Steel-Concrete Composite Shells for Enclosing Large Spaces
J.G. Teng
Department of Civil and Structural Engineering
The Hong Kong Polytechnic University
ABSTRACT
This paper describes a new structural system, namely steel-concrete composite shells, for
enclosing large spaces. These composite shells are formed by pouring concrete on a thin
stiffened steel base shell which serves as both the permanent formwork and the tensile steel
reinforcement. The thin steel shell, constructed by bolting together semi-box modular units
consisting of a base plate with surrounding edge plates, is a steel shell with thin ribs in both
directions. In this paper, the background to the development of this new structural system is first
given, followed by a detailed description of its structural features. Advantages of this new
structural system over existing systems are next discussed and possible failure modes of the new
structural system are also outlined.
INTRODUCTION
Many thin concrete shells have been built around the world as large span roofs, but their use
has gradually declined over the past few decades. This decline has been due mainly to the high
cost of construction and removal of temporary formwork and associated falsework for concrete
casting in the construction of a thin concrete shell. This labour intensive and costly process of
construction, coupled with the increasing ease in analysing complex skeletal spatial structures
offered by advances in computer technology, has made concrete shells much less competitive than
they were a few decades ago.
Over the years, there have been several attempts aimed at eliminating the need for
temporary formwork in constructing thin concrete shell roofs, but these have met with only
limited successes. An excellent review of these attempts and other developments in thin concrete
shell roofs has been given by Medwadowski (1998). One of the better recognised and more
successful attempts has been the use of inflated membranes as forms. In general, this method is
limited to the construction of circular-based domes with a spherical or nearly spherical meridian
and requires the use of special construction equipment. A major disadvantage of this method is
that the shape and thickness of the dome and steel bar positions are difficult to control in the
construction process. As shells, particular these domes, depend on their shape for their buckling
strength, even small shape deviations can be dangerous. A well-known example of this kind of
construction are Binishell domes. A Binishell dome collapsed in Australia in August 1986, with
the origin of the failure being poor shape control (IEAust 1991). A previous collapse of a similar
Binishell dome occurred in 1975 (IEAust 1991).
Another well known method for eliminating formwork is to have the concrete shell
prefabricated in small panels in a factory and then transported to the site for assembly into a shell.
Joints between these panels are sealed by in-situ casting of concrete. Although this method
eliminates the need for formwork, extra work is required in transportation and assembly. The
overall cost savings, if any, are limited.
Medwadowski (1998) in his recent article concluded that forming "remains the great,
unsolved problem of construction of concrete thin shell roofs. Any and all ideas should be
explored, without prejudice." This paper presents the new patented structural system of steelconcrete composite shells recently developed by the author which is believed to have ultimately
solved this problem. This system is referred to as the COMSHELL system for ease of future
reference.
STEEL-CONCRETE COMPOSITE CONSTRUCTION
Before describing details of the new structural system, it is appropriate to briefly review
recent developments in steel-concrete composite construction so that the new COMSHELL
system is set in the proper context of technological development.
A major recent development in steel-concrete composite construction has been the use of
cold formed thin steel sheeting as both the permanent formwork and the tensile steel
reinforcement. Examples include composite deck slabs (eg Wright et al. 1987, Uy 1997),
composite profiled beams (eg Oehlers 1993, Uy and Bradford 1995) and composite profiled walls
(eg Wright 1998, Bradford et al. 1998). Because the need for temporary formwork is eliminated in
this form of construction, the construction time can be shortened, leading to overall economy of
the structure. The development which is most relevant to the new COMSHELL system is profiled
deck slabs using profiled thin sheeting. Another system of composite construction for plates and
complete cylinders is the double (or dual) skin composite (DSC) construction (eg Jefferson and
Wright 1996, Link and Elwi 1995), in which concrete is sandwiched between two skins of steel
plates. This technique was developed for structures sustaining heavy external loading, such as
submerged tube tunnels or ice-resisting walls, so normal steel plates of much greater thickness are
used rather than cold formed thin sheeting.
STEEL-CONCRETE COMPOSITE SHELLS
An obvious approach for constructing steel-concrete composite shells is to use coldformed thin curved sheets with corrugations running in one direction and with shear studs
welded on before concrete casting. This is a simple extension of composite deck slab systems
but has a number of disadvantages: (a) the buckling strength of the sheeting may be quite low at
the construction stage, requiring the use of large curvatures and/or extensive shoring; (b)
concrete may flow off the sheeting if the slope is too steep; (c) the axial stiffness and strength of
the sheeting in the direction perpendicular to the corrugations are rather low; (d) only cylindrical
shell roofs can be constructed as it is difficult to produce corrugated sheets in the form of a
doubly curved surface; and (e) big curved sheets have to be transported to and handled on site.
