Seismic Retrofitting Techniques Research Articles

Seismic retrofit of beam–column joints using prestressing wire

An effective external prestressing technique is presented for the seismic retrofit of reinforced concrete beam–column joints with prestressed high-strength steel wires. This method is to improve the seismic performance and enhance the shear strength of beam–column joints with non-seismic design as well as to overcome the shortcomings of traditional seismic retrofitting techniques for the joints. Six large-scale interior beam–column joints were tested under reversed cyclic loading, where five were strengthened by the proposed technique and one was un-retrofitted as a control specimen. Particular emphasis was placed on the effects of the ratio of prestressed high-strength steel wires, prestressing level of wires and polymer grout layer. Test results showed that the joint shear capacity was increased significantly and cracks in beam–column joints were controlled effectively after retrofitting. All retrofitted joint specimens failed in a ductile mode of beam flexural failure, whereas the control specimen failed in a brittle mode of joint shear failure. In addition, the energy dissipation capacity as well as ductility of the retrofitted joint specimens was enhanced significantly compared with the un-retrofitted specimen.

11.32: Linked columns with friction dampers as a technique for seismic retrofit of steel moment resisting frames

ABSTRACTThe existing steel moment‐resisting frames (MRFs) designed to seismic codes of older generation typically have limited local ductility capacity and may not perform to the desired “strong‐column‐weak‐beam” concept due to the lack of capacity design provisions. All these deficiencies require modifications for reducing their seismic vulnerability.The paper proposes a seismic retrofit technique for steel moment frames based on the linked column concept. The linked columns are dual piers connected throughout their height with short horizontal replaceable ductile link elements. Developed in the last decade for new seismic‐resistant steel structures, the linked column frame structural system combines two sub‐systems – a primary lateral force resisting system composed of linked columns and secondary moment resisting frames mainly involved in resisting gravity loads.During a seismic event, the relative deformations of the closely spaced piers activate the links which are designed to yield in shear or bending for seismic energy dissipation, but they also help in controlling interstorey drifts and limiting the forces transferred to the surrounding structural members.The authors present a new innovative design of the linked columns. Instead of short steel beams, rotational friction dampers (RFDs) are used as seismic links in order to avoid the damage expected in conventional link elements. The RFDs are supplied by a Danish company and have been used for seismic protection in many projects for the last decade. They have proven energy‐dissipation capacity and can be delivered in different shapes and slip capacities at relatively low cost.In this study three‐, six‐ and nine‐storey four‐bay steel MRFs have been analyzed using the capacity spectrum method. The added linked columns are arranged symmetrically on both sides of the first interior columns of the MRFs.The results obtained for three different arrangements of the seismic links are presented and summarized. They indicate that the seismic retrofit using linked columns with rotational friction dampers is a promising technique for seismic upgrade of existing steel moment frames.

Smart hybrid isolation of a case study highway bridge exploiting seismic early warning information

Abstract An innovative technique for seismic retrofit of a case study highway bridge is presented herein. It is done by means of the friction pendulum isolators (FPS) combined with smart magnetorheological dampers (MR), the latter calibrated through the use of a Seismic Early Warning System (SEWS). Hybrid systems, widely investigated by researchers in last years, generally represent a suitable solution in case of base-isolated bridges. As a matter of facts, the introduction of an isolation system allows to reduce shear and bending moment in piers, but may increase decks’ relative displacements. A possible effective strategy for limiting this undesired effect may be the adoption of a dissipative system. Herein the passive smart use of MR devices is proposed. The MR dampers are smart in the sense that their mechanical properties can be tuned almost in real time, once before the earthquake strikes the site. A properly designed control algorithm modifies dampers' characteristics according to the intensity of the seismic event is going to occur, as forecasted by the SEWS. This retrofit strategy makes the structure someway adaptive, leading to a significant enhancement of the seismic performance against earthquakes even very different one each other. The case study is an existing reinforced concrete highway bridge built in the 70’s, located in the city of Naples, southern Italy.

Cyclic Stress–Strain Behavior of Concrete Confined with NiTiNb-Shape Memory Alloy Spirals

AbstractRecent studies showed that concrete confinement using shape memory alloy (SMA) spirals is a promising technique for seismic retrofitting of reinforced concrete columns that lack flexural du...

