D-BAUG - Departement Bau, Umwelt und Geomatik - ETH Zürich

Highlights ▪ Structures
Elastic modului of spruce wood
Comparison between experimentally and
theoretically determined elastic moduli of
spruce wood depending on the growth
ring angle.
When wood is mechanically loaded perpendicular to the grain,
its elastic modulus depends on the so called growth ring angle.
This angle is defined by the load direction and the anatomical
structure of wood in the radial-tangential plane (Fig. 1a). Elastic
moduli depending on the growth ring angle were,so far,ascertained
either theoretically according to the tensor theory for
anisotropic materials on basis of elastic engineering parameters
for the principle growth directions (L,R,T).Or a dynamic approach
was employed (measurement of direction-depending
ultrasound velocities).
The goal of a project in the Wood Physics Group (Institute for
Building Materials) was to determine these values experimentally
using a static approach suitable also for angles between
the principle planes. For this purpose, miniature specimens
from spruce wood were produced (surface area in the RT
plane: 10mm x 10mm; thickness in the longitudinal direction:
4mm) and tested with a micro compression stage. The small
specimen size (compared tostandardspecimens) allowed to assume
an orthotropic wood structure, curved growth ring
boundaries were avoided.The growth ring angles of the specimens
were varied in 15° steps (Fig.1a:0° = radial load direction,
90° = tangential load direction) and for each angle 13 specimens
were tested. The results (Fig. 1b) show a high correlation between
experimentally and theoretically determined values,
and confirm the sufficient accuracy of the theoretical approach.
48 ▪ D-BAUG Annual Report 2009
Heat and moisture transport of
building envelopes based on wood
How is the behaviour of heat and mois-
ture transport in building elements made
of timbered wood compared to conventio-
nal wood frame constructions?
by Daniel Keunecke, Joszef Garab, Peter Niemz / IfB by M. Joscak and P. Niemz / IfB
To answer this question, comparative measurements were
carried out at the IfB in the group of Wood Physics. Because
of the novelty of this type of solid wood constructions, the
practical experiences concerning the behaviour under nonsteady-state
conditions are scarce. Additionally, further investigation
and calculation of the heat and moisture transport
is needed because of the extremely high material
consumption for the manufacturing of such building elements.
For the experiments under natural weathering conditions
at the campus of the ETH Zurich, small test buildings without
openings were constructed. For further experiments,
wood-based building components were created and are
currently tested in cooperation with the Fraunhofer Institute
for Building Physics in Holzkirchen (Germany).
The conventional wood frame constructions with different
types of insulation (low density fibreboard, cellulose flakes
and glass or rock wool) improved with thin layers of
wooden panels or solid wood are compared to those made
of timbered wood, like layered board construction joined
with wooden dowels, a construction with slotted wooden
boards and a log construction.
For the calculation of the relevant physical parameters
such as heat and moisture transport under non-steadystate
conditions based on measured data, the simulation
software Delphin is employed. (Fig. 2) shows the calculation
of the heat transport inside the wall of a small test building
made of a layered timber construction joined with
wooden dowels. The aim of the investigations is to discover
new building physics parameters and their advantages and
disadvantages, respectively, for building envelopes made of
solid wood. Additionally, the simulation should answer the
question, if contemporary calculation models of heat and
moisture transport are suitable to calculate building elements
based on wood.