The purpose of the Heritage Structure Annex is to provide additional considerations to the application of the ISO 13822 Standard to heritage and historic structures. This annex must be read in conjunction with all of the sections of this standard.
This annex is based on the premise that structure has cultural value in itself. Heritage structures should be preserved for their own sake and not merely as supports for the rest of the historic material. It follows that the integrity of the existing structure should be respected in any interventions.
Discussion of these issues is provided in documents listed in the bibliography.
[The second paragraph is still to be improved]
I.2 Fundamentals I.2.1 Assessment of heritage structures
The assessment of a heritage structure will include two aspects: that concerning its structural performance, familiar to engineers, and that concerning its value as a cultural resource. These two aspects must both be taken into account in any decision involving possible structural interventions and should therefore be carried out in tandem.
The heritage value of an historic structure resides in the authenticity and integrity of its character-defining elements. Contemporary interventions must not compromise this authenticity and integrity. To retain authenticity, the structure is to be preserved, as far as possible, with its original materials and structural concept.
NOTE 1- The structure itself often represents an important aspect of the culture of its period: the construction knowledge, technology, and skills of a given time, and as such, represent a legacy to future generations. There are numerous examples of exceptional heritage structures while other heritage structures are typical structural designs of their time, but nonetheless are critical to the cultural resource in their supporting role to other character-defining elements, such as architectural material or paintings.
NOTE- 2 Judgments about heritage value and authenticity may differ from culture to culture, and thus, there are no fixed criteria. In some geographical areas, keeping alive traditional construction practice is privileged over the conservation of original materials.
I.2.3 Limitation of structural intervention
An over cautious approach to structural assessment should be avoided because it can lead to unnecessary intervention on the structure and result in loss or major alteration of heritage character defining elements, and ultimately in the loss of authenticity and historic significance of the cultural resource. Furthermore, excessive scope of intervention can add unnecessary cost and compromise the viability of a conservation project, and eventually jeopardize the existence of a cultural resource.
Note: In the case of historic resources the commemorative integrity of an historic site can be threatened by unjustified structural interventions.
For the purpose of this annex the following terms and definitions have been added, supplementing those in the core document, to reflect activities related to heritage structures.
I.3.1 heritage structure
Existing structure, or structural component of a heritage resource, that has been recognized by the appropriate authorities for its heritage value.
NOTE- Heritage structures may include all kinds of buildings, bridges and civil engineering works
I.3.2 character-defining elements
Historic materials, forms, locations, spatial configurations, morphology, concept and details, structural design, uses and cultural associations that contribute to the heritage value of a structure, and which must be retained in order to preserve its heritage value
Note 1: Intangible aspects, such as ancient construction knowledge, technologies, and skills may be identified as character-defining elements.
Note 2: Significant subsequent historic changes and alterations, imperfections and damage may be identified as character-defining elements, and must be respected provided that they do not compromise the safety requirements.
I.3.3 heritage value
Aesthetic, historic, scientific, cultural, social or spiritual importance or significance for past, present, and future generations; heritage value is embodied in its character-defining elements
I.3.4 cultural resource
Structure, building, landscape, archaeological site or other engineering works that has been formally recognized for its heritage value
All actions or processes that are aimed at safeguarding the character-defining elements of a cultural resource so as to retain its heritage value and extend its physical life. Conservation includes repair, and upgrading.
Note 1: Several words are used over the world to describe the action and process of safeguarding and extending the life of a cultural resource: conservation, preservation, and restoration. Conservation has been used by UNESCO and World Heritage.
Note 2: conservation is a particular case of rehabilitation as defined in the core document, in which the respect for cultural value is considered as a constraint.
I.3.6 emergency stabilization
Action or process implemented urgently to temporarily secure a structure which has insufficient reliability until conservation/preservation can begin.
I.3.7 minimal intervention
Intervention that balances safety requirements with the protection of character-defining elements.
