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![[Dept of Engineering]](http://www.eng.cam.ac.uk/images/house_style/engban-s.gif){kind=small}
Steel and Composite Steel-Concrete Structures
Reinforced Concrete Structures
Partially Bonded Non-metallic Prestressing Tendons
Lateral Stability of Precast Concrete Beams
Polyester and Aramid Fibre Properties
Non-Metallic Reinforcement for Concrete
Application of Geometric Alegbra to Beam Stability
Repair and Strengthening of Existing Structures
Joining of Advanced Composite Materials
Bridge Engineering
Actuators and Adaptive Structures
Structural Vibration and Control
Deployable Structures
Analysis of Symmetric Structures
Dynamics of Nonlinear Elastic Systems
Long Span Bridges Aeroelasticity
Shell Structures
Pipeline Problems
Mechanics of Historical Construction; Curriculum Development
References
Dr R.E. McConnel
A project to investigate the use of asymmetric hot-rolled steel sections ("ASB") in a stub-girder arrangement with deep-troughed concrete-filled floors ("slimfloor") for large-span buildings has been completed. A span of 13.2 m has been tested and the results obtained will be published in 2000.
An investigation into the use of different constructional materials in large-span shallow domes of reticulated, composite construction continues. Numerical models for shear weak members have been derived, which include the de-stabilising effects of significant axial force. These models are being incorporated in a Dynamic Relaxation routine, and will be used to study shear weak (in plan and out of plan) shallow domes with rectangular grids. Suitable experimental studies will be conducted.
Dr C.T. Morley
Research has continued on the application (with appropriate caution and safeguards) of the upper-bound theorem of plasticity theory to predict the collapse loads of reinforced concrete structures _ most recently beam-and-slab bridges in shear(H10). Work has also continued on simplified approaches to the analysis of structures made of materials exhibiting marked strain-softening after little or no plastic flow. Progress is being made using the Galerkin method. Tests are in progress on heavily-reinforced slabs with a single developing yield-line and limited rotation capacity, for comparison with the theoretical approach.
Research continues on compressive membrane action in reinforced concrete slabs. A conference paper(H1) has been published on model tests on reinforcement layouts near columns, one designed as a torqueless grillage.
Dr C.J. Burgoyne
Dr J.M. Lees
The project on beams prestressed with partially bonded non-metallic tendons has now been completed.
It showed that it is possible to achieve both high strength (normally associated with fully bonded systems), and high rotation capacity (normally associated with unbonded systems), by judiciously debonding the tendons. This has been achieved in two ways; by alternately bonding and debonding the tendon along its length, or by coating the tendon with a surface layer of known, low, shear strength. The tendons were made from aramid fibres, in the form of resin-bonded pultrusions or braided rods. Associated work has been investigating the bond behaviour between the rods and the concrete, since the mechanism of bond breakdown is critical to the behaviour of the tendons(H16,H17).
Dr C.J. Burgoyne
Recommendations have been produced to allow designers to take into account stability considerations for precast concrete beams(H24,H25). These beams can become unstable under their own self-weight, especially when lifted, since in this configuration the restraint to lateral deflection is removed, and a toppling behaviour can result. Also of interest is a problem that arises during transportation, when the beam is supported on turntables at both ends. In this configuration, the primary problem arises from lateral loads caused by uneven ground or by centrifugal forces during cornering. The study has included the effects of initial imperfections, and has investigated the minor axis bending stresses that are induced. Design recommendations for beams on rotationally flexible bearings are also shortly to be published; work continues in the form of undergraduate projects to test predictions.
Dr C.J. Burgoyne
The long-term mechanical properties of polyester and aramid fibres are being studied, with a view to drawing up safe levels of tension load for long-term structural applications. In the case of polyester, the uses are normally as soil-reinforcement, while for aramids, the normal uses would be as prestressing tendons. In both cases, a proper understanding of the stress-rupture, creep and relaxation properties can have a significant impact on the economics of using such materials.
Dr C.J. Burgoyne
Although the normal use of non-metallic systems in concrete will be as prestressing tendons, such systems can also be used to reinforce against shear, and to provide restraint against compressive failure. Both end-uses require modifications to the conventional wisdom about structural behaviour.
