Bracing Of Steel Bridges During Construction; Theory, Full .
Bracing of steel bridges during construction; theory, full-scale tests and simulationsMehri, Hassan2015Link to publicationCitation for published version (APA):Mehri, H. (2015). Bracing of steel bridges during construction; theory, full-scale tests and simulations.Total number of authors:1General rightsUnless other specific re-use rights are stated the following general rights apply:Copyright and moral rights for the publications made accessible in the public portal are retained by the authorsand/or other copyright owners and it is a condition of accessing publications that users recognise and abide by thelegal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private studyor research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portalRead more about Creative commons licenses: https://creativecommons.org/licenses/Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will removeaccess to the work immediately and investigate your claim.LUNDUNIVERSITYPO Box11722100Lund 46462220000Download date: 28. Apr. 2021
Bracing of steel bridges duringconstructionTheory, full-scale tests, and simulationsHassan MehriDOCTORAL DISSERTATIONby due permission of the Faculty of Engineering, Lund University, Sweden.To be defended at lecture hall MA3 at Mathematics Annex building,Sölvegatan 20, Lund, on 22 January 2016 at 10:15 AM.Faculty opponentProfessor Emeritus Torsten HöglundKTH Royal Institute of Technology
Organization: LUND UNIVERSITYDocument name: Ph. D. dissertationDate of issue: December 2015Sponsoring organization: J. Gust Richert StiftelseThe Lars Erik Lundbergs StipendiestiftelseByggrådet- Britek AB- Structural Metal Decks Ltd.Title and subtitle: Bracing of steel bridges during construction - Theory, full-scale tests, and simulationsAbstract A number of steel bridges have suffered lateral-torsional failure during their construction due to theirlacking adequate lateral and/or rotational stiffness. In most cases, slight bracing can be of great benefit to the maingirders involved through their controlling out-of-plane deformations and enabling the resistance that is needed tobe achieved. The present research concerned the performance of different bracing systems, both those ofcommonly used types and pragmatic alternatives. The methods that were employed include the derivation ofanalytical solutions, full-scale laboratory testing, and numerical modeling. The results of a part of the studyshowed that the load-carrying capacity of The Marcy Bridge that collapsed in 2002 could be improved by addingtop flange plan bracing at 10-20% of its span near the supports. Theoretically, according to Eurocode 3, providingeach bar of an X-type plan bracing having cross-sectional area as small as 8 mm serves to enhance the loadcarrying capacity of the bridge by a factor of 1.28, which is sufficient to prevent failure of the bridge during thecasting of the deck. The research also included the derivation of a simplified analytical approach for determiningthe critical moment of the laterally braced steel girders at the level of their compression flange, which otherwisecan usually not be predicted without the use of finite element program. The model employed related the bucklinglength of the compression flange of steel girders in question to their critical moment. An exact solution and asimplified expression were also derived for dealing with the effect of the rotational restraint of the shortersegments on the buckling length of the longer segments in beams having unequally spaced lateral bracings. Theeffects of this sort are often neglected in practice and the buckling length of compression members in such systemsis commonly assumed to be equal to the largest distance between the bracing points. However, the present studyshowed that this assumption can provide an unsafe prediction of buckling length for relatively soft bracings andcan also lead to a significant overdesign in regard to most bracing stiffness values in practice. Full-scaleexperimental study on a twin-I girder bridge together with numerical works on different bridge dimensions werecarried out on the stabilizing performance of a type of scaffolding that is frequently used in the construction ofcomposite bridges. Minor improvements were discussed which found to be needed in the structure of thescaffoldings that were employed. Findings showed the proposed scaffoldings to have a significant stabilizingpotential when they were installed on bridges of differing lateral-torsional slenderness ratios. Axial strains in thescaffolding bars were also measured. Indications of the design brace moment involved were also presented whichwas approximately between 2 and 4% of the maximum in-plane bending moment in the main girders. Three fullscale experimental studies were also performed on a twin I-girder bridge in which the location of the cross-beamacross the depth of the main girders was varied. The effects of several different relevant imperfection shapes onthe bracing performance of the cross-beams were of interest. It was found that the design