CHAPTER 5 INTERVENTION STRATEGIES

Transcription

1 CHAPTER 5 INTERVENTION STRATEGIES Dan LUNGU, Cristian ARION Structural Safety for Natural Hazard Research Centre, Technical University of Civil Engineering Bucharest, Romania 5.1 GENERAL & TERMINOLOGY The value of heritage cannot be measured simply in terms of price. Conservation professionals and decision-makers increasingly must confront economic realities. Policy makers decisions are many times outside the domain of conservation practice for the cultural heritage. The PROHITECH project seeks the intervention strategies for historical buildings taking into accounts the economic and cultural background of the specific countries. The following web sites contains criteria and methodology for seismic rehabilitation of historical heritage buildings: Who is implementing the project? The project strategy should contain informations that can be used by building owners, public clerks, architects, engineers and policy makers in order to decide on the adequate solutions for necessary rehabilitation interventions on constructions. Implementing the concept of a target performance, the rehabilitation should allow a comprehensive understanding of the parameters essential in the analysis that can ultimately provide the optimal intervention solutions. Who is responsible? The responsibility of safeguarding the existent individual building falls on the owners, while in the case of public constructions that exhibit a high level of vulnerability the responsibility extends to the authorities. Planning the retrofit of historical buildings before an earthquake strikes is a process that requires teamwork of engineers, architects, code officials, NGO, administrators and authorities. Policy development should also include consideration of other urban and environmental factors affecting the future of a place/location of building. What is the target? 1

2 The two major goals of the seismic rehabilitation/strengthening in historic buildings are life safety and the protection of historical value of the building. Because rehabilitation should be sensitive to historic materials and the building's historic character, it is important to put together a team experienced in both seismic requirements and historic preservation. Team members should be selected for their experience with similar projects, and may include historians, architects, engineers, code specialists, contractors, and preservation consultants. Because the typical seismic codes are written for new construction, it is important that both the historians & architect and structural engineer be knowledgeable about historic buildings and about meeting building code equivalencies and using alternative solutions. The rehabilitation programs might be active i.e. directly intended for strengthening of the building and passive i.e. due to the change of occupancy of the buildings. The reinforcement of the historical buildings to meet new construction requirements often, can destroy much of the historical building's appearance integrity and value. Important preservation principles Three important preservation principles should be kept in mind when undertaking seismic retrofit projects: Historical materials should be preserved at the largest extent possible and not replaced wholesale with other new materials in the process of seismic strengthening; New seismic retrofit systems, whether hidden or exposed, should respect and should be compatible to the architectural and structural integrity of the historical building; Seismic work should be "reversible" to the largest extent possible to allow future removal for the use of future improved systems. While seismic upgrading work is often more permanent than reversible, care must be taken to preserve historical materials to the historical appearance of the building. In addition to the above principles, the general aim of building conservation is to preserve the cultural significance of the place where the building is culturally aggregated. Places of cultural significance should be safeguarded and not left at risk in a vulnerable state. The different intervention works are defined in various manners documents from various countries, as well as in general literature, TABLE 5.1. Adaptation Conservation Consolidation TABLE 5.1 Terminology for seismic rehabilitation of historical buildings FEMA 2004, and The Australia ICOMOS charter for the conservation of places of cultural significance modifying a place to suit the existing use or a proposed use the operations which maintain the building as it is today, even if limited interventions are accepted to improve the safety levels the rebuilding or renewal of any parts of the construction (of some elements or an assembly of elements) with the purpose of obtaining an enhanced structural capacity; for example, highresistance capacity, enhanced stiffness, better Oxford popular English dictionary suitable for new use or conditions Make or become secure and strong 2

3 Examination Intervention Maintenance Preservation Repair Reshaping Rehabilitation Restoration Reconstruction Strengthening FEMA 2004, and The Australia ICOMOS charter for the conservation of places of cultural significance ductility the visual part of an investigation that excludes material testing, structural analysis, structural testing, and other more sophisticated investigative techniques a concept that involves standards/norms regarding consolidation, repair and reshaping of structural and/or non-structural elements continuous protective care of the fabric/aggregate and setting of a place, and is to be distinguished from repair Oxford popular English dictionary enter a situation to change its course or resolve it process of maintaining something maintaining the fabric/aggregate of a place in its keep safe, unchanged, or in existing state and retarding deterioration existence rebuilding or renewal of any damaged or faulty part put into good condition after of the construction to obtain the same level of damage; make amends resistance, stiffness and/or ductility with the those previous to its degradation Repair involves restoration or reconstruction. the rebuilding or renewal of any parts of the construction resulting in the change in function or in the occupancy rebuilding or renewal of a damaged construction in order to ensure the same level of functioning, similar to the that previous to the degradation of the building returning the existing fabric/aggregate of a place to a known earlier state, by reassembling existing components without the introduction of new material returning a place to a known earlier state and it is distinguished from restoration by the introduction of new material into the work. judicious modification of the strength and/or stiffness of structural members or structural systems to improve their performance in future earthquakes. Strengthening generally includes increasing the strength or ductility of individual members or introducing new structural elements to significantly increase the lateral force resistance of the structure. On occasion, strengthening can also involve making selected structural members weaker to improve the interaction of the structural members and prevent premature failure of a weaker adjacent member. redevelop into certain condition restore to a normal life or good condition bring back to its original state reconstruct; restore to its original form The diagram of used terminology is presented in FIGURE