Axial Compression Capacity of Shear-Damaged Reinforced Concrete Columns and Lashing Belt Prestressing

The compressive strength and ductility of concrete can be considerably improved by lateral confinement. In this study, an emergency seismic retrofit technique using lashing belt prestressing is used as to manually retrofit damaged reinforced concrete (RC) columns. The initial prestressing is an important aspect of this technique and is introduced by the ratchet buckle. Thus, this technique offers active and passive confinement as well as shear strengthening. Furthermore, diagonal cracks in the damaged RC columns can be closed by using the active confinement of lashing belts, and the lateral and vertical load-carrying capacity and ductility of the damaged RC columns are recovered. In this study, the recovered axial compression capacity of the retrofitted RC columns and repaired RC columns using epoxy resin was investigated. Finally, the hysteretic behavior of the shear-damaged RC columns after the proposed emergency retrofit was investigated.

Ecosmart Reinforcement for a Masonry Polycentric Pavilion Vault

In the cultural life of modern societies great importance has acquired the preservation of existing and, in particular, ancient architectural heritage. With the inherent historical aspects, the economic implications have to be taken into account as well. Indeed, especially European cities and countries receive significant economic advantages by the existence of monuments and ancient suburbs. In this context, structural maintenance, strengthening and monitoring has gained an important academic and professional impulse. The present paper aims to present the results of a real scale experimental work regarding the application of an innovative seismic retrofitting technique for masonry walls and vaults by Hydraulic Lime Mortar strengthened by Glass Fiber Reinforced Polymer textile grids (HLM-GFRP) embedding new sensing systems as fiber optical sensors. The real scale specimen is a masonry polycentric pavilion vault that was damaged during the L’Aquila earthquake of April 2009. The need of eco compatibility of bonding material with masonry support implies the use of HLM-GFRP as strengthening system. On the other hand, the use of Fiber Bragg Grating (FBG) has a large number of advantages in opposite to electrical measuring methods. Example are: small sensor dimensions, low weight as well as high static and dynamic resolution of measured values, distributed sensing feature allowing to detect anomalies in load transfer between reinforcement and substrate and the location of eventual cracking patterns. A suitable Finite Element (FE) model is developed both to assess the effectiveness of the HLM-GFRP strengthening layers in retrofitting of the masonry vault and to define the strain field essential to the design of the FBG sensors network.

Seismic reliability of base-isolated structures with friction pendulum bearings

The friction pendulum system (FPS) is becoming a widely used technique for seismic protection and retrofit of buildings, bridges and industrial structures due to its remarkable features such as the stability of physical properties and durability respect to the elastomeric bearings. Experimental data also showed that the coefficient of friction depends on several effects (i.e., sliding velocity, apparent pressure, air temperature, cycling effect) so that it can be assumed as a random variable. The aim of the study consists in evaluating the seismic reliability of a base-isolated structure with FP isolators considering both isolator properties (i.e., coefficient of friction) and earthquake main characteristics as random variables. Assuming appropriate density probability functions for each random variable and adopting the Latin Hypercube Sampling (LHS) method for random sampling, the input data set has been defined. Several 3D non-linear dynamic analyses have been performed considering both the vertical and horizontal components of each seismic excitation in order to evaluate the system response. In particular, monovariate and multivariate (joint) probability density and cumulative distribution functions have been computed and, considering the limit state thresholds and domains (performance objectives) defined respectively on mono/bi-directional displacements, assumed as earthquake damage parameter (EDP) according to performance-based seismic design, the exceeding probabilities (structural performances) have been evaluated. Estimating the reliability of the superstructure, substructure and isolation level led to define and propose reliability-based abacus and equations useful to design the FP system.

An Experimental Study on Hybrid Noncompression CF Bracing and GF Sheet Wrapping Reinforcement Method to Restore Damaged RC Structures

We describe a novel technique for restoration of reinforced concrete (RC) structures that have sustained damage during an earthquake. The reinforcement scheme described here is a hybrid seismic retrofitting technique that combines noncompression X-bracing using CF with externally bonded GF sheets to strengthen RC structures that have sustained damage following an earthquake. The GF sheet is used to improve the ductility of columns, and the noncompression CF X-bracing system, which consists of CF bracing and anchors to replace the conventional steel bracing and bolt connections, is used to increase the lateral strength of the framing system. We report seismic restoration capacity, which enables reuse of the damaged RC frames via the hybrid CF X-bracing and GF sheet wrapping system. Cyclic loading tests were carried out to investigate hysteresis of the lateral load-drift relations, as well as the ductility. The GF sheet significantly improved the ductility of columns, resulting in a change in failure mode. The strengthening effect of conventional CF sheets used in columns is not sufficient with respect to lateral strength and stiffness. However, this study results in a significant increase in the strength of the structure due to the use of CF X-bracing and inhibited buckling failure of the bracing. This result can be exploited to develop guidelines for the application of the reinforcement system to restore damaged RC structures.

