Retrofitting Structures of Historic & Heritage Importance

The definition of historic and heritage structures may vary from country to country and from culture to culture. But the need for the existence of these assets of historic values is nowhere denied, rather they are worth preserving. The design, construction and the materials used for these structures are often drastically different to survive against the threat of present-day hazards. The technique that is used to make the life of the structures longer is called retrofitting – a general term that consists of a variety of treatments: preservation, rehabilitation, restoration and reconstruction. It is not that retrofitting is done in case of the structures of historic importance only. But in such cases the technique of retrofitting is a lot more challenging, with many restrictions imposed. Retrofitting of the historic structures generally involve an array of diverse technical considerations such as fire safety, remedies against weathering and water infiltration, chemical attack, geotechnical hazards, structural performance under earthquake and wind loads etc.

Introduction

Retrofitting (Kelly et al.,2005), the general term in broader sense means preservation which is the process of applying measures necessary to sustain the existing form, integrity and materials of a historic structure. Rehabilitation refers to the process of creating new application for a structure through repair, additions and alterations while preserving those features which convey its historical, cultural, or architectural values.

Restoration is the process of accurately restoring a structure as it existed at a particular period of time. Reconstruction is described as the act of replicating a structure at a specific period of time. Having said that, retrofitting especially aims to enhance the structural capacities (strength, stiffness, ductility, stability and integrity) of a structure that is suspected to be deficient or vulnerable. In the specific context of enhancing the resistance of a vulnerable structure to earthquake the term seismic retrofit is used. The building need not be deteriorated or damaged as a seismic retrofit is intended to mitigate the effect of a future earthquake.

The principal difference between a heritage structure and a regular or traditional structure is that a retrofitting technique cannot indiscriminately be applied with the sole aim of improving structural response to earthquakes and other components in case of a historic structure. Application of techniques and materials should be chosen very cautiously and judiciously so that the authenticity and heritage value of the structure in its entirety is maintained.

Basic principles of retrofitting of structures of historic and heritage importance: Structures of heritage and historic importance present a number of challenges in restoration and retrofit which limit the application of codes and building standards of modern times. However, recommendations and guidelines are necessary to enable rational methods of analysis and restoration work fitting to the cultural context. Some relevant recommendations of International Council on Monuments and Sites (ICOMOS) are listed were:

ICOMOS, 1964

1. The restoration of monuments must have recourse to all the techniques which can contribute to the safeguarding of the architectural heritage. (Article: 2)
2. The intention in conserving and restoring monuments is to safeguard them no less as works of art, than as historical evidence. (Article: 3)
3. The aim of restoration is to preserve and reveal the aesthetic and historical value of the monument and is based on the respect for original material and authentic documents. (Article: 9)
4. Where traditional techniques prove inadequate, the restoration of a monument can be achieved by the use of any modern technique of construction, the efficacy of which has been shown by scientific data and proved by experience. (Article: 10)
5. The valid contributions of all periods to the building or a monument must be respected, since unity is not the aim of restoration. When a building includes the superimposed work of different periods, the revealing of the underlying state can only be justified in exceptional circumstances. (Article: 11).

ICOMOS, 2003

1. The removal of the inner structures maintaining only the facades does not fit the conservation criteria. (Article: 1.3)
2. No action should be undertaken without having ascertained the achievable benefit and harm to the architectural heritage, except in cases where urgent safeguard measures are necessary to avoid the imminent collapse of structures (for example, after seismic damages). Those urgent measures, however, should not be irreversible. (Article: 1.7)
3. Therapy should address root causes rather than symptoms. (Article: 3.1)
4. Safety evaluation and an understanding of the significance of the structure should be the basis for conservation and reinforcement measures. (Article: 3.3)
5. The choice between “traditional” and “innovative” techniques should be weighed upon a case-by-case basis and preference given to these that are least invasive.
6. At times of the difficulty of evaluating the real safety levels, the possible benefits of interventions may suggest “an observational method”, that is an incremental approach, starting from a minimum level of intervention. (Article: 3.8)
7. Where possible, any measure should be “reversible”, so that they can be removed and replaced with more suitable measures when new knowledge is acquired. Where they are not completely reversible, interventions should not limit further interventions. (Article: 3.9)

Degree of Intervention: Retrofitting of historical building is a difficult compromise between requirements of structural theories and conservation principles. Intervention should be reversible to give room to incorporate better solutions with technologies to come in future. That way, intervention must be “as much as necessary, but as little as possible”. Temporary interventions must be carried out to prevent catastrophic collapse if such situation occurs. Before the final retrofit procedure is tested and arrived at, safety against collapse such as during the post-earthquake investigation may be ensured by temporary interventions.