4 value 100 requirements maintenance strengthening ageing without maintenace damage repair restoration ageing with maintenace FIGURE 5.1. Value alteration diagram of a building time t 5.2 INTERVENTION SOLUTIONS AND CRITERIA FOR SEISMIC UPGRADING & REHABILITATION OF CONSTRUCTIONS The intervention solutions must rely on cost-benefit analyses and take into account their socioeconomic impact on society. An obvious requirement is to minimize as much as possible the disturbance of the owners of the building during the building rehabilitation. The financial resources available decisively influence the intervention solutions for the particular purposes, including labor work capacity, equipments, materials, duration of the work etc. It is also compulsory to have alternative strategies for intervention and to evaluate the decrease in building vulnerability with various strategies. The following excerpts from various international organizations should be considered for intervention decisions. Examples of international organizations are given in TABLE 5.2. Traditional techniques and materials are preferred for the conservation of old building aggregates. However, modern techniques and materials, which offer substantial conservation benefits, may sometimes be also appropriate. Restoration and reconstruction should reveal significant cultural aspects of the place. Restoration is appropriate if there is sufficient evidence of an earlier state of the fabric. Reconstruction is appropriate only a place is incomplete through damage or alteration, and where there is sufficient evidence to reproduce an earlier state of the fabric. In rare cases, reconstruction may also be appropriate as part of a use or practice that retains the cultural significance of the place. 4

5 Demolition of significant fabric of a place is generally not acceptable. In some cases minor demolition may be appropriate as part of conservation. Removed significant fabric should be reinstated when circumstances permit. TABLE 5.2. International organizations related to historical heritage preservation ICOMOS is an international non-governmental organization of professionals, dedicated to the conservation of the world's historic monuments and sites. World Heritage International Centre for the Study of the Preservation and Restoration of Cultural Property The Getty Conservation Institute ICCROM Via di San Michele 13, Rome, Italy Tel: World Monuments Fund is the foremost private, non-profit organization dedicated to the preservation of historic art and architecture worldwide through fieldwork, advocacy, grant making, education, and training The intervention strategy and the intervention techniques must take into account the following criteria: (i) (ii) (iii) (iv) (v) (vi) Seismic hazard level at the construction site; Characteristics of the building intended use (architectural constrains, original occupancy of the building, building structure, technical equipments within the building, etc); Building safety as a response to daily activities, mainly related to the seismic safety (structural vulnerability, vulnerability of non-structural elements, appliances or/and equipments, building exposure or value, etc.; Required level for building performance (life safeguarding, immediate occupancy after earthquake, preventing building-collapse, etc); Economical criteria, including insured & reinsured value of the building; Technological capability available at the site The Repair and Strengthening Design Process Even though the main aim of this workpackage is the reviewing of intervention strategies, some basic principles on current approaches to damage assessment and definition of vulnerabilities of structural types will be cited for completeness. 5

6 Criteria of Repair and Strengthening A post earthquake evaluation of the seismic parameters of the region and the individual sites is required for the earthquake affected region and structures. The definition of the seismic parameters for the region is a prerequisite for successful accomplishment of the repair and strengthening of damaged/undamaged structures. The study should include the expected maximum acceleration of bedrock for different return periods, amplification factors and to propose adequate time histories and average spectra for design of repaired structures. For more important structures, group or typical structures it is necessary to determine the seismic parameters for the considered sites and to perform field, soil investigations and geophysical studies to be used as input design parameters. The definition of seismic criteria comprises the correlation of the seismic force design parameters with the structural characteristics in terms of strength, deformability, ductility, etc. Each country or governmental area should establish its own criteria based on specific conditions related to the seismology and probable seismic events of the region. The criteria for repair and strengthening should preferably not be fully encompassed by the current building code for new construction, although showing respect to its provisions. The designer must use established criteria or Codes and Regulations of the area as the minimum standard for repair and strengthening projects. For selected projects such as historical structures, the designer may have to use criteria in excess of the established ones, based on the particular circumstances regarding the project. The designer may also have to establish criteria or methods to assign strength values to traditional materials whish are not covered by modern Building Codes or Regulations Structural investigations During the preliminary investigation, the nature and general degree of damage is determined. In order to design repair and/or strengthening measures, it is necessary to perform additional investigations and gather supplemental data while fully utilizing the preliminary investigation ones. Documents regarding the original construction should be compiled to the extent that they are available. This includes the designs, drawings, specifications, construction details, data on original construction material strengths, foundation and soil condition data, previous repairs or alterations, coeds under which the original design was prepared, etc. The information gathered should be compared with the actual structure to confirm that it was built in conformance with that information. Deviations should be noted and recorded. If information is not available or considered untrustworthy, field measurements and observations must be taken into account to verify the characteristics of the original construction. More specifically, the problems that arise while treating rehabilitation tasks of historical buildings concern the laborious work demanded for evaluating material characteristics and structure of bearing elements, as well as the lack of relevant written material. It is often required studying specific structures under loading conditions which are considered probable to have occurred, instead of designing on purpose of fulfilling safety demands under well-defined loading cases. In addition, interpretation of the actual structural performance is expected to be based on existing damage, the full record of which is non achievable. Completion of the detailed site inspection, which begun in the preliminary investigation, must be accomplished. This operation is an essential and important phase in the process of designing repair and strengthening measures. Damage due to seismic forces most often appear in structural elements as 6

