intTypePromotion=1
zunia.vn Tuyển sinh 2024 dành cho Gen-Z zunia.vn zunia.vn
ADSENSE
 » 
 » 

Thép Hình

Chia sẻ: 3 3 | Ngày: | Loại File: PDF | Số trang:169

Thêm vào BST
Báo xấu
108
lượt xem 53
download
 
  Download Vui lòng tải xuống để xem tài liệu đầy đủ

Tham khảo sách 'thép hình', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả

Chủ đề:

Bình luận(0) Đăng nhập để gửi bình luận!

Lưu

Nội dung Text: Thép Hình

  1. A single copy of this Steel Construction Institute Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI publication is licensed to fm03 on 15/08/2007 This is an uncontrolled copy Faber Maunsell This is an uncontrolled copy. Ensure use of the most current version of this document by searching the Construction Information Service at www.tionestop.com
  2. P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005 SCI PUBLICATION P295 Commentary on BS 5400-3: 2000 Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI Code of practice for the design of steel bridges To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ Editors: C W BROWN MA FICE D C ILES MSc ACGI DIC CEng MICE Published by: The Steel Construction Institute Silwood Park Ascot Berkshire SL5 7QN Tel: 01344 623345 Fax: 01344 622944
  3. P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005 Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ © The Steel Construction Institute, 1991, 2000 Apart from any fair dealing for the purposes of research or private study or criticism or review, as permitted under the Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the UK Copyright Licensing Agency, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organisation outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to the publishers, The Steel Construction Institute, at the address given on the title page. Although care has been taken to ensure, to the best of our knowledge, that all data and information contained herein are accurate to the extent that they relate to either matters of fact or accepted practice or matters of opinion at the time of publication, The Steel Construction Institute, the authors and the reviewers assume no responsibility for any errors in or misinterpretations of such data and/or information or any loss or damage arising from or related to their use. Publications supplied to the Members of the Institute at a discount are not for resale by them. Publication Number: SCI P295 ISBN 1 85942 112 1 British Library Cataloguing-in-Publication Data. A catalogue record for this book is available from the British Library. ii
  4. P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005 Order before 31st August 2005 to receive 15% discount off any SCI publication in the 2004/05 Edition of the SCI Publication Catalogue FREE postage & packing within the UK Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ Commentaries to Standards Connections Fire Design Architectural Design Portal Frame Design
  5. P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges SUMMER SPECIAL This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005 15% Off any SCI publication 0rder up to 10 PUBLICATIONS with 15% off the total price. (Minimum Order 2 Publications) Offer open to Members & Non Members. Free p & p within the UK. Discount applies to this form only. This offer is not available in the e-shop. Offer ends 31st August 2005 (All orders received after this date will not be processed with discount) Non Pub Member Total Title Qty Member Ref. Price £ Price Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ Total no. of copies Sub-Total Less 15% P & P Rates* st U.K. Free Europe £12 for the 1 Item.Additional items +£2 per item st *Postage & Packing Rest of World £15 for the 1 Item Additional Items +£5 per item Membership No. Grand Total £ Customer Order ref: Name__________________________________________________________________________________ Company_______________________________________________________________________________ Delivery address _______________________________________________________________________________________ _______________________________________________________________________________________ Postcode __________________Telephone _____________________ Fax __________________________ E-mail________________________________ Please tick as appropriate Please Note: We do not accept American Express I enclose a Cheque/Postal Order made payable to The SCI I wish to pay by Credit/Debit Card: VISA SWITCH (Issue no.___) MASTERCARD Card number ________________________________________________Expiry date___________________ Name on card ___________________________________________________________________________ Registration address for card if different from delivery address: _______________________________________________________________________________________ ________________________________________________________Postcode________________________ FAX BACK (01344 622944) The Steel Construction Institute Publication Sales: Silwood Park, Ascot Telephone: (01344) 872775 Berkshire SL5 7QN Fax: (01344) 622944