Seismic Retrofitting Techniques for Historic Adobe Buildings

This article presents a comprehensive overview of the techniques available for the seismic retrofitting of adobe buildings. It analyzes these techniques’ viability from a structural engineering standpoint and evaluates their suitability in the context of historic preservation. Analysis was based on an exhaustive review of studies addressing retrofitting techniques of earthen structures, as well as consultation with field experts. The guiding principles of conservation relating to the structural reinforcement of monuments were taken into account. Traditional techniques that enhance the stability of the building, such as wooden ring beams, wooden ties interconnecting parallel walls, corner keys, and the addition of buttresses were found to be effective solutions since they employ compatible and low-cost materials such as earth and wood. When these techniques prove insufficient, minimally invasive measures such as introducing a plywood diaphragm, horizontal steel rods, a geomesh covered with mud rendering or a strapping system can be appropriate.

Experimental and Numerical Study of Enhancing the Seismic Behavior of Rammed Earth Buildings

Rammed earth structures are widely used as farmers dwellings in the southwest of China, however, they are extremely vulnerable to earthquake loadings. An economic, environmental-friendly and less-intervention seismic retrofitting technique is required to reinforce these dwellings so as to increase regional seismic capability. A preliminary laboratory testing was conducted to investigate the enhancement of the seismic behavior of the rammed earth wall with externally bonded fibers. Different retrofitting materials and adhesives were tested to characterize their mechanical properties and bonding performance when externally glued on rammed earth blocks. The most suitable and practical retrofitting material and adhesive were chosen based on the experimental results. Furthermore, a numerical analysis was performed to investigate the improvement of the shear capacity of the rammed earth wall with the proposed technique. It was verified that the proposed retrofitting technique is a promising option for seismic retrofitting of rammed earth walls.

Out-of-plane strengthening of unreinforced masonry walls using near surface mounted fibre reinforced polymer strips

The development of cost effective and minimally-invasive seismic retrofit techniques is required for clay brick unreinforced masonry (URM) buildings because of their poor performance in past earthquakes. A laboratory-based experimental program was implemented to address this need, having well-defined but artificial boundary conditions, and utilising constituent construction materials that replicated the characteristics of masonry found in historic URM buildings. The purpose of this experimental program was to investigate the performance of near-surface mounted (NSM) carbon fibre reinforced polymer (CFRP) strips as a seismic retrofit solution for improving the out-of-plane performance of walls in URM buildings. In the experimental testing program nine masonry beams and five full-scale masonry walls were tested. Details of the design methodology for flexural retrofit of URM walls using the NSM CFRP retrofit technique and laboratory test results are reported. The results of the experimental tests confirmed that the NSM CFRP retrofit technique is a minimally-invasive option for seismic strengthening of URM walls to resist out-of-plane earthquake induced lateral forces. Two recent case-study projects that implemented the NSM CFRP technique are also briefly presented.

Investigation of in-plane seismic retrofit of unreinforced masonry walls by means of vertical steel ties

In this paper, a technique for seismic retrofit of unreinforced masonry walls is studied which comprises addition of two vertical steel ties on both edges of the walls. In total, four specimens have been tested under cyclic lateral load in combination with vertical constant load: two specimens with ties and two without ties. The tests show that vertical ties cause significant increase in seismic capacity in terms of both strength and ductility not only at yield but also at maximum. The ties also cause the failure modes to transform from either shear slip or diagonal tension into a combination of diagonal tension and toe-crushing.

Cyclic Response of Nonductile Reinforced Concrete Frames with Unreinforced Masonry Infills Retrofitted with Engineered Cementitious Composites

AbstractA new seismic retrofit technique for unreinforced masonry infills in nonductile reinforced concrete frames is presented. The objective was to develop a light and easy to apply retrofit technique for the infills that both delays shear failure at the columns of the nonductile bounding frame when subjected to an earthquake by preventing severe deterioration of the infill wall, and enhances the ductility of the system. The retrofit technique uses sprayable engineered cementitious composites (ECCs), a micromechanically designed cement-based material reinforced with short, randomly distributed polymeric fibers. Four small-scale unreinforced masonry infilled nonductile reinforced concrete frames were tested under in-plane quasi-static cyclic lateral loading. One unretrofitted wall and three different retrofit schemes were evaluated. Continuous steel reinforcement was embedded in the ECC layer and a bonding agent was applied between the masonry wall and the ECC layer. In two retrofits the impact of two di...

