Strengthening Schemes Research Articles

Experimental study of RC deep beams strengthened by NSM steel bars

Abstract In this work, an experimental investigation on the behavior of reinforced concrete deep beams strengthened by near surface mounted steel bars is presented. Thirteen beams were cast and strengthened by steel bars with different strengthening schemes. The specimens were categorized into three groups according to the shear span over depth (a/d) ratio. Each group contained one unstrengthened beam as a control and other strengthened beams using different strengthening schemes of near surface mounted steel bars. Some parameters that might affect the performance of strengthened deep beams were studied, such as bar spacings, bar sizes, and their inclinations. All beams were tested under two points loading, adopting various shear span over effective depth ratios. The test results showed considerable enhancement in the ultimate capacity of strengthened beams by about 7.35–20.6%. Besides that, ductility and diagonal crack width were improved when more intensive strengthening schemes were adopted.

Damage assessment and mitigation measures of underwater tunnel subjected to blast loads

The damage prediction and protective measures of tunnels subjected to blast loads have gained importance in recent years due to increasing accidental events and terrorist attacks. This paper performs numerical modeling to assess the damage characteristics of an underwater tunnel subjected to blast loads and explore the potential mitigation measures. Firstly, a three-dimensional numerical model of an underwater tunnel based on coupled Lagrange and Euler (CLE) method is developed. The accuracy of the CLE method and material models is verified with blast testing data. Subsequently, the dynamic response and damage evolution of the tunnel subjected to underwater explosions are investigated. Potential failure patterns due to various underwater blast loads are explored. Intensive numerical simulations are then carried out to investigate the damage levels of the tunnel. Based on numerical data, empirical relations are suggested to predict the tunnel damage levels based on charge weight and standoff distances. CFRP (carbon fiber reinforced polymer) cloth is used to improve the resistance of the tunnel to underwater explosions. Mitigation effects of different strengthening schemes are investigated, and the optimum strengthening thickness of the CFRP cloth is recommended. The results show that the rigidity and load carrying ability of the tunnel are significantly improved by bonding the CFRP cloth. The recommended thickness of the CFRP cloth is 0.5–0.835 mm.

Fiber-reinforced polymer plates for strengthening web opening in steel I-beams under cyclic loading

This article is interested in finding the most appropriate strengthening scheme for web opening in steel I-beam under cyclic loading. Different opening shapes and locations were strengthened using two different fiber-reinforced polymer plates: carbon fiber–reinforced polymer and basalt fiber–reinforced polymer. A three-dimensional nonlinear finite element model was adopted using ANSYS program. The results showed that the high strength of carbon fiber–reinforced polymer plates helped the steel beam to recover its original strength; however, noticeable reduction was observed in beam ductility and in the number of inelastic excursions that took place before failure. Moreover, remarkable difference in stress concentration existed at the position of the carbon fiber–reinforced polymer plate edges. Under cyclic loading, fiber-reinforced polymer composites with low modulus of elasticity such as basalt fiber–reinforced polymer managed to eliminate these deficiencies using longer strengthening length. The most appropriate strengthening length and scheme for each opening case were outlined.

A Multiple Response-Based Structural Capacity Reserve Assessment Model for Long-Span Single-Layer Lattice Shells under Strong Earthquakes

ABSTRACT A new structural capacity reserve (SCR) model is proposed based on the selected response-based indicators for single-layer lattice shells. The modified residual seismic capacity ratio is defined as the normalized relative distance with respect to the collapse-critical state. A reserve performance level matrix is developed under the performance-based design framework. To achieve a desirable SCR, the proportions of plastic components are applied to identify vulnerable areas for determining strengthening schemes. The case results reveal that the desirable SCR cannot be achieved if the components are not strengthened properly. Obviously, the proposed SCR can lead to a more comprehensive balanced design.

Flexural behaviour of RC beams strengthened with combinations of two CFRP systems and P-SWRs

Externally bonded (EB) carbon fibre-reinforced polymer (CFRP) sheets, near-surface-mounted (NSM) CFRP bars and prestressed steel wire ropes (P-SWRs) are widely used for strengthening reinforced-concrete (RC) beams. Existing research has shown that it can be inefficient or infeasible to provide sufficient load-carrying capacity using one strengthening method because of the CFRP strain limitation at debonding or the upper bound on the amount of strengthening materials due to limited space for strengthening. Moreover, some already strengthened RC beams need to be further strengthened under heavier loading conditions. Therefore, using a combined strengthening scheme for one beam should be considered in both these situations. In this study, five concrete beams were strengthened and tested to explore the effects of different combined schemes on beam flexural behaviour. The results demonstrate that P-SWRs are of crucial importance in improving the cracking behaviour and ductility of beams originally strengthened with EB-CFRP sheets or NSM-CFRP bars. The strengthening scheme of NSM-CFRP bars and P-SWRs is found to be better than that of EB-CFRP sheets and P-SWRs. A recommendation for the appropriate selection of strengthening schemes is given based on the test results.