7 columns, shear and infill walls, beams, beam-column joints, staircase towers, floor slabs and the connection between floors and walls and foundations. Each structural member must be inspected and the damage or lack of damage must be recorded, sketched and photographed. Damage and location of non-structural elements should also be recorded and present status, characteristics and strength of original construction materials should be estimated during the additional investigation Damage evaluation and selection of a repair and strengthening solution Utilizing the investigation data which has been documented and the criteria for repair and strengthening, the designer must typically evaluate the damage, perform analysis to determine why the damage occurred, and develop alternative schemes to repair and/or strengthen the structure. These alternative schemes must be evaluated and the most appropriate solution should be selected. The engineer must first analyze the damaged structure and thoroughly understand the causes of damage occurrence. The force resistant paths in the building must be determined, explaining why certain members sustained damage while others were practically untouched. It must be determined whether the structure suffered due to discontinuities in strength or stiffness, due to torsional moments within the structure, due to hammering with adjacent structures or due to improper connections or details. The effects of non-structural elements such as infilled walls and appendages on the structural performance must be considered. It must be determined if members failed due to shear, compression, tension, flexure, bar anchorage, etc. This analysis is essential before any repairs can be assigned. Calculations and analysis must be performed in order to evaluate the existing strength and stiffness of the damaged structure. The decision of the need to strengthen the structure will generally follow these analyses. If the repaired structure without strengthening conforms to the design criteria, then strengthening will generally not be required. If the repaired strength is less than the requirements of the criteria, the strengthening will generally be desirable in addition to repairing the damage. Based on results obtained by this analysis, alternative solutions for repair and/or strengthening can be determined for evaluation of their feasibility. Imagination and ingenuity should be exercised by the designer utilizing professional experience, as the best and most economical solution is seldom the first one conceived. Analysis for every alternative scheme must be performed to evaluate the effects of the strengthening measures and provide a reasonable basis for comparison among them. It must be recognized that despite the specific design criteria used, possible weak links in the structure can be detected during the analytical procedure. A future earthquake strong enough to cause damage will result in some elements of the repaired structure exceeding their yield strength and sustaining inelastic response. With a thorough understanding of the potential weak links in the structure, the designer can propose repair and strengthening measures whish will improve the response of the structure in future earthquakes. The repair and strengthening measures should establish an improved structure for seismic performance by avoiding irregularities in plan, abrupt changes in stiffness between floors and elements subjected to shear or brittle failure. The effects of strengthening elements added to the structure must be carefully evaluated to insure that they will not cause increased damage in a future earthquake. Conclusively, in order to select one solution for implementation, apart from the factors discussed above, the feasibility evaluation of alternative solutions must include the following aspects: Compatibility with the functional requirements of the structure Feasibility of construction, including availability of materials, construction equipment and personnel with specialized training and the ability to implement the solution 7

8 Economical considerations Sociological considerations Aesthetics 5.3 OPTIONS FOR INTERVENTION STRATEGY The strategy of interventions for improvement of the seismic performance of a structure or for the reduction of seismic risk might contain technical or/and managing approach. The technical strategies approach use the interventions on the bearing structure for the repair of possible deficiencies, on the increase of strength and stiffness of the structure, on the increase of the deformation assumption capacity, on the increase of the energy dissipation capacity and the reduction of seismic requirements. The basic intervention selection criterion is the limitation of damages caused to the primary and the secondary structural elements to acceptable levels for a given performance level. There are also managing strategies that can be taken into consideration during the design of interventions. Managing strategies comprise: (a) the decision of realizing interventions while the building remains in use or of evacuating it until the end of the strengthening works, (b) the acceptance of the existing seismic risk of the building and the omission of interventions or the alteration of its use in a way that renders the seismic risk acceptable, (c) the demolition of the existing building and its substitution with another, (d) the realization of the proposed interventions progressively in a very large time margin or the taking of temporary strengthening measures until the replacement of the structure, (e) the accomplishment of interventions on the external of the building in order to minimize the negative consequences for the inhabitants or in the internal in order not to change the characteristics of its external aspect. Managing strategies should be co-assessed by engineers and owners in order to select the suitable intervention strategy. Generally, the best solution for a building is related to the taking of decisions of a managing and technical nature Technical Strategies The technical strategies might apply when the structure should dispose a complete system for seismic load assumption, able to limit deformation in extents that correspond to acceptable damage levels for the targeted structural performance level. The main parameters which define the efficiency of the seismic loading bearing system are: (a) the mass, stiffness, damping and configuration of bearing and not bearing structural elements, (b) the deformation capacity of bearing and not bearing structural elements and (c) the energy and character of the seismic excitation imposed to the structure. 8