I.3.8 incremental approach
A step-by-step procedure in which the behaviour of the structure is monitored at each stage and the data acquired then used to provide the basis for further action.
[Text in next sections I.3.9, I.4 and I.5 proposed by L. Fontaine and D.Yeomans Discussion pending.] I.3.9 heritage recording and documentation Heritage recording is different from heritage documentation in that the former is a record made in the present while the latter is the assembly of pre-existing documents. [Lyne is to provide the Getty definition.]
I.4 General framework of assessment
The main objective of a structural assessment is to ascertain the current and future structural capacity with sufficient reliability to minimize the impact of potential structural interventions on the character defining elements. The required structural performance shall consistent with objectives listed in section 4.1 of the core text:
safety performance level for the users,
performance level related to the protection of the cultural resources.
I .4.2 Procedure
For heritage structures the procedure listed in 4.2 of the core document is appropriate provided that the particularities listed in section I.4.3 to I.4.7 are taken into consideration.
Structural assessment for heritage structure should be typically carried out in collaboration with a multi-disciplinary team that includes specialists such as engineers (structural, geotechnical, etc.), architect, landscape architect, heritage recorder, archaeologist, historian, cultural resource manager, interpreter, material scientist, conservator, geophysicist, etc. While the structural engineer should deliver a specific structural evaluation report, the ramifications of this report should be discussed within the multi-disciplinary team and decision typically reached in a consensus process.
I. 4.3 Specification of assessment objectives
In heritage structures the principal objective of the assessment must be the determination of the current, and future structural performance, any limitations of that performance and therefore any limitations on allowable actions. The typically uncertain nature of heritage structures means that a precise specification of investigative procedures and analysis is often impossible at the outset and that the agreement between the client and engineer must allow for revisions and extensions as required.
I 4.4 Scenarios
Scenarios for interventions must respect the heritage values. The heritage values will normally place severe restrictions on possible scenarios and therefore more than one scenario will need to be assessed.
I 4.5 Preliminary assessment
I 4.5.1 Historical record
It is essential that an historical report be produced for heritage structures. This should identify the nature of the original construction, all subsequent alterations and any significant events that have caused structural damage. Typically cultural resource specialists (such as historians or archaeologists) should be employed to produce this report. The engineer must assist them in the identification and interpretation of structurally meaningful records. Where no such specialist is employed the engineer will need to include historical information in his assessment report and will therefore be responsible for the identification and interpretation of historical records. Note: Historical records were not produced for structural purposes. Although they typically provided useful information, they could also be misleading and contain information difficult to interpret. Their reliability must be ascertained carefully.
I.4.5.2 Preliminary inspection
Where insufficient documentation exists describing the structure the engineer needs to identify those areas where some opening up of the structure is required. This might also be to determine its condition. The engineer must consult the relevant heritage professionals to obtain approval prior to such opening up. I 4.5.3 Monitoring
Monitoring is often necessary for heritage structures, and must be carried out when there are conditions or actions affecting the structural behaviour over time and whose effects or magnitude are not fully understood. Monitoring may be initiated as part of the ongoing maintenance or as an early step in the preliminary assessment. Recommendations for further monitoring should be provided where its initial design has not provided sufficient data for a detailed understanding of the condition and its effects.
In addition to the safety and serviceability of the structure the preliminary check needs also to identify critical deficiencies related to the conservation of character defining features.
I 4.5.4 Immediate actions
Where character defining features are in immediate danger this should be reported to the client and to any heritage resource manager. Any interim interventions necessary to ensure the stability of the structure must not cause damage to the heritage character.
I 4.5.5 Recommendations for detailed assessment
Recommendations for detailed assessment need to be made when there is evidence that character-defining elements may be at risk in the long term.
I 4.6 Detailed assessment
I 4.6.1 Detailed documentary search and review.
In addition to the activities listed in the core document, a history of the use of a heritage structure affecting its past environmental conditions should be documented. This may need to draw upon such sources as oral history, historic photographs, as well as maintenance or other records.