Strengthening the MSME Through Institutional Cooperation Improvement Between MSME and Sharia Microfinance Institutions (SMFI)

Micro, Small and Medium Enterprise (MSME) is one of the pillars of the national economy because of the magnitude of the contribution made by MSME. It is even mentioned that MSME is an integral part of the country's economy. MSME in Indonesia is able to absorb the labor up to 97.02%. The contribution of MSME to GDP (57.08%) and investment (53.24%) also has a larger value when compared to large businesses. But in general, MSME has limited access to financial institutions. By these problems, the government has provided special financing of MSME through Microfinance Institutions (MFI). But in its implementation, externally MFI has obstacles of institutional aspects that have an impact on the variety of the MFI forms. While SMFI has Islamic financing methods that give priority to moral, ethical, and fair so efficiently it can encourage the participation of MSME entrepreneurs to succeed. This study formulates schemes or strengthening models of MSME to increase institutional cooperation between MSME and SMFI. The institutional strengthening scheme offered is expected to increase the contribution of MSME to the national economy.

Stability analysis and performance comparison of large-scale hyperbolic steel cooling towers with different latticed shell systems

The cooling tower is a very important building for the thermal power plant. The large steel cooling tower is proposed to overcome the serious challenges that arise from modern cooling towers being higher, larger, and more complex. In this study, for the large-scale hyperbolic steel cooling tower, seven different latticed shell systems with different grid forms, including five double-layer systems and two single-layer systems, were designed and studied in detail. For different heights, grid sizes, and shell thicknesses, numerous parametric analyses (792 models) of the structures were performed. The static behaviors—including the steel quantity, peak displacement, and peak stress—of the seven structural systems were obtained and compared. The variation laws of the steel quantity and peak displacement with respect to the parameters were presented, and the reasonable range of the parameters for the steel cooling tower with different heights were obtained. The results show that the optimal structural system is different for the steel cooling towers with different heights. Additionally, based on the nonlinear stability analysis, a reasonable strengthening scheme was proposed for the single-layer system with poor stability. The results will be of great help for further research on large steel cooling towers and provide guidance for actual engineering design of the structures.

Axial strength of steel circular hollow section (CHS) X-joints strengthened by flanged larger pipe

In steel design, enhancing the structural joints’ capacity is considered a challenge that faces the designer. This challenge becomes more difficult when it comes to enhancing the capacities of circular hollow section (CHS) joints due to their closed nature that complicates the strengthening process. Recent research related to strengthening T-joints by utilizing two outer hollow ring flanges welded to additional pipe showed that this technique can significantly improve the joints’ strength. In this study, the utilization of this technique is extended for enhancing the axial strength of CHS X-joints. In this regard, a parametric study using finite element models was carried out to investigate the different design aspects that might affect the behavior of strengthened X-joints. The examined parameters included, the ring flange diameter, the stiffening pipe thickness and length for different brace diameter-to-chord diameter ratios and chord diameter to double chord thickness ratio. The results demonstrated that these strengthened X-joints gained significant axial strength that reached up to three times the axial strength of the unstrengthened joints. Guidelines for proper detailing of such strengthening scheme were provided. Finally, an equation that estimates the axial strength of strengthened joints was established based on the achieved results.

Experimental and analytical study of strengthening schemes for shear deficient RC deep beams

Abstract This paper investigates experimentally and analytically the effectiveness of adopted schemes for strengthening of reinforced concrete (RC) and fiber reinforced concrete (FRC) deep beams in shear. Six specimens were cast in two groups of three specimens each. First group was cast in concrete without fibers, whereas the second was cast half in concrete without fibers and the remaining half-length in concrete having steel fibers. The steel web reinforcement was provided in half span of all beams and the other half was strengthened using different schemes. The shear strengthening schemes employed for the first group were: (i) near surface mounted (NSM) carbon fiber reinforced polymer (CFRP) U-wrap strips and externally bonded horizontal CFRP strips, and (ii) welded wire mesh. For shear strengthening of the FRC of the second group in shear, externally bonded CFRP U-wraps and horizontal CFRP strips were used. The strengthening schemes upgraded the shear strength of deep beams to greater than that provided by steel web reinforcement. FRC showed excellent shear resistance although it caused reduction in the beam deformation capacity. A methodology is proposed to assess the shear strength of concrete (without fibers) and FRC deep beams. The shear strength prediction of the tested deep concrete beams was consistent with the test results.