9 Deciding the intervention strategy on building in terms of structure resistance, deformation and ductility performance, as well as in terms of non-structural elements performance should be made explicit through the following relation: SEISMIC REQUIREMENT~ CAPACITY The measures of intervention for the accomplishment of the above relation are: (i) Reducing seismic requirements; (ii) Improving the mechanical characteristics of the construction; (iii) Combining modalities: (i) and (ii). In other words, the options the designer has are either to ensure an elastic behavior of the structure or to increase the hysteretic energy through plastic behaviour which involves the degradation of the structural elements. The strategy of improving a structure through local interventions is applied to structures which, although dispose the basic elements of an adequate seismic loading bearing system, lack of some constructional details which are necessary for the system optimization and the ensuring of the required functionality. The deformation capacity of such a structure may be adequate in comparison with the given seismic requirements, all the same before reaching this deformation it is probable that local damages in different locations of the structure occur. The most usual defects which are to blame for such local damages are the inadequate seating length at abutment locations of precast elements and the inadequate anchoring or connection between structural elements, either primary or secondary. Local interventions aiming at reforming these defects would allow a satisfactory structural behavior. Very often the strategy of local interventions is used in combination with other strategies in order to reach an adequate structural behavior Reducing seismic requirements Reducing seismic requirements can be achieved by the following methods: (i) Reducing resistance requirements. The modalities of reducing seismic inertia forces developed in the structure can be identified by analysis of the structural elastic response acceleration spectrum. The objective is to stay away, as much as possible, from the maximum amplification of the spectrum by altering the characteristics of the structure. Reduction of resistance requirements may also be achieved through a reduction of building mass. This can be obtained by eliminating unnecessary weights: getting out one or several storeys, reducing the weight of partition walls, replacing a heavy roof with a light one etc. (ii) Reducing drift requirements. The reduction of drift requirements may be generally achieved by reducing the natural period of building, either by increasing the stiffness or by decreasing the mass of the construction Improving the mechanical characteristics of the construction Improving the mechanical characteristics of the construction can be done by, FIGURE 5.2.: a) Increasing the stiffness to lateral forces. Increasing stiffness certainly reduces the effective drifts under values given by the lateral forces; b) Increasing of the plastic deformation capacity, i.e. increasing of ductility; 9

10 c) Increasing the resistance capacity. In many situations of insufficient ductility increasing the ductility is difficult to be achieved in practice and seems to be much easier increasing the building resistance; d) Increasing the dissipation/damping capacity. In several cases increasing the dissipation capacity can be achieved by elements that may be sacrificed on purpose. The infilled walls (partitions) may be specially designed to fit this purpose. In most cases the intervention on a structural characteristic will inevitably modify other properties that the structure has. The increase in stiffness, for example, may enhance the resistance of the structure as well as the decrease in drift and ductility requirements. FIGURE 5.2. Concept of seismic retrofit and seismic performance upgrading of existing building Choice of intervention systems The seismic strengthening methods can be classified as follows, as far as their target is concerned: 1. In case the target is the increase of stiffness and structural strength, the most effective method is the addition of walls in the existing frames. What follows is the method of adding truss bracings, the method of adding walls as an extension of existing columns and the use of composite materials. 2. In case the target is the increase of plasticity, the method that is recommended is the application of jacketing on a number of selected columns, as well as the use of composite materials. 3. In case the target is the simultaneous increase of strength, stiffness and plasticity of the structure, any seismic strengthening method can be used taking into consideration the required degree of increasing each of the above mentioned characteristics. In case that the necessary increases are very high for all three characteristic, it is generally inevitable the addition of new vertical elements Repair & Strengthening of Existing Buildings As the general awareness of the earthquake risk increases, and standards of protection for new buildings become higher, the safety of the older, less earthquake-resistant construction becomes an increasingly important concern. In many earthquakes, damage is concentrated in the older building stock, while recently constructed buildings stiller comparatively lightly. The problem can be expected to diminish over time if improved standards of new construction can be achieved, because the 10

11 proportion of unsafe buildings in the total building stock will diminish. This is true particularly where there is a high rate of turnover of the building stock. But such cases are rare; in many industrialized countries the rate of new building construction is only about 1% per annum, and even in more rapidly developing areas with sustained growth rates of 8% and more, the total number of older buildings does not diminish very rapidly. Indeed, rather than being taken out of use, the older building stock is increasingly being modified and adapted, sometimes in ways which significantly increase the loading or reduce its inherent resistance to earthquake ground shaking. And where old buildings are not being modified, lack of maintenance may lead to decay of already weak and poorly integrated structures, resulting in a continual decline in earthquake resistance possibly made worse by the cumulative effect of low levels of damage in previous earthquakes. For all these reasons, strengthening existing buildings is assuming increasing importance in earthquake regions. For most types of buildings, strengthening is a cheaper way of bringing earthquake resistance up to acceptable levels than rebuilding; depending on the situation and construction type, costs of strengthening typically range from 5% to 40% of the cost of a new building. Careful consideration needs to be given to the type of strengthening best suited to achieve the desired safety level. Factors which need to be taken into consideration in deciding whether to strengthen, and which method to use, will include: The required level of structural resistance; The general structural form and any changes needed; The materials and degree of connection in the existing structure; Foundation conditions and the effect of strengthening on them; The effect of strengthening on the appearance and functioning of the building; Required strengthening of non-structure and services; The time during which the building will be unusable; The cost of the work. The main objective in strengthening is to achieve a structure, which satisfies the principles of good earthquake-resistant design. The actual methods used are different for different types of structure Strengthening unreinforced masonry buildings Many cities, towns and villages in earthquake zones consist primarily of unreinforced masonry buildings of a great variety of types and ages which experience has shown to have poor resistance against earthquakes. Damage to masonry structures usually takes the form of tension and/or shear cracks caused by imposed deformations e.g. differential settlement, excessive lateral forces e.g. from arch thrusts or incipient bursting due to buckling of ashlars or expansion of rubble fill in structures which consist of relatively thin ashlars faces with a core of rubble and mortar. The principal sources of the weakness of these buildings are: Low-strength masonry units and mortar inadequately bonded together. Insufficient interconnection between inner and outer leaves of external walls, insufficient connection at the junctions of perpendicular walls. Insufficient rigidity of floor and roof slabs in their own plane, and inadequate connection between these slabs and the bearing walls. 11