In the COMSHELL system, the steel base shell is constructed by bolting together semibox modular steel units which consists of a flat or slightly curved base plate surrounded by edge
plates. By adopting different shapes for the base, different shell forms can be achieved as
explained in the next section. For ease of explanation, the structural functions of such modular
units are explained here referring to such units with a square or rectangular base for constructing
cylindrical shell roofs as shown in Fig. 1.
Fig. 1 Semi-box modular units and bolted connection
Fig. 2 Steel base shell formed from modular units
The modular units for cylindrical shell roofs are made with two of the edge plates slightly
inclined, so that when they are assembled using bolts, a curved profile of desired curvature
results (Figs 1 and 2). Holes are drilled on the edge plates for bolt connections to adjacent units,
and the bolted-together edge plates form stiffeners in both directions (Fig. 2). The circular
profile of the shell can be either approximated by flat bases, or exactly followed by slightly
curved bases with all edge plates being perpendicular to the base. In fact, the base can be convex
or concave, and in both situations, the deformations of the base under wet concrete loading can
be substantially reduced by membrane actions in the base.
This system has many advantages: (a) mass factory production of the units of a few
standard sizes and curvatures is possible, and shells of desired spans and curvatures can be built
easily; (b) transportation to and handling on site is easy, and units can be bolted together to form
big panels (eg half-span arch panels) for lifting; (c) the steel shell so built has many rib stiffeners
formed from edge plates, as a result its buckling resistance is greatly enhanced so shoring during
construction will be substantially reduced; (d) the ribs can prevent wet concrete from flowing
down the shell surface; (e) the ribs together with bolts act as shear connectors between the steel
shell and the concrete, so additional welded shear connectors are unlikely to be required; (f) as
the top of the ribs is either near the top surface (below the necessary concrete cover) or in
contact with the top reinforcing bars, the ribs also provide a substantial amount of hogging
moment resistance. Additional steel reinforcement is unlikely to be required unless large
hogging moments are present, and if they are required the ribs can be used as spacers to ensure
accurate positioning of reinforcing bars.
It may be noted that the most interesting aspect of the COMSHELL system is the edge
plates which fulfil six important roles: (a) connection flanges between the modular steel units;
(b) barriers to prevent concrete from flowing; (c) shear connectors; (d) stiffeners to the steel
shell; (e) reinforcement for hogging moments; (f) spacers for positioning additional reinforcing
bars.
Steel sheets of 1-2 mm in thickness are sufficient for the base plate of the modular unit to
carry wet concrete loading without excessive deformations and stresses. Due to this small
thickness, the units can be press formed easily with precise geometric control. The units can be
assembled into arch panels on ground before lifting, ensuring a tight control of the geometric
shape of the assembled shell and limiting shoring requirement.
CONSTRUCTION OF DIFFERENT FORMS OF SHELLS
Although the COMSHELL system is the simplest to implement for cylindrical shell roofs,
its benefits are equally attractive for other forms of shell roofs. Here, interesting possibilities of
pattern cutting arise. For example, for the construction of conical and spherical domes, modular
units with a trapezoidal base plate can be used. In general, a shell needs to be cut into a suitable
pattern so that it can be constructed from a minimum number of different modular units which
can be produced from a factory production line.
It should also be noted that the COMSHELL system is not only suitable for roofs, but also
for other large span applications. One such example is the construction of large span floor slabs
of small curvature. Such curved slabs can be both structurally efficient and aesthetically
pleasing.
ADVANTAGES OF THE COMSHELL SYSTEM
Advantages over In-Situ Cast Concrete Shells
The COMSHELL system possesses all the advantages of the widely used thin concrete shell
roofs, such as good thermal and sound insulation properties and aesthetically pleasing shapes. The
material costs for the new system and for concrete shells are expected to be similar. The shell roof
resists external loading (dominantly its selfweight) mainly by membrane actions. Compared to a
concrete shell, the use of the steel skin, in many cases, only means moving the reinforcing steel to
the bottom surface, so the material cost of the composite shell is similar to that of a concrete shell.
For shells under membrane forces, this repositioning of steel reinforcement has only a small
structural consequence. Where bending resistance is important, as for the buckling strength of
shells, the steel skin on the bottom surface provides a high resistance to sagging moments. While
the bolted joints between modular units are the weak links when subject to sagging moments, at
least part of this loss of material efficiency is compensated by the steel shell being able to resist
stresses in both directions, compared to the less efficient one-dimensional action of reinforcing
bars. In the COMSHELL system, a significant amount of hogging moment resistance also exists
due to the presence of steel ribs, so additional steel reinforcing bars will be limited or unnecessary
in most cases.
There are two very important advantages of the new system over in-situ cast thin concrete
shells. The first is that it eliminates completely the need for temporary formwork for concrete
casting and greatly reduces shoring requirement. There are also substantial labour cost savings in
the placement of steel reinforcement because steel reinforcing bars may be eliminated altogether
or much fewer/smaller steel bars need to be bent and fixed.
