Retrofitting Of Structures Research Articles

New Composite Material for Masonry Repair: Mortar Formulations and Experimental Studies.

The need for retrofitting existing masonry structures is progressively becoming more important due to their continuous deterioration or need to meet the current design requirements of Eurocodes. Textile-Reinforced Mortar (TRM) composite systems have emerged as a sustainable repair methodology suitable for structure retrofitting. Nevertheless, their mechanical performance is still far from being fully investigated. This paper presents an experimental study on the tensile and bond behaviors of a new mortar-based composite consisting of mineral additives, blended cement mortar, and stainless-steel grid. Three different mineral additives (silica fume, fly ash, and blast furnace slag), in binary and ternary systems were used. The experimental study included uniaxial tensile coupon testing on composite specimens and bond tests on composite material applied to clay-brick substrate. The results obtained with the different textile-reinforced cement-based mortars were compared and are discussed here. It was found that, for mortar formulations containing mineral additives—such as fly ash or blast-furnace slag—with high tensile and bond strengths, an adequate adherence between the constituents was obtained. The developed mortar presents mechanical performances equivalent to traditional mortars without additives. The study contributes to the existing knowledge regarding the structural behavior of TRM and promotes the development of a low impact carbon cementitious matrix.

Integration of CFRP strips as an internal shear reinforcement in reinforced concrete beams exposed to elevated temperature

In this study, an innovative application was implemented in which CFRP strips were integrated as internal shear reinforcement for reinforced concrete beams. The intent was to assess the CFRP strips' contribution to the shear strength and thus evaluate the effectiveness of using them as main or supplemental shear reinforcement before and after exposure to elevated temperature. Internal application of CFRP strips does not require an epoxy for attaching them, as they can be cut without difficulty to any width/length and then attaching them to the main steel bars. Similar to the steel shear stirrups, the CFRP strips work integrally with the beam concrete and steel reinforcement as the concrete confines them. The proposed technique applies only to new reinforced concrete (RC) elements and not as retrofitting existing members' strategy. Instead, externally bonded CFRP strips represent a popular strategy for structural retrofitting purposes. The investigated parameters include the area and number of layers of CFRP strips and different elevated temperatures. By demonstrating an outstanding structural performance with significant enhancement in the ultimate strength, ultimate deflection, stiffness, and toughness, this paper's findings strongly attest that CFRP strips can be effectively utilized as internal shear reinforcement in reinforced concrete beams exposed to elevated temperatures. A new guideline is proposed for predicting the damage level and shear strength of heated damaged RC beams internally reinforced with CFRP composites taking into consideration the influence of the number of CFRP strips and elevated temperature

An extended multiple-strut model to estimate infill effects on multi-storey steel frames with different connection rigidities

The experiences of previous earthquakes highlight the necessity of modelling infills in the analysis of structures. In the 2017 Sarepol-e-Zahab earthquake, infills were the cause of most damages inflicted on structures. In this paper, a previously proposed multiple-strut model for infills in steel frames with rigid connections is further developed for non-rigid connections. A comprehensive sensitivity analysis is carried out on the single-storey, single-bay frames with pinned and semi-rigid connections. The parametric study includes the evaluation of the aspect ratio, friction coefficient, and infill-to-column relative stiffness in the models. The global and local effects of infills on multi-bay infilled frames are also explored. This study also presents an investigation of the application of the proposed model in multi-storey infilled frames. Finally, a method consisting of a few simple steps is proposed to optimally take into account the effects of infill panels in multi-storey frames. The accuracy of the model in predicting the base shear and the internal forces of the components of the frame shows that it is a reliable model in the retrofit and seismic design of the steel structures.