I 4.6.2 Heritage recording
Heritage recording must be carried out by specialists (commonly surveyors, archaeologists or architectural technologists). This will normally be carried out before or during the preliminary assessment.
Records should be made in such a way that they can be used as the basis for reporting structural condition observations, structural modelling and for conceptual design drawings of construction interventions. Note: Numerous recording methodologies exist to document heritage structure: survey, photogrammetry, rectified photography, photo mosaic, hand record, laser scanning, shape capture, etc.
I 4.6.3 Detailed inspection
A detailed visual and tactile inspection should be carried out. Representative samples of hidden elements should be inspected. Openings should be carried out in stages, increasing the number only as analysis shows further openings to be necessary. The potential gain in information obtained of either dismantling or opening to access hidden features must outweigh the loss of heritage value.
Dismantling and rebuilding to ascertain the capacity of hidden elements may be carried out if:
it is recognized by the cultural heritage management authorities part of the traditional method for repair and maintenance, or
if it is otherwise absolutely necessary to ensure safety and serviceability.
I 4.6.4 Sampling
As above, samples taken for materials testing should be kept to a minimum and generally restricted to those parts of the structure that have least heritage value.
I.4.6.5 Preferred testing methods
Non-destructive testing (NDT) and minor destructive testing (MDT) should be used. Historic structural assemblies are typically complex and non-homogeneous. NDT tests should be carried out by qualified specialists to ensure proper calibration of instrumentation and interpretation of the results. In some cases, destructive tests may be necessary to calibrate NDT Note: NDT aims to determine physical, mechanical and chemical characteristic of materials, in-situ stresses, mechanical properties of assemblies, and the presence of discontinuities. Recent developments in software and instrumentation have increased significantly the effectiveness of NDT.
I.4.6.6 Destructive testing
Destructive tests, causing significant damage, should only be used when absolutely necessary either to obtain information that cannot be acquired by other means, or to calibrate NDTs, preferably in inconspicuous areas and without affecting character-defining elements. However, as described above, the potential gain in information obtained must outweigh the loss of heritage value, i.e. the losses resulting from the tests should be less than those that would be caused by the more severe interventions necessitated by poorer understanding of the structure.
I 4.6.7 Monitoring
The results of the structural monitoring at this stage can be used to validate the structural analysis model. Further monitoring should be specified:
where the monitoring results are inconclusive in predicting structural performance,
where intervention has been delayed or
where present concerns do not yet justify intervention.
I 4.6.8 Determination of actions
Special provisions within various national codes can be made for enhanced or reduced values for the actions on heritage structures balancing safety and heritage values.
I.4.6.9 Determination of structural properties
Testing to determine the properties and/or load bearing capacity of the structure shall be planned in consultation with the heritage management professionals. The need for the test and any possible adverse effects on the historic fabric shall be clearly identified.
Structural analysis should take into effect the uncertainties resulting from:
greater age factors
more complex load paths
limited knowledge of archaic structural systems
and therefore it may be necessary to consider more than one structural model. When the models fail to demonstrate sufficient reliability of the structure it may be necessary to carry out additional testing of materials and structural properties or monitoring, incorporating the results in a revised structural analysis.
Note: The information gained through this incremental procedure can be used to gradually improve the understanding and modelling of the structure.
I 4.6.11 Verification
[Note: provide key messages relative for verification relative to heritage structure.]
I 4.7 Results of assessment
I 4.7.1 Report
In addition to the report format presented in Annex G the following points should be added:
Further to section 4.7.2 of the core document recommendations that safeguard the character defining elements of the structure should be provided. In some cases an incremental approach, i.e. a strategy combining minimal intervention with subsequent monitoring, may be the optimum solution.