For a monumental building, eight degrees of interventions in the ascending order of intrusion are possible:

1. Prevention of deterioration
2. Preservation of existing state
3. Consolidation of the fabric
4. Restoration
5. Rehabilitation
6. Reproduction
7. Reconstruction
8. Translocation

Investigation and collection of information: Investigation and collection of information can be broadly classified into four categories:

1. Foundation and geologic
2. Architectural
3. Structural
4. Services such as heating, ventilation, air conditioning (HVAC), sanitary plumbing, fire fighting, illumination (energy retrofitting) etc.

Foundation and geologic investigation: This part deals with the investigation of soil strength and stiffness parameters for consideration of foundation-load deformation characteristics, collection of specified data for site characterization, outlining procedures for mitigation of geologic site hazards and specific requirements for seismic rehabilitation of foundations.

Load-Deformation characteristics for foundations: Foundation-deformation behavior of foundation characterized by both stiffness and capacity can have significant effect on both structural response and load distribution among structural elements. Load-deformation characteristics are required where the effect of foundations are to be taken into account in Linear Static and dynamic procedures, non-linear static (pushover) procedures (NSP) or nonlinear dynamic procedures. Because of the difficulties in determining the properties of soil and the possible variability of soil supporting foundations, an equivalent elasto-plastic representation of load-deformation behavior is recommended recognizing the load-deformation behavior of foundation being non-linear. The sources of uncertainty include variations due to rate of loading, assumed elasto-plastic soil behavior, level of strain, cyclic loading, variability of soil properties.

Collection of Specified data for site-characterisation: Site characterisation shall include collection of information on the structural foundation, sub-surface soil and condition and seismic geologic site hazards. The following structural information should be obtained for the foundation of the structure to be rehabilitated:

1. Foundation type.
2. Foundation configuration, including dimensions, locations, depth of embedment of shallow foundations, pile tip elevation, and variations in cross-section along the length of the pile or belled caissons.
3. Material composition and details of construction.

Information on sub-surface soil conditions should be obtained as required by the selected performance level:

1. For collapse prevention and life safety performance levels, the type, composition, consistency, relative density and layering of soils shall be determined to a depth at which the stress imposed by the building is less than or equal to 10% of the building weight divided by the total foundation area. For buildings with friction piles, the depth so calculated shall be increased by two-third of the pile length. For end bearing piles, the depth of the investigation shall be the pile length plus 10 feet.
2. Location of water table and its seasonal fluctuations beneath the building should be determined.
3. For enhanced rehabilitation objectives, the unit weight of soil g, soil cohesion C, soil friction angle j, Soil Compressibility Characteristics, Soil Shear Modulus G and Poisson’s ratio µ, for each type shall be determined.

Seismic Geologic Site Hazards: Seismic rehabilitation shall include an assessment of earthquake induced hazards at the site due to fault rupture, liquefaction, differential compaction, land sliding and an assessment of earthquake induced flooding or inundation.

Fault Rupture: A geologic fault shall be denied as a plane or zone along which earth materials on opposite sides have moved differentially in response to tectonic forces. Geologic site information shall be obtained to determine if an active geologic fault is present under the building foundation.

Liquefaction: It is denied as the earthquake induced process in which saturated, loose, granular soils loose shear strength and liquefy as a result of an increase in pore-water pressure during earthquake shaking. Subsurface soil and ground water information shall be obtained to determine if liquefiable materials are present under building foundation. If present, the information required are: soil type, soil density, depth of water-table, ground surface slope, proximity of free-face conditions and lateral and vertical differential displacement.

Differential Compaction: Differential compaction shall be denied as an earthquake induced process in which foundation soils compact and the foundation settles in an inform manner across a site.

Land Sliding: It is denied as down-slope mass movement of earth resulting from any cause. Subsurface soil information shall be obtained to determine if soils susceptible to a landslide that will cause differential movement of the building are present at the site.

Flooding or Inundation: For seismic rehabilitation of buildings for performance levels higher than life safety, site information should be obtained to determine if the source of earthquake induced flooding or inundation are present such as:

1. Dams located upstream, subject to damage by earthquake shaking or fault rupture.
2. Pipelines, aqueducts and water storage tanks located upstream, subject to damage by fault rupture, earthquake induced landslides or strong shaking.
3. Coastal areas within Tsunami zones or areas adjacent to bays or lakes subject to seismic waves.
4. Low-lying areas with shallow ground water, subject to regional subsidence and surface pounding of water resulting in inundation of site.

An unique example of restoring a very famous historic structure by mainly solving the geotechnical issues is the Leaning Tower of Pisa. The person who was instrumental behind the great and challenging job is Prof. John Burland. It entailed drilling out slivers of soil from beneath the northern side of the tower-away from the lean-and allowing gravity to coax the structure back upright. By this process the tower itself was not touched, thus, the famous heritage structure could be restored with all of its glory, still leaning but stabilised.


Architectural Investigation: Architectural investigation of an historic or heritage structure is the critical step in planning an appropriate treatment after assessing how a structure has changed over time causing the deterioration. For retrofitting of the historic structures, the need of a meticulous planning prior to work on the irreplaceable cultural resources cannot be denied. It actually necessitates proper investigation. Whether investigation will be undertaken by professional-architects, conservators, historians or by property-owners, the process should essentially be comprised of a four-step procedure: Historical Research, Documentation, Inventory and Stabilization.