12 These weaknesses are frequently compounded by the deterioration of the structure due to weathering and rot, and to extensive structural modification during the lifetime of the building. According to the weaknesses identified in particular cases, strengthening may involve any or all of the following interventions: Modifying the plan form of the building to improve symmetry, improving the connections between perpendicular walls; Strengthening or replacing floor and roof structures, and improving their connection with the load-bearing walls; Strengthening the walls themselves; Strengthening the foundations. The range of techniques available is wide, and details depend on the type of masonry involved. Common strengthening techniques include: Stiffening existing wooden floors and roofs by covering them with a thin layer of reinforced concrete; Insertion of reinforced concrete ring beams into the inner face of external walls at floor and caves level to tie vertical and horizontal elements together; Extensive strapping of masonry walls to each other and to slabs using both horizontal and vertical steel straps; Strengthening of walls (mainly when cracked) by cement injection or by adding a thin layer of cement render reinforced with steel mesh on either side of the wall; Adding plywood sheathing; Strapping parapets. Where repair and strengthening by these means is not considered feasible, an alternative is to introduce a new independent concrete frame to carry the earthquake loads, and attach the masonry to it. Methods of evaluating the earthquake resistance of existing unreinforced masonry building have been developed both in California and New Zealand. These methods can also be used to assess the effectiveness of proposed strengthening interventions, and so to consider the cost-effectiveness of strengthening as against alternative mitigation measures such as reconstruction or change of use Strengthening reinforced concrete buildings The need for strengthening reinforced concrete buildings has become more urgent in recent years in countries where recent earthquake damage has indicated that the resistance requirements in previous codes were inadequate or where buildings have been found to be below code standard. The principal causes of weakness in reinforced concrete buildings are: Insufficient lateral load resistance, as a result of designing for too small lateral loads. Inadequate ductility, caused by insufficient confinement of longitudinal reinforcement, especially at beam-column or slab-column junctions. A tendency to local overstressing due to complex, and irregular geometry in plan and elevation. Interaction between structure and non-structural walls resulting in unintended torsional forces and stress concentrations. Weak ground floor due to lack of shear walls or asymmetrical arrangement of walls. High flexibility combined with insufficient spacing between buildings resulting in risk of neighboring structures pounding each other during shaking. Poor-quality materials or work in the construction. Unrepaired damage from previous earthquakes may also be a reason for requiring strengthening. 12

13 The principal objective in most strengthening interventions is to increase the lateral load resistance of the building; usually increasing the ductility of the structure will be an additional objective. The strengthening may also involve removing or redesigning non-structural walls which may affect the performance of the building, and sometimes the strengthening effect may be achieved by removing load from a structure (by, for example, reducing the number of stories). For tall buildings on soft soil deposits, an increase in stiffness may also help to improve a building's performance by reducing its natural period to a value below that of the subsoil. Often the intervention may require simultaneous strengthening of the foundations. The principal technical options for improving the lateral loadcarrying ability of existing reinforced concrete structures include: Adding concrete shear walls, Buttressing, Jacketing, Adding cross-bracing or external frames. Adding Shear Walls The most common method of strengthening of reinforced concrete frame structures is the addition of shear walls. These are normally of reinforced concrete, or may exceptionally be of reinforced masonry. In either case, they are reinforced in such a way as to act together with the existing structure, and careful detailing and materials selection are required to ensure that bonding between the new and existing structure is effective. The addition of shear walls substantially alters the force distribution in the structure under lateral load, and thus normally requires strengthening of the foundations. Buttressing Buttresses are braced frames or shear walls installed perpendicular to an exterior wall of the structure to provide supplemental stiffness and strength. This system is often a convenient one to use when a building must remain occupied during construction, as most of the construction work can be performed on the building exterior. Sometimes a building addition intended to provide additional floor space may be used to buttress the original structure for added seismic resistance. Buttresses typically require the construction of foundations to provide the necessary overturning resistance. Even considering the extra foundation costs, the cost of buttressing an occupied building may be substantially lower than that for interior shear walls or braced frames. The aesthetic impact and the availability of building space adjacent to the existing building are obvious factors affecting choice of this solution. Jacketing An alternative technique is to increase the dimensions of the principal frame members by encasing the existing members in new reinforced concrete. The technique is known as jacketing. Adequate reinforcement of the new encasing concrete can increase both strength and ductility, and concrete damaged in a previous earthquake can be replaced at the same time. Again careful consideration needs to be given to achieving an adequate bond between the existing and new concrete. Jacketing of beams is much harder than columns; jacketing the beams and columns may be ineffective if the beam/column joint is inadequate, and retrofitting joints is also difficult. Jacketing may be a viable option where a significant improvement is available from increasing the strengthening and ductility of some or all of the columns, without substantial intervention to the beams and joints. It may be attractive where there are architectural difficulties in adding shear wails. Jacketing is a valuable technique when complex or deep foundations make the change in the lateral load-bearing system required by a shear wall system impossible or very costly. Addition of Cross-bracing Both of the above techniques involve major interventions to the structure. An alternative technique, involving a less drastic intervention and smaller increase in foundation loads, is the addition of steel 13