  6. P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005 FOREWORD Foreword to Commentary on BS 5400-3: 1982 This publication is one of a range of publications produced by The Steel Construction Institute which relate specifically to the design of steel and composite bridge structures and the use and interpretation of the various clauses within BS 5400: Part 3: 1982. It has been edited by Dr D M Martin (The Steel Construction Institute) and Dr J Tubman (Scott Wilson Kirkpatrick). The following Engineers contributed to the commentary on the various clauses: Dr M J Baker Imperial College of Science and Technology Mr A Bannister British Steel Swinden Laboratories Mr C W Brown The Steel Construction Institute Mr B D Cheal Consultant Dr P Davidson Imperial College of Science and Technology Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI Mr K Goodearl Consultant Mr D C Iles The Steel Construction Institute Dr W Manners University of Leicester To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ Dr D M Martin The Steel Construction Institute Dr R Narayanan The Steel Construction Institute Dr G W Owens The Steel Construction Institute Dr J Spindel Consultant Dr J Tubman Scott Wilson Kirkpatrick Our particular thanks are due to Mr S Chakrabarti of the Department of Transport for his valuable advice and comments on the text. During the preparation of the Commentary many points of interpretation were resolved by discussion. The document, therefore, represents a consensus of opinion on the various clauses within the Code and it should act as both an authoritative guide and a reference document for practising Bridge Engineers. The work leading to this publication was funded by British Steel General Steels and the project was managed by Dr D M Martin (The Steel Construction Institute). Foreword to Commentary on BS 5400-3: 2000 The publication of a revised version of BS 5400-3 in 2000 has necessitated an update of the Commentary. The editors, Mr D C Iles and Mr C W Brown of The Steel Construction Institute, have endeavoured to retain as much as possible of the original text, but have brought it into line with the new clauses. Major revisions have been necessary to Sections 6 and 9, and the opportunity has been taken to add a commentary on Clauses 9.10 and 9.15, which were not covered in the commentary on BS 5400-3: 1982. A summary of the clauses that have been amended in the 2000 version of BS 5400-3 is given in an Appendix. The type of amendment (e.g editorial, major technical change, etc.) is indicated in that Appendix. Further guidance on the use of BS 5400-3: 2000 can be found in two other SCI publications, Design guide for composite highway bridges (P289) and Design guide for composite highway bridges: Worked Examples (P290) iii
  7. Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI iv P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005
  8. P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005 CONTENTS Page No. FOREWORD iii 1 INTRODUCTION 1 2 HISTORICAL BACKGROUND 2 2.1 General 2 2.2 Scope of BS 5400 relating to steel bridge construction 3 2.3 Safety factors 5 2.4 Load factors 5 2.5 Combinations of loads 6 2.6 The limit state design process 6 Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI 2.7 References 6 3 PREAMBLE TO COMMENTARY 7 To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ 4 DESIGN OBJECTIVES 8 4.1 General 8 4.2 Limit states 8 4.3 Partial safety factors to be used 10 4.4 Structural support 12 4.5 Corrosion resistance and protection 12 4.6 Clearance gauges 13 4.7 References 13 5 LIMITATIONS ON CONSTRUCTION AND WORKMANSHIP 14 5.1 Workmanship 14 5.2 Robustness 14 5.3 Handling and transport 14 5.4 Composite steel/concrete construction 14 5.5 Built-up members 14 5.6 Diaphragms and fixings required during construction 14 5.7 Camber 15 5.8 End connections of beams 15 5.9 Support cross beams 15 6 PROPERTIES OF MATERIALS 16 6.1 General 18 6.2 Nominal yield stress 18 6.3 Ultimate tensile stress 19 6.4 Ductility 19 6.5 Notch toughness 19 v
  9. P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005 6.6 Properties of steel 23 6.7 Modular ratio 23 6.8 References 23 7 GLOBAL ANALYSIS FOR LOAD EFFECTS 25 7.1 General 25 7.2 Sectional properties 25 7.3 Allowance for shear lag 25 8 STRESS ANALYSIS 26 8.1 Longitudinal stresses in beams 26 8.2 Allowance for shear lag 26 8.3 Distortion and warping stresses in box girders 29 8.4 Shear stresses 29 Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI 8.5 Imperfections 29 8.6 Residual stresses 29 8.7 References 31 To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ 9 DESIGN OF BEAMS 32 9.1 General 32 9.2 Limit states 32 9.3 Shape limitations 33 9.4 Effective section 39 9.5 Evaluation of stresses 44 9.6 Effective length for lateral torsional buckling 51 9.7 Slenderness 60 9.8 Limiting moment of resistance 69 9.9 Beams without longitudinal stiffeners 71 9.10 Flanges in longitudinally stiffened beams 79 9.11 Webs in longitudinally stiffened beams 84 9.12 Restraints to compression flanges 89 9.13 Transverse web stiffeners other than at supports 95 9.14 Load bearing support stiffeners 98 9.15 Cross beams and other transverse members in stiffened flanges 101 9.16 Intermediate internal cross frames in box girders 105 9.17 Diaphragms in box girders at supports 105 9.18 References 105 10 DESIGN OF COMPRESSION MEMBERS 108 10.1 General 108 10.2 Limit states 108 10.3 Limitations on shape 108 10.4 Effective lengths 109 vi