Historical Research: The primary resource for carrying out historical research on a structure of heritage importance are generally drawings, maps, plates, paintings, photographs, legal transaction documents, account books, insurance policies, letters, diaries etc. Oral statement from people’s remembrances can also be taken. Secondary resources comprised of research or history already complied and written about a subject, are also important for providing a broad contextual setting or a project. Historical research should be conducted well before physical investigation because it cannot be over-estimated that proper historical research can give many important clues for structural investigation and solutions as well. For example, the historical research on a structure through deed records may not only determine the sequence of owners but also in turn, aids to investigate the chronology of ownership and the changes each owner had made on the structure. A letter may indicate that an owner / occupant of the building / structure painted that in which year.

Documentation: A simple but comprehensive method of documentation is to take photographs of every wall elevation (interior and exterior), as well as general views and typical and unusual details. The systematic numbering of rooms, windows and doors on the floor plan will help organizing this task and also be useful for labeling the photographs. Video coverage with annotated sound may supplement still photographs. Additional methods of documentation include written descriptions, sketches and measured drawings. It is very important to note that the documents created during investigation might play an unforeseen role in future treatment and interpretation. Documentation is particularly valuable when a feature will be removed or altered.

The date of original construction and later changes have to be searched and documented. In absence of such information, architectural histories and field guides to architectural style can help identifying a structure’s age through its form and style.

The architectural investigation usually determines original construction details, the chronology of later alterations and the physical condition of a structure. In detailed investigation of the fabric of historic building, its materials and features are to be documented properly such as:

Masonry & Mortar: Studying historic brickwork can provide important information about methods of production and construction. From colour, size, shape and texture of brick ideas and information can be had whether, it was hand molded and traditionally fired or machine molded and fired in a kiln using modern fuels. Similarly, whether the mortar is lime or cement, if lime, whether hydrated lime or hydraulic lime, can give information about time, place and human variables of construction such as type of bond, special brick shapes, decorative uses of glazed or rubbed brick, joints, coatings and finishes. The same types of questions related to production and construction characteristics can be applied to all types of masonry work, including stone, concrete, terra cotta, adobe (sun dried earthen brick) and coquina construction. During “surface mapping” a complete survey can outline the materials and construction practices for the various periods of a structure, distinguishing the original work as well as the additions, alterations and replacements.


Wood & Timber: A considerable number of historic structures or a part are constructed with timber or wood. As it is light, easy and fast to build, resistant against the horizontal forces, easy to curve craft works, timbers are preferred construction material for structures which were built in historic times. Investigations may be carried out on type of construction – timber frame structure or otherwise, evidence seen on wooden surface indicating whether production was by axe, pit saw, mill saw or band saw, what are the varying dimensions of timber used, whether the timbers are fastened by notching, mortise or tenon pegs or nailing. From the mails also, important information can be obtained about the period of construction. Irrespective of region or era, the method of joining, framing and finishing a wooden structure will reveal something about the original construction, its alternations and the practices of its builders.


The roofs of many historic buildings are constructed as wooden single roof. In such a case, the following information is required:

• Original wood type.
• Size of shingle (length, width, butt thickness, taper etc.)
• Exposure length and nailing pattern.
• Type of fabrication.
• Distinctive details (hips, ridges, valleys, dormers etc.)
• Decorative elements (trimmed butts, variety of pattern, applied colour coatings, exposed nails etc.)
• Type of substrate (open shingle lath or sheathing, closed sheathing, insulated attics, sleepers etc.)

Steel & Concrete are two important construction materials which are generally of lesser use in historic structures.

Roofs: Exterior features are especially prone to alteration due to weathering and lack of maintenance. Roof covering typically lasts not more than fifty years. If roof covering is done with some layers in different times, clues can be obtained from that. Or if earlier coverings were removed and new causing were done, clues can be obtained from that too. The roof pitch itself can be a clue to stylistic dating and unlikely to change unless the entire roof has been rebuilt. Apart from wooden shingles, metal shingles, sheet metal, asphaltic or asbestos shingles were also used in historic building roofs. Slate, clay tiles and lime-terraced roofs are also found.

Floors: Floor finishes, covering, pattern & floor tiles can give sufficient clues.

Walls & Plasters: Inside and outside walls along with their associated trim and plaster over them, ornamental plaster, stucco etc. hold many clues of building’s construction and changes made over time.

Attics & Basements: It is very common that Attics & Basements have been used as collection points of out-of-date, out of style, cut-off pieces, rejected furniture, furnishing, family records, & architectural fragments. These and out of the way places of a structure provide an excellent opportunity for non-destructive investigation because these are the areas where structural and framing members may be exposed and escaped from alterations done in more lived parts of a building.