14 cross-bracing to increase lateral load resistance. Tin's generally also involves the strengthening of adjacent columns, which will have to carry increased axial loads, although this is offset by a reduction in column moments and ductility demand. Strengthening of columns may be achieved by the addition of an external steel cage surrounding each of these columns. The addition of steel bracing considerably alters the appearance of a building, but is particularly suitable for comparatively low-cost strengthening of buildings which have not been damaged in a previous earthquake. Other Methods Other methods sometimes adopted to improve the performance of reinforced concrete buildings in earthquakes include the addition of separate external frames (ATC-40), or the removal of one or more storeys to reduce the lateral load. New techniques such as bonding of steel plates to the concrete frame have been proposed but are as yet little tested. In rare instances, base isolation has been used to protect the superstructure from the ground shaking, but this is very expensive. The addition of supplemental damping devices is becoming increasingly used as a retrofit measure for concrete frame buildings. This is generally suitable for special cases; it would not be recommended unless a high level of the relevant engineering expertise is available. In a few cases strengthening through the use of advanced fiber-reinforced plastic (FRP) has been used. While this method has the drawback of cost, it offers the advantage of quick installation, minimum disruption, and no weight increase in the structure Repair and strengthening of historical buildings In what way do monumental buildings differ from other buildings that constitute the world s architectural heritage? The international framework of principles for the protection and restitution of historical buildings is known as the International Charter of Venice. The International Charter of Venice was drafted in May 1966 on the occasion of the second international conference of architects and building technicians involved in the restoration and maintenance of monuments that was held in Venice. It consisted of a re-examination of a similar Charter of 1931 and summarized the experiences and lessons of almost twenty years of intense postwar restoration activity. For obvious reasons the creators of the Charter were almost entirely concerned with medieval and later buildings that were in use immediately before the war and not with the special case of monumental buildings of Greco-Roman civilization. Nevertheless, the Charter of Venice has become widely accepted because of its concise, uncomplicated character. Speaking about structural restoration, from all the 16 articles included in the Charter, the followings have a significant meaning: Article 2: The conservation and restoration of monuments must have recourse to all sciences and techniques, which can contribute to the study and safeguarding of the architectural heritage. The involvement of structural engineering faculty in the restoration practice is fully justified from this article. Article 9: The process of restoration is a highly specialized operation. Its aim is to preserve and reveal the aesthetic and historic value of the monument and is based on respect for original material and authentic documents. It must stop at the point where conjecture begins, and in this case moreover any extra work, which is indispensable, must be distinct from the architectural composition and must bear a contemporary stamp. The restoration in any case must be preceded and followed by an archaeological and historical study of the monument. Article10: Where traditional techniques prove inadequate, the consolidation of a monument can be achieved by the use of any modern technique for conservation and construction, the efficacy of which has been shown by scientific data and proved by experience. 14

15 Another principle, deriving indirectly from the Charter of Venice, is the reversibility that is the acceptance of the possibility of the restitution of a monument to the condition it was in prior to the intervention. This principle stems from the view that a monument is a source of scientific evidence and from the premise that mistakes may occur during the execution of the work. It takes as its starting point the intention to preserve the monument as a source of evidence after the work is completed and also to ensure that any mistake is rectifiable. Historical buildings constitute a case of special importance. A distinction has to be made between: (1) Historical monuments and (2) Historical urban centers. Each is valued for different reasons and the strengthening techniques necessary to retain those values arc different, and have different costs and constraints. It is important in planning the repair of older buildings to consider which approach and level of budgeting suits each building. Usually strengthening of historical structures will be done as a result of minor damage from an earthquake, or they may have been damaged in other ways. In either case, the techniques for repair and strengthening are the same. Historical Monuments Historical buildings are valued for their cultural associations and interesting physical construction. In restoration and strengthening, the physical fabric of the structure must remain essentially the same as before the earthquake. Strengthening elements that are added should be unobtrusive and. where possible, reversible, i.e. removable by future renovators. This type of restoration work requires specialist skills and is expensive. There may be only a few buildings for which this sort of expense can be justified. Restoration and strengthening techniques used on historical monuments include: Dismantling damaged masonry and reassembling it with improved mortar and concealed reinforcement (e.g. metal cramps, reinforcing bars, mesh, etc.). Addition of concealed tension bars, as anchor bolts, ring beams, corner ties, splay members, arch chords, and other structural connections. These maybe drilled through masonry using extended bit drills, capped and grouted into place. Internal grout or chemical injection into wall cores where poor-quality rubble has to be stabilized and bonded without altering the external wall finish. Strengthening or stiffening foundations. There may often, however, be some conflict between the desire for reversibility and the effectiveness of the intervention. Grouting can be gravity fed or pressure injected, but is irreversible and often unpopular with renovators. Historical Urban Centers The historical centers of many earthquake-prone cities consist of dense residential and commercial districts whose buildings are usually of unreinforced stone or brick masonry, much altered in unrecorded ways over the centuries and often in poor condition. Although they represent a valuable and irreplaceable part of the urban heritage, they are often under threat from general decay and deterioration and from the pressure for redevelopment in addition to the earthquake risks they face. To date little has been done to protect any of such buildings from future earthquake damage, and upgrading strategies are needed which will fulfill the sometimes conflicting criteria of life safety for occupants and functional upgrading, limitation of damage from future earthquake, and limitation of alteration to the fabric and appearance of the buildings. Criteria governing the choice of upgrading strategy for historical centers are: first, that interventions should make a significant improvement to the earthquake resistance of the buildings in a way which is both identifiable and measurable; secondly, that interventions should cause only very limited alteration to the external appearance of the building; and thirdly, that interventions should be consistent with existing programs of upgrading for the 15

