changes in the bearing capacity and the nature of the destruction of the samples. The span of steel beams is 5 meters, the span of the arch is 1.1 meters, the rise of the arch is 120 mm, the thickness is 120 mm. The load on the vaults is applied evenly distributed. The material of the vaults is ceramic solid brick on a lime-cement mortar. The experimental results on unreinforced structures are compared with those obtained on the strengthened ones in terms of failure mode, maximum load, stiffness and ductility.

Failure mechanism and strengthening effect of shield tunnel lining reinforced by steel plates with corbels

This study investigates the failure mechanism and strengthening effect of a shield tunnel lining structure reinforced via steel plates with corbels. A detailed three-dimensional (3D) numerical model is established for the staggered-joint segmental lining structure of an actual shield tunnel in the Kunming metro-system, which is then validated via a full-scale simulation experiment of the shield tunnel segmental lining structure. Then, the actual loading conditions of the staggered-joint shield tunnel segmental lining structure in the Kunming metro-system are simulated, and the mechanical responses of the segmental lining are analysed in terms of the variation in its diameter, opening amount of the joints and stress of the bolts. The ultimate load-bearing capacity of the lining structure reinforced using steel plates with corbels is improved to 610 kN, which is 41.9% higher than the unreinforced structure with an ultimate load-bearing capacity of 430 kN. Moreover, the strengthening effect of the reinforcement using steel plates with corbels is discussed considering actual field monitoring data and other reinforcement methods. It is concluded that the results of this paper can provide an effective reference for similar reinforcement schemes of shield tunnels. The defined classification criteria can be adopted to determine the deformation stage of reinforced shield tunnels.

Tribological behavior of bioactive multi-material structures targeting orthopedic applications

The following study proposes a multi-material solution in which Ti6Al4V cellular structures produced by Selective Laser Melting are impregnated with bioactive materials (hydroxyapatite or β-tricalcium phosphate) using press and sintering technique. To assess the tribological response of these structures, an alumina plate was used as a counterpart in a flat-on-flat reciprocating sliding test. Ti6Al4V cellular structures impregnated with bioactive materials displayed the highest wear resistance when compared with the unreinforced structures. Among the bioactive structures, Ti6Al4V cellular structures impregnated with βTCP were the ones with higher wear resistance, having the lowest weight loss. Hence, these structures are promising multifunctional solutions for load-bearing applications by gathering suitable mechanical properties (strength and stiffness); bioactive properties and in addition an improved wear performance.

Experimental investigation into brick masonry arches’ (vault and rib cover) behavior reinforced by FRP strips under vertical load

The current experimental study is the reinforcement of the simple curvature vault masonry structures. In this study, we discuss complex structure include vault and rib cover with two radii and actual dimensions under a vertical load. The unreinforced structure data were compared with analysis data. The analysis data are in good agreement with experimental data. In the first experiment, a structure without reinforcement is tested and according to the test results, the second structure was reinforced using the carbon polymer fibers and the same test is done to see the effects of reinforcement. Based on the test results of the first structure, the first cracks are created in the vault. Moreover, the reinforcement with carbon fibers will increase the loading capacity of the structure around 35%.

Ancient masonry arches and vaults strengthened with TRM, SRG and FRP composites: Experimental evaluation

Composite materials made with textile fibers both with polymeric and cementitious matrices are often adopted for the retrofitting of masonry arches and vaults. A specific project that analyzes the performance of ancient masonry arches and vaults strengthened with composite systems has been recently concluded at Politecnico of Milan. The project involves the experimental evaluation and the development of numerical and analytical simulations. In this paper the experimental campaign is described, whereas the numerical validation is provided in an accompanying paper (Bertolesi et al., 2017). The tests were performed in situ on ancient masonry arches and vault elements. In particular, three barrel vaults and two arches either unreinforced or reinforced with Steel Reinforced Grout (SRG), Textile Reinforced Mortar (TRM) and Fiber Reinforced Polymer (FRP) were tested.The arches had a span equal to 3.30 m, a rise equal to 0.83 m and were built with common Italian bricks regularly spacing out two bricks laid edge on (thickness of the arch 12 cm) with two bricks (one over the other) disposed in single leaf. Barrel vaults had the same geometry of the arches but were made with a single leaf. In all cases, an eccentric vertical load was applied at 1/4 of the span and was increased up to failure. The experimental results on unreinforced structures are compared with those obtained on the strengthened ones in terms of failure mode, maximum load, stiffness and ductility.