Modelling of strengthened hollow brick infills

Strengthening the existing hollow brick infill walls by bonding high-strength precast concrete panels or applying steel fibre reinforced mortar was found to be an occupant-friendly seismic retrofitting technique for the buildings in use with deficient reinforced concrete structural systems. Both techniques convert the existing non-structural hollow brick infill walls into load-carrying structural members. To verify the effectiveness of the techniques, 20 reinforced concrete frames with hollow brick infill walls were tested under reversed cyclic lateral loading simulating earthquake. In the present study, hollow brick infill walls strengthened by these techniques were modelled by means of two equivalent diagonal struts in the analytical studies. Analytical results matched well with the experimental results. A performance-based rehabilitation case study of an existing building showed that both techniques offer a sound and practical solution for rehabilitation studies.

鉄筋コンクリート造そで壁付き柱の簡略的骨格曲線の評価

This paper proposes a seismic retrofitting technique for existing RC columns with cast-in-place RC sidewall using polymer-cement mortar (PCM). In the technique, additional shear reinforcement (deformed steel bar: prefabricated reinforcing unit) is adhered to the surface of existing RC columns with RC sidewall by PCM. Shear-loading tests were conducted to clarify the structural performance of the columns with sidewall strengthened by polymer-cement mortar. The results suggested that the capacity and ductility of the strengthened RC column with RC sidewall could be controlled by choosing the strengthened area and the amount of shear reinforcement. Furthermore, the shear force Q &mdash drift angle R curve of RC columns with RC sidewalls retrofitted by RCM was proposed by considering the shear resistance mechanism.

Numerical Investigations of Different Seismic Retrofit Techniques for Flat-Slab Structures

The construction of reinforced concrete (RC) buildings with flat-slab systems has become widely used in some high seismicity European countries. This type of structure is particularly common both for office and residential buildings, taking advantage of the reduced floor height to meet economical and architectural demands. Even though some national codes may include rules for the design of these structures, the provisions of Eurocode 8 don’t cover flat-slab frames used as part of the lateral load resisting system. Main results of seismic analyses performed on a six-story RC flat-slab structure are reported in this study. The structural response was predicted using both nonlinear static and dynamic analyses with artificial ground motion records with increasing intensity levels. Different seismic retrofit techniques were applied to enhance the seismic performance of the structure including: 1) insertion of shear wall; 2) confinement of column plastic hinge regions using FRP wrapping; 3) addition of RC column jackets. The predicted seismic performance of the retrofitted models was compared to that of the unretrofitted counterpart. Numerical investigations provide information about the seismic performance of a common type of RC structure not covered by the provisions of Eurocode 8 as well as the potential to mitigate the damage for varying earthquake intensity levels through different retrofit techniques.

Seismic Rehabilitation of a Three-Storey R/C Flat-Slab Prototype Structure Using Different Techniques

This paper presents some results of a numerical study on the seismic performance and retrofitting of a R/C flat-slab prototype structure. The study was based on an experimental program carried out at the JRC ELSA Laboratory with the aim of assessing the seismic behaviour of flat-slab structures. Numerical models of the test structure were developed and nonlinear static and dynamic analyses were carried out. Two seismic retrofit techniques were numerically simulated to enhance the seismic performance of the structure. First the structure was rehabilitated by concrete jacketing of columns with added longitudinal and transverse reinforcement. The second intervention was based on confinement of column ends by using FRP wrapping. The results of this study provide information about the seismic performance of a common type of R/C structure not covered by the provisions of Eurocode 8, as well as the potential to mitigate the expected damage for varying earthquake intensity through retrofit. The comparison of the overall seismic response of the bare and rehabilitated structures showed the seismic performance improvements provided by the retrofitting interventions.