Structural-capacity-reserve-based performance assessment of long-span single-layer lattice shell structures

To achieve a well-balanced structural design, a customized performance assessment approach based on the structural capacity reserve (SCR) is proposed for commonly used long-span single-layer lattice shells. The SCR is calculated by integrating the normalized residual seismic capacity (RSC) over the entire range of possible earthquake-induced damage levels. Six typical RSC ratio curves and the corresponding ranges of the SCR index are discussed and verified for several spherical single-layer lattice shells. The impact of the arrangement of the components on the structural deformability, failure mode, plasticity and damage development as well as the RSC and SCR is clarified for two Schwedler shells. Based on observations from vulnerability and sensitivity analyses, several alternative strengthening schemes are generated for an S12 Schwedler single-layer lattice shell, and the optimal one is determined by means of a multiple-objective evaluation system that includes the SCR. The investigation indicates that normalized RSC ratio curves can reflect the design rationality of structural layouts and that they can clearly capture the dynamic instability of lattice shells. The ability of lattice shells to retain their post-earthquake resistance reserves is found to depend primarily on the locations where damage develops. The localization of fully yielded components has the potential to create several weak regions, leading to a high possibility of substantial loss of resistance reserves. The case study of the S12 Schwedler shell demonstrates the necessity of considering the SCR in a comprehensive structural performance evaluation system.

Enhancing network synchronizability by strengthening a single node.

In improving the stability of complex dynamical systems, an outstanding problem is how to achieve the desired performance at a low cost. For engineering and biological complex systems whose performance and functionality rely on the synchronous motion of their units, an important question related to the performance-cost-balance problem is how to improve efficiently the system synchronizability when a small amount of additional coupling resource is available. Here, employing a complex network of coupled chaotic oscillators as the model, we address this question by introducing a small amount of coupling intensity to only a single oscillator and investigate how the improvement of the network synchronizability is dependent on the location of the target oscillator. Theoretical analysis shows that, to achieve the maximum network synchronizability, the target oscillator to be strengthened should be chosen according to the eigenvector of the most unstable mode. Based on the theoretical finding, we further propose a single-node-based scheme for improving synchronization: the eigenvector-centrality-based strengthening scheme. We describe in detail how to apply this scheme under different synchronization scenarios and justify its efficiency in various network models by numerical simulations. The performance of the new scheme is compared with the conventional ones based on betweenness, closeness, and degree centralities, and it is shown that the new scheme has a clear advantage over the conventional ones. Furthermore, by a brute-force search of the target oscillator over the network, it is verified numerically that the oscillator identified by the new scheme indeed gives the best synchronization performance.

Experimental and Numerical Studies on Effectiveness of Hybrid FRP Strengthening on Behavior of RC Columns under High Eccentric Compression

This paper discusses the efficacy of different fiber-reinforced polymer (FRP) techniques on the behavior of RC columns under uniaxial high eccentric compression. Eight RC columns were strengthened using three FRP strengthening schemes: (1) near surface mounting (NSM), (2) external bonding (EB), and (3) hybrid strengthening, which uses a combination of NSM and EB. All the columns were tested under an eccentricity (e) to column depth (h) ratio of 0.63. The results obtained from the experiments were compared with the results of numerical analysis using the finite-element (FE) software developed at the University of Stuttgart. The hybrid FRP-strengthened specimens showed better performance compared with only NSM or EB in enhancing the peak strength and ductility by 51% and 277%, respectively, when compared with the control RC columns. Moreover, the brittle bond failure typically observed in NSM-strengthened columns was effectively prevented through FRP confinement in hybrid strengthening. The FE modeling approach developed in this work effectively captured the overall behavior of RC columns under eccentric compression. An extensive parametric analysis using the validated modeling approach was performed to quantify the effectiveness of the different strengthening techniques for a wider range of design parameters.