  10. P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005 10.5 Effective section 110 10.6 Compression members without longitudinal stiffeners 110 10.7 Compression members with longitudinal stiffeners 113 10.8 Battened compression members 115 10.9 Laced compression members 118 10.10 Compression members connected by perforated plates 119 10.11 Compression members with components back to back 120 11 DESIGN OF TENSION MEMBERS 122 11.1 General 122 11.2 Limit states 122 11.3 Effective section 122 11.4 Thickness at pin-holes 124 11.5 Strength 124 Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI 11.6 Battened tension members 125 11.7 Laced tension members 126 To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ 11.8 Tension members connected by perforated plates 126 11.9 Tension members with components back to back 127 12 DESIGN OF TRUSSES 130 12.1 General 130 12.2 Limit states 130 12.3 Analysis 130 12.4 Effective length of compression members 132 12.5 Unbraced compression chords 132 12.6 Lateral bracing 133 12.7 Curved members 133 12.8 Gusset plates 133 13 DESIGN OF BASE, CAP AND END PLATES 134 14 DESIGN OF CONNECTIONS 135 14.1 General 135 14.2 Limit states 135 14.3 Basis of design 136 14.4 Splices 142 14.5 Connections made with bolts, rivets or pins 145 14.6 Welded connections 150 14.7 Hybrid connections 153 14.8 Lug angles 153 14.9 Other attachments 153 14.10 References 153 APPENDIX A Summary of Changes to BS 5400-3 155 vii
  11. Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI viii P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005
  12. P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005 1 INTRODUCTION The use of steel and composite construction for bridges is now recognised as economical over a wide range of spans and is versatile in the many forms in which it may be applied. This Commentary is one of several documents published by The Steel Construction Institute to provide guidance for the designers of steel and composite bridges. The guides are intended for practising engineers, to enable them to apply the design codes with ease. The Commentary discusses in detail the provisions in the clauses of BS 5400-3:2000, provides background information where appropriate and comments on their practical application. The comments and discussion represent the considered opinion of experienced engineers and researchers. In some cases Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI information about the basis of the clauses has been difficult to obtain; for such cases, the commentary presents the consensus view of the background to these clauses. To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ The scope of this publication is limited to those aspects of BS 5400-3: 2000 most commonly used by bridge designers. Comments are given on a clause-by-clause basis. The design of cross frames and diaphragms in box girder design (Clauses 9.16 and 9.17) is not discussed. The Commentary is intended as a reference document to be used in conjunction with the Code. References and comparisons are made to other codes where appropriate (viz. BS 5950, BS EN 10025 and BS 153). References are given at the end of each principal Section and are referred to in the text in the form[2.3], where ‘2’ indicates Section 2 and ‘3’ is the third reference in that Section. 1
  13. P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005 2 HISTORICAL BACKGROUND 2.1 General The British Standard, BS 5400, was conceived at an informal meeting held at the British Standards Institution in July 1967, under the chairmanship of the late Dr Oleg Kerensky. At this meeting it was proposed that a comprehensive standard replace BS 153, on Steel Bridges, and CP 117: Part 2, on Composite Construction in Structural Steel and Concrete. The new Code was conceived as being applicable to steel, concrete and composite construction. BS 153 was restricted in its scope to simply supported steel bridges of up to 300 ft in span and was primarily intended for plate girders and truss construction. By 1967, continuous steel bridges, box girder bridges and cable stayed systems had been designed and numerous long span bridges were in prospect. The Code was Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI therefore being applied, and sometimes misapplied, beyond its intended bounds. The group of engineers referred to above recommended that limit state philosophy To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ should be adopted, so that there would be uniformity of reliability of service. This was the first time a British code was to be drafted using limit state concepts. The work of the drafting panels commenced in 1968 and an embryonic draft of the Steel Design Code existed in 1970. The collapse during erection of four steel box girder bridges (two of them British designed) triggered much research effort, which resulted in the production of the Interim Design and Workmanship Rules by the Merrison Committee[2.1]. These rules were rather severe - though at the time they were regarded as essential, largely because of the anxiety about the safety of the bridges in service. Following the publication of the Interim Design and Workmanship Rules, the Steel Bridge Sub-Committee was reconvened in 1973. The intention was to base the new Design Code on the Merrison Rules as regards the treatment of box girder structures. When