Prestress and its application to shell, fabric, and cable net structures
Doktorsavhandling, 2021

Prestressing and shells provide means to create material-efficient and well-functioning structures, as do their combination, offering opportunities for increased material efficiency within the built environment. Prestressing introduces stresses in an object to enhance its performance, and shells include concrete shells, masonry vaults, fabric structures, cable nets, and timber or steel gridshells. Both prestressing and shell structures come with technical and practical considerations that need attention during the design, or there is a risk of wasted opportunity. However, successful attention to and combination of these aspects, resulting in a material-efficient prestressed shell, is not enough to make a high-quality architecture. There is a need for additional project-specific considerations, requiring ways to study design choices' impact on structural and architectural aspects.

This thesis aims for an increased understanding of prestressing and its application to shell, fabric, and cable net structures and improved means for their design. It provides a broad overview of prestressing, expanding beyond the common perception of prestress being limited to concrete structures, and shell structures, focusing on applications within architecture. The scope is the combination of prestressing and shells, and it addresses three main research questions: (1) Can any shell be prestressed? For those that can, what is the meaning and influence of prestressing?; (2) How can prestressed shells be form-found using analytical and numerical approaches?; and (3) How can prestress in shells be represented and chosen, aspiring for efficient structural performance?

Appended papers A-F help answer these questions, and the thesis contributes to architectural and structural design and structural optimisation and applies differential geometry. It provides approaches for the form-finding of gridshells containing both tension and compression elements (Paper A) and of minimal surfaces (Paper C and D). Paper B concludes that a sphere cannot be prestressed, but a torus can. Paper E extends the Williams and McRobie (2016) discontinuous Airy stress function from flat structures to curved shells, allowing moments and shear forces in edge beams of shell structures to be quantified and appropriate prestressing chosen. Paper D uses a discrete Airy stress function and discusses the structural behaviour of shells with negative Gaussian curvature loaded with patch loads. Paper D studies Eduardo Torroja's prestressed concrete Alloz aqueduct, concluding that longitudinal prestressing may reduce the wall bending moments and that, at the limit, the channel act as a cylindrical membrane-action shell rather than of an Euler-Bernoulli beam, enabling thinner cross-sections.

Structuraldesign

Engineering

Conceptual design

Stress pattern

Prestress

Architecture

Form finding

Geometric stiffness

Opponent: Caitlin T. Mueller, Associate Professor, Digital Structures Research Group, Department of Architecture, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA

Författare

Alexander Sehlström

Chalmers, Arkitektur och samhällsbyggnadsteknik, Arkitekturens teori och metod

Prestressed gridshell structures

Proceedings of the IASS Symposium 2017,; (2017)

Paper i proceeding

Unloaded prestressed shell formed from a closed surface unattached to any supports

Form and Force - IASS Symposium 2019 and Structural Membranes 2019,; Vol. 2019(2019)p. 167-174

Paper i proceeding

Tensioned principle curvature cable nets on minimal surfaces

Advances in Architectural Geometry 2021,; (2021)p. 84-107

Paper i proceeding

The analytic and numerical form-finding of minimal surfaces and their application as shell structures

Proceedings of the IASS Annual Symposium 2020/21 and the 7th International Conference on Spatial Structures,; (2021)

Paper i proceeding

Sehlström, A. Olsson, K.-G. Williams, C. J. K. Design of tension structures and shells using the Airy stressfunction. (Accepted for publication)

Sehlström, A. Olsson, K.-G. Williams, C. J. K. Does Torroja’s prestressed concrete Alloz aqueduct act as a beam or a shell? (Under review)

Shells are material-efficient and beautiful structures. They are formed from thin curved surfaces and include concrete shells, masonry vaults, fabric structures, cable nets, and timber or steel gridshells. Sometimes shell structures are prestressed. Then stresses are deliberately introduced into the structure before it is loaded by, for example, its weight, wind, snow, and earthquake. These stresses improve the structure's performance, giving stiffness to tension structures and preventing cracking in concrete. In the case of masonry structures, the prestress is actually induced by its weight, enabling an arch to carry live loads. Spiderwebs, violin strings, and plant stems are prestressed, and the vitreous humour and aqueous humour maintain the shape of our eyes through fluid pressure.

This thesis aims for an increased understanding of prestressing and its application to shells and thereby improve their design. It provides a broad overview of prestressing and shell structures, focusing on those within architecture. It summarises research presented in six appended papers, which propose design approaches for prestressed shells and insights on their structural behaviour. Some develop excising form-finding methods whereby the curved geometry of a prestressed shell can be determined using a computer algorithm. Others focus on graphical design methods that allow the analysis of shells by 'folding' and 'cutting' a surface representing the internal stress state. One paper investigates the structural behaviour of the elegant prestressed concrete Alloz aqueduct designed by Eduardo Torroja in 1939.

Ämneskategorier

Arkitekturteknik

Teknisk mekanik

Arkitektur

Geometri

ISBN

978-91-7905-605-6

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5071

Utgivare

Chalmers tekniska högskola

Opponent: Caitlin T. Mueller, Associate Professor, Digital Structures Research Group, Department of Architecture, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA

Mer information

Senast uppdaterat

2021-11-26