Design Methodology for Seismic Upgrading of Substandard Reinforced Concrete Structures

This article presents a design methodology for seismic upgrading of existing reinforced concrete (RC) buildings. The methodology is based on the modification of the deflected shape of the structure so as to achieve a near-uniform distribution of interstorey drift along the building height, thereby eliminating damage localization. Yield Point Spectra are utilized for the definition of demand and a direct displacement-based design approach is implemented. The fundamental steps of the method are described in detail, including a systematic evaluation of assumptions and limitations. A full-scale tested structure is used as a case study for assessment and verification of the proposed methodology. Alternative retrofit scenarios are set according to target response and performance levels. The role of the target deflected response shape and its influence on the outcome of the retrofit strategy is investigated. The viability of the alternative retrofit scenarios is studied for different ground motions including near-fault earthquake records.     

Cost-Benefit Analysis of Alternative Retrofit Strategies for RC Frame Buildings

Monetary losses induced by earthquakes in Reinforced Concrete (RC) buildings are mainly due to damage to non-structural elements (infills, partitions, finishes, etc.). In this study, alternative retrofit strategies for reducing monetary losses in RC frame buildings are examined. They include local strengthening of infills (and partitions) and seismic isolation. The Expected Annual Loss (EAL) of a number of RC frame buildings, pre- and post-rehabilitation, is evaluated, following the time-based assessment approach proposed in the FEMA P-58 guidelines. The breakeven time of each retrofit intervention is then computed, considering the initial cost of the intervention and the expected benefit in terms of EAL reduction.     

Seismic Retrofitting of Non-Seismically Designed RC Beam-Column Joints using Buckling-Restrained Haunches: Design and Analysis

Many existing reinforced concrete (RC) structures around the world have been designed to sustain gravity and wind loads only. Past earthquake reconnaissance showed that strong earthquakes can lead to substantial damage to non-seismically designed RC buildings, particularly to their beam-column joints. This paper presents a novel retrofit method using buckling-restrained haunches (BRHs) to improve the seismic performance of such joints. A numerical model for RC joints is introduced and validated. Subsequently, a new seismic retrofit strategy using BRHs is proposed, aimed at relocating plastic hinges and increasing energy dissipation. The results indicate the retrofit method can effectively meet the performance objectives.     

Thin-folded Shell for the Renewal of Existing Wooden Roofs

In this article, a technique for the renewal of historic building wooden roofs is presented. The solution can be used for the strengthening of existing wooden roofs against excessive lateral thrusts on the peripheral wall or for the recovery of the attics, as it allows removing the existing structural elements, such as possible wooden truss-works of no artistic value. With minor adaptations, the solution can be addressed to enhance the building seismic performance. The technique is minimally impairing on existing buildings and can be applied also in new constructions.

The technique is based on the construction of a thin folded shell, overlaying the existing pitches. Emphasis is given to lightweight folded shells, obtained by overlaying thin plywood panels on the existing roof rafters and planks, without modifying the overall architectural layout.

The technique conceptual design is discussed and a simplified analytical method is proposed, which allows for the clarification of the role of each structural component and can be adopted for the folded shell proportioning and design. The analytical results are validated against numerical results obtained with reference to some case studies. Ultimately, emphasis is given to the detailing, whose correct execution is mandatory for the success of the proposed structural intervention.     

Full-Scale Experimental Verification of Soft-Story-Only Retrofits of Wood-Frame Buildings using Hybrid Testing

The FEMA P-807 Guidelines were developed for retrofitting soft-story wood-frame buildings based on existing data, and the method had not been verified through full-scale experimental testing. This article presents two different retrofit designs based directly on the FEMA P-807 Guidelines that were examined at several different seismic intensity levels. The effects of the retrofits on damage to the upper stories were investigated. The results from the hybrid testing verify that designs following the FEMA P-807 Guidelines meet specified performance levels and appear to successfully prevent collapse at significantly higher seismic intensity levels well beyond for which they were designed. Based on the test results presented in this article, it is recommended that the soft-story-only retrofit procedure can be followed when financial or other constraints limit the retrofit from bringing the soft-story building up to current code or applying performance-based procedures.     

