38 research outputs found
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Problems associated with direct displacement-based design of concrete bridges with single-column piers, and some suggested improvements
Currently available displacement-based design (DBD) procedures for bridges are critically evaluated with a view to identifying extensions and/or modifications of the procedure, for it to be applicable to final design of a fairly broad class of bridges. An improved direct DBD procedure is presented, including a suite of comprehensive design criteria and proper consideration of the degree of fixity of the pier top. The design of an overpass bridge (originally designed to a current European Code), applying the improved ‘direct’ displacement-based design (DDBD) procedure is presented and both ‘conventional’ and displacement-based designs are assessed using non-linear response-history analysis (NLRHA); comparisons are made in terms of both economy and seismic performance of the different designs. It is seen that DDBD provided a more rational base shear distribution among piers and abutments when compared to the force-based design procedure and adequately captured the displacement pattern, closely matching the results of the more rigorous NLRHA
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Performance-Based Seismic Design and Assessment of Bridges
Current trends in the seismic design and assessment of bridges are discussed, with emphasis on two procedures that merit some particular attention, displacement-based procedures and deformation-based procedures. The available performance-based methods for bridges are critically reviewed and a number of critical issues are identified, which arise in all procedures. Then two recently proposed methods are presented in some detail, one based on the direct displacement-based design approach, using equivalent elastic analysis and properly reduced displacement spectra, and one based on the deformation-based approach, which involves a type of partially inelastic response-history analysis for a set of ground motions and wherein pier ductility is included as a design parameter, along with displacement criteria. The current trends in seismic assessment of bridges are then summarised and the more rigorous assessment procedure, i.e. nonlinear dynamic response-history analysis, is used to assess the performance of bridges designed to the previously described procedures. Finally some comments are offered on the feasibility of including such methods in the new generation of bridge codes
Homogeneous M2 duals
Motivated by the search for new gravity duals to M2 branes with
supersymmetry --- equivalently, M-theory backgrounds with Killing superalgebra
for --- we classify homogeneous M-theory
backgrounds with symmetry Lie algebra for . We find that there are no new backgrounds
with but we do find a number of new (to us) backgrounds with . All
backgrounds are metrically products of the form , with riemannian and homogeneous under the action of
, or with lorentzian and homogeneous
under the action of . At least one of the new
backgrounds is supersymmetric (albeit with only ) and we show that it can
be constructed from a supersymmetric Freund--Rubin background via a Wick
rotation. Two of the new backgrounds have only been approximated numerically.
(The second version of this paper includes an appendix by Alexander~S.~Haupt,
closing a gap in our original analysis.)Comment: 56 page