Mechanical Failures of the Timber Structural Systems

From Material to Structure -Mechanical Behaviour and Failures of the Timber Structures
ICOMOS IWC -XVI International Symposium – Florence, Venice and Vicenza 11th -16th November 2007




Mechanical Failures of the Timber Structural Systems
Gennaro Tampone University of Florence, Dept. of Conservazione e Restauro dei Beni Architettonici, Collegio degli Ingegneri della Toscana
The stability of the existing timber structures is commonly studied with the general methodology of the Engineering Science, the Theory of Construction, the Theory of Materials applied to the wood, the Technology of the Timber Constructions (see for instance the European Codes). Whilst detailed studies can be found for deformation, rupture, collapse of members or small samples of clear wood and solid timber in structural size, only a few specialized publications deal with structures for the same problems.
The most common causes of failure of the timber structural systems are inadequacy of configuration (geometry of the structure, sizing of the members, kind of connection of the members, bracing etc.) in relation to the actions, both static and dynamic loading; besides slenderness, instability, …, defects of the wood laid in place, severe biotic damages, accidental factors.
No doubt that the decay of the wood caused by beetles and fungi is a major problem both for members and structures too, especially in the tropical countries; in fact mechanical failures can also be caused by biotic factors that reduce the strength of the material as tothe designed actions, thus reducing or cancelling their efficiency and safety.
The failures can hit the structure at any hierarchic level, i.e. that of the members, the units, the whole structural system and the connections.
The widespread habit of replacement of the damaged parts of the ancient structures or their total demolition and rebuilding is, unfortunately, extended up to the present days though only by a restricted number of culturally unacquainted operators who rely on a supposed tradition. This habit has the effectual alternative, required by the contemporary cultural instances, of the repair and strengthening with minimum disturbance of the ancient complex.
This new vision demands for an updated scientific and technical approach. If repair and strengthening must be planned in order to recover the lost efficacy of the system and, at the same time, avoid modifications or at least minimize them, in any case without significantly altering the general concept of the same system, the mechanical failures as well as the causes of their presence must be known and interpreted; a general evaluation of the efficiency and of the safety of the system will then be possible. Planning the strengthening will subsequently follow in a natural way as the activity meant to take the right measures and put in place only those remedies judged adequate to counteract the recorded effects of each failure, avoiding the assumption of the generic remedies featured and recommended in the handbooks and in the dépliants of companies without a specific reference to the real nature of the damage, and cancel or at least neutralize the causes that are responsible of the problem.
The author wants also to stress out that this kind of comprehensive approach and this widening of the field of appraisal will lead to a deeper understanding of the existing
From Material to Structure -Mechanical Behaviour and Failures of the Timber Structures
ICOMOS IWC -XVI International Symposium – Florence, Venice and Vicenza 11th -16th November 2007