Parametric study on buckling stability of CFRP-strengthened cylindrical shells subjected to uniform external pressure

Stability performance of cylindrical shells and providing a potential method to enhance the buckling stability of these structures are major concerns for designers. Carbon Fiber Reinforced Polymer (CFRP) composites are promising materials for application in the design and retrofit of such thin-walled structures. On this basis, this paper presents a comprehensive numerical study on the buckling behavior of CFRP-strengthened cylindrical shells under uniform external pressure and evaluates the effects of different parameters in the strengthening process. Numerous CFRP-strengthened cylindrical shells with distinct reinforcement layouts in three groups of models having different slenderness ratios are investigated through nonlinear stability analyses using the ABAQUS finite element package. The application of all considered reinforcement layouts for strengthening purposes is found to be effective in improving the buckling stability of cylindrical shells. It is demonstrated that the circumferential reinforcement of the middle region of the shell using CFRP strips with [0°] fiber angle (in the hoop direction), in particular, is the most effective approach for improving the buckling performance of the CFRP-strengthened cylindrical shells under uniform external pressure.

Behaviour of buckling-restrained brace equipped with steel and polyamide casing

Steel bracings are one of the most widely used systems for resisting transverse loads in the design and retrofit of concrete and steel structures. This paper reports on an experimental and numerical study of the effects of core steel and the distance of the core from the casing on the behaviour of buckling-restrained braces. Laboratory models of six braces with a core made from conventional ST37 steel or ST37-n steel (softened at a temperature of 1000° using the O'Neill method) with a core–casing distance of 0, 2 or 3 mm were designed. The experimental results showed that, compared with the brace made from ST37 steel, the use of ST37-n steel resulted in a reduction of 32% in the tensile and compressive strengths of the brace. An increase in the core–casing distance in the ST37 and ST37-n braces resulted in reductions in the brace bearing capacity and energy dissipation by 26% and 52%, respectively. An increase in the core–casing distance to more than 2 mm resulted in loss of symmetry and stability of the hysteresis behaviour of the brace; in addition, distortion due to brace local buckling under pressure also intensified.

Experimental study on steel hysteretic column dampers for seismic retrofit of structures

In this research, the seismic performance of a steel column damper is evaluated using cyclic loading tests of two one-story one-bay reinforced concrete (RC) frames before and after retrofit. The theoretical formulation and design procedure of the damper are explained first and then the details of the tests are described. The seismic performances of the test frames are evaluated in terms of hysteretic behavior, energy dissipation, crack pattern, failure mechanism, and damper behavior. The analytical model of the damper is established and verified using the experimental data. In order to further investigate the applicability of the developed damper for seismic retrofit, a case-study structure is chosen and retrofitted using the proposed damper. The seismic performance of the structure is evaluated and compared before and after retrofit in detail using pushover, nonlinear time-history, and fragility analyses. The results show that the presented damper can efficiently reduce inter-story drifts and damage of the structure. The details of modeling techniques and simulations given in this study can provide guidelines and insight into nonlinear analysis and retrofit of RC structures.

Investigation and numerical analysis of steel beams strengthened with CFRP

One of the most common methods of strengthening concrete structures is the use of composite fibers such as FRP. These fibers have found a special place in structural retrofitting methods due to a number of specific ability including lightness and ease of performance. Reinforced concrete structures have long been considered by design engineers for the possibility of retrofitting high strength composite steel structures. In this study, twenty-two steel beam models were modeled by ANSYS software. Variable parameters studied in the modeled steel beams can be referred to the number, orientation and thickness of composite layers.

Experimental free vibrations test of steel braced frames reinforced with NiTi shape memory alloy wires

Shape memory alloys (SMA) have just recently made their way into structural design, and through studies have proved to be quite effective in increasing a structures strength and performance. Earthquakes are one of the most chaotic natural disasters that occur around the world, significantly impacting infrastructure to the point where cities lose their ability to function as bridges and hospitals are destroyed. The main objective of this paper is to investigate the natural behaviour of steel braced frames reinforced with pseudoelastic (PE) NiTi SMA wires. A free vibrations test was conducted to determine the natural frequency and damping ratio of two different systems. The tests of the steel and PE SMA frames showed that the stiffness of the brace does not affect the natural frequency of the system but rather, the connection of the system to the rigid floor controls the natural behaviour. Findings of this research are expected to add valuable knowledge to the field of seismic retrofitting of structures and widen the potential applications of the SMA in the structural engineering field.