recommendationscurrently employed can provide uncertain and incorrect predictions of the brace forces present in the crossbracings. Both the tests and FE investigations carried out showed the shape of the geometric imperfectionsinvolved to have a major effect on the distortion that occurred in the braced bridge cross-sections. It was alsofound that significant warping stresses could develop in cross-beams having asymmetric cross-sections, theavoiding of such profiles in the cross-beams being recommended. Finally, seven full-scale laboratory tests of theend-warping restraints of truss-bracings and of corrugated metal sheets when they were installed on a twin I-girderbridge were also performed. The load-carrying capacity of the bridge was found to be enhanced by a factor of 2.53.0 when such warping restraints were provided near the support points. Relatively small forces were developed inthe truss-bracing bars in order to such significant improvements in the load-carrying capacity of the bridge to beachieved. Moreover, bracing the bridge in question by means of the metal sheets that were employed was found toresult in a significantly larger degree of lateral deflection at midspan than use of the utilized truss bracings did.Author: Hassan MehriKey words: brace, stability, steel, bridge girder, constructionClassification system and/or index terms (if any):Supplementary bibliographical information: ISRN LUTVDG/TVBK-1049/16-SEISSN and key title: 0349-4969, Report TVBK-1049Recipient's notesNumber of pagesSecurity classificationLanguage: EnglishISBN 978-91-87993-04-6PriceI, the undersigned, being the copyright owner of the abstract of the above-mentioned dissertation, hereby grant toall reference sources permission to publish and disseminate the abstract of the above-mentioned dissertation.SignatureDate 4th December 2015
Bracing of steel bridges duringconstructionTheory, full-scale tests, and simulationsHassan Mehri
Copy right Hassan MehriFaculty of Engineering, Division of Structural EngineeringP. O. Box 118, SE-221 00 Lund, SwedenReport TVBK-1049ISRN LUTVDG/TVBK-1049/15-SE(250)ISBN 978-91-87993-04-6ISSN 0349-4969Printed in Sweden by Media-Tryck, Lund UniversityLund December 2015En del av Förpacknings- ochTidningsinsamlingen (FTI)
PrefaceOne of the major concerns in the design of steel bridges is the global and localinstability of structural members, both during construction and in service.Catastrophic failures resulting in fatalities have occurred at times when stabilityprinciples have been violated during construction. The stability of steel-bridgesduring construction is highly dependent upon the adequacy in terms of bothstiffness and strength requirements of the bracings that are provided. Winter [1]presented a dual brace criterion, his showing experimentally that the load-carryingcapacity of an approximately 3.5 m long I-shape column (having a depth of 100mm, a width of 50 mm and a thickness of 1.6 mm) was enhanced by a factor offifteen by use of bracings as weak as cardboard strips. The efficiency of suchslight bracings was impressive. It is possible that some of the bridge tragedies thathave occurred could have been avoided by use of very inexpensive bracings. Idecided here to investigate how common bracings function in bridge applicationsduring what is the most critical stage in terms of possible instability, namely theconstruction phase.The thesis is being submitted for a degree of Doctor of Philosophy at the Divisionof Structural Engineering of Lund University. It is based on research carried outby the author between May 2011 and December 2015. The thesis itself, theappended papers excluded, is 121 pages in length. No part of the dissertation workhas been submitted for a degree at any other university. The research work wassupervised primarily by Prof. Roberto Crocetti, to whom I am extremely grateful. Iwould also like to thank Dr. Eva Frühwald Hansson for her assistance. Specialthanks go as well to Dr. Miklos Molnar, the Head of the Division, for his endlesssupport and his kindness. I appreciate too the assistance provided by Per-OlofRosenkvist (from LTH) and Göran Malmqvist (from SP) during the conducting ofthe tests. Jamie Turner (from SMD Ltd in the U.K.) and Thomas Lindin (fromBritek AB) provided the corrugated metal sheets and scaffoldings that wererequired during the tests that were employed, I am highly appreciative of theirsupport. I would also like to thank Fredrik Carlsson (from Reinertsen Sverige AB)and Ola Bengtsson (from Centerlöf & Holmberg AB) for the consultancy advice Ireceived from them in our meetings and the discussions we had. I take thisopportunity as well to thank my fellow researchers for the great times and thediscussions we have had.i
Most importantly, thank you Parvaneh for your patience and encouragement.Thanks for believing in me more than myself, and being there supporting meunconditionally.Hassan MehriDecember 2015ii