Evaluation of Experimental and Numerical Simulation of Triaxial Geogrid Reinforcement on the Strength of Road Structures

The purpose of this research consists in analyzing the contribution to the bearing capacity of the road structures as brought by triaxial rib geogrids. The objective of the present study lies in diminishing the effects of void production under road structures and respectively in finding new ways of improving their bearing capacity with the help of geosynthetic materials reinforcing the structures. The experimental study involved performing a series of laboratory tests on the unreinforced and geogrid reinforced models. The test results demonstrate that the geogrid reinforced structure undergoes three times smaller deformations relatively into unreinforced structure The experimental results were also compared to the data obtained by using finite elements methods.

Seismic and vibration tests for assessing the effectiveness of GFRP for retrofitting masonry structures

Full-scale shake table seismic experiments and low-amplitude vibration tests on a masonry building are carried out to assess its seismic performance as well as study the effectiveness of a new multifunctional textile material for retrofitting masonry structures against earthquakes. The un-reinforced and the retrofitted with glass fiber reinforced polymer (GFRP) strips masonry building was subjected to a series of earthquake excitations of increasing magnitude in order to progressively induce various small, moderate and severe levels of damage to the masonry walls. The performance of the original and retrofitted building states is evaluated. Changes in the dynamic characteristics (lowest four modal frequencies and damping ratios) of the building are used to assess and quantify the damage states of the masonry walls. For this, the dynamic modal characteristics of the structure states after each earthquake event were estimated by performing low-amplitude impulse hammer and sine-sweep forced vibration tests. Comparisons between the modal results calculated using traditional accelerometers and those using Fiber Bragg Grating (FBG) sensors embedded in the reinforcing textile were carried on to investigate the reliability and accuracy of FBG sensors in tracking the dynamic behaviour of the building. The retrofitting actions restored the stiffness characteristics of the reinforced masonry structure to the levels of the original undamaged un-reinforced structure. The results show that despite a similar dynamic behavior identified, corresponding to reduction of the modal frequencies, the un-reinforced masonry building was severely damaged, while the reinforced masonry building was able to withstand, without visual damage, the induced strong seismic excitations. The applied GFRP reinforcement architecture for one storey buildings was experimentally proven reliable for the most severe earthquake accelerations. It was easily placed in a short time and it is a cost effective solution (covering only 20% of the external wall surfaces) when compared to the cost for full wall coverage by GFRPs.

Liquefaction during the 1990 Manjil, Iran, earthquake, I: Case history data

Abstract During the 1990 Manjil, Iran, earthquake (MS = 7.7), an estimated 35,000 people lost their lives and more than 300,000 were left homeless. The earthquake ground shaking caused enormous destruction of unreinforced structures. In addition, widespread liquefaction contributed significantly to building damage in towns as far away as 85 km from the ruptured fault. Following the earthquake, the authors surveyed the liquefaction regions, conducted geotechnical field explorations, and documented case histories on liquefaction of level ground, liquefaction-induced building settlement, permanent ground displacement, and performance of piles and piers in liquefied soils. This article presents and discusses the data of the case histories investigated. In addition, based on the evidence from the Manjil earthquake, the importance of geologic input in mapping of liquefaction potential in earthquake-prone regions is demonstrated. In the companion article, the authors present the results of their analyses of liquefaction-related case histories.