Proceedings e report - Firenze University Press

WHAT ONE NEEDS TO KNOW FOR THE ASSESSMENT OF TIMBER STRUCTURES
2. Overall structural system evaluation
The original structural system behaviour, although sometimes complex and difficult to understand, can
in most cases be handled by structural engineers.
One should have present that ancient timber structures are not always structurally sound, even
disregarding possible degradation. Some exhibit an enormous degree of improvisation, some have
basic conceptual/structural errors, some were poorly made and others have been altered disrespecting
safety considerations.
Common problems in roof structures are erroneous geometry, eccentric loading at the truss supports
and due to rafters placed away from the truss nodes, lack of bracing between trusses and missing
elements due to previous interventions. In the case of floor systems, insufficient support length at the
beam ends, lack or sloppy bracing between beams, removal of support walls and introduction of
intermediate loading partitions are quite common.
Joints frequently have some kind of damage (metal corrosion, sloppiness, timber splitting or crushing)
and original defects like missing plates or fasteners, minute edge and end distances of fasteners, too
small washers, gaps between elements that should be in contact. The engineer should carefully inspect
the joints, be able to understand them, identify failure or other dangerous situations like the
consequences in the internal forces distribution of a failure in a joint, and propose remedial measures.
Structural defects must receive due consideration and the interventors must be aware of the need to
guarantee suitable safety levels (sometimes by restraining the use of the structure to less demanding
activities) despite the historical or architectural interest of the building.
3. Estimation of timber basic properties
Estimation of timber properties begins with the identification of wood species. This task although
highly specialized, can be easily performed, especially when historical information is available (date
of construction, region and wood species normally used in that period and region) thus the wood
anatomist only has to confirm the species from a limited range of suspected species.
After wood species identification, the tricky task of allocating characteristic values (strength, stiffness)
to the different timber structural members should be performed. In general three different types of
approach are adopted by contractors. These are discussed below.
3.1. Sampling
Building contractors are often tempted with sampling and testing timber from the structure, copying
the approach followed for other construction materials. Their sampling may involve one or two
structural members that for some reason are to be replaced; or involve a small amount of wood taken
from different locations within the structure: either to check clear wood properties, or state of
conservation, as an attempt to evaluate its effects in strength.
Due to the natural variability of timber, such sampling can’t be considered representative. To account
for variability, the determination of characteristic values for the mechanical properties of timber is
generally based in several samples of 40 timber pieces each, of structural dimensions from a certain
timber grade (exhibiting representative defects and features of that grade). This is obviously not
feasible within the assessment of existing structures, but explains why strength values obtained by
sampling from the structure can hardly be used with confidence. Besides, sampling of clear wood
disregards the major influence of defects like knots, slope of grain or fissures, and the effect of local
biological damage, which may vary considerably within the structure.
Sampling is however very useful to provide information on species, moisture content and density. It
can also be used to check if strength values of clear wood fall within the expected range or to restrain
the quality of wood in a certain visual strength grade; but in order to do that the collected information
shall be used in conjunction with methods specially developed to derive strength values appropriate
for design.
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