Structural and thermal analysis of an innovative baking furnace for carbon anode production

The urgent demands for the high efficiency and low consumption of carbon anode baking process stimulate the structure optimization of the baking furnace. In this paper, a closed-top innovative anode baking furnace (CTABF) with periodically arranged burners and anode crucibles is proposed, and the thermal process in the CTABF is studied via a coupled multi-physics model and the in situ measurement. Results show that the multi-channel design changes the one-way high-temperature flue gas path and increases its residence time in the furnace, while the temperature distributions in the inner and outer crucibles show good uniformity. The average convective heat flux accounts for about 68% of the total heat flux around the crucible surface, and thus the baking rate of anodes can be controlled by adjusting the power of driven fan. Therefore, due to the above structural retrofits, the baking time and energy consumption are reduced and the baking quality is improved, showing the superiority of the proposed CTABF over the traditional ABF.

Design Considerations for Retrofitting of Historic Masonry Structures with Externally Bonded FRP Systems

ABSTRACT Historic masonry buildings often require structural repair or retrofit to attain a satisfactory level of seismic resistance. This paper analyses the effect of externally bonded fibre-reinforced polymer (FRP) retrofitting systems on the global seismic response of such buildings in order to achieve the fundamental code-based requirements that refer to the structure’s state of damage for a specified limit state. As a case study, a three-storey historic unreinforced masonry (URM) building was selected. The first part of the paper presents an FRP design procedure for determining which vertical structural elements should be retrofitted and to what extent. As a result, six different FRP retrofitting layouts have been selected and analysed with the aid of non-linear static analyses in the second part of the paper. The paper concludes with a cost-feasibility study by introducing a cost-effectiveness index for FRP reinforcement that identifies the effectiveness of an FRP layout by accounting for the amount of retrofitting material used and comparing it with its effectiveness in improving the global seismic resistance.

Progressive collapse capacity of a gravity-load designed RC building partially collapsed during structural retrofitting

The problem of progressive collapse has been a major topic in structural forensic engineering since its inception, but it has recently received a growing interest. In the last two decades, the occurrence of extreme events and their catastrophic impact on the built environment and people has promoted several research programmes and guidelines for structural design and assessment, spotlighting the importance of back-analysis and numerical prediction of progressive collapse phenomena. This paper presents a numerical study aimed at investigating the ability of existing reinforced concrete structures to prevent progressive collapse during structural retrofitting. The progressive collapse capacity of a real, reinforced concrete framed building constructed in the 1950s and partially collapsed in 2001 is discussed in relation to a couple of structural retrofitting operations that were found to be the primary causes of the accident. Existing information from past forensic investigations was integrated with a simulated design procedure to develop the capacity model of the structure, according to design codes and practice rules used at the time of construction. Nonlinear pushdown analysis with displacement control was carried out both in intact conditions and during retrofitting operations. In addition to global capacity models of the structure, the use of partial capacity models representing the building corner under retrofitting was evaluated, highlighting a very high computational efficiency of those simplified models. Analysis results also indicate that the load contribution from infill walls can significantly influence the progressive collapse resistance, particularly in relation to retrofitting works involving the corner of the building plan as actually happened. Retrofitting interventions on the real building included the concrete cover removal from ten ground-floor columns and soil excavation around column bases. The removal of concrete cover from single and multiple ground-floor columns together with the loss of supports due to soil excavation around column bases was found to significantly affect the collapse capacity of the structure. The major impact of retrofitting operations in terms of location and extent provides a simulation-based proof of the progressive collapse suffered by the case-study building, evidencing that soil excavation around more than three columns was able to produce the accident.