Another major advantage of the new system lies in its robustness in shape. Creep of the
concrete will only lead to stress redistribution between the concrete shell and the steel shell, but
will not lead to appreciable changes in the shape of the structure, in contrast to concrete shells
where creep deformations have a strong influence on the buckling strength through continuous
shape changes.
Advantages over Other Shell Structures
The new system is similarly advantageous over precast concrete shells as the need to precast
and transport heavy concrete panels and the difficulty in assembling and joining together the
panels no longer exist. Compared to steel reticulated shell structures, the new system uses less
steel and does not require additional roofing materials. Compared with corrugated sheeting roofs,
the new system has a much higher resistance to buckling and corrosion, and much better thermal
and sound insulation properties which are generally necessary for enclosing spaces for human
occupancy.
FAILURE MODES
For the COMSHELL system to be widely used in practice, many issues have to be studied
to establish a sound understanding of the behaviour and strength of these shells, and to develop
suitable design methods. At the present, a number of failure modes have been identified, and
each needs a great amount of research. These failures modes are associated with either the
construction stage or the service stage and are currently being investigated at The Hong Kong
Polytechnic University.
Construction Stage
The key of the new system is the construction of a steel base shell from modular units.
This steel base shell is required to be strong enough to resist the wet concrete loading during
construction. First, the base of modular semi-box units needs to be stiff enough so that it does
not deform excessively under wet concrete loading. This base deformation is one of the
controlling criteria for the selection of sheeting thickness. While the use of thinner sheets can be
made possible by adopting smaller modular unit sizes, the reduction in size will increase on-site
assembly time. Typical sizes are expected to range between 1 m by 1 m to 0.5 m by 0.5 m for
square bases. The use of a curved base is believed to a more desirable option for reducing base
deformations. Another failure mode is the buckling of edge plates, and this is another
controlling criterion for the sheeting thickness. A possibility to increase the buckling strength of
edge plates is to add a horizontal stiffer to the top of the edge plate, but this increases difficulty
in press forming of modular units.
The most important mode of failure in the construction stage is the overall buckling of the steel
base shell. The base shell is already stiffened with ribs formed from edge plates, but when a
large span is considered, the buckling strength of the stiffened shell under wet concrete loading
is likely to control the sheeting thickness. There are a number of possible ways of increasing the
buckling strength without increasing the sheeting thickness. One is to cast concrete in stages, by
forming stiff arches/beams over selected rows/columns of modular units. Another is to cast over
the whole shell only part of the total thickness in the first stage, with the second stage casting
carried out after the first stage concrete has gained sufficient strength. Yet another possibility is
to provide vertical support to selected locations to enhance the buckling resistance. For example,
the provision of supports to one or more circumferential lines of a cylindrical steel base shell
can substantially increase its buckling resistance against wet concrete loading. With these
measures, it is possible to prevent this failure mode from controlling the sheeting thickness,
thereby minimising the cost of the COMSHELL system.
Service Stage
Once the shell has been constructed, the main loading on the structure is its self weight,
snow loading, wind loading, and seismic loading if the shell is located in a seismically active
area. The wind loading is uplifting, provided the shell is not a deep one, so in general, it is
beneficial. Self weight, together with snow loading if the structure is located in a cold region, is
probably the most important loading condition for a large span COMSHELL roof. Seismic
loading is another important consideration.
Under self weight with or without snow loading, the structure is likely to fail by buckling.
Creep deformations of the concrete under self weight loading are still important in a
COMSHELL system, but they will only lead to redistribution of stresses between the steel shell
and the concrete shell above it. They will not have the devastating consequence of changing the
shape of the shell, as the steel base shell controls its shape. The bolted joints are weak links in
bending resistance in the shell and are likely to play a major role in determining the buckling
resistance of a COMSHELL roof.
CONCLUSIONS
This paper has presented in detail a new structural system, namely steel-concrete
composite shells, for enclosing large spaces. These composite shells are formed by pouring
concrete on a thin stiffened steel base shell which serves as both the permanent formwork and
the tensile steel reinforcement. The thin steel shell, constructed by bolting together semi-box
modular units consisting of a base plate with surrounding edge plates, is a steel shell with thin
ribs in both directions. These modular units can be prefabricated in factories and then
transported to the construction site for assembly. The new system has all the advantages of thin
concrete shell roofs, but eliminates the need for the construction and removal of temporary
formwork which has been a major obstacle to the wider application of the latter. It is hoped that
the new system will revive the use of shell roofs by reaping their benefits without the
disadvantages associated with thin concrete shells.
ACKNOWLEDGEMENTS
This work has been supported by the Research Grants Council of the Hong Kong Special
Administrative Region, China (PolyU: 5059/99E). The author is grateful for this support and to
Mr. H.T. Wang and Dr. Z.C. Wang for producing the graphical work for the paper.
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