I 4.7.3 Control of risk
In addition to the requirements of the core document, an assessment of the level of risk to both the structural performance and to the heritage value should be undertaken whenever there is a contemplated change of use. For heritage structures, because of the uncertainties listed in I 4.6.10, monitoring after completion of the work should be used to verify the long-term performance of the intervention.
I.5 Data for assessment
Sufficient data must be obtained investigation and documentation must be carried out to reduce the level of uncertainty in order to achieve minimal intervention.
As specified in clause 5 of the CD, investigation, must encompass materials, actions, environment, existing documents, and other relevant aspects. Particular attention must be given to investigation of records on the historical performance of the structure.
I 5.2 Actions??????
Special provisions within various national codes can be made for enhanced or reduced values for the actions on heritage structures balancing safety and heritage values.
I.5.3 Monitoring data
I.5.3.1 monitoring of structural performance
Because crack and deformation may be of seasonal nature, monitoring should ascertain the extent of progressive movement.
I.5.3.2 Monitoring of environmental conditions
Monitoring of structural movements must be accompanied by monitoring of the environmental conditions that might be causing those movements.
I 5.4 Determination of dimensions and properties
The dimensions and materials properties of heritage structures must always be measured. No reliance should be placed upon early drawings and specifications.
I 5.5 Site specific data
Historical records need to be consulted to determine any significant changes that may have affected site conditions.
I 5.6 Foundations
The assessment of the foundation of heritage structures is complex and requires the services of a specialist geotechnical engineer. For example these foundations may be subject to extremely long and complex consolidation processes. Information on previous interventions should be obtained. When excavating for foundation investigations prior approval and often archaeological supervision are required.
I 5.7 Site drainage
Site drainage patterns and water table levels may have changed with time and fundamentally affected the performance of the foundations. Where this is suspected historical data must be sought.
I.6 Structural analysis I.6 STRUCTURAL ANALYSIS [Need to mention limit states, seviciability, and considering limit states on account of possible damage to artistic contents. Normal serviceability is not applicable, but some situations need to be considered (artistic paintings…) deformations in case of paintings or artistic material]
I.6.1 Purpose of structural analysis
In addition to the purposes identified in 4.6.5 of the core document Structural analysis of heritage structures may also be required to explain the effect of past actions on the structure and the effect of previous alterations. This requires the analysis of past states of the structure and may require qualitative as well as quantitative methods of analysis.
Models of heritage structures should ideally satisfy the requirements of sections 4.6.5 and 6.1 of the core document regarding the accurate representation of actions, geometry and material properties, while also taking into account the alterations and deterioration, including those caused by both natural phenomena and human interventions.
Models shall be calibrated and validated using the evidence available on the performance and condition of the building. Historical information, as well as inspection and monitoring shall be used to validate the predictions of analytical models.
Note: NDT or MDT as described in I.5.5 may be useful for model validation. For instance, flat jack tests may be used to verify stress levels predicted by models.
I.6.3 Deterioration models
When deterioration of heritage structures analyzed, complex time dependent deterioration processes such as long-term related damage and other complex combination of various actions must be taken into consideration.
I.6.4 Model uncertainties
A significant amount of uncertainty may remain, in spite of the validation effort, due to various factors such as heterogeneity of materials, construction details, extent of deterioration, or limitation on the information gathered by direct inspection.
The adoption of partial factors, or the possibility of probabilistic model factors discussed in section 6.4 of the core document may not be realistically applicable to heritage structures.
In such cases, additional evidence obtained from complementary activities (comparative analysis, analysis of historical information) may, through engineering judgment, reduce the uncertainty.
Engineering judgment may take into consideration the comparative approach and historical approach.
I.6.5 Comparative approach.
Comparison with a number of similar constructions whose behavior is already understood can be used to obtain insight on the performance and capacity of a structure. The reliability of the analysis will increase with the number of structures observed and the similitude between these and the case into examination. Similitude should encompass both structure and actions.
Note: Comparison with structures that have already experienced severe effects is particularly useful in assessing seismic performance.