The Influence of Masonry Infill Walls in the Framework of the Performance-Based Design

In this article, a performance-based seismic design (PBD) methodology is proposed for the design of reinforced concrete buildings, taking into account the influence of infill walls. Two variants of the PBD framework are examined: The first is based on the non-linear static analysis procedure (NSP) while the second relies on the non-linear dynamic analysis procedure (NDP). Both design approaches are compared in the context of structural optimization with reference to the best possible design achieved for each case examined. Life-cycle cost analysis is considered a reliable tool for assessing the performance of structural systems and it is employed in this study for assessing the optimum designs obtained. The optimization part of the problem is performed with an Evolutionary Algorithm while three performance objectives are implemented in all formulations of the design procedures. The two most important findings can be summarized as follows: (i) if structural realization follows the design assumptions, then total expected life-cycle cost of the three type of structures, bare, fully infilled and open ground story, is almost the same and (ii) if an open ground story building is designed as bare or as fully infilled frame, real performance will be much worse than anticipated at the design stage.     

General Seismic Load Distribution for Optimum Performance-Based Design of Shear-Buildings

An optimization method based on uniform damage distribution is used to find optimum design load distribution for seismic design of regular and irregular shear-buildings to achieve minimum structural damage. By using 75 synthetic spectrum-compatible earthquakes, optimum design load distributions are obtained for different performance targets, dynamic characteristics, and site soil classifications. For the same structural weight, optimum designed buildings experience up to 40% less global damage compared to code-based designed buildings. A new general load distribution equation is presented for optimum performance-based seismic design of structures which leads to a more efficient use of structural materials and better seismic performance.     

Nonlinear Modeling and Mitigation of Seismic Pounding between R/C Frame Buildings

A multi-link finite element contact model reproducing Jankowski’s nonlinear viscoelastic relationships is devised and implemented for a time-history analysis of seismic pounding. The mechanical and algorithmic parameters are calibrated with a numerical test case study, represented by two single-story R/C frames. A real case study, concerning two adjacent buildings representative of a stock of R/C multi-story frame structures with insufficient separation joints at rest, is then examined. A damped interconnection-based mitigation solution consisting in linking the two structures with fluid-viscous dissipaters is proposed. The benefits of this retrofit strategy are discussed by comparison with the response in original conditions.     

On the Design of Viscous Dampers for the Rehabilitation of Plan-Asymmetric Buildings

In this article, one of the procedures proposed in literature for the design of viscous dampers to be inserted in existing buildings is examined and extended to 3D eccentric buildings. The proposed procedure has been verified through a case study characterized by a six-story RC building. Both plan-symmetric and plan-asymmetric configurations have been considered for comparisons. The effect of considering the plan-asymmetry in the design has been studied. Moreover, also the importance of considering the higher modes has been investigated. The effectiveness of the design procedure has been then evaluated through the comparison with nonlinear dynamic analyses.     

Seismic Vulnerability Assessment of the Urban Building Environment in Nablus,Palestine

ABSTRACT

This article describes a specifically developed framework to produce a seismic physical vulnerability model of residential buildings in Nablus (Palestine) within the European project SASPARM2.0. Based on the structural taxonomy of the territory, two different forms were defined to collect geometrical and structural data of buildings by different stakeholders (citizens and practitioners). This data was then employed to produce fragility curves using the mechanics-based procedure SP-BELA. To estimate seismic risk, the developed fragility model was combined with a hazard curve for the corresponding location. The described procedure is implemented in a WebGIS platform that allows to georeference and assess the surveyed buildings and define retrofitting strategies. Finally, the article carries out a comparison between the fragility curves of buildings in Nablus and the ones calculated for similar building typologies within a UNDP Jordan project aiming at the integrated risk assessment in Wadi Musa and surroundings.     

ECONOMIC ASSESSMENT OF THE SEISMIC RETROFITTING OF LOW-COST APARTMENT BUILDINGS

When structural retrofitting of buildings is required due to seismic safety considerations, building owners or government officials are faced with a crucial decision whether to demolish and rebuild their buildings, or retrofit them. Simple decisions based on fixed proportions, such as demolish and rebuild if retrofit costs exceed 40% of the replacement cost, may be misleading. A financial analysis should be carried out considering all the revenues and costs of the building during its life time. A low-cost residential apartment building that has a construction cost of $140/m² is selected in this study to represent the vulnerable buildings in developing countries under seismic risk. Life-cycle cost analysis approach is employed to value the rebuild or retrofit alternatives to compare economically. Economic assessment has been carried out for various parameters. It has been found that the age of the building and the retrofit ratio are dominant parameters among the others. The critical retrofit ratio (percent of retrofit cost to initial construction cost) varied from 25% for 40 year old buildings to 67% for 10 year old buildings. It is suggested that economical decisons should not be made simply by comparing the retrofit costs to initial construction costs.     

