Preamble

The safety of the historic load-bearing structures is a performance expectation, which requires an acceptable behaviour of the historic load-bearing structures in the resistance and stability limit state. This behaviour is anticipated through (i) investigations for the determination of the extant safety level, (ii) interventions needed for the correlation of the extant level with the contemporary safety requirements, (iii) the maintenance of the acceptable behaviour during use.

The acceptable behaviour of the historic load-bearing structures in the resistance and stability limit state is necessary because: (i) human lives and material goods have to be protected, (ii) heritage values are to be preserved.

An important remark has to be made on the heritage values: the historic load-bearing structures have intrinsic heritage values, beside the worn ones, such as artistic elements (murals, painted wooden elements, carved stone elements etc.) or architectural details (carpentry, wainscot, floors etc.).The intrinsic heritage values are connected both with the empirical-intuitive concept of the load-bearing structure and the historic materials and traditional technologies, through which they are put in operation [5].

Thus the three phases of ensuring the acceptable behaviour - the result of which is the safety of the historic load-bearing structures - take the heritage values always into consideration, including the intrinsic values of the historic load-bearing structure.

2 THE SAFETY OF THE ENGINEERED LOAD-BEARING STRUCTURES [1]

The major problems regarding the safety of the engineered load-bearing structures are the ones related to the form of idealization (determinist, statistical or strategic). Not only the bases of the problem, but also the decisions regarding the solutions have to be in direct connection with the applied form of idealization.

The semi-probabilistic methods - that use an information infrastructure brought almost to perfection -create a more and more adequate framework for the handling of safety issues regarding the engineered load-bearing structures.

The major problems, which this field has to confront, are ethical or related to the insufficiencies in the technical-scientific knowledge, or result from the errors due to the inappropriate quality of the materials and from the omitted, unpredictable loads etc.

3 PREMISES FOR THE SAFETY OF HISTORIC LOAD-BEARING STRUCTURES [2] [6]

3.1 The objective premises of the safety of historic load-bearing structures

The characteristics of the historic load-bearing structures, as well as those of the historic and/ or current load-bearing structural environment are considered to be the objective premises.

Figure 1. The structural sensitivity of the masonry to moisture, freezing cycles.

Figure 1. The structural sensitivity of the masonry to moisture, freezing cycles.

3.1.1 The characteristics of the historic load-bearing structures

These characteristics are well-known, including their share in the safety of historic buildings; for example:

(a) the masonry structural sub-units:

- are rigid to gravitational actions, if these lead to compression and if due to these no differential settlements are produced;

- have minimum reserves for taking over the tension, thus they are very sensitive to dynamic actions;

- are sensitive to climatic actions, moisture, freezing cycles etc.;

- are sensitive to natural or artificial corrosion factors;

- the fatigue, the seismic energy-absorption of the masonry mortars and the consequences due to corrosion can be tested in the mass of the masonries with difficulty, the evaluation of their state parameters being possible only in quality.

(b) the timber structural sub-units:

- are load-bearing structures, which work in an elastic limit state, being suitable for both static and dynamic actions;

- are subject to biological decay, some of the damages - like the ones caused by the moisture of the supporting structure of the floors - causing an immediate, unforeseeable collapse.

(c) being hundreds of years old, the durability of the historic load-bearing structures is a certainty.

(d) the state of the historic load-bearing structures, the safety reserves are continuously changing, the periodic re-evaluation of safety being compulsory.

3.1.2 The historic and/or current characteristics of the historic load-bearing structural environment

These are of great significance, influencing to a great degree the safety of load-bearing structures.

The current characteristics of the historic load-bearing

Figure 2. The structural sensitivity of the timber structure to biological decay.

structural environment differ from those of the historic environment:

- the artificial corrosion factors changed essentially both in quality and quantity - the historic buildings being extremely vulnerable to some of these actions;

- the level of the ground water was essentially modified in the 19th century, influencing the foundation conditions and the historic foundations to a great degree;

- the impermeable flooring and pavement basically changed the hydrostatic balance of the load-bearing structural environment surrounding the buildings, increasing the humidity (and dissolved salts) absorbed by the load-bearing structures due to the lack of horizontal waterproofing;

- the vibrations due to traffic are very damaging, especially for the masonry mortars - operating almost constantly, making also the efficiency of the interventions questionable;

- the vibrations due to industrial or household appliances have an effect similar to the one of the vibrations due to traffic.