Overall, to investigate, identify and record the architectural features, fabric and character of a historic building, some checklists are necessary which involve the steps:

1. Overall visual aspects
2. the visual character at close range
3. Visual character of spaces, features and finishes.

Considering the intangible aspects of a building or structure are already enough to establish the building or structure with historic or heritage status, the checklist of the tangible aspects may be short listed on the following items:

Step – I :-

1. Shape
2. Roof and roof features
3. Openings
4. Projections
5. Trim and Secondary features
6. Materials
7. Settings (alignment or orientation etc.)

Step – II :-

1. Materials at close range
2. Craft details

Step – III :

1. Individual spaces
2. Related spaces and sequences of spaces
3. Interior features
4. Surface finishes and materials
5. Exposed structures.

It should be remembered that the documents created during investigation might play an unforeseen role in future treatment and interpretation. Documentation is particularly valuable when a feature will be removed or altered.

Inventory: The historic building and its components should be carefully inventoried prior to taking up any action. Premature clean-up of structure or site may be a mistake which should be avoided. Inventory list should also include material which have fallen off due to deterioration, fragments removed and stored in basement, attics or out of buildings, parts of fragile components and even materials which have seemingly been discarded. From the very beginning, anything that seems even remotely meaningful should be saved. Even if the period of significance or interpretation is known from the beginning, evidence from all periods should be protected.

Stabilization: In urgent cases, immediate stabilization is necessary to ensure that a structure does not continue to deteriorate prior to a final treatment or to ensure safety of current occupants, investigator or visitors. Very severe cases may call for structural remedies, but generally preliminary stabilization would be undertaken on a maintenance level. Such work would involve making arrangements for keeping water away from the structure particularly from roof and foundation, removing plants that are too close to the structure that hold water or securing structures against insects, animals and vandals. Stabilization may add to the cost of any project, but human safety and protection of historical evidence are well worth the extra money.

Structural Investigation The condition of any structure typically deteriorates with age. Moreover, if there is inherent design or construction defect, then there is detrimental effect on performance. Any rehabilitation in past may have boosted the performance. But that may not be sufficient to cater present day’s hazard, the required performance may be even higher. For historic structures which are supposed to survive through generations, this requirement is rather more critical. Thus, proper condition assessment is necessary before retrofit is undertaken to assess the actual condition of the structure in relation to the current requirement. The actual condition or performance of a building with time is depicted in figure 1.


After the retrofit scheme is implemented, the structural performance is expected to improve to meet the requirement throughout the remaining life of the structure.

Condition assessment includes the following steps:

1. Initial inspection and appraisal
2. Review documents
3. Detailed investigation
4. Reporting and recommendations

In initial inspection and appraisal, a visual inspection helps to plan a strategy to inspect the structure further, using more sophisticated techniques.

Review of documents involve after collection of documents mainly available structural drawings, architectural drawings, geotechnical report, construction specifications site plan, inspection reports, reports on previous investigation, previous repair works, any complain letter and inspection reports after site visit and with the technique of Rapid visual Screening (RVS) as per FEMA (Federal Emergency Management Agency) 154 and 155, taking the right decision in which way the detailed investigation should be carried out.

Detailed investigation includes the following:

1. Obtaining properties of the structural materials used in the structure.
2. Locating deteriorated materials and other defects, and identifying their causes.
3. Determining the type and disposition of reinforcement in reinforced concrete members if at all present in historic structures.

Reporting & Recommendation: After completion of detailed investigation, a report should be prepared and suitable recommendations should be prescribed.

Materials for Structural Inspection: The construction materials for inspection are:

Masonry

• Quality of bricks, masonry units such as stone mortar.
• Cracking and differential movement.

Concrete

If concrete is used at all in historic structure

• Pattern, location and orientation of cracks
• Scaling, sapling, staining, disintegration of the surface, honey-combing.
• Express reinforcement and corrosion.

Steel:

• Corrosion.
• Stress concentration (evident from crack in the point).
• Crippling, misalignment, deformation, twisting.
• Cracks in welds or missing welds.
• Missing bolts / rivets.

Timber:

• Defects in wood.
• Insect damage.
• Decay.
• Cracking.

Detailed Structural Investigation: For conducting detailed structural investigation, three types of tests are generally carried out

1. non-destructive
2. intrusive and
3. destructive test

Non-destructive Tests: First, some basic tests may be done, which are to follow visual inspection.

Key Test: Scrapping by a key or sharp knife will enable to identify porous portion of mortar at the joints of masonry wall. The week portions of mortar can be identified and suitable further investigation at these locations can be carried out.

Spray Test: Cleaning the concrete or masonry surface by water spray which will clearly show the cracks and their pattern.

Push Test: A weak portion of wall may be identified sometimes by pushing it. Weak portion, if any, can be identified by some movements.