structures extended to all the components and including their behaviour, this way allowing a wider appreciation of the specimens which reached safely the present days or those disappeared or simply described in reports and treatises. Furthermore, the study of every unsuccessful outcome is, beyond any doubt, very fruitful for the conception of new systems and the design of new structures in the sense that it allows direct corrections and changes of trend: if one looks at the history of engineering, he discovers that it is what really happened.
A different consideration of the whole process of the treatment of the existing structures will start the passage from the appraisal of the building material to that of the structures, from the concern for single elements to the consideration of a rational combination of components that are connected by relations the nature of which is either material or immaterial. This updated, enlarged attitude marks a significant achievement.
* * *
The structural systems are, in general, designed as a combination of different elements; the basic ones are the Members that are organized and connected in a way to define Units of different kinds (frames, trusses, floors,……); a series of Units, connected and stabilized by auxiliary Members, determine the System; hence their organization is of hierarchic type. All the components are arranged in such a way as to allow the ensemble to assume a configuration capable to withstand the actions (loads, settlements of the construction, dimensional variations caused by hygro-thermal fluctuations, ……..) more or less cooperating to stiffness, strength and inhibition of motion of the adjacent parts.
Connections and joints, being designed according to various conceptions and ways of working, more or less engaging, more or less limiting the movements of the concurring members or units, can be seen as relations (see before) started between the various levels of components.
Significant examples of complex structural units are the floors, generally woven with principal beams, extended from one long wall to the opposite, which support a row (if any) of secondary beams placed in the orthogonal sense, thus forming a net so close to allow the placement of a co-operating boarding. The hierarchical conception is well expressed, amongst other matters, by the different size of the members, with a constant decrease in the ratio of the diminution of span and bay: for instance, the diminution from principal to secondary beams. Connections are usually made with the aid of nails.
Important and widespread examples of structural systems are the traditional roofs of the buildings, specially those of the Christian churches, which also fulfil a symbolic role: there the roof carpentries are in a way, with their hierarchical and specialized organization, expression of the ecclesial system, conceived in the same way.
The lead sheets covering of the St Catharine church in the Sinai, Egypt, rests on a roof of the VI c. composed by a series of trusses, the earliest known, with king post and struts, surmounted by purlins, joists etc. The rafters are connected to the chord by deep joints of the type tenon-mortise. The king post is fully perforated at the upper end so as to allow the insertion of the tenon-shaped extremity of the rafters.
The cotto (earth ware tiles) covering of the St. Mark’s church in Florence rests on a series (built in the XIV c.; two of them were built in the following century.) of traditional king post timber trusses, composed by members connected by traditional joints and iron fittings. The trusses are connected to each other by purlins (the auxiliary members) and joists which provide stability in the vertical plane to each unit and reduce the mesh to the size of a large and thin brick (pianella). The front and back pediments of the building provide stability to the series of units as well. The appropriate grading of the sizing of each member, from the massive chords and rafters of the trusses, passing trough the purlins and joists of medium size to the thin struts and the small rulers of the upper texture, ensures each role being pursued efficiently and the structural system equilibrated. The system is resistant to the static and dynamic actions, steady and balanced. The St Mark’s roofing, a simplified and standardized product of the tradition, is similar to
From Material to Structure -Mechanical Behaviour and Failures of the Timber Structures
ICOMOS IWC -XVI International Symposium – Florence, Venice and Vicenza 11th -16th November 2007

hundreds of timber roofs in Italy and Near East. A few minor failures hit the roof of St Mark’s building over the time but it arrived safe and genuine to our consideration until the rough and destructive replacements operated a few years ago by the Authority responsible for the preservation of the Florentine Monuments. The late gothic roof of the Cathedral of Nicosia, Sicily (De Francisco, Tampone, Copani, Funis etc.), is made in a much more complicate way but one can recognize the application of the same structural concept.
2. Objectives
Main purpose of the present paper is to present a set, systematic though abridged and simplified, of the most frequent failures of mechanical nature that occur to timber structures. The situations presented here are extreme but it is possible to distinguish different levels: failure as reduction of serviceability or safety, presence of cracks, breaking, collapse. In any case all the cited phases are preceded by peculiar symptoms.
Doing so the author intends to contribute to the building up of a systematic atlas of failures, the frequent and the rare ones, to help in the appraisal methodology of the mechanical characteristics and behaviour of the ancient timber structures. The study itself has its own strong appeal but we ought to remember that it is essential as a preliminary research for the assumption of the measures of collapse prevention and strengthening.
3. Failures of the members
Members, especially the beams stressed by bending, that are not attacked by fungi and beetles, break with the crack of single fibres, therefore with a grinding and in a progression. Decayed members break more easily than healthy ones, with a neat and sudden crack; the time of the event is unpredictable.
Due to the fibrous nature of the wood tissue, the way of breaking of the members is influenced by the kind of stress.
(For a description of the most recurrent types of member, their names, role etc., and of the causes and kind of their decay and failure, see Tampone, 1996, cit., and UNINormal….)
3.1 Compression
The longitudinal compression stress mainly occurs in pillars, columns, posts, stakes, in rulers of the lattice girders etc.; in a king post truss the struts are compressed (see over), the rafters are subjected to bending and compression.
The peculiar way of deforming, and therefore the typical failures in general, of the wooden pieces subjected to compression has been well explained by Curioni (Turin, 1885), Giordano (Milan, 1993, 2001, …), Wille (Oldenburg, 1950) (Figures 1,2 3,4).
The failures are transversal lines which represent the crumpling and buckling of the cells, generally visible to the unaided eye, sometimes to be looked for with optical aids. When occurring at one end of the member, the base or the regions close to it can undergo a general swelling of the wood with longitudinal fissures; when occurring at the base of the piece, the enlargement of the bottom sections produces fissures which are the effect of splitting.
The failures to transversal compression, to which wood is much weaker, are featured (F. Wille, cit.) by depression of the compressed area, permanent if the stress is adequate, and of a large surrounding region with bulging of the cross section if this is narrow (see figure ).