Self prestressing concrete composite with shape memory alloy

The infrastructure sector along with developing new ideology for constructing new facilities have equal importance in maintain the existing structure in making them serve the intended use for which it has been designed over the design period. In this context retrofitting of structures plays a major role in reviving the strength of the structural members. There are various methods used in the retrofitting process. But mainly they are passive strengthening techniques. Active technique like prestressing is costly. The use of Shape memory Alloy (SMA) has proved to be a boon to the civil engineers in improving the flexural strength of the existing beams. This paper discusses the test results of using Fe alloy as SMA for retrofitting beam. It has been noted that the FE alloy SMA when thermo mechanically treated improves the load carrying capacity by 60% and a self prestressing force is induced due the activation of the Fe alloy SMA. The mid span deflection and ultimate load carrying capacity has proved invariably by the use of this SMA.

Characterization study on recycled coarse aggregate for its utilization in concrete - A review

Industrialization and urbanization are two major factors that contribute to the scarcity in construction materials furthermore leading to dependency on alternative materials in construction. Consequently, rehabilitation and retrofitting of structures leads to the generation of Construction and Demolition(C&D) waste which is to be effectively reutilized/disposed. Such C&D wastes can be reutilized as coarse aggregates in concrete termed as recycled aggregate concrete to achieve sustainability in construction. Presence of adhered mortar on the surface of recycled aggregates tends to reduce the quality of recycled aggregate concrete. This paper reviews the statistical data on the generation of C&D wastes around the globe and its percentage of reutilization with a comparison on physical characteristics of C&D wastes with natural coarse aggregate in concrete. Comparative studies on the physical characteristics of recycled aggregates performed by various researchers embark with conclusive remarks to specify its limitation on utilization in structural applications due to the presence of adhered mortar on its surface.

Experimental evaluation of out-of-plane strength of masonry walls retrofitted with oriented strand board

Structural retrofitting is carried out as an economical alternative to demolishing and rebuilding existing masonry structures. Retrofitting is provided to offer structural upgrade and damage-control. This paper presents the application of oriented strand board type 3 (OSB/3) for retrofitting unreinforced masonry walls. Out-of-plane bending test in form of four-point loading was performed on six masonry walls. The test results show that OSB/3 can considerably increase the load and flexure capacity by (1.4 & 1.8), limiting-toughness by (1.6 & 2.4) and overall-toughness by (16 & 10) times compared to plain wall subjected to out-of-plane loading for (single & double-sided) application respectively.

Local Site Effects in Definition of Seismic Design Parameters for Historical Monuments

Presented in this paper are investigations related to local site conditions carried out for seismic assessment of two historical structures in Macedonia: the Church of the Holy Mother of God Peribleptos dating back to the 13th Century and located in the old town of Ohrid, and the Mustafa Pasha Mosque in Skopje dating back to the 15th Century. The objective of the described procedure has been to take into consideration the regional and microlocation geological and seismological parameters in order to define the input seismic parameters for dynamic analysis of the historical structures. The obtained results have pointed out the significance of involving local site conditions in seismic assessment and retrofitting of historical structures in general. The procedure has been applied to many buildings of historical importance requiring strengthening and rehabilitation with great success and efficiency.

VULNERABILITY ASSESSMENT OF A CIVIC TOWER USING AMBIENT VIBRATION TESTS

This paper focuses on the vulnerability assessment of a civic tower built in 1512, which is now considered a national monument. It is the original bell tower of S. Ambrogio church that was destroyed in 1809. Experimental investigations have been carried out on this historical tower. First, detailed investigations have been carried out to identify the geometry of the tower as well as the mechanical features of the constituting materials. Then, ambient vibration tests have been applied using five Micro Electro-Mechanical Systems (MEMS) sensors to detect of the main dynamic features, e.g., modal parameters and damping. Two output-only identification methods, including Frequency Domain Decomposition and Random Decrement Techniques, have been used. The outcomes of the modal identification have been employed to inform the FE model. The numerical analysis can be used for vulnerability assessment, providing a valuable picture of possible damage evolution, tower collapse mechanism, and subsequently, useful hints for the execution of structural retrofitting strategies.