3.1.3 Special historic conditions relating to the historic load-bearing structures In the course of history the ecclesiastic constructions (especially the churches) have been built according to prototypes. These «typical projects» are made for exactly formulated functional requirements, but are elaborated also (more or less consciously) for technical requirements, demanded by the climatic, seismic, corrosive historic conditions etc. of the place, where the prototype was created. The prototype being generally propagated - for example by the Cistercian monks -, both the functional and technical characteristics are entirely transmitted, without being adapted to the local technical climatic or soil conditions (shrinkable clays,

Figure 3. The level of the historic technical knowledge -the empirical-intuitive concept in the Czech Republic.

slope stability etc.), as well as to the seismic requirements. If the technical conditions of the location are very different from those of the prototype, the safety of the constructions will be reduced. Due to the fact that the ecclesiastic constructions are built by a community quite rarely (once in 3-800 years), the local experience is much harder to gain.

Much more secular constructions are built - the imported model is even adapted to the local conditions - even if the initial "imports" are entirely transposed. Due to the very frequent subsequent transformations, as well as the large number of similar constructions built in a relatively short period of time, the local (regional) experience is much easier to gain, creating (intuitively) load-bearing structures that are resistant to seismic actions and actions occurring due to shrinkable clays and unstable slopes etc.

3.2 The subjective premises for the safety of historic load-bearing structures

The category of the subjective premises consists of the social-political conditions, the level of technical knowledge, research possibilities, quality control system, available materials, aptitude and others.

These premises are equally present in the building of the constructions, the former subsequent interventions

Figure 4. Historical skill in England.

(«historic» ones) and those present in our time («contemporary» ones).

The subjective premises influence the safety of the load-bearing structures both in a positive and a negative way.

3.2.1 The historic subjective premises

(a) the social-political conditions:

- cheap and abundant manpower, often compelled to work;

- wars that produce loads, which were not taken into consideration at the conception of the load-bearing structure;

- the disposition of the society to protect the heritage values (a more and more intense attitude since the Renaissance, the doctrinal approach of which was not always favourable to safety).

(b) the level of technical knowledge:

- the empirical-intuitive concept of the historic load-bearing structures - without engineered calculations;

- the scientific basis of the technical regulations in the historic period of interventions.

(c) available materials:

- mainly local materials, limited transportation possibilities (the load-bearing structures built of these materials are not always suitable for local loads);

- building materials with a reduced tensile stress (the historic load-bearing masonry structures are very sensitive to dynamic actions).

- building materials with a reduced resistance to corrosion - the historic conditions were not familiar with contemporary corrosion.

- the traditional technologies are developed for the processing of the local materials (there are situations, in which the traditional processing offers a higher quality);

- the manual operations present a development in time, going into decline only after the great industrial revolution from the 19th century.

3.2.2 Contemporary subjective premises

(a) the social-political conditions:

- wars - producing loads, which were not taken into consideration at the conception of the load-bearing structures;

- the consumer society - the contemporary expectations regarding the durability are different from the historical ones;

- globalization - can also have positive effects: the most recent standards spread quickly all over the world;

- the disposition of the society to protect the heritage values - laws, which protect (to a higher or lesser degree) the historic load-bearing structures;

- the (higher or lower) level, at which society acknowledges the research and testing activities on the load-bearing structures that require a qualified manpower.

(b) the level of technical knowledge:

- the level of the general technical culture and the knowledge related to historic buildings (the high level of knowledge within the contemporary construction industry also requires scientific sensitivity towards the historic buildings);

- the level of technical knowledge related to historic buildings has to refer to the load-bearing and non-bearing structural issues, building physics and biology etc. as well;

- the average specialist (researcher, designer, contractor) is expected to have technical-scientific knowledge regarding both the empirical-intuitive concept, historic building materials and the traditional technologies; an information infrastructure has been available for 12-15 years, through which the modelling issues can be widely mastered (especially in the case of wooden load-bearing structures) - these have to be known and used;

- the history of the load-bearing structural materials and subunits has to be studied: floods, seismic events, fires (the micro-fissures in the stone of the masonry resulting from the fire hundreds of years ago etc.);

- the conceptual compatibilities of the interventions demand the knowledge of the empirical-intuitive concept of the foundations, the supporting structures, the historic floor and roof

Figure 5. The use of the traditional technologies in Romania.

structures, extending it in space and time over the regions where the specialist wants to pursue his activity (strong and weak points, frequent deteriorations, characteristic historic interventions etc.);

- the features of the historic building materials have to be identified - the incompatibility of the materials can be the cause of inefficient interventions - the historical stone-quarry have to be identified, so as to use materials similar to the original ones;

- the traditional technologies have to be known as well, the interventions need to be compatible, in the technological field included.

