Sustainable Construction A Life Cycle Approach in Engineering

Although a number of papers can be found in the literature related to the analysis of connections
with dowels, not many researches were concerned with the behaviour of joints with timber
pegs. With specific regard to connections with timber pegs, simplified orthotropic elastoplastic
bilinear material model have been used by some authors, as in Miller 2004. In this case,
the plasticity model is based on the von Mises yield criterion, with an associated flow rule and
with isotropic hardening.
The aim of this numerical modeling is to investigate the possibility to use Hashin criterion as
material constitutive law in the modeling of connections with timber pegs by finite elements.
This type of failure criterion presents several advantages. It is available in commercial codes
and includes the possibility to set different strength values in tension and compression along
longitudinal and radial directions, with different values of fracture energy.
In particular, the aim of numerical investigation is to extend the results of experimental activities
by a parametric analysis devoted to characterize the behaviour of connections with particular
regard to the peg and to the interaction area with the plates.
The connection considered was tested by Ceraldi et al. 2008 and was assembled using different
materials. In particular, ash was used for pegs and fir for plates. The different materials were
selected with the aim of evaluating the effects of various material densities on the type of collapse
mechanism of the pegs.
The numerical model of the connection was calibrated on the basis of experimental tests on
wood materials in compression, bending and cutting shear. Afterwards, the predicted behaviour
of the double shear joint has been validated against experimental results in terms of loaddisplacement
curves and failure modes.
Figure 3. Finite element models of pegs and prismatic specimens used for the calibration of ash material
properties, and Finite element model of a quarter of double shear connection.
The implemented material model is an anisotropic damage model which takes into account different
modes of failure.
The undamaged response of the material is linearly elastic. The anisotropic behaviour in the
elastic range is defined by the engineering constants, that allow different values of elastic modulus
E and shear modulus G in longitudinal 1-1, radial 2-2 and tangential 3-3 directions to be
set. In performed analysis it was assumed that the elastic properties along radial and tangential
directions are equal due to the hypothesis of orthotropic behaviour.
The damage initiation criterion is based on Hashin theory. This criterion is based on the definition
of four damage variables: fiber collapse in tension d tf , fiber collapse in compression d cf ,
matrix collapse in tension d tm , matrix collapse in compression d cm .
The post-damage behavior is computed on the basis of a damaged elasticity matrix whose elements
are derived on the basis of different energies dissipated during damage for fiber tension,
fiber compression, matrix tension, and matrix compression failure modes, respectively.
Different finite element models were implemented for calibration and numerical investigation.
A model of ash prismatic specimen and of a peg were developed to calibrate elastic and ultimate
material properties in selected connections, respectively. On the basis of previous results,
the numerical model of a double shear joint was generated.
In particular, the longitudinal elastic modulus, tensile and compressive strength of ash material
were calibrated on the basis of compression and four point bending tests on prismatic specimens.
189