The Structural Behaviour of Shell Structures

By Cliff Button – QUT - Aug 2007 (an extract of the publication)

Dome structures have inherently many structural advantages This form of structure has evolved over many thousands of years from the “heavy” masonry solutions used predominantly in church structures through to the Eskimo’s form of ice winter shelter. These historic structures relied heavily on the thickish compressible materials translating the dual forces of natures’ extreme events and its own self-weight, directly to their footings.   Under extreme natural events, particularly large earthquakes, these structures were found wanting.

In recent times, the introduction of materials with significantly much higher bending capacities has allowed dome structures to rapidly evolve Over the last ten (10) years, the use of light weight composites has meant the dome (certainly at domestic scale applications) is rapidly returning to be a competent and attractive structural solution.

This process has been significantly aided with materials applications techniques now possible with thin in-situ shell materials

Structurally, a modern dome shell structure will be able to withstand axial reasonable tension and compressive stresses, particularly with a coexisting competency to transmit reasonably significant bending stresses. This modern day ability presents a very valuable tool

Lateral wind loading on the dome structure, for comparable floor areas (for a conventional single storey structure compared with a dome structure) are significantly reduced.   This decreased load can be calculated from the reduced “target” area and the reduced drag associated with an appropriately curved (dome) structure.    The downside effect comes with increased uplift (and consequential overturning) that will be marginally increased.   With appropriate hold down, this downside feature is not a significant handicap.

The relatively thin materials used by DomeShells Australia (varies but commonly 65mm) with 6.5mm inner and outer layers of glass fibre reinforced concrete bonded to a 60mm Polyurethane core, results in a very competent material for domestic scale shell structures.

A structural “downside” of thin light weight shell structures such as those used by DomeShells Australia, is that under extreme loading events, stress concentrations may be created, if inappropriate “holes” are formed in the shell structure.     If, however, these “holes” – in the form of doors / windows / skylights are controlled in number, size and proximity to each other – the downside of impairment to the structural integrity will not be significant.

The structural analysis of DomeShells shell structures (with their desirable (and liveable) openings) has effectively only been possible in the last 10 years with the development of high-speed super computers and very sophisticated specialised software.

The structural analysis effectively breaks the three dimensional dome structure into small interconnected structural elements and “tests” for structural distortions (under the maximum design loadings).   The effects of these distortions are then “accumulated” by the sophisticated computer software from all the loading and support conditions and the consequential resulting stresses are able to be determined.    The Structural Engineer then merely ensures the final shell material (and shape) has the appropriate physical properties and dimensions.