Structural shear retrofitting of reinforced concrete beam: multilayer ferrocement technique

Most of the existing buildings in developing countries lack proper reinforcement and are in danger of collapse under seismic and extreme events. Therefore, a sustainable and feasible retrofitting technique is necessary to ensure the safety of the infrastructure and its occupants. This paper describes a broad experimental programme, in which a total of 20 reinforced concrete beams with shear deficiency were constructed, 4 of them considered as control beam, while the rest of the beams were retrofitted with ferrocement in four different techniques after loading up to initial crack. All the specimens were tested up to the failure, and beam deflection at different points along with the load versus displacement response was recorded for each case. It allowed the determination of elastic stiffness, peak strength and associated displacement capacity of the retrofitted beam, compared with control beams. From the observed results, a plausible technique of retrofitting using ferrocement is suggested. A preliminary finite element model is developed for future study, which compared the experimental and FE results.

Simplified seismic loss assessment for optimal structural retrofit of RC buildings

This study deals with selecting optimal seismic retrofit solutions for reinforced concrete (RC) buildings. To this aim, multi-criteria decision-making (MCDM) is implemented explicitly considering earthquake-induced economic loss as a decision criterion. Fragility (i.e. likelihood of damage levels vs intensity measure ( IM) levels) and vulnerability (i.e. likelihood of loss levels vs IM levels) relationships are derived by using three increasingly refined analysis methods: Simple Lateral Mechanism Analysis; numerical pushover; time-history analysis. A seismically deficient RC school index building, with construction details typical of developing countries, is used for illustrative purposes. Concrete jacketing, addition of concrete walls, and addition of steel braces are the considered retrofit alternatives. Intensity-based expected loss and expected annual loss are adopted in the MCDM, among other criteria, independently derived with the three analysis methods. It is shown that, given the adopted loss-analysis methodology, the ranking of the retrofit alternatives is insensitive to both analysis methods and loss metrics, even when the weight for the seismic loss criterion is high. These findings suggest that simplified methods can be effectively employed in the conceptual/preliminary design of retrofit alternatives.

Seismic Retrofitting of RC Structures with Exterior Shear Walls and Bracing

Seismic Retrofitting of RC Structures with Exterior Shear Walls and Bracing

Size Effect on FRP External Reinforcement and Retrofit of Concrete Structures

The size effect on the strength of reinforced concrete (RC) beams flexurally strengthened by surface bonded sheets of fiber-reinforced polymer (FRP) is studied. As the failure is neither ductile nor brittle, but quasibrittle, occurring after stable growth of large cracks with large damage zones, the transitional size effect governed by the energetic size effect law (SEL) must be expected, which the present analysis confirms. Owing to scarcity of size effect experimental data, finite element (FE) analysis is used. A powerful microplane damage constitutive model M7 for concrete, coupled with the crack band model to suppress spurious mesh sensitivity, is calibrated by fitting the existing test data to FRP-strengthened RC specimens of diverse types, and consequently is trusted to simulate the size effect. The analysis captures the debonding near the concrete–FRP interface starting at midspan or at the end of the FRP sheet. It also captures the debonding between adjacent concrete shear cracks, and the delamination of concrete cover. The causes of these different delamination modes are explained. Simple beam-type formulas, in which the size characteristic is not the beam depth but the length of the shear span, are proposed and validated by FE results. The use of single-lap shear tests to determine the bond strength is critically discussed. Finally, a good fit of a few existing limited test data on the size effect of FRP-strengthened specimens is demonstrated.