AbstractA number of steel bridges have suffered lateral-torsional failure during theirconstruction due to their lacking adequate lateral and/or rotational stiffness. Inmost cases, slight bracing can be of great benefit to the main girders involvedthrough their controlling out-of-plane deformations and enabling the resistancethat is needed to be achieved. The present research concerned the performance ofdifferent bracing systems, both those of commonly used types and pragmaticalternatives. The methods that were employed include the derivation of analyticalsolutions, full-scale laboratory testing, and numerical modeling.The results of a part of the study showed that the load-carrying capacity of TheMarcy Bridge that collapsed in 2002 could be improved by adding top flange planbracing at 10-20% of its span near the supports. Theoretically, according toEurocode 3, providing each bar of an X-type plan bracing having cross-sectionalarea as small as 8 mm serves to enhance the load-carrying capacity of the bridgeby a factor of 1.28, which is sufficient to prevent failure of the bridge during thecasting of the deck.The research also included the derivation of a simplified analytical approach fordetermining the critical moment of the laterally braced steel girders at the level oftheir compression flange, which otherwise can usually not be predicted without theuse of finite element program. The model employed related the buckling length ofthe compression flange of steel girders in question to their critical moment. Anexact solution and a simplified expression were also derived for dealing with theeffect of the rotational restraint of the shorter segments on the buckling length ofthe longer segments in beams having unequally spaced lateral bracings. Theeffects of this sort are often neglected in practice and the buckling length ofcompression members in such systems is commonly assumed to be equal to thelargest distance between the bracing points. However, the present study showedthat this assumption can provide an unsafe prediction of buckling length forrelatively soft bracings and can also lead to a significant overdesign in regard tomost bracing stiffness values in practice.Full-scale experimental study on a twin-I girder bridge together with numericalworks on different bridge dimensions were carried out on the stabilizingperformance of a type of scaffolding that is frequently used in the construction ofcomposite bridges. Minor improvements were discussed which found to be neededin the structure of the scaffoldings that were employed. Findings showed theiii
proposed scaffoldings to have a significant stabilizing potential when they wereinstalled on bridges of differing lateral-torsional slenderness ratios. Axial strains inthe scaffolding bars were also measured. Indications of the design brace momentinvolved were also presented which was approximately between 2 and 4% of themaximum in-plane bending moment in the main girders.Three full-scale experimental studies were also performed on a twin I-girderbridge in which the location of the cross-beam across the depth of the main girderswas varied. The effects of several different relevant imperfection shapes on thebracing performance of the cross-beams were of interest. It was found that thedesign recommendations currently employed can provide uncertain and incorrectpredictions of the brace forces present in the cross-bracings. Both the tests and FEinvestigations carried out showed the shape of the geometric imperfectionsinvolved to have a major effect on the distortion that occurred in the braced bridgecross-sections. It was also found that significant warping stresses could develop incross-beams having asymmetric cross-sections, the avoiding of such profiles in thecross-beams being recommended.Finally, seven full-scale laboratory tests of the end-warping restraints of trussbracings and of corrugated metal sheets when they were installed on a twin Igirder bridge were also performed. The load-carrying capacity of the bridge wasfound to be enhanced by a factor of 2.5-3.0 when such warping restraints wereprovided near the support points. Relatively small forces were developed in thetruss-bracing bars in order to such significant improvements in the load-carryingcapacity of the bridge to be achieved. Moreover, bracing the bridge in question bymeans of the metal sheets that were employed was found to result in asignificantly larger degree of lateral deflection at midspan than use of the utilizedtruss bracings did.iv
licationsAppended papersPaper I)Paper II)Paper III)Paper IV)Paper V)Contribution of the authorsOther scientific contributions of the authorConference paperSupervision of M.Sc. thesis1 Introduction1.1 Objectives1.2 Limitations1.3 State-of-the-art1.4 Terminology1.5 3462 Examples of bridge failures during construction associated with instability92.1 Examples of steel-truss bridge failures during construction associatedwith problems of instability102.2 Examples of failures of built-up steel girder bridges during the noncomposite stage associated with their instability112.2.1 The collapse of Bridge Y1504 in Sweden132.3 Steel bridge accidents during concreting that were associated withproblems of instability in their timber falseworks162.4 Conclusions17v