Next step follows the followings –

Rebound Hammer Test: It is of two types in

1. Pendulum type which is applicable for lower strength concrete and masonry block
2. Schmidt rebound hammer type, suitable to assess the strength and quality of concrete near the surface. Depending on volume and strength of concrete, the types may be ‘M’, ‘N’ or ‘L’ type however the procedure of the test is as per IS:13311 (1992) Part:2.

Penetration Techniques: Such as Windsor probe method work on the principle of resistance to penetration of a probe that is shot into the concrete surface with a definite amount of energy. The depth of penetration of the hardened steel alloy probe is empirically related to compressive strength of concrete. The procedure of the test is as per ASTMC803-03. This test is also able to assess the surface condition of concrete.


Thermal Methods: It is as per ASTMC 1046-95. By infrared thermography technique, the detection of heat flow through a body can indicate the presence of flows or defects. However, there are limitations such as accuracy is somewhat limited to near surface area and application depends on particular weather condition.

Radiography: In radiography, X-rays or neutrons are passed through the structural element and resultant image is captured on a film. This film is then studied to find the location of defects as per ASTME 748-02. Internal flaws are easy to detect using radiography. The limitation of this method is the hazard associated with the test and higher cost.

Ultrasonic Pulse Velocity Test: The procedure of this test has been laid down in IS: 13311 : 1992, part : 1 UPV method is used to establish Homogeneity of concrete, pressure of cracks, voids and imperfections, change in structure of concrete with time, quality of concrete in relation to standards, quality of one element of concrete in relation to other, values of elastic modulus of concrete.


In this method, the velocity of a pulse traveling through concrete is measured and co-related to different characteristics of concrete. It is conducted in three transmission modes: direct, semi-direct and indirect. By this method, although on overall assessment of the quality of concrete can be obtained, it may be difficult to point out the exact location of a defect.

Impact Echo / Pulse Echo Method: In the impact echo method, sound waves are created by impacting device such as hammer. The waves those reflect off defects or other features are picked up by a receiving transducer and conveyed to a processor, where analysis is done to evaluate the amplitude and travel time. In pulse echo system, the pulses are generated by a pulsing transducer (IS : 3664 : 1981).

In this method, defect lying under other defects are not easy to detect. Also, reflections from sides, edges and corners can confuse the data.

Measurement of corrosion of reinforcing bar by half cell potential test: Corrosion in reinforcing steel bars inside concrete is detected by half cell potential test. By this method, electrostatic potential of the bar is measured with respect to reference electrode. Depending on the potential values, the likelihood of corrosion of the bar is judged as per provision of ASTMK C 876-91.


Intrusive Test : Where the non-destructive tests have limitations in suitably detecting the defects, intrusive tests are carried out. These tests can cause some damage to the structural members, which are needed to be repaired.

Core Tests: In order to ascertain the compressive strength of concrete or masonry, a cylindrical core is usually drilled out from the structural member. IS: 1199 (1959) describes the procedure of obtaining cores from hardened concrete. Proper care should be taken so that no damage is caused to the member for drilling core. For an R.C.C. member, the location of reinforcing bars inside the concrete should be carefully mapped before drilling a core from it. Concrete core can be tested as per the procedure of IS : 516 (1959). Compared to non-destructive determination of the in-place compressive strength, the result obtained from core tests yield a much more reliable data. In general, cores removed from a concrete structure give lower strength compared to cylinder specimens cast along with the structure and cured under specified condition. According to American Concrete Institute Guideline, if at least 3 cores are removed from a representative part of concrete and none of them shows less than 75% of the characteristic strength and the average is not less than 85% of characteristic strength, then the concrete is considered to be in sound condition.

For masonry buildings, samples taken along the horizontal and vertical directions tend to give different compressive strengths.

Apart from determination of compressive strength and elastic modulus, core samples may be used to further study the material proportions and microstructure. By chemical analysis, the cement content and water-cement ratio of concrete may be determined. The microstructure can be studied for the presence of excess voids in case of durability problems.

Few other tests are also generally done on cores

• Determination of depth of carbonation: Phenolphthalein solution is sprayed immediately on the outer surface of extracted core. The depth of carbonation is measured as the depth up to which the concrete does not develop any pink colouration. When the colour turns to pink (pH above 10), the concrete is ok. No colour change indicates, the concrete is affected by carbonation.
• Sulphate, chloride and pH are also determined from the extracted cores.

In Situ Shear Test : The in-situ shear test is used to find the actual shear strength of the mortar joint in a masonry wall. First, the mortar joints around one or two masonry units are removed. Next, a small jack is inserted in the wall and the in-situ shear test on mortar joint is carried out. The procedure of the test is described in ASTM C 1531-03.


Flat Jack Test : The flat Jack Test is used to find the in-situ compressive stress in a masonry wall. Flat jacks are inserted into horizontal slots cut in a wall. The pressure required to bring back the deformation across the opening is used to arrive at the in-situ compressive stress in the wall. The procedure of the test is given in ASTM C 1196-04. The flat jack can also be used to measure deformability properties of masonry (ASTM C 1197- 04) and shear strength of mortar joints (ASTM C 1531-03).