The history of the structure is often a valuable source of information on its performance. For this purpose, historical investigation must seek for information on the significant events experienced (including earthquakes and other extraordinary natural or anthropogenic actions). Effort must be carried out to understand and characterize, both the historical causes (or actions) and the building’s responses.
Information on historic repairs or strengthening which might have altered its performance is also needed.
The use of quantitative methods which simulate historical events should be considered as a means of model calibration.
I.7 VERIFICATION Mention the need to verify both ultimate and serviciability limit states
I.7.1 Reliability assessment
Heritage structures do not have a defined working life and therefore the approach recommended by section 7.2 of the core document is inappropriate. Conservation should therefore be regarded as a continuous or incremental activity.
I.7.2 Plausibility check
The plausibility check must be carried out as part the activities leading to the validation of the models as mentioned in section I.6.2 of this annex. Explanation of discrepancies shall lead to improvement of the models.
I.7.3 Target reliability level
The target reliability level chosen for verification shall balance safety considerations with the protection of heritage value.
In some cases, the protection of heritage value may require the acceptance of a reliability level lower than that implicit in accepted design codes. In these cases, parallel measures should be adopted to limit the consequences of a failure on people or non-movable artistic contents. For example, appropriate or restricted uses may limit the number of people at risk.
NOTE: Sections 7.5 of the core document and Annex F are also applicable to heritage structures as they recognize the fundamental differences between the design of new buildings and the assessment of existing structures. Furthermore, Annex F specifically identifies the need to consider the heritage values when establishing the target reliability level.
I.8 Assessment based on satisfactory past performance Past performance based assessment is acceptable in structures with historic damage and alterations if inspection and / or monitoring shows no continuing distress.
Consider both ultimate + serviceability limit states.
To be further developed and discussed.
I.9 Interventions [Basic guidance for interventions is presented herein.
Refer back to the structure of the core document
Text in blue still to be developed and discussed].
I.9.1 Root causes
Intervention should address the root cause of the observed symptoms and be based on a full understanding of those causes. I.9.2 Demonstrated to be indispensable
Interventions should not be undertaken without demonstrating that they are indispensable I.9.3 Minimal intervention
Intervention should be kept to the minimum level that meets structural requirements in order to ensure the least harm to heritage values. I.9.4 Compatible materials
Material used for interventions should be compatible with the original material in terms of mechanical, chemical and other characteristics, and should maintain these characteristics in the long term. They should not have harmful effects, such as causing corrosion or decay. I.9.5 Incremental (step by step) approach
When more time is required to define optimum conservation strategy, intervention should adopt an incremental or step-by-step approach beginning with a minimum level of intervention, with the possible adoption of subsequent supplementary or corrective measures. I.9.6 Removable measures
Where possible, any measures adopted should be ‘reversible’ so that they can be removed and replaced with more suitable measures if new knowledge is acquired. Where they are not reversible, interventions should not compromise later interventions. I.9.7 Proven technologies
Intervention should use technologies that possess a proven performance record over a long period of time.
I.9.8 Structural concept and construction methods
Each intervention should, as far as possible, respect the original structural concept and heritage value of the structure and of the historical evidence that it provides. I.9.9 Integrated plan
Intervention should be the result of an integrated plan to meet architectural, structural, and functional requirements. I.9.10 Distinctive historic features
The removal or alteration of any historic material or distinctive architectural features should be avoided wherever possible. I.9.11 Repair rather than replacement
Repair is always preferable to replacement. I.9.12 Dismantling
Dismantling and reassembly should only be undertaken when required by the nature of the material and structure and/or when conservation by other means is more damaging. I.9.13 Controllable methods
Measures that are impossible to control during execution should not be allowed. Any proposals for intervention should be accompanied by a programme of monitoring and control, to be carried out, as far as possible, while the work is in progress. I.9.14 Documentation
All interventions including control and monitoring activities should be documented and retained as part of the history of the structure. References