3 Theory of beam stability193.1 Introduction193.2 Effects of material inelasticity on bracing requirements203.3 Effects of residual stresses on buckling load213.4 Lateral-torsional buckling of doubly-symmetric simply supported beamssubjected to uniform bending223.5 Modifications required in the basic approach to the critical bendingmoment value233.5.1 Effects of different boundary conditions243.5.2 Effects of different loading conditions243.5.3 Effects of lateral restraints (Paper II)253.5.4 Effects of cross-sectional asymmetry263.5.5 Effects of inelasticity on lateral-torsional buckling273.5.6 Effects of variable cross-section on lateral-torsional buckling(unpublished work)284 Fundamentals of beam bracing4.1 Introduction4.2 Lateral bracing of beams4.3 Torsional bracing of beams4.4 “Column-on-elastic-foundation model”assessments in steel bridgesforcross-brace5 Lateral-torsional instability concerns during construction of steel bridges5.1 Bracings required during concreting of the deck5.2 Bracings required for skewed bridges5.3 Bracings required for in-plane curved bridges5.4 Bracing required prior to the concreting stage31313334stiffness3641414344456 Bracing options in steel bridges476.1 Intermediate cross-bracings476.2 Support bracings536.3 Bracing of half-through girders546.4 Full-span plan bracings556.5 Partial-span plan bracings (Papers I, and V)576.6 Bracings required in open trapezoidal girders586.6.1 The equivalent plate concept and forces generated in planbracings by torsion596.6.2 Forces generated from distortion in the cross-bracings606.6.3 Forces in the plan bracings due to the web inclinations oftrapezoidal girders61vi
6.6.4 Forces generated in intermediate external cross-bracings fromtorsion616.6.5 Support cross-diaphragms626.7 Bracing potential of stay-in-place corrugated metal sheets (Paper V) 636.7.1. Stiffness requirements of the metal sheets666.7.2. Strength requirements of the metal sheets666.7.3. Connection requirements676.7.4. Use of corrugated metal sheets in Twin-I girder bridges686.8 Scaffolding bracing of steel girders (Paper III)686.8.1 The common practice in design aimed at providing stability forthe steel girders during construction stage696.8.2 Shortcomings of the common bridge timber-falseworks716.8.3. The concept of scaffolding bracing of steel bridges726.8.4. Effects of the ledgers on the load-carrying capacity of the testbridge756.8.5 The effects of the ledgers on the bracing forces786.9 Bracing potential of precast concrete slabs817 The effects of initial imperfections on the performance of bracings (Papers III,IV, & V)837.1 Effects of the magnitude and the shape of the initial geometricimperfections on the load-carrying capacity of steel girders847.2 Effects of the shape and the magnitude of initial geometric imperfectionson brace forces868 Laboratory tests8.1 Background8.2 Tests performed by the author8989909 Numerical simulations9510 Conclusions and future research10.1 Conclusions from the appended papersPaper I:Paper II:Paper III:Paper IV:Paper V:10.2 Future endix IDetails regarding the test setup105105vii
Appendix IITest data not reported directly in the appended articles111111Appendix III113AIII.1 Bracing analysis; AASHTO recommendations [82]113Summary of the recommendations regarding the use of crossbracings113Summary of the recommendations regarding the use of lateralbracings114AIII.2 Bracing analysis; Eurocode recommendations [76]115Effects of imperfections in analyzing a bracing system115Lateral-torsional buckling of structural components116Appendix IVEquivalent plate thickness of typical plan-bracings [2]117117References119viii
NotationsThe following symbols are used in the present report:,,,Area of a cross‐section;The enclosed area defined by the wall‐midline in a thin‐walledclosed section;Cross‐sectional area of a single top flange, a bottom flange, andthe web of a steel girder;/4 2 ;,,,,,,,Cross‐sectional area of a diagonal and transversal strut bar;Distance between the struts in a truss bracing system;Thickness of a web stiffener;Moment gradient factors of a beam in its entirety, and of theunbraced and the braced spans, respectively;Top flange loading modification factor;1for I
Title and subtitle: Bracing of steel bridges during construction - Theory, full-scale tests, and simulations . One of the major concerns in the design of steel bridges is the global and local instability of str
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UDC Wall Bracing Provisions Permanent Rule effective September 1, 2014 A ‘How To’ guide for use of the new provisions Summary: Forget what you knew about the previous wall bracing provisions – this method is a different concept. The provisions are generally based on the 2012 IRC Simplified Wall Bracing Provisions.
APS 240 Interlude Ð Writing Scientific Reports Page 5 subspecies of an organism (e.g. Calopteryx splendens xanthostoma ) then the sub-species name (xanthostoma ) is formatted the same way as the species name. In the passage above you will notice that the name of the damselfly is followed by a name: ÔLinnaeusÕ. This is the authority, the name of the taxonomist responsible for naming the .