Destructive Testing : In most of the cases of historic structures, nothing more than historical research, surface examination and non-destructive tests are required. In very rare instances, the investigation may require a sub-surface examination and removal of fabric. Such destructive testing should be carried out in most accessible spaces, such as retrofitted service and mechanical chases, loose or previously altered trim, ceiling or floor boards. This type of subsurface testing and removal is sometimes called “architectural archeology because of its similarity to the more well-known process of trenching in archeology. The analogy is apt because both forms of archeology use a method of destructive investigation.

Investigation for services to be provided such as heating, ventilation, air conditioning (HVAC), sanitary plumbing, fire fighting, illumination (energy retrofitting) etc.

If the historic structure is a building, then suitability of incorporation of mechanical electrical and sanitary plumbing systems for provision of creating, ventilation, cooling (air conditioning), fire fighting etc. has to be taken up with due care. As such, no set of formula exists for determining what type of mechanical and electrical system is best for a specific building. For a historic building, it is rather more critical. The difference of temperature and humidity level in interior and exterior of a historic building has considerable effect on damage. Methods of controlling interior temperature and humidity and improving ventilation must be considered in any new or upgraded HVAC or Climate Control System. Certain energy retrofit measures will have a positive effect on the overall building but installing vapour barrier in the walls of historic building often results in destruction of significant historic material.


Retrofit techniques and measures:- Various retrofit techniques and measures for vulnerable heritage structures include intervention at a component level and intervention at the structural level.

Component intervention include strengthening of walls, arches, vaults, domes, towers and spires while structural level intervention will include strengthening of soil and foundation, reduction of forces by based isolation and energy dissipation and seismic retrofitting.


Strengthening of masonry walls:- Various techniques are available for strengthening of different types of masonry walls, while choosing the method of strengthening, the type and quality of the masonry material and the structural integrity of the building are main criteria to be considered.

Many methods are there like repairing of cracks by grout injections, coating with a cement concrete and with a wire mesh in addition to grout injection, re-pointing of bad joints by replacing parts of the existing deteriorated mortar in bed-joints and sometime placing steel bars, stainless steel bars, fiber reinforced plastic bars along the bed joints, reinforced concrete jacketing, grout injection, wall reconstruction where excessive bulging or collapse has occurred, strengthening using fiber reinforced polymer (FRP) which is non-invasive and reversible, thus, suitable for historic building, strengthening using Shape Memory Alloys (SMA), repairing of wall corners and intersections by stone stitching or metal stitching. Figures of some of the repairing techniques are shown in Fig. 2.


Strengthening of Arches, Vaults and Dames:- Dry stone masonry offers very high strength in compression, but their joints provide limited shear and tensile resistance as they depend purely on friction. Shear and tensile strength can be improved by inserting dowels, cramps or special type bars or structural connections inserted through specially prepared holes in joints without being visible from outside.

Strengthening of Soil and Foundation :- The methods of strengthening soil and foundation of historical buildings can be grouped into the following (Przewlocki et al, 2005):-

• Increasing the area of foundation, lowering the foundation level and strengthening the existing foundation.
• Inclusion of structural elements such as piles and different ground improvement techniques like micro piles, underpinning, nailing etc.
• Modifying the effective stress of soil by drainage or consolidation.
• Improving the subsoil by chemical or cement grouting or electro-osmosis.
• Replacement of entire sub-structure.

Seismic Retrofitting

Minimum Seismic Resistance:- The required minimum seismic resistance (MSR) is expressed quantitative by design seismic co-efficient as per IS : 1893-2002 which includes –

• The basic seismic co-efficient of the zone
• The fundamental period of the building and
• Importance factor of the building.

Available Seismic Resistance:- The available seismic resistance (ASR) is expressed quantitatively by the earthquake force under which the first of the columns of any building storey will reach its ultimate limit strength, when the remaining structure remains in the undamaged state.

Theoretically, if ASR equals MSR, no damage should be expected, provided a reliable value of ASR could be determined. If reliable value of ASR is determined analytically, the ratio ASR/MSR would be a safe indication of expected damage. But to determine the reliable value of ASR is a challenge because of the dynamic character of the problem, the inelastic behaviour of structure, the materials, the infills and many others. The retrofitting of an existing structure, therefore, must be based on ASR/MSR.

Level of Retrofitting:- Three level of ratio ASR/MSR are adopted for decision making purposes.

• ASR/MSR is more than 0.8
  In such cases, the seismic resistance is considered satisfactory with the probability of somewhat deeper excursions into the inelastic range, without approaching the failure limits retrofitting may not be needed.
• ASR/MSR in the range 0.8 to 0.5
  If enough ductility exists, the building could have safety against collapse in a strong earthquake, but this type of structure could reach the failure limits. The structure needs strengthening by retrofitting.
• ASR/MSR is less than 0.5
  The safety of the structure is clearly unsatisfactory, hence, it will require retrofitting / or upgrading the strength as well as ductility.

