Civil Engineering Reference
In-Depth Information
2.2.2 Tensioned Membranes and Conventional Roo
ng
Forms
The design of tensioned membrane structures is a relatively new branch of archi-
tecture and engineering and is based on different assumptions. Compared with
conventional rigid structures, membrane structures de
ne their own contour on the
basis of the boundary conditions, the material properties and the level of pre-stress
into the surface. In addition, membrane structures accommodate loads through
changes in surface tensions and very large displacements, producing a geometri-
cally nonlinear behaviour which requires speci
c design software. The level of pre-
stress is fundamental for an adequate resistance to the applied external loads and is
a tradeoff between the risk of ponding and
fluttering on one hand, and the risk of
tearing on the other. Cables and membranes are generally characterised by an
intrinsic nonlinear behaviour mainly due to the geometrical layout of yarns and
wires. This results in relatively high costs for the computational analyses which
should be added to the intrinsic cost-effectiveness of this type of structures.
These features highlight the fundamentally different approach between conven-
tional roo
fl
ng forms and tension membranes which require a tight collaboration
between the subjects involved in the design and realisation of this type of building.
Considering the design phase, it can be seen that ordinary structures are generally the
outcome of two different protagonists: the architect, in charge of the initial proposal
and its development from the aesthetical point of view up to the singular detail, and
the engineer responsible for the static equilibrium of the structure, which has to meet
the aesthetic requirements within a reasonable cost and feasibility of the structure.
This approach is practicable because the rigid-type constructions are characterised by
small deformation, thus there are no considerable consequences on the geometrical
shape, which is generally chosen at priori with limited attention to the structural
problems investigated by the engineering of
ce once the project is approved. The
same organisation characterises the building erection with reduced collaboration
between the companies involved, who are generally responsible for the execution of a
single activity, with no considerable consequences for the other phases.
This methodology is totally inadequate for tension membranes, the aesthetic
issues go hand in hand with the structural aspects because the overall shape of the
structure depends on its equilibrium. Thus, the membrane shape cannot be imposed,
but it has to be found by working on the boundary conditions and the internal stress
distribution due to pre-stress. This imposes a tight cooperation between architects
and engineers in the design phase, which should consider the technical limitations
due to material production, erection and maintenance. For this reason the manu-
facturer and material producer are generally involved at the early stage of the
project development, which should consider aspects related to the chosen material
and the technology available at the workshop in charge of the manufacture of the
structure (Fig.
2.2
).
The comparison between conventional roo
ng forms and tensioned membranes is
generally based on the mere confrontation of a few parameters which generally comes