Strengthening and Retrofitting Precast Concrete Buildings to Mitigate Progressive Collapse Using Externally Bonded GFRP Strips

Compared to cast-in-situ RC buildings, precast concrete (PC) buildings may have lower redundancy and integrity depending on their construction methods and the types of beam-to-column connection. In this study, a series of multipanel PC beam-column-slab substructures were tested to evaluate the performance of a typical existing PC building with bolted connection, which is a typical dry connection in industrial buildings, subjected to the loss of a single penultimate column scenario. It was found that an existing PC substructure with bolted connections exhibited very low ductility and load-resisting capacity. The deficient connection between the PC slab and beams led to the specimen failing in a brittle manner and gaining fewer benefits from tensile membrane action. To overcome the poor performance of existing PC buildings with bolted connections, improvement in detailing and strengthening schemes relied on glass fiber–reinforced polymer (GFRP) strips were proposed. Based on the test results, the improvement could enhance the redundancy of PC substructures effectively. Moreover, the proposed strengthening scheme further enhances the flexural capacity, initial stiffness, and the postpeak behavior in large deformation stage significantly.

BEHAVIOUR OF VERTICAL CYLINDRICAL TANK WITH LOCAL WALL IMPERFECTIONS

Design of modern thin-walled metal structures is widely used around the world. In recent decades, more comprehensive research is carried out to investigate the behaviour of various thin-walled structures. Generally, the structure with regular geometry is investigated. In various countries such as USA, Russia, and the European Union issued the standards on regulation of the construction, design and maintenance of thin-walled structures. The actually used period of tanks usually is longer than recommendatory period. Recommendatory maintenance period of metal tanks is 15–20 years. Therefore, for such structures one of the most considerable questions is the residual load bearing capacity beyond the end of the maintenance period. This phase of using of structures is associated with complex investigation and numerical analysis of thin-walled structures. In this paper the load bearing capacity of the steel wall of the existing over-ground vertical cylindrical tank in volume of 5,000 m3 with a single defect and with a few contiguous local defects of the shape is analyzed. Calculations carried out are taking into account all the imperfections of the wall geometry. A major goal of the research – developing a realistic numerical model of the object analyzed, taking into account all the imperfections, determining the wall stress and strain state, exploring the places of extreme points, calculating the residual load bearing capacity of the tank and scrutinizing possible strengthening schemes for defective areas.

Analytical Studies on Debonding Behaviour of RC Beams Strengthened with UHPFRC Overlay

The debonding characteristics of ultra-high-performance fibre-reinforced concrete (UHPFRC)-strengthened structures under certain loading conditions have been investigated in this paper. Debonding of UHPFRC-strengthened structures seldom occurs owing to the fact that the tensile strength and the stiffness of UHPFRC are similar to that of the concrete substrate. This work concentrates mainly on the strengthening scheme where UHPFRC is provided in the form of strips at the tension zone of concrete. The studies on the effect of normal and shear stresses at the interface of the beam and UHPFRC strip with load increments are essential to determine the scheme of strengthening. The effect of interfacial shear and normal stresses near the strip end was studied by using the Tounsi model which is a modification of Teng model. A comparative study on the variation of the interfacial shear and normal stresses for the beams retrofitted with UHPFRC strip and other repair materials has been carried out. In the present study, the variation of stresses is investigated at two load levels with one load close to the pre-damage load given to the beam and the other load close to the ultimate load. Analytical studies are carried out to study the debonding behaviour of damaged RC beams strengthened with UHPFRC strips under static and fatigue loading. Parametric studies have also been carried out to determine the optimum configuration of the strips to be bonded to the beams.

Static performance analysis of single-layer steel cooling tower

For a hyperbolic steel-structure cooling tower, a single-layer reticulated shell system was designed. The static behaviors of the single-layer reticulated shell system were studied under different load combinations. By analyzing the stability of the system, a whole load-displacement curve of the structure was obtained, and a reasonable strengthening scheme was proposed. Taking the grid form and grid size as the optimization parameters, the single-layer reticulated shell system for a steel-structure cooling tower was optimized at heights of 90, 124, and 220 m. The static behaviors of the single-layer reticulated shell system with different parameters, including the quantity of steel, maximum displacement of the nodes, peak stress, and ultimate bearing capacity, were compared. The optimum grid form and grid size of the single-layer reticulated shell system under different heights were obtained.