(c) The level of knowledge regarding the heritage values:

- those have to have a knowledge of the heritage values, who wish to perform interventions regarding the safety of the load-bearing structures - the issues regarding the safety and heritage values have to be solved simultaneously in the case of historic buildings;

- we have to know the historic load-bearing structures both with the intrinsic and the worn heritage values (artistic elements: murals, coffered ceilings, stuccoes, stone frames etc., building structures: windows and doors, flooring, tinning etc.);

- the worn heritage values have to be considered, as they can eliminate a part of the consolidation works - the allowed limit states have to take into consideration the expectations regarding the heritage values;

- the intrinsic heritage values of the load-bearing structures (related to the empirical-intuitive concept, the historic materials and the traditional technologies) can reduce the intervention possibilities;

- we have to know the doctrinal issues regarding the interventions carried out on historic buildings, especially the ones related to the historic

Figure 6. The use of the traditional technologies, intervention on a fractured masonry vault.

load-bearing structures; we have to choose between preservation, restoration, retrofitting or re-construction - only that specialist can choose, who knows the terminology;

- the doctrinal attitudes have to be adopted carefully: for example the Mediterranean fashion of natural surfaces of the 1950s and '60s has always had negative results in continental Europe, as the number of freezing cycles is incomparable in these places.

(d) available materials:

- we have to take into consideration that the current materials do not reach the quality level of the historic ones (the solid brick with the old format, the lime or the timber from the planted forests are only a few examples for the poor quality of the recently produced traditional materials);

- the contemporary building materials are produced so as to be compatible with other recently produced materials - their compatibility with the historic building materials has to be studied in each case; laboratories, specialists, the awareness of the issue, as well as the will to take measures are thus needed.

- one of the tasks is the studying of the traditional technologies, avoiding the technological incompatibilities between the traditional technologies and the intervention;

- we need to study the technological conditions for the reinforcement to begin immediately to work (reinstating the original geometry with the introduction of counter-bending etc.).

(f) research possibilities:

- the basic research programs are necessary [the slow transformation of the slopes, the mechanics of the earth banks and of the poor-quality foundation soils that have been loaded for centuries (the soil mechanics regulations deal

Figure 7. The use of the traditional technologies, intervention on a historic timber roof construction.

only with "virgin lands"), the behaviour of the historic timber - rheological aspects etc.];

- the use of the research results of the different fields in the interventions carried out for the safety of the load-bearing structures: art history (the building of a construction in phases - the source of damages in the load-bearing structures), archaeology (the former foundations -damaging factors), building physics (moisture containing dissolved salts - causing damages) etc.;

- providing laboratories for those industrial branches that carry out the interventions on historic buildings, where the quality of the load-bearing structural materials would be studied, creating conditions for the routine experiments: e.g. stone is generally used without a quality check;

- in-situ testing of the building materials and a qualified personnel.

(g) the quality control system:

- the historic load-bearing structures have to be levelled with the current safety standards - this is impossible without a quality control system, taking into consideration the aforementioned deficiencies;

- the personnel designated for the quality control has to have thorough knowledge of the historic load-bearing structures;

- the quality control laws have to deal with the characteristics of the historic load-bearing structures in detail;

- uniform measures are recommended regarding the safety of the built environment - taking of course into consideration the characteristics of the historic buildings (it is not efficacious, if a group of institutions takes care of the contemporary buildings and another group watches over the historic ones).

4 ENSURING THE SAFETY OF HISTORIC LOAD-BEARING STRUCTURES [6], [7], [8], [9]

The acceptable behaviour of the historic load-bearing structures in the resistance and stability limit state -safety measurement of load-bearing structures - is anticipated through: (i) investigations for the determination of the extant safety level, (ii) interventions needed for the correlation of the extant level with the contemporary safety requirements, (iii) the maintenance of the acceptable behaviour in the resistance and stability limit state during use.

4.1 Investigations for the determination of the extant safety level

The investigations for the determination of the extant safety level of the historic load-bearing structures are related to technical, as well as to heritage issues.