AUTOMATIC MINIMIZATION OF THE RIGIDITY ECCENTRICITY OF 3D REINFORCED CONCRETE BUILDINGS

The objective of this paper is to obtain the optimum design of 3D reinforced concrete buildings in terms of their performance under earthquake loading. This goal is achieved by considering the minimisation of the eccentricity between the mass centre and the rigidity centre of each storey layout as the optimisation objective in order to produce torsionally balanced structures. This problem is considered as a combined topology and sizing optimisation problem. The location and the size of the columns and the shear walls of the structure of each storey layout constitute the design variables. Apart from the constraints imposed by the seismic and reinforced concrete structure design codes, architectural restrictions are also taken into account. The test examples showed that a reduction in the structural cost of the building is achieved by minimising the eccentricity between the mass centre and the rigidity centre of each storey layout. Evolutionary optimisation algorithms and in particular a specially tailored algorithm based on Evolution Strategies is implemented for the solution of this type of structural optimisation problems.     

Probabilistic Performance Assessment of Retrofitted Skewed Multi Span Continuous Concrete I-girder Bridges

The unique dynamic response of skewed bridges causes them to experience more noticeable damage compared to straight bridges during seismic events. The effectiveness of different retrofit strategies on the fragility of skewed bridges can change with the skew angle. This article assesses the impact of skew angle and various retrofit strategies on the fragility of multi span continuous concrete I-girder bridges. The results indicate that the level of effectiveness of a retrofit strategy is highly dependent on the skew angle and damage state of interest and an appropriate retrofit strategy should be chosen based on the vulnerability of the components.     

Building survey forms for heterogeneous urban areas in seismically hazardous zones. Application to the historical center of Valparaíso,Chile

ABSTRACT

Building survey is an essential data-collection procedure to feed large-scale seismic vulnerability assessment. The available strategies usually consider survey forms to gather information about the urban buildings. The application of the available survey forms poses important challenges for the case of the heterogeneous urban centers including different structural typologies. This work proposes four specific survey forms for traditional structural typologies constructed with masonry, reinforced concrete, mixed steel-reinforced concrete, and timber. The proposed forms request essential information on the parameters necessary for seismic vulnerability assessment, by evaluating the lateral-load resistant system, regularity, condition of conservation, and existing damages. The survey forms were applied to the study of 111 buildings of the historical center of Valparaíso, Chile. The proposed methodology was complemented with the use of Geographic Information Systems to obtain a complete database with the structural characterization of the most representative typologies for future works of large-scale seismic vulnerability assessment.     

Confidence Factor?

Eurocode 8 Part 3 (EC8-3) is devoted to assessment and retrofitting of existing buildings. In order to take into account the uncertainty in the knowledge of structural properties, EC8-3 defines, analogously to the ordinary material partial factors, an adjustment factor, called “confidence factor (CF),” whose value depends on the level of knowledge (KL) of properties such as geometry, reinforcement layout and detailing, and materials. This solution is plausible from a logical point of view but it cannot yet profit from the experience of its use in practice, hence it needs to be substantiated by a higher level probabilistic analysis accounting for and propagating epistemic uncertainty (i.e., incomplete knowledge of a structure) throughout the seismic assessment procedure. This article investigates the soundness of the format proposed in EC8-3. The approach taken rests on the simulation of the entire assessment procedure and the evaluation of the distribution of the assessment results (distance from the limit state of interest) conditional on the acquired knowledge. Based on this distribution, a criterion is employed to calibrate the CF values. The obtained values are then critically examined and compared with code-specified ones. The results pinpoint a number of deficiencies that appear to somewhat invalidate the approach. The methodological significance of the work extends beyond the assessment procedure in EC8-3, since similar factors appear in other international guidelines (e.g., the knowledge factor of FEMA356).     