the draft was presented in 1975, the Committee found itself dissatisfied with its own brief for the Code. Extensive modifications were made under the direction of Steering Panels. Continuity of style was achieved by getting the drafts vetted by one person before the final draft was reviewed by a panel of designers. The draft for public comment was eventually published in 1979 followed by a one-day Seminar (organised by IStructE) in January 1980 and a three-day Conference (organised by the University College, Cardiff) in March 1980. The Code of Practice for Steel Bridges (i.e. BS 5400-3) finally appeared (after some modifications) in 1982. It should be noted that this was a new Code based on the limit state approach; it was not a revision of an old one. Two official amendments have been issued since publication, AMD 4051 in August 1982 and AMD 6488 in April 1991. The former was comparatively minor; the latter was a major revision, incorporating much (but not all of) of the material published in the Department of Transport’s (subsequently Highways Agency’s) Standard BD 13/90. By 1992 it was becoming apparent that a complete overhaul of the Code was necessary. It had been hoped to avoid this pending the issue of a Eurocode for steel bridges, but when it became obvious that such a 2
  14. P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005 Eurocode was a long way from completion, the BSI Bridge Code Committee B525/10 set up a Working Group (WG3) to update BS 5400-3. A draft of the proposed revisions was issued for public comment in 1995; after taking these comments into account a completely new version of BS 5400-3 was issued in 2000. The complete Standard is as shown in Table 2.1, produced in 10 parts which cover various facets of bridge design. Table 2.1 Various Parts of BS 5400 Part Title Current Latest Issue Amend’t 1. General Statement 1988 2. Specification for Loads 1978 1983 Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI 3. Code of Practice for the Design of Steel Bridges 2000 4. Code of Practice for the Design of Concrete Bridges 1990 To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ 5. Code of Practice for the Design of Composite Bridges 1979 1982 6. Specification for Materials and Workmanship, Steel 1999 7. Specification for Materials and Workmanship, 1978 Concrete, Reinforcement and Tendons 8. Recommendations for Materials and Workmanship 1978 9. Bridge Bearings 9.1 Code of Practice for the Design of Bridge 1983 Bearings 9.2 Specification for Materials, Manufacture and 1983 Installation of Bridge Bearings 10. Code of Practice for Fatigue 1980 1999 In addition to Parts 1 and 2, steel bridge designers are specially concerned with Parts 3, 6, 9 and 10. Notice that Parts 2, 6 and 9.2 are Specifications, whereas Parts 3, 9.1 and 10 are Codes of Practice. 2.2 Scope of BS 5400 relating to steel bridge construction Part 1 of the Standard is a General Statement of the concepts and limit state principles common to the other parts. It should be referred to for definitions, the general procedures for limit state partial safety factor design and other common requirements, such as robustness and design life. Part 2 covers all aspects of design loading, other than fatigue, which is covered by Part 10, for highway, railway and foot bridges including partial safety factors on loads and load combinations. Its scope is suited to the design of modern forms of bridge having been particularly extended from BS 153 to provide more guidance 3
  15. P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005 on the assessment of wind loading and temperature effects. Various combinations of loads together with the load factors to be used in the ultimate limit state, as well as in the serviceability limit state, are prescribed. For bridges in the UK under the control or approval of the Highways Agency or for which the Highways Agency has a responsibility, the values of some loads (and in particular highway loads) have been substantially increased in the Departmental Standard BD37/88, which effectively replaces Part 2. Part 3 contains recommendations for the design against collapse and unserviceability of steel bridges. The design objectives (including partial factors on strength) and limitations in design on construction and workmanship are set out. Guidance is given on steel properties to be assumed in design or specified, including requirements for notch toughness. The major part of the Code treats the design of tension and compression members, of beams (including plate and box girders), of trusses and of connections. Part 4 is the Code of Practice for the design of concrete bridges. The Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI specifications for the materials used in such bridges are contained in Part 7 and Part 8. To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ Part 5 lays down extensive requirements for the design of composite bridges and covers both serviceability and ultimate limit states. Guidance is given about crack widths allowed in concrete, the use of shear connectors, the influence of shear lag etc. The requirements of composite box girders, cased and filler beams, composite columns and permanent formwork are all features covered in a codified form for the first time. The Highways Agency