Proceedings e report - Firenze University Press

WOOD SCIENCE FOR CONSERVATION OF CULTURAL HERITAGE
The ultimate strength and the global stiffness of the joint are insensitive to the variation of the
compressive strength of wood in the parallel direction. Fig. 5b indicates higher sensitivity of the
ultimate strength of the joint to the variation of the compressive strength of wood in direction
perpendicular to the grain, as expected: with a reduction of 50%, the ultimate strength of the joint,
given by an offset of the linear stretch by 2‰, decreases from 130 kN to 100 kN (-30%); multiplying
by a factor of 2 the ultimate strength of the joint, given by an offset of the linear stretch by 2‰,
increases from 130 kN to 160 kN (+23%). However, the global stiffness of the joint is insensitive to
the variation of the compressive strength perpendicular to the grain.
6. Conclusions
From the results obtained in the first part of the study no significant different was found between the
mechanical characteristics of the old wood and new wood, although the former shown a slightly higher
values (7-12%) is could be easily allocated to the natural variability of wood.
Comparing the characteristic tensile strength and modulus of elasticity values with the characteristic
compressive results one can conclude that the characteristic strength values are slightly higher in
tension parallel to the grain than in compression parallel to the grain (≈ 32%). The elastic modulus of
elasticity results are also slightly higher in tension parallel to the grain than in compression parallel to
the grain (≈ 25%).
The second part of the study shows that the difference in mortise-tenon joints results for the ultimate
load between the two groups is very low, which is in agreement with the values of density found for
the sample. Thus, the results seems to support, although more studies should be conducted on timber
removed from members in-service, that safety assessment of existing timber structures can be made
using mechanical and physical properties data from current available chestnut wood.
Regarding the third part, the different failure mechanisms observed in the experiments are well
captured by the numerical model, which is the most important validation of any simulation. It is
striking that such excellent agreement is obtained also in the load-displacement diagrams.
A preliminary analysis considering an infinite stiffness of the interface, assuming a fully rigid
connection, indicates that such an assumption provides too stiff results. Another conclusion is that the
normal stiffness of the interface elements has considerable influence in the yield strength of timber
joints. The numerical results, in terms of force-displacement diagrams, with the adjusted stiffness for
the interface elements, provide very good agreement with the experimental results both in the linear
and nonlinear parts. The influence of the experimental horizontal restraint, simulated by a linear
spring, is only marginal.
It has been shown that the parameters that affect most the ultimate load are the compressive strength
of wood perpendicular to the joint and the normal stiffness of the interface elements representing the
contact between rafter and brace. The tangential stiffness of interfaces and the Young’s moduli of
wood have only very limited influence in the response. The compressive strength of wood parallel to
the grain has almost no influence in the response.
References
1. CEN; 1991 – “EN 26891 – Timber structures. Joints Made With Mechanical Fasteners General
principles for the determination of strength and deformation characteristics”. Office for Official
Publications of the European Communities. Brussels, Belgium.
2. Lourenço, P.; 1996 – Computational strategies for masonry structures. PhD thesis, Delft
University of Technology.
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