Base Isolation Technique: Isolation of superstructure from the foundation in known as base isolation technique. It is a very powerful tool for passive structural vibration control technique. It gives significant protection to the structure of building, non-structural components and contents but is a costly option.

Seismic Isolation are classified as follows

Elastomeric Base Isolation Systems:-

• This is most widely used base isolator.
• The elastomer is made of either natural rubber or neoprene.
• The structure is decoupled from horizontal components of the earthquake ground motion.
• A layer with low horizontal stiffness is introduced between the structure and the foundation

Sliding Base Isolation Systems

• This is the second basic type of isolators.
• This works by limiting the base shear across the isolator interface.

Advantages of Base Isolation

• Isolates building from ground motion, hence causes lesser damage to the structure and minimal repair of superstructure.
• Building can remain serviceable throughout construction.
• Does not involve major intrusion upon existing superstructure.
• This technique is suitable for historic building because it reduced the extent and intrusion of seismic modifications on the historic fabric of the building.

Disadvantages of Base Isolation

• Expensive.
• Cannot be applied partially to structures unlike other retrofitting.
• Challenging to implement in an efficient manner.
• Allowance for building displacement is required.
• Not effective technique for high rise building.
• Not suitable for buildings rested on soft soil.

Energy Dissipation System:- Energy dissipation System are considered in a somewhat broader context than isolation system. For the taller buildings where isolation system may not be very effective, energy dissipation system should be considered as a design strategy. Certain energy dissipation devices are economical and might be practical for performance goals that address only limited safety. Energy dissipation devices may also be useful for control of building response due to small earthquakes, winds or mechanical loads.

These devices are used in forms of seismic dampers in place of structural elements like diagonal bracings, for controlling seismic damages in structures. It partly absorbs the seismic energy and reduces the motion of buildings.

Types of Seismic dampers are :-

• Viscous Dampers: Energy is absorbed by silicon-based fluid passing between piston-cylinder arrangements.
• Friction Dampers: Energy is absorbed by surfaces with friction between them rubbing against each other.
• Yielding Dampers: Energy is absorbed by metallic components that yield.

Tuned Mass Damper (TMD)

• It is also known as an active mass damper (AMD) or harmonic absorber.
  It is a device mounted in structures to reduce the amplitude of mechanical vibrations.
• Their application can prevent discomfort, damage or outright structural failure.
• This type of dampers is suitable for tall buildings and other taller structures like transmission tower.

[Largest TMD sphere in the world and weighs 660 metrictonnes with a diameter of 5.5 metre and costs US$4 million (total structure costs US$ 1.80 billion).]

Case Studies

Case:A

Mani Mandir (Temple) Complex (100 m x 100 m in plan) is an important historic monument of the town of Morbi in the State of Gujarat which suffered significant damage during Bhuj earthquake (M7.7) in India during 2001. Morbi is situated in Western Region of Gujarat at a distance of about 125 km from the epicenter of the 2001 Bhuj earthquake. The Mani Mandir is located in the Central Courtyard of Willingdon Secretariate Building in the western bank of river Macchu just outside the fortified walls of the old Morbi Town. The complex was built during 1930s by the ruler of Morbi. It comprises of a very ornate masonry building built in yellow sandstone in the tradition of Indo-Saracenic style of architecture.

As a part of earthquake reconstruction program, Govt. of Gujarat decided to seismically retrofit the complex. A detailed condition survey was carried out and measured drawings were prepared. A comprehensive retrofit program was formulated. In the retrofit program due weightage was put on conservation principles, minimum intervention and consonance with the heritage character of the building. The complex was modeled using finite elements and behaviour was studied of the existing structure as well as retrofit structure. The recommended retrofit measures included discriminate use of internal reinforced concrete skin walls, providing a rigid diaphragm behaviour mechanism in existing slabs, introducing stainless steel reinforcement bands in the existing masonry walls, cross-pinning and end pinning in walls and pillars and strengthening of arches and elevation features. The structural retrofit program has tried to limit the intervention to most minimum. The total area of the new reinforced concrete skin wall introduced is less than 10% of the area of the existing masonry walls. The estimated cost of retrofit program worked out to less than Rs.4300/m2 compared to a standard new reinforced concrete building of Rs.3000/m2. The cost of constructing a similar ornate masonry building would be many times more, if at all it could be built.

Case: B

The region of the Sikkim and Darjeeling Himalayas is known for seismic activities and the state Sikkim is located in the Seismic Zone IV of India’s Seismic Zoning Map. On 18th September’ 2011, earthquake struck Sikkim with a magnitude of 6.9, the epicenter located near the India-Nepal border about 68 km NW of Gangtok, the capital of the State of Sikkim. The tremor was felt across a wide region including India, Nepal, China, Bhutan and Bangladesh. The most affected areas with the highest level of ground shaking were Chungthang and Lachung with maximum intensity of VIII in North Sikkim and greater area of Ganghtok with an intensity of VI on MSK Scale. The earthquake, aftershocks and heavy seasonal rains triggered more than 300 aftershocks and heavy seasonal rains triggered more than 300 landslides which caused most of the fatalities and damage to the infrastructures including several buildings out of which many are of heritage status.