The technical investigations take place during [9] the diagnosis phases carried out on the historic load-bearing structure (characterized by the approach of the structural issues from the point of view of building mechanics, physics and biology, as well as of soil mechanics and building services engineering), focusing their attention on the safety aspects: (i) the identification (survey and presentation) of the historic load-bearing structure; (ii) finding the insufficiencies in the historic load-bearing structure; (iii) the formulation and testing of the performance expectations at the historic load-bearing structure; (iv) the determination of the causes of insufficiencies at the historic load-bearing structure.

The heritage values have to be recognized - the investigations are linked in this respect with the artistic elements or architectural details, taken over from archaeology or art history studies, as well as from the expert opinion on the artistic elements or the non-bearing structures. The intrinsic heritage values of the load-bearing structure are determined by experts in the field of historic load-bearing structures.

4.2 Interventions needed for the correlation of the extant level with the contemporary safety requirements

The interventions needed for the correlation of the extant level with the contemporary safety requirements are also approached from a technical point of view, as well as with a view to the heritage values.

The technical interventions take place during [9] the diagnosis phases carried out on the historic load-bearing structure (characterized by the approach of the structural problems from the point of view of building mechanics, physics and biology, as well as of soil mechanics and building services engineering), focusing their attention on the safety aspects: (i) the elimination of the causes of insufficiencies in the load-bearing structure, (ii) ensuring the performance expectations at the load-bearing structure and (iii) preserving the performance expectations in the case ofthe load-bearing structure - ensuring the durability of the intervention.

From the principles adopted for the safety interventions the following are to be stressed:

- the interventions are in connection with the load-bearing structure concept: the correct concepts allow conservation or restoration works, in the case of the wrong ones retrofitting or possibly reconstruction is needed; safety is provided in the case of both permanent and temporary, as well as extraordinary (special) actions;

- reversible (that can be dismantled) interventions are preferred, especially when the compatibility of the intervention with the heritage values is uncertain;

- the efficiency of the interventions has to be checked; in this respect it is recommended to test the dynamic characteristics of the load-bearing structure before and after the interventions: acceleration, speed etc.;

- the consequences of the interventions regarding the improvement of safety at all performance expectations: in connection with building physics and biology, as well as soil mechanics;

- the reconditioning of the quality of historic materials is also recommended, with a view to regaining their initial safety level (for example re-freshening the mortar from the vaulted slab masonries through its moistening with lime milk [8]).

The interventions carried out on the heritage values should be minimal. The problem consists in, whether the intrinsic heritage values of the load-bearing structure are recognized or not: these values have to be protected or not, or the load-bearing structure has only the role to support, to carry the heritage values of the non-bearing structures (artistic elements and architectural details). If the intrinsic heritage values need to be protected, then the diagnosis and prediction of the historic load-bearing structures have to focus on the identification and protection of the intrinsic heritage values, going through each phase of the diagnosis and prediction, mentioning what happens to the intrinsic and worn heritage values every time, thus: (i) survey

- the heritage values are surveyed; (ii) insufficiencies: it will be described to what degree the insufficiencies affect the heritage values; (iii) testing the way in which the performance expectations are provided: whether the heritage values have got deficiencies in this respect or not, (iv) the determination of the causes of insufficiencies: whether the causes dispose of heritage values or not; (v) the elimination of the causes of insufficiencies: whether they are destroyed through elimination in the case of heritage values as well, (vi) providing the performance expectations, (vii) preserving the performance expectations - whether the heritage values are affected through the suggested interventions or not.

4.3 The maintenance of the acceptable behaviour of load-bearing structures during use

The maintenance of the acceptable behaviour of load-bearing structures during use requires - among others - the following:

- the functional and technical (conceptual, material and technological) compatibility of the interventions has to be provided (shear thresholds, compressions, counter-bending etc.);

- the incompatibilities in connection with building mechanics, physics, biology and soil mechanics have to be avoided;

- specifying certain measures for the perpetuation of the effects of the interventions carried out on the load-bearing and/ or non-bearing structure (avoiding heat bridges that bring about vapour, ventilation conditions, anticorrosive treatments etc.);

- the maintenance of the interventions, with a view to obtaining the response of the load-bearing structure to extraordinary actions at any time (for example earthquakes);

- reducing the aggressiveness of the environment.

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