16 buildings in terms of cost, appropriate techniques and the process of design and management. Repair and strengthening techniques suitable for use in these situations include: Use of steel tie rods passing through the floors and external walls with external anchorage plates or bars to connect the walls and the floors together. Improving the stiffness of floors in their own plane by adding new timber members - for instance, two layers of floorboards laid perpendicular to each other or by cross-bracing with steel straps. The monolithic floors can themselves be made into strengthening diaphragms for their supporting walls by chasing in and casting skirting beams around the edge of the floor. Jacketing the walls by application of layers of wire mesh on each face, tied together through the wall at intervals and covered with a layer of dense plaster. Where upper storeys are badly cracked and lower floors are relatively sound, the upper storey may be demolished, a reinforced concrete ring beam cast on top of the remaining wall, and a new identical upper storey constructed. New masonry should be reinforced and may be in solid brick, high-quality concrete block work, or cut stone. Original wall thicknesses should be retained, and a reinforced concrete ring beam at roof level should top all walls. In reconstruction, the plan of some buildings - for example, 'L'-shaped plans - may be compartmented into interlocking rectangular structural units, by means of ring beams and cross-ties, for greater seismic rigidity. In cases of moderately damaged walls with elaborate stucco decoration work, it may be possible to save the wall and its original decoration by using cement injection grout injected into the core of the wall. This should only be used in conjunction with extensive 'stapling', i.e. drilling and grouting steel reinforcing bars as connector reinforcements between walls and from walls to floors. The skills needed for repair and restoration of the buildings of historical urban centers are general building skills. Techniques of grouting, stapling and mesh reinforcement are relatively straightforward to learn and can be carried out by almost any building professional. Skilled craftwork is needed for repair and for renovation of any original interiors that owners wish to preserve Materials and construction techniques Repair materials should be compatible with the original construction. This applies to the stone, as well as the mortar. Repair and strengthening techniques are conveniently considered under the headings of the defects they are intended to remedy. The selection of a suitable solution for repair or strengthening of a structure presupposes that the engineers involved have a satisfactory knowledge on materials and techniques available for such interventions. Conventional materials of construction are often insufficient at providing solutions, even though they still play a major role in the procedure. The need of using new materials and innovative technologies combined with modified traditional ones arises very often. As a result, there would be established a system of quality control and cooperation insuring between traditional and new materials. Replacement and Strengthening of wall Intersections: Wall intersections are particularly vulnerable to earthquake damage, resulting frequently in large vertical cracks or separations as the walls are insufficiently interconnected and lack adequate strength to allow proper interaction. Considering the basic weakness of masonry construction under earthquake conditions, repair procedures are most often combined with a local strengthening of the wall intersection. 16

17 Stone stitching, or adding stones across the crack is a method, which can be used. A reinforced concrete corner column properly tied into the intersecting walls could be added to strengthen the wall intersection. Strengthening Walls by Confinement with steel sections: structural steel sections can also be used to strengthen masonry walls. Steel sections can be quickly installed and are often used when urgency of repair and strengthening is necessary. Depending on conditions, light steel sections may also be more readily available than shotcreting equipment or other suitable alternatives. The steel sections must be attached to tie beams or belts and the horizontal diaphragms at both top and bottom. Strengthening Rubble Core Stone Walls with Injection: Strengthening of rubble core stone masonry structures can be summarized as connecting the walls of the building at the level of the floor and roof by steel ties on both sides of the wall, anchored at their ends, and by grouting the walls with cement emulsion or other adhesives. This method of strengthening stone structures can be used in combination with other procedures such as adding shear walls or strengthening shear walls by jacketing of some wall parts within the structure. In this case, when new elements are introduced into the structure, the foundation structure should be checked and strengthened, if necessary. Steel profile jacketing: steel profile skeleton jacketing consists of four longitudinal angle profiles placed one at each corner of the existing reinforced concrete column and connected together in a skeleton with transverse steel straps. Jacketing with steel profiles (angles and straps) is used at the strengthening of separate members, mainly columns. The joint beam-to column is difficult to strengthen by this technique. Jacketing by steel encasement is implemented by gluing of steel plates on the external surfaces of the original members. This technique does not require any demolition, it is considerably easy for implementation and there is a negligible increase in the cross section size of the strengthened members. Steel encasement: Steel encasement is the complete covering of the existing column with thin steel plates. Special measures must be provided for fire and corrosion protection. Steelwork for structural restoration Steelwork can be conveniently used for the consolidation of all kind of structures, both old and new, made of all common constructional materials, i.e. masonry, timber, reinforced concrete and also steel itself. In the case of consolidation of historical structures, the use of steel gives further advantages to the designer who has undertaken this delicate operation. The use of steelwork is widely used in many consolidation operations which are framed in restructuring. Comparing to the use of metal sheets for structural reinforcement, the alternative application of FRPs introduces some significant advantages, such as the excellent weight to strength properties, the material availability in an unlimited length, the comparatively easier installation and the strength against corrosion. These advantages render composite materials to be a very attractive alternative proposal. Glued Metal or Fiber Reinforced Polymer straps The use of glued straps of steel or Fiber Reinforced Polymers, for strengthening of reinforced concrete elements, is nowadays a very popular technique due to the ease of its implementation. The traditional way of practice suggests the use of steel plates but the implementation of FRP straps has been growing competitively. 17