After this earthquake an investigative study was carried out, based on the inventory “Cultural Properties of Sikkim” which was complied in 2003/2004 by INTACH in collaboration with the Cultural Department of Sikkim. The total number of 287 heritage properties were documented including precincts, settlements, buildings, sacred structures, places of worship etc. The principal aim of the study was to evaluate the degree and kinds of damage to the cultural heritage properties of Sikkim caused by earthquakes. Restoration proposals were also suggested in particular cases where repair, restoration and retrofit were required immediately.


The study revealed two distinct damaged property categories. The first one are the old stone masonry buildings such as Buddhist monasteries and kothis which are an integral part of Sikkim’s heritage. Though those structures have shown decays related to again combined with survival against several earthquakes during countries, the performance has been considered as moderate, however, immediate repair and retrofit works are required. The second group of major damaged buildings are recently built religious structures such as Gumphas and Mandirs and institutional buildings, constructed predominantly by R.C.C. Major structural damage was occurred in this buildings by 2011 earthquake which proves a rather poor seismic performance mainly due to low quality standards involved in the construction work. For example, the old Rumtek Gompha has stone masonry walls in ground and first floor, which suffered only superficial damage such as cracks in plaster due to 2011 earthquake. The roof level and second floor built as RCC frame suffered severe damage such as column failure and separation of the staircase. As per the study, the assessment was done as –

• Damage occurred mostly to R.C.C. structures in roof level.
• Several damages occurred to R.C.C. Pillars and R.C.C. Staircase prone to collapse.
• Slight cracks in walls.
• Several collapsed R.C.C. pillars were replaced by brick pillars.
• Non Earthquake related damage: impermeable cement concrete plaster at plinth caused dampness in lower wall portions and Damage in columns above roof level in 2011 Earthquake.

Suggested recommendations as per the study are –

• Consolidation of the ground around the building and the courtyard.
• Structural consolidation of the building: dismantling of temporary brick pillars and replacing missing structural members of roof level and grouting of slight cracks.
• Retrofit: additional reinforcements for R.C.C. pillars in roof level.
• Replacing the R.C.C. structure by wooden frame structure where possible / necessary.
• Replacing cement plaster of plinth level.
• Provision of drainage.
• Replacing of damaged moulding in second floor.
• Retouching of wall paintings.

References :-

1. FEMA 356
2. Preservation Briefs (1-47), National Park Service, U.S. Department of Interior.
3. Handbook on Seismic Retrofit of Buildings (April’ 2007) CPWD & Indian Building Congress in Association with IIT, Madras.
4. Seismic Evaluation of Existing RC Buildings by Prof. D. K. Paul, Emeritus Fellow, IIT, Roorkee.
5. NDT Evaluation : Durability Tests by Dr. U. K. Sharma, IIT, Roorkee.
6. Seismic Retrofitting of Mani Mandir Complex at Morbi, Gujrat, India by Alpa Sheth, R. D. Chaudhari Ejaz Khan, Divay Gupta and Malvika Saini.
7. Improving the Structural Performance of Heritage Buildings : New Zealand Historic Places Trust. Pauhere Taonga.
8. Strengthening of Historic Buildings in Past-disaster Cases by Banu Çelebioğlu* Department of Architecture, Yıldız Technical University.
9. Methods of Seismic Retrofitting: MIT (Part-5) & Sevgül Limoncu
10. Seismic Retrofitting of Heritage Buildings : Conservation Interventions by T. S. Brar, M. A. Kamal & R. K. Jain (ISET Golden Jubilee Symposium, Dept of Earthquake Engineering, IIT, Roorkee)
11. Controversial Aspect in Seismic Assessment and Retrofit of Structures in Modern Times : Understanding and Implementing Lessons from Ancient Heritage by Stefano Pampanini, Senior Lecturer, University of Canterbury, Christchurch, New Zealand.
12. Seismic Retrofitting Techniques : A Powerpoint Presentation by Aritra Banerjee.
13. Earthquake Damage Assessment – Vulnerability of Sikkim Built Heritage : Indian National Trust for Arts and Cultural Heritage, New Delhi (March’ 2012)
14. Web Portal of ICOMOS.
15. Retrofit of Historical Monuments and Principles of Base Isolation : Prof. Antonello De Luca, Dept. of Structural Analysis and Design, University of Naples.
16. Retrofitting of Reinforced concrete structures: A power point presentation by Dr. Satyendra Mittal, Dept. of Civil Engg., IIT, Roorkee.
17. Wikipedia: Photographs of Leaning Tower of Pisa.