Civil Engineering Reference
In-Depth Information
A strain-controlled biaxial test aims to reproduce strain histories assigned to the
central zone of the cruciform specimen. It is considered one of the most recent
procedures developed in this
field and has scarcely been investigated due to the
higher complexity of the testing equipment and the controlling software required.
Instead of considering the value measured by force or displacement transducer, the
procedure is based on the strains measured by the extensometers which are com-
pared to the imposed values and, in case of the discrepancy, the controlling software
requires the actuators to have a force or a displacement adjustment.
The results obtained by using strain-controlled biaxial procedures can provide
further insight into the mechanical behaviour of coated fabrics and can give valu-
able information in connection with membrane installation processes such as the
stress level required to achieve certain desired strains.
5.1.1 A Simple Case of Installation of a Flat Membrane
in Terms of Strains
The load bearing capacity of a membrane structure is mainly related to its level of
double curvature and pre-tension. Due to the progressive use of tensioned mem-
brane roofs and facades in direct competition with traditional and more conven-
tional solutions, the demand of the market is mainly focused on structures
characterised by a low level of curvature which leads to a more rational use of the
building
s volume. This trend imposes a better understanding of the materials
behaviour in order to ensure that the membrane remains in tension under all loading
conditions, and to reduce de
'
ections (Bridgens and Birchall
2012
). The testing
campaign carried out at Politecnico di Milano (Beccarelli et al.
2013
) investigated a
simple case of installation of a
fl
fixed to a rectangular
rigid frame such as a modular lightweight panel for a textile fa
fl
at membrane that must be
ade. This simple
application requires an accurate knowledge of the material properties in order to
estimate how much smaller the unstressed textile panels must be with respect to
their
รง
final dimensions (compensation factor).
It has been assumed that the fabric is initially mounted on the frame without
tensioning the material. Then the coated fabric is stretched along one of the material
directions (warp or
fixed to the rigid support. Later the membrane is
stretched parallel to the other weaving direction and
fill) and tightly
fixed to the boundary frame. In
terms of a strain-controlled biaxial test, the installation is reproduced by the two
different strain-paths represented in Fig.
5.1
a, c and the respective strain histories
represented in Fig.
5.1
b, d.
The
e
f
, corresponding to an
expected level of pre-stress equal to 2 kN/m, has been estimated by carrying out a
traditional load-controlled biaxial test on a cruciform sample loaded with
e
w
and
final strain value in warp and
fill direction
five cycles
from 0.1 to 2 kN/m at a constant rate (0.1 kN/s) and using a load ratio of 1:1. The
results are shown Fig.
5.2
and have been used to estimate the value of
e
w
and
e
f
:33
and 0.21 % respectively.