In the case of shear reinforcement, yielding of the reinforcing links can no longer be relied on, which means that plasticity theorems no longer hold. This, in turn, means that an explicit relationship is required between the shear deformation and the forces in the links. As with the partially-bonded longitudinal tendons discussed above, the bond behaviour is critical, since this controls the way in which the elements move relative to one another. A model of the equilibrium of the "tooth" of concrete between shear cracks is being developed.
The ability of systems based on fibres to provide restraint against compressive failure is well-known; but it has been ignored in the current design philosophy that "under-reinforced" failure is desirable, so that the tendon fails before the concrete. If beams are to be built with non-metallic tendons, and those tendons are brittle, that would imply that "over-reinforced" failure, where the concrete crushes before the tendon snaps, is desirable. Thus, a significant increase in structural ductility can be achieved by providing hoop reinforcement in the compression zone. The consequence is not to increase the overall strength significantly, but rather to enhance the strain capacity, so that substantial amounts of energy are dissipated within the concrete when it crushes.
A review of the most logical applications for composites in concrete has been undertaken. It has been concluded that resin-free rope systems are the most suitable material for use as external, unbonded, post-tensioning tendons; while partially-bonded pultrusions would be sensibly applied as internal pretensioning strands. It has also been concluded that these materials are not suitable for the reinforcement of concrete. It will be important for there to be a sensible appreciation of the costs of the various materials, including whole-life costs, before these materials can be satisfactorily applied. Novel forms of bridge construction should also be considered.
Dr J. Lasenby
Dr C.J. Burgoyne
Dr J.M. Lees
Research is being carried out in the field of strengthening and repair. A particular focus has been the use of advanced composite materials as a means of increasing the shear and/or flexural capacity of existing structures.
A project which investigates the shear enhancement of concrete using external carbon fibre reinforced plastic (CFRP) straps has been initiated in collaboration with the Swiss Federal Laboratories for Materials Testing and Research (EMPA). The CFRP straps are prestressed and will potentially compensate for an existing structure which otherwise has insufficient shear reinforcement. The work includes both theoretical and experimental studies of the behaviour of the composite straps and the performance of the strengthened concrete beams.
Research which studies flexural strengthening using tensioned CFRP laminates has also commenced. Key issues are the anchorage of the stressed laminate and the development of an all-composite anchorage system. Both of these aspects are currently under consideration.
Dr J.M. Lees
Dr S.D. Guest
Although advanced composites have been used extensively in the aerospace and manufacturing industries, to date their potential has not been fully exploited in civil engineering. One contributing factor is the difficulty associated with the joining of these novel materials.
Current research activities include an investigation of a number of innovative systems as a means of achieving
an optimum joint configuration. A preliminary experimental study has considered combined mechanical and bonded connections. Initial work has also been carried out on the design of a longitudinal seam in bistable composite tubes, in association with Rolatube Ltd.
Dr C.R. Middleton
Dr C.T. Morley
Research on bridge assessment is continuing, with emphasis on the development and evaluation of collapse-analysis methods. The COBRAS generalised yield-line collapse-analysis program, developed within the group, has now been successfully released through the University's technology transfer company, CUTS Ltd, and is believed to be the world's most advanced such program commercially available. It has been used by numerous bridge authorities and consultants, resulting in substantial financial savings. Courses for bridge engineers on using the program have been put on in Cambridge.
Research ancillary to COBRAS, on checking failure-mode compatibility, on examining ductility requirements and corrosion effects, and on the strength of cantilevered slabs, continues. A paper has been published(H10) on collapse analysis for shear in beam-and-slab bridges.
Examination of whole-life performance, and of the limits of application of reliability-analysis methods in bridge engineering, continues under contract with the Highways Agency.
Investigation of the progress of corrosion, and of its influence on collapse- and reliability-analysis, continues. Non-destructive testing techniques for determining location and corrosion of reinforcing bars in concrete are being investigated and developed.
Dr S. Pellegrino
Dr S.D. Guest
A study of polyurethane shape-memory polymer foams under cyclic stress-strain-temperature loading has been carried out. This is a preliminary study of a material that is being proposed for use in space exploration vehicles. Further work in this area with Composite Optics International, in San Diego, and NASA JPL is being planned.