have issued a modified version of this Part for use on bridges in the UK under their control or approval or for which they have a responsibility. Part 6 is a mandatory Specification for steel materials and workmanship. The materials clauses cover requirements for structural steels, steels for rivets, pins, shear connectors and bolts, welding consumables, steel castings and forgings and cast iron. Fabrication tolerances, preparation of materials, requirements for bolting, rivetting and welding, and other manufacturing limitations are defined. Inspection and testing procedures and general instruction on handling, transport and erection are provided. Part 9 is in 2 parts: The first part is a Code for design of bearings and the second is a Specification for the materials, manufacture and installation of all types of bridge bearings. Part 10 is a Code of Practice, concerned with fatigue life appraisal for steel, concrete and composite bridges. Following general guidance on the approach to design against fatigue, structural details are classified for defining their fatigue characteristics, with explanatory notes. Design loading spectra are specified for highway and railway bridges in the United Kingdom. Acceptable methods of stress calculation are set down. The Code then sets out methods of life assessment based on the Palmgren-Miner damage summation. Amendment No. 1 was issued in 1999. It should be noted that, for all bridges under the control or approval of one of the four overseeing Organisations (the Highways Agency in England, the Scottish Executive Development Department, the Welsh Assembly and the Department for Regional Development in Northern Ireland), the Departmental Standards in the 4
  16. P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005 Design Manual for Roads and Bridges [2.2] effectively supersede certain clauses of the design Code(s). 2.3 Safety factors BS 5400 is based on a partial safety factor/limit state philosophy. The specified value of each type of load (given in Part 2) is multiplied by a partial factor to obtain a design value which has an appropriately small probability of being exceeded in the design life of the bridge. When two or more loads act together, the partial factors to be used allow for the fact that the peak values of the time- varying loads are unlikely to occur at the same instant in time. The factored loads, in different combinations, are applied to the structure and the load effects determined. The latter are then compared with the design strength of the elements. Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI This is expressed mathematically in BS 5400-3 as:  1   function of σ y  To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ (the effects of (fLQk) #     γ  f3 γ m1 γ m 2  and other geometric variables  where (fL is a partial factor for load; (f3 is a factor that takes account of inaccuracies in load assessment, stress distributions and construction; (m1(m2 is replaced by a single factor (m which takes account of uncertainty in material strength and quality, and manufacturing tolerances; and Qk is the specified nominal load. It should be noted that this expression differs from that given in Part 1 and Part 4, where the partial factor (f3 is included with the load effects rather than the assessment of strength. This difference arose due to separate drafting of Parts 3 and 4. 2.4 Load factors It is clear that live loads will in general vary more than dead loads and hence a higher load factor is assigned to live loading. The exception to this is superimposed dead load due to deck surfacing, which has a very high load factor. This is to allow for the possibility of resurfacing work being carried out on top of the original surfacing. 5
  17. P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005 2.5 Combinations of loads The Code recognises that when loads are in combination, particularly combinations involving wind or HB loads, they are unlikely to exceed their specified values simultaneously by large amounts, and hence load factors for such combinations are reduced. Table 2.2 Typical combinations of loads [Source: BD 37/88] Load (fL at ULS Combination 1 Combination 2 Dead (steel) 1.05 1.05 Dead (concrete) 1.15 1.15 Superimposed dead (deck surfacing) 1.75 1.75 Superimposed dead (other) 1.20 1.75 Wind Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI with dead + superimposed dead 1.40 with dead + superimposed dead + Live 1.10 To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ Vertical live load HA alone 1.50 1.25 HA with HB or HB alone 1.30 1.10 2.6 The limit state design process The purpose of limit state design is to identify all the possible modes of failure for each structure and all the ways in which it could become unserviceable and then to proportion the structure so that these events are sufficiently unlikely to occur. In practice, the engineer does not have to carry out risk or probability calculations directly since these were done by the Code drafting committee. Appropriate levels of safety and serviceability are achieved by using the partial factors given in Parts 2 and 3 of the Code. 2.7 References 2.1 Inquiry into the basis of design and method of erection of steel box girder bridges. Interim design and workmanship rules. HMSO, 1973. 2.2 Design Manual for Roads and Bridges. The Highways Agency, the Scottish Executive, the Welsh Assembly and the Department for Regional Development, Northern Ireland. 6
