Nicholas O'Brien Melin
Doctor of Philosophy, Magdalen College, University of Oxford, Trinity Term, 2004
Application of Bennett Mechanisms to Long-Span Shelters
Abstract
Rapidly assembled tent structures are temporary enclosures used to
house people or goods. Their uses vary to include recreation, refugee
housing, and military shelters. The structural concepts applied in these
shelters are as variable as their uses. Some make use of a tensioned
fabric and pole system to provide structural strength. Others have a
load-bearing frame with attached fabric skin. Further variants make
use of inflatable arches or consist of modular containers. Analysis
of a number of different types of rapidly assembled tent structures
reveals an area where innovation can occur. Conflicts in the last ten
years suggest that rapidly assembled shelters for both military purposes
and humanitarian relief have the greatest need for innovative solutions.
Existing shelters used by the military lack the versatility and speed
of deployment necessary in modern conflict. The lack of scalability
in the designs makes it difficult to use an existing tent in different
situations. They are slow to construct, heavy, and difficult to transport
in large numbers. These problems suggest that there is a need for new
shelters that better meet the needs of the military. The application
of deployable structures technology meets military's needs for structures
with the advantages of a small compacted volume, rapid assembly, and
ease of deployment. This makes them ideal for application to shelter
structures.
The aim of this dissertation was to develop a new type of deployable,
long-span shelter frame based upon tiled Bennett mechanisms. An overlapping
combination of equilateral Bennett mechanisms yields a structure that
opens into a half-cylinder shape, providing an enclosed space useful
and applicable to the problem of deployable shelters. The specific application
considered in the design portion of this process will be a long-span
deployable shelter capable of housing military helicopters.
This report details the development of the Bennett Shelter concept.
Its deployed and compacted geometries are explored, and a procedure
for determining structural properties and dimensions is presented. The
full concept for the structure, from outer covering to foundation support
is then detailed. Loads affecting the structure are determined, and
the process of modelling and analysing the structure is then considered.
Optimisation of the structure with respect to weight and serviceability
requirements is conducted using a number of different materials, and
full analysis of the optimal geometries is completed. As no method exists
for evaluating the effect of imperfections on the deployment of overconstrained
mechanisms, a procedure is derived. The effects of manufacturing imperfections
on deployment of the Bennett mechanism are then explored using the method.
A full examination of the variation of energy within the Bennett Shelter
during deployment provides valuable insight into the performance of
the structure. With the above analysis complete, it is shown that the
Bennett Shelter is viable as a long-span deployable shelter.
This thesis can also be downloaded from the ORA website