18 Shear connectors anchors Metal connectors which are anchored on existing concrete elements can act either as studs or as anchors, depending on the type of load applied. Studs are subjected to shear stress while anchors are stressed by axial load. There is a variety of industrialized connectors which are anchored on concrete elements in a mechanical or chemical way. Chemical fixing of connectors, constituting the most popular choice in practice, is almost always performed by epoxy resins. Anchorage and welding of new reinforcement bars Anchorage of reinforcement bars on hardened concrete is accomplished in chemical way, by the use of an epoxy resin. FIGURE 5.3 indicates that by adding wing wall increases strength and ductility, ductility is increased remarkably by RC jacketing, steel jacketing, FRP wrapping and installing seismic slit. FIGURE 5.3. Strengthening effect in the case of columns observed in structural test Repairing Gravity Load Capacity of Beams: Steel rods can be used for improving the shear resistance of damaged or undamaged beams. If load reversals are anticipated, four-sided jacketing is the preferred method of strengthening. Steel plate reinforcement is a new technique which can be used for beams subject primarily to static loading to improve their shear strength or midspan flexural strength. This procedure is not recommended for beams subject to cyclic loading due to earthquakes. FRP jacketing: The implementation of this kind of materials results in the increase or, even better, the alteration of bending, shear and axial strength of the member on which they are applied. The use of FRPs should generally be avoided when the bedding conditions are unknown or poor, there is in progress a significant corrosion of the reinforcement bars or there is no reinforcement ensuring the plastic behavior of the member to be strengthened. Among the most important advantages of the use of composite materials for structural repair and strengthening, in comparison to relevant traditional methods, the following ones can be stated: Insignificant repair is required in the work site. The evacuation of the area is not necessary and the annoyance to the users is minimal. The preparation of the elements to be strengthened is small and brief. The application of composite materials is simple. 18

19 The dimensions of the strengthened structural element remain practically unaltered, due to the small thickness of the composite material. The placing of composite materials is feasible even when there are working space restraints. The weight of composite materials is small and for their placing no heavy or specific equipment is required. Composite materials can be coated and colored according to the aesthetical needs of the work. The architectural characteristics of the structures remain practically unaltered. The application cost of composite materials is analogous to that of the traditional repair/strengthening methods. In FIGURE 5.4 is presented a comparison study based on structural tests regarding the strengthening methods applied to the RC frames. It is indicated an 3.5 to 5.5 times increase in strength in case of infilling wall. 0.6 to 1.0 times in strength and a little bit increased ductility can be seen in case of infilling wall compared with monolithic RC wall. FIGURE 5.4. Strengthening effect observed in structural test Introduction of new structural elements/systems: New vertical trusses can be constructed with steel members, cast-in-situ reinforced concrete members or a combination of the two. If reinforced concrete members are used, all members should be confined with closely spaced ties for their full length to provide adequate member ductility. Concentrically braced frames: This traditional form of bracing is, of course, widely used for all kinds of construction such as towers, bridges, and buildings, creating stiffness with great economy of materials in two-dimensional trusses or three-dimensional space frames. Concentrically braced frames are constructed from steel, timber, and concrete and composite forms are frequently met such as timber beams and columns with steel diagonals. Eccentrically braced frames: This system conforms in part to the requirement for good earthquake design of failure mode control, insofar as post-elastic behavior of the frame is largely confined to selected portions of the beams and sudden failure modes are suppressed. Hybrid structural systems: The most common of these hybrid systems are those in which momentresisting frames are combined with either structural walls or diagonally braced frames. While hybrid systems are often unavoidable and can provide good seismic resistance, care must be taken to ensure that the structural behavior is correctly modeled in the analysis. 19

20 FIGURE 5.5. Strengthening effect observed in structural test Foundations It may be desired that the strengthening of the foundation is not required in seismic retrofit pf buildings. In general, strengthening of foundation shall be performed only when the retrofit scheme is simple, practical, cost effective, and reliable for drastic improvement of seismic performance. Also, when adverse effects to the structural performance of building concerned is expected in future due to settlement of ground, negative friction of pile, and liquefaction of sandy soil at the time of earthquake, we have to improve the soil performance appropriately. FIGURE 5.6.Seismic strengthening method of pile foundation 20