Civil Engineering Reference
In-Depth Information
considerably different characteristics as described in Sect
2.1
. ETFE foils are tra-
ditionally used for multilayer pneumatic cushions, however, in recent projects, such
as the Unilever headquarters in Hamburg Hafecity (Stimp
e
2010
), the application
of single skin ETFE foils has been successfully investigated. This innovative
application opens new areas of interest in the area of the building envelope, such as
a protective secondary facade. In addition, the growing demand for reducing the
heating\cooling costs has recently increased the demand for open mesh coated
fabrics which are able to modulate the amount of radiation re
fl
fl
ected, absorbed and
transmitted through different weaving patterns and coatings.
2.2.1.2 Pneumatic Structures
The term pneumatic structures includes all the lightweight structures in which the
load bearing capacity is achieved by means of air under pressure. They are mainly
subdivided into two categories: the buildings characterised by a single layer, sta-
bilised by a slight difference in pressure between the inside and the outside of the
structures, and the building envelopes stabilised by air under pressure enclosed
between two or more membrane layers.
The basic idea of single layer pneumatic structures is quite simple, an internal
volume delimitated by the thin membrane is maintained under pressure by means of
fans, a low level of pressure leads to a distributed force on the membrane surface,
which receives the support necessary to compensate for the self-weight and the
external loads, assuming the classical synclastic curvature. This type of structure is
generally designed to maintain an internal pressure between 0.2 and 0.55 kN/m
2
,
which assures the necessary stability for wind loads but can be inadequate for heavy
snow falls. The snow loads, which can reach a value on plan from 0.2 to 2.4 kN/m
2
,
represent a critical aspect of these structures which can be overcome heating the
internal air space. If the stabilising forces do not exceed the external loads, due to
overloading or dysfunctions in the internal control system, dimpling occurs in the
membrane and the reversion of the surface curvature can lead to the collapse of the
structure, with consequent damages to the structure and its occupants. Air-supported
structures provide a cost effective alternative for seasonal wide span coverings,
nevertheless, the reduced resistance under bad weather conditions combined with
high costs due to great pressure losses, reduced insulation, maintenance and the
seasonal mounting and dismounting costs can progressively reduce the initial
convenience over the entire life span (Shaffer
2013
).
Some critical aspects can be reduced by adopting a multiple layer solution. It has
been widely demonstrated that envelopes with cushions realised with two or more
layers of material show a higher thermal insulation (Devulder et al.
2007
; Schmid
2007
;Ward et al.
2010
) and lower pressure losses despite an increase in the complexity
of the pressure control system. Moreover, the load bearing capacity of structural
elements can be increased by including cables and steel bars, which can increase the
resistance in tensioned and compressed areas. The synergetic combination of pneu-
matic fabric structures with cables and struts is at the basis of the Tensairity
®
system,