Work is beginning, with funding from EPSRC and an industrial consortium, on new technologies for structures that can change their shape in a controlled fashion. It is planned to build upon previous work on bi-stable composite shells and shape-memory actuators that has been done in the Deployable Structures Group.
Dr S. Pellegrino
Shape and vibration control of flexible structures by means of inertial stick-slip actuators has been investigated. A non-linear model representing the interaction between the structure and a 6DOF Stewart platform system containing 6 actuators has been derived, to investigate the closed-loop stability and performance of the controlled system. Quasi-static alignment and vibration control of a payload attached to the platform are possible with this system(H2).
Dr S. Pellegrino
Dr S.D. Guest
Work is continuing, on behalf of the European Space Agency, on a new generation of deployable membrane reflectors. These reflectors consist of an expandable hub connected to a series of thin-walled foldable ribs, supporting a precision cut membrane. It had been previously thought that only symmetric configurations of these reflectors are possible, but a methodology for the design of reflectors with offset configuration has now been developed. Two different configurations of offset reflectors have been introduced, with comparable surface errors but different prestress distributions.
The first configuration can be used to design reflectors that have a conventional shape and acceptable prestress. The second configuration requires a non-standard shape of the surface, but makes it possible to achieve fully biaxial prestress distributions, equivalent to those previously obtained for symmetric CRTS reflectors(H14).
An extensive study of tape springs, i.e. straight, thin-walled strips with a curved cross-section, has been published. Springs of this kind are increasingly being used as deployment actuators in spacecraft structures. It has been shown that these springs deform by forming elastically deformed regions with zero transverse curvature and uniform longitudinal curvature. The process of formation and growth of a fold belongs to a wide class of propagating instabilities. It is characterised by a high peak moment and a lower propagation moment. A compact model of the moment-rotation relationship for an elastic fold has been derived; a key feature is that the bending moment on either side of a fold moving along a uniform tape spring, away from any end supports, is constant, whereas this moment increases near a support. Compact and accurate two-dimensional theories have been developed to simulate the self-actuated deployment of tape springs. It has been shown that conservative energy formulations are
appropriate for coiled springs, where the velocity field is smooth, but not for springs with localised folds. To simulate the motion of such localised folds a non-conservative impulse-momentum formulation has been proposed, and it has been found that this model can accurately predict both the steady motion of the folds along the tape spring and their rebound against the end supports(H23).
Work continues on gravity compensation systems for deployable structures. The purpose of such systems is to reduce, or eliminate gravity loading on large spacecraft structures during ground testing. An impressive active suspension system has been set in the laboratory and an extensive experimental program is under way.
New research on deployable synthetic aperture radar has begun, funded by a research contract from Matra Marconi Space UK. The key ideas for this work were developed during Dr Pellegrino's secondment to the Center for Aerospace Structures, at the University of Colorado in Boulder.
A new concept for retractable roof structures has been proposed. It is based on a new mechanism invented at Cambridge a few years ago. The new structures consist of a foldable lattice of beams connected by cylindrical joints, to which covering panels or membranes are attached. These roof structures fold towards their perimeter and there is practically no limit to their shapes. Solutions to the key problems that have to be solved in the course of the kinematic design of this new type of structures have been obtained. These include two different ways of connecting these structures to fixed foundation points while maintaining their internal degree of mobility, and how to identify the shapes of the covering panels, to avoid interference during retraction(H13).
There is a growing interest in movable and deployable architectural structures(H20). Possible ways of transferring to Architecture solutions obtained in Aerospace have been outlined in an invited paper at the conference Engineering a New Architecture, held in Aarhus(H21).
Three former members of the Deployable Structures Group have been appointed to Lectureships at Bath, Oxford and UMIST.
Dr S.D. Guest
Work has continued on the use of symmetry in structural analysis(H11,H12). An exciting development this year has been the development of tools and methods for the design of "overconstrained mechanisms". These have initially been exploited to develop deployable support structures for synthetic aperture radars.
Mr F.A. McRobie
Research is being conducted into the dynamical behaviour of nonlinear elastic systems, focusing on the application of dynamical systems theory(H19,H22). In collaboration with Dr J. Lasenby, the use of Clifford Algebra has been developed as the appropriate mathematical framework in which to construct a large-rotation rod theory suitable for finite-element implementation(H18). Work has also been undertaken on the development of dimension-reduction algorithms for extracting low-dimensional dynamic models from massive numerical simulations, by the application of techniques of inertial manifold approximation(H15).