Performance of externally strengthened RC beams with side-bonded CFRP sheets

The conventional method of strengthening reinforced concrete (RC) beams in flexure is via bonding carbon-fiber reinforcement polymer (CFRP) laminates to the beam’s soffit. However, the beam’s soffit could be narrow or inaccessible for strengthening. To overcome such obstacles, this paper explores the feasibility of strengthening RC beams in flexure by side-bonded CFRP composite sheets. Accordingly, a total of nine RC beams have been cast, eight of which were strengthened in flexure with different configurations of bottom-bonded and side-bonded CFRP sheets, and tested under four-point bending till failure. The load-deflection response curves, failure modes, and ductility of the tested specimens were recorded and compared. Overall, it is observed that specimens strengthened with similar amount of reinforcement are comparable with percent increase in the flexural strength over the control beam ranged from 62 to 92% for bottom-bonded and 39.7–93.4% for side-bonded strengthened beams. In addition, an analytical model based on ACI 440.2R-08 guidelines is developed and the predicted flexural strength was in good agreement with experimental results with differences ranging between 2.4% and 6.8%. It is concluded that the proposed side-bonded strengthening scheme is a valid alternative to the bottom-bonded scheme of strengthening of RC beams in flexure.

Structural efficiency of various strengthening schemes for cold-formed steel beams: Effect of global imperfections

Cold-formed steel (CFS) has a great potential to meet the global challenge of fast-track and durable construction. CFS members undergo large buckling instabilities due to their small wall thickness. CFS beams with corrugated webs have shown great resistance towards web buckling under flexure, when compared to the conventional I-sections. However, the magnitude of global imperfections significantly affects the performance of CFS members. This paper presents the first attempt made to experimentally study the effect of global imperfections on the structural efficiency of various strengthening schemes implemented in CFS beams with corrugated webs. Different strengthening schemes were adopted for two types of beams, one with large global imperfections and the other with small imperfections. Strength and stiffness characteristics of the beams were used to evaluate the structural efficiency of the various strengthening schemes adopted. Six tests were performed with simply supported end conditions, under four-point loading conditions. The load vs. mid-span displacement response, failure loads and modes of failure of the test specimens were investigated. The test results would compensate the lack of experimental data in this area of research and would help in developing numerical models for extensive studies for the development of necessary guidelines on the same. Strengthening schemes assisted in enhancing the member performance significantly, both in terms of strength and stiffness. Hence, providing an economic and time saving solution to such practical structural engineering problems.

Web crippling of stainless-steel tubes with bolt-anchored carbon-fibre-reinforced polymer

This paper presents a numerical investigation and design equation for web crippling of cold-formed stainless-steel tubes strengthened with bonded and bolt-anchored plates of carbon-fibre-reinforced polymer. The bolt-anchored polymer strengthening scheme was proved by previous experimental study to delay or totally prevent de-bonding failure and significantly improve web-crippling behaviour of stainless-steel tubes. A non-linear finite-element model was developed and verified against existing experimental results. An extensive parametric study was then carried out using different web slenderness, bearing-plate length and polymer-plate thickness to investigate the increased resistance of strengthened stainless-steel tubes to web crippling. Two loading conditions of end two-flange loading and interior two-flange loading were considered. The test data and numerical results were then compared with US and European web-crippling design rules for stainless-steel structures. A comparison was also made with a web-crippling design equation proposed by other researchers for polymer-strengthened stainless-steel tubes. Finally, a unified web-crippling equation is proposed for bolt-anchored polymer-strengthened cold-formed stainless-steel tubes under end and interior two-flange loading conditions.

Innovative strengthening schemes of concrete cantilever beams using CFRP sheets: End anchorage effect

The potential of improving the structural performance of concrete cantilever beams by extending the attachment of CFRP sheets to concrete columns through chamfers and fillets, constructed at beam-column connection, was investigated; while considering the impact of end anchorage using U-wraps of CFRP sheets or steel plates. Fourteen beam-column connections with 750-mm-long cantilever beams (150 × 150 mm) were load-tested after the attachment of CFRP sheets along the negative moment zone of the cantilever beams only or extending attachment of CFRP sheets’ to the columns surface through creating chamfers and fillets at the beam-column joints. CFRP sheets’ bond length and width, and number of layers, as well as the use of concrete chamfers and fillets had a significant impact upon mechanical performance of tested cantilever beams. End anchorage using U-wrap CFRP sheets was beneficial in delaying CFRP sheets debonding hence improving mechanical response; especially for cantilever beams, repaired by extending CFRP sheets to columns surface using large concrete chamfers.