도시철도 개착터널의 확률적 내진성능평가

최근 한반도 주변국들이 강진으로 인해 큰 피해를 입었고, 국내에서도 중진이 몇 차례 발생하여 우리나라에서도 지진재해에 대한 관심이 높아졌다. 특히 지진재해대책법의 제정으로 사회기반시설물의 내진대책이 의무화되어 내진성능평가 및 내진보강기술이 절실히 필요하게 되었다. 지중구조물에 대한 내진성능평가는 지반 동특성의 영향을 받게 되는데, 만일 지반의 동특성이 불확실하면 내진성능평가의 결과도 불확실하게 된다. 본 연구에서는 불확실한 지반의 동특성이 지중구조물의 내진성능평가에 미치는 영향을 분석하기 위해 FOSM방법을 이용하는 방법을 제시하였다. 불확실한 지반 동특성을 확률변수로 가정하고 FOSM방법을 통해 불확실성이 전파되어 구조물의 내진성능평가 결과가 확률변수로 계산되도록 하였다. 이 방법을 서울시 도시철도 개착터널의 일부구간에 적용하고, N값의 불확실성을 고려한 결과 설계하중에 대한 구조물의 내진성능 만족여부를 확률로 계산할 수 있었다. In addition to recent strong earthquakes in neighboring countries, mid-size earthquakes in Korea have brought up attention. Since the Earthquake Disaster Measures Act has been established, the needs for researches on the seismic performance evaluation and seismic retrofit techniques have increased in Korea. Dynamic soil properties affect the seismic performance of underground structures. Therefore, uncertain soil properties will cause an uncertain seismic performance of underground structures. The objective of this study is to investigate the effect of uncertain soil properties to the seismic performance evaluation of underground structures using the First-Order Second Moment method. Uncertain soil properties are considered as random variables and uncertainty is propagated to the decision variable of the performance evaluation process. The proposed methodology is applied to an open-cut tunnel of a part of Seoul Metropolitan Subway system where the N-value is considered as a random variable. The example shows that the proposed methodology is simple, and yet practical.

1980년도 표준설계도에 준하여 건설된 학교건축물의 내진보강기법 연구

내진보강기법에 따른 성능평가 수치해석 결과 철근콘크리트 전단벽이 가장 우수한 것으로 평가되었다. 또한 X형 철골가새, K형 철골가새 및 knee형 철골가새의 순으로 내진성능이 우수한 것으로 나타났다. 학교건축물의 평면 기능성을 유지시키면서 내진보강 구조물이 설치가능한 곳에 보강을 하더라도 기존의 보 및 기둥골조에 부재별 별도의 보강이 필요한 보강기법은 4층 학교건출물에서는 K형과 knee형 철골가새로, 3층 학교건출물에서는 knee형 철골가새로 나타났다. 내진보강기법에 대한 성능평가 및 장단점에 대하여 종합적으로 분석해 보면 전단벽 방식이 효율성 측면에서 가장 우수한 것으로 나타났다. 설치 후 외관의 형태에 변화를 주거나 개구부의 크기 및 위치에 제약조건이 수반되는 철골가새 보강방식과는 달리 기존 개구부의 크기와 위치를 그대로 유지시키면서 철근콘크리트 전단벽 구조로 보강함으로써 외관에 영향을 주지 않는다. 또한 기존 벽체의 중량에 변화를 주지 않도록 두께를 선정함으로써 기초보강의 필요성이 배제되었다. We considered the practical aspects to the seismic performance evaluation method and adopted a numerical approach using a universal program, MIDAS-GENw applied the KBC-structural design standards 2009. The four types such as reinforced concrete shear walls and X-type, K-type, and knee-type steel braces were chosen to the seismic retrofit techniques. The results were summarized as follows: First, the results of a numerical approach to the performance evaluation according to the seismic retrofit techniques, the excellence of the seismic performance was in the order of reinforced concrete shear walls and X-type, K-type, and knee-type steel braces. Second, when installing the shear walls in 24 places among 65 places that can enhance in the direction of X and in 11 places among 49 places that can enhance in the direction of Y, in the case of the four-story school building the structural reinforcement by the action of the earthquake loads in the existing beams and the columns framing may not be require.

Seismic Retrofit of RC Beam-Column Joints Using the MF-EBR Strengthening Technique

The strengthening technique based on the application of multi-directional laminates of CFRP (MDL-CFRP) simultaneously glued and anchored to the surface of the elements to be strengthened has been recently proposed. This technique was designated Mechanically Fastened and Externally Bonded Reinforcement (MF-EBR) and combines the fasteners from the MF-FRP technique with the externally glued properties from the EBR. With the aim of assessing the potentialities of this technique for seismic retrofitting, three interior RC beam-column joints were strengthened according to the MF-EBR technique and tested. This work presents the entire test program executed, including test configuration, results and corresponding analysis.