Mr F.A. McRobie
Work is in progress assessing the applicability of computational fluid dynamics (CFD) to the design of long-span bridges. Using a fully-coupled fluid-structure finite-element code and inhouse discrete vortex and finite volume codes, the various aeroelastic instabilities such as flutter and vortex-induced oscillations can be modelled in the time domain, providing detailed insights to supplement wind-tunnel tests and simpler mathematical descriptions(H3,H4).
Professor C.R. Calladine
Work continues on various aspects of the buckling of thin shell structures. A computational analysis of stresses induced in a silo wall on account of local shrinkage in axial and circumferential welds, and of other lack-of-fit constructional defects, has been completed(H9).
A long-standing paradox in the buckling of plain thin-walled cylindrical shells under axial loading has been resolved. The key to the situation is that in experimental assays the buckling strength corresponds to certain post-buckling modes which are inextensional except in narrow boundary-layers surrounding "dimples". These modes and corresponding loads are well-defined in open-topped shells loaded by self-weight, giving buckling stress clearly proportional to thickness raised to the power 1.5. However, in shells having both ends closed the situation is not so transparent; and experimental evidence, while showing the same power law, also includes a lot of "scatter". This new analysis successfully addresses issues which are not satisfactorily resolved by the traditional emphasis on "imperfection sensitivity" in theories of buckling of thin shell structures.
Work has begun on the response of masonry domes to earthquake loading. The initial aim is to produce analyses of static stresses in masonry arches and domes subjected to non-vertical "gravity" loading.
Professor C.R. Calladine
Professor A.C. Palmer
Laboratory-scale modelling has been used to study the mechanics of buckling of a pipeline under construction up a hillside in Columbia. The pipeline was represented by a slender silicone-rubber rod, and its temporary supports by small wooden cubes. The model "hillside" could be tilted to arbitrary inclinations. The apparatus enabled the mechanics of a particular failure to be clarified. [See PETROLEUM ENGINEERING, ref. I12]
Mechanics of Historical Construction; Curriculum Development
Work continues on the application of mechanics to masonry and geotechnical construction(H5,H6,H7). It may seem strange that there is anything new to say about the methods of construction that were used two or three centuries ago. But the conventional structural wisdom of today is firmly rooted in nineteenth-century elasticity; and it is largely inappropriate to masonry and geotechnical construction, just as it was inappropriate to the understanding of the actual behaviour of real steel structures, as pointed out by Lord Baker in the 1950s(H8). Our undergraduate and post-graduate courses on the theory of structures and solid mechanics continue to place a proper emphasis on the development of appropriate theory and the importance of experimental investigation.
H1. Chung Kam Chung, D.M., Morley, C.T. Model tests on different layers of slab reinforcement near columns. 9th British Cement Association, Concrete Communication Conference, Cardiff (July 1999).
H2. Darby, A.P., Pellegrino, S. Modeling and control of a flexible structure incorporating inertial slip-stick actuators. Journal of Guidance, Control and Dynamics (AIAA), 22, (1), 36-43 (1999).
H3. Frandsen, J.B., McRobie, F.A. Computational aeroelastic modelling to guide long-span bridge cross-section design. Wind Engineering into the 21st Century: Proceedings, 10th International Conference on Wind Engineering, Copenhagen, Denmark (June 1999); Edited by A. Larsen, G.L. Larose, F.M. Livese , 2, 1277-1284 (A.A. Balkema, 1999).
H4. Frandsen, J.B., McRobie, F.A. Finite element simulation of wind-induced bridge motion. Computational Methods for Fluid-Structure Interaction: Proceedings, International Symposium on Computational Methods for Fluid Structure Interaction, Trondheim, Norway (February 1999); Edited by T. Kvamsdal, et al, 271-282 (Tapir Press, 1999).
H5. Heyman, J. The assessment of strength of masonry arches. Arch Bridges: Proceedings, 2nd International Conference, Venice, Italy (October 1998); Edited by A. Sinopoli, 95-98 (A.A. Balkema, Rotterdam, 1998). ISBN 9058090124.