A New Proposed Passive Mega-Sub Controlled Structure and Response Control

It is still a serious challenge for structural engineers to effectively reduce the seismic responses of tall and super tall buildings to further improve these structural safeties. In order to solve this problem, in this article a new kind of structural configuration, named passive mega-sub controlled structure (PMSCS), is presented, which is constructed by applying the structural control principle into structural configuration itself, to form a new structure with obvious response self-control ability, instead of employing the conventional method. In the analysis of PMSCS the equations of motion of the seismically excited system are developed, based on a realistic analytical model of the complete mega-structural system. Expressions of the displacement and acceleration response of the structure, resulting from simulated earthquake ground motions represented by stationary and nonstationary random processes, are derived. These responses are then determined for both the PMSCS and its conventional mega-sub structure (MSS) counterpart, whose configuration was modeled after the traditional mega-frame that was used in the construction of the Tokyo City Hall. A parametric study of the structural characteristics that influence the response control effectiveness of the PMSCS is presented and discussed. The region over which these structural characteristics yield the optimum seismic response control of the PMSCS is identified and serves as a very useful design tool for practitioners. The study illustrates that the proposed PMSCS offers an effective means of controlling the seismic displacement and acceleration response of tall/super-tall mega-systems. It also overcomes shortcomings exhibited in earlier proposed mega-sub controlled structural configurations.     

Retrofit Design Methodology for Substandard R.C. Buildings with Torsional Sensitivity

Recent earthquakes have revealed the susceptibility of non-ductile reinforced concrete (R.C.) buildings with deficiencies related to stiffness and/or mass irregularities in plan and elevation. This paper proposes a design methodology for the seismic upgrading of rotationally sensitive substandard R.C. buildings. The methodology aims to first eliminate the effect of torsional coupling on modal periods and shapes and then modify the lateral response shape of the building in each direction so as to achieve an optimum distribution of interstory drift along the building height. A case study is used to illustrate practical application of the proposed methodology.     

A Modified Capacity Spectrum Method with Direct Calculation of Seismic Intensity of Points on Capacity Curve

A direct methodology for solving the seismic intensity of each point on the capacity curve is proposed. By utilizing the procedure, a continuous curve between the structural response and the seismic intensity, the structural response function, can be easily generated. Unlike previous procedures that search for the performance point of a determined seismic intensity, the proposed methodology easily draws the full curve without iterations. The procedure is applicable to both a smooth design spectrum and an actual response spectrum. Examples indicate the methodology is accurate and fast, and convenient to be combined with existing procedures, such as Modal Pushover Analysis.     

ON THE SEISMIC VULNERABILITY OF EXISTING UNREINFORCED MASONRY BUILDINGS

A large part of the building population in Switzerland is made of unreinforced masonry. For the assessment of the seismic risk the evaluation of the seismic vulnerability of existing unreinforced masonry buildings is therefore crucial. In this paper a method to evaluate existing buildings, which was developed for the earthquake scenario project for Switzerland, is briefly introduced and discussed in more detail for unreinforced masonry buildings. The method is based on a non-linear static approach where the seismic demand on the building is compared with the capacity of the building. In-plane and out-of-plane behaviour are considered. Comparisons with test results from model buildings show that the proposed method suitably forecasts the capacity of a building. Finally, a numerical example of the application of the method to a building in the city of Basel is given.     

INDEPENDENT HARMONIC CONTROL FOR STRUCTURAL ENGINEERING

In a previous paper [Baratta and Corbi, 1999] one has defined a procedure allowing to identify a closed-ioop control algorithm with feedback based on the whole record of the response time-history rather than on instantaneous response parameters. The control force results from control of each harmonic component of the forcing function, simply integrated over the frequency domain. Every harmonic is controlled, independently of each other, by a classical linear control whose coefficients are calibrated in way to make the relevant response component a minimum compatibly with the control effort one wants to apply at the corresponding frequency. The distribution of this control intensity over the frequency range remains a arbitrary choice; such a choice however lends itself to be effectively assisted by intuition, much more than similar choices in other procedures (e.g.: the coefficients of the quadratic norms in the J-index optimization). The result is that every harmonic remains controlled by a different couple of optimal coefficients (corresponding to the proportional and to the derivative terms in the linear control law), and the overall control force for an arbitrary disturbance, after Fourier inverse transformation, is produced by feedback integration over the whole response time-history.

The procedure, tested with reference to simple and composed harmonic excitations incoming a s.d.o.f. structural system, has proved a good agreement of the numerical results with the theoretical treatment; furthermore it has shown that the main limit of such an approach consists of referring the dynamic equilibrium solution to a particular solution, that, neglecting the initial conditions, may introduce some unstable components in the oscillation. In the paper the effects induced in the controlled structural system response by the adoption of the proposed procedure are deepened and an improved strategy is presented, able to overcome the detrimental transient effects determined by the original algorithm. The final adopted control law is shown to achieve an improved time response, both in the transient and in the steady-state field, in comparison to a control strategy based on classical linear control minimizing the response norm conditioned by a bounded control.