H6. Heyman, J. El Esqueleto de Piedra. (CEHOPU, Juan de Herrera, Madrid, Spain, 1999). ISBN 84-89977-73-9. (Translation into Spanish of: The Stone Skeleton, CUP, 1995).
H7. Heyman, J. Mechanical behaviour of arches and vaults. In: Structural Analysis of Historical Constructions II; Edited by P. Roca, J.L. González, E. Oñate, P.B. Lourenço, 1-6 (CIMNE, Barcelona, Spain, 1998). ISBN 8489925267
H8. Heyman, J. Navier's straightjacket. Architectural Science Review, 42, (2), 91-95 (1999).
H9. Holst, J.M.F.G., Rotter, J.M., Calladine, C.R. Imperfections in cylindrical shells resulting from fabrication misfits. Journal of Engineering Mechanics (ASCE), 125, (4), 410-418 (1999).
H10. Ibell, T.J., Morley, C.T., Middleton, C.R. Strength and behaviour in shear of concete beam-and-slab bridges. ACI Structural Journal, 96, (3), 386-391 (1999).
H11. Kangwai, R.D., Guest, S.D. Detection of finite mechanisms in symmetric structures. International Journal of Solids and Structures, 36, (36), 5507-5527 (1999).
H12. Kangwai, R.D., Guest, S.D., Pellegrino, S. An introduction to the analysis of symmetric structures. Computers and Structures, 71, (6), 671-688 (1999).
H13. Kassabian, P.E., You, Z., Pellegrino, S. Retractable roof structures. Proceedings of the Institution of Civil Engineers, Journal of Structures and Buildings, 134, (1), 45-56 (1999).
H14. Lai, C.-Y., Pellegrino, S. Deployable membrane reflectors with offset configuration. Collection of Technical Papers, 40th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference, St Louis, MO, USA, 3, Paper AIAA-99-1477, 2290-2299 (April 1999).
H15. Laing, C.R., McRobie, F.A., Thompson, J.M.T. The post-processed Galerkin method applied to non-linear shell vibrations. Dynamics and Stability of Systems, 14, (2), 163-181 (1999).
H16. Lees, J.M., Burgoyne, C.J. Experimental study of influence of bond on flexural behaviour of concrete beams pretensioned with aramid fiber reinforced plastics. ACI Structural Journal, 96, (3), 377-385 (May-June 1999).
H17. Lees, J.M., Burgoyne, C.J. Transfer bond stresses generated between FRP tendons and concrete. Magazine of Concrete Research, 51, (4), 229-239 (August 1999).
H18. McRobie, F.A., Lasenby, J. Simo-Vu Quoc rods using Clifford algebra. International Journal for Numerical Methods in Engineering, 45, (4), 377-398 (1999).
H19. McRobie, F.A., Popov, A.A., Thompson, J.M.T. Auto-parametric resonance in cylindrical shells using geometric averaging. Journal of Sound and Vibration, 227, (1), 65-84 (1999).
H20. Pellegrino, S. An introduction to deployable structures. Proceedings, Textile Roofs'99: 4th International Workshop on the Design and Practical Realisation of Architectural Membrane Structures, Berlin, Germany (June 1999).
H21. Pellegrino, S., Miura, K. Deployable structures: from spacecraft to buildings. Proceedings, International Conference, Engineering a New Architecture, Aarhus, Denmark; Edited by P. Domebernowsky, T. Wester, 61-70 (May 1998).
H22. Popov, A.A., Thompson, J.M.T., McRobie, F.A., Chaotic energy exchange through auto-parametric resonance in cylindrical shells. Cambridge University Engineering Department Technical Report CUED/D-STRUCT/TR.182 (1999).
H23. Seffen, K.A., Pellegrino, S. Deployment dynamics of tape springs. Proceedings of the Royal Society of London, Series A: Mathematical, Physical and Engineering Sciences, A455, (1983), 1003-1048 (1999).
H24. Stratford, T.J., Burgoyne, C.J. Lateral stability of long precast concrete beams. Proceedings of the Institution of Civil Engineers, Structures and Buildings, 134, (2), 169-180 (May 1999).
H25. Stratford, T.J., Burgoyne, C.J., Taylor, H.P.J. Stability design of long precast concrete beams. Proceedings of the Institution of Civil Engineers, Structures and Buildings, 134, (2), 159-168 (May 1999).
Last modified: July 2000