  18. P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005 3 PREAMBLE TO COMMENTARY The Commentary is intended to be read in conjunction with BS 5400-3: 2000. References to tables and figures in the Code are made in italics, e.g. “See Figure 10”. Figures and tables within this Commentary are numbered by reference to the commentary clause to which they refer; for example Figure 9.5.5 is the fifth figure in the commentary to Clause 9.5. Reference should be made to BS 5400-3: 2000 for details of definitions and nomenclature. The Commentary refers only to the Code of Practice published by the British Standards Institution. It does not deal with Departmental Standards in the Design Manual for Roads and Bridges unless they are specifically relevant to the background of a particular aspect. Armed with the background information to the original clauses, users should be able to assess for themselves the implications of any modest changes demanded by implementation documents. Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI Even where no comment is considered necessary, the clause heading is nevertheless given for reasons of uniformity of presentation. Clauses 9.16 and To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ 9.17 relate primarily to box-girder construction and are outside the scope of the present work. 7
  19. P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005 4 DESIGN OBJECTIVES 4.1 General Design objectives are stated in Part 1 of the Code. 4.2 Limit states There are generally two, closely related, reasons for introducing the concept of limit states into Codes of Practice: (a) to clarify the structure of the code, and hence to assist the designer by informing him of the type of failure each section of the code is addressing, Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI (b) to provide a basis for prescribing different levels of safety for different types of failure. The Code lists three limit states: the ultimate limit state, the serviceability limit To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ state, and fatigue. Leaving fatigue on one side for later discussion (Section 4.2.3 below), the other two limit states can be defined as follows. The ultimate limit state represents conditions of total collapse which endanger the safety of people and/or require major or total reconstruction. The serviceability limit state represents the limit of acceptable performance in service. Hence the key distinction between these limit states in practice is that serviceability limit state calculations are made with loads factored to represent the worst loads that the bridge is expected to experience during its design life, while ultimate limit state calculations are made with significantly greater factored loads representing extremely improbable occurrences. Since, in general, structures are susceptible to several different types of failure, and are subject to several different types of limitation on acceptable performance, it is common in more recent codes (e.g. BS 5950-1 and the design Eurocodes) to use the plural form and refer to ultimate limit states and serviceability limit states. Different ultimate limit states can be defined in terms of different failure mechanisms. The most basic is the overturning or sliding of the whole structure, or some part of it, when considered as a rigid body; this applies to structures which rely on only their self-weight for stability. The more common failure mechanisms are those of yielding, buckling and brittle fracture. Although it can sometimes be useful to think of these as separate limit states, the possibility of structural failure involving a combination of these failures mechanisms must also be considered. The serviceability limit states can include any requirement relevant to the satisfactory performance of the structure in service, and are usually concerned with limiting deflections and distortions, either permanent or temporary, or vibrations. The way in which the Code handles these various limit states will be discussed in more detail in the following pages, but it should be noted that, because of the long period of time over which BS 5400 was written, different parts of the Code refer to limit states in different ways. In particular, Part 1 uses the plural form ultimate limit states and serviceability limit states, and list individual limit states under each heading; however it goes on to list certain deflection limits, i.e. those concerned 8
  20. P295: Commentary on BS 5400-3: 2000, Code of practice for the design of steel bridges This material is copyright - all rights reserved. Reproduced for IHS Technical Indexes Ltd under licence from The Steel Construction Institute on 15/8/2005 with the appearance of the bridge, with clearances and with drainage of water from bridge decks, under “Further Requirements”, rather than as serviceability limit states. 4.2.1 Ultimate limit state The first consideration in creating and organising the rules for the ultimate limit state is the extent to which different types of failure can be handled separately. One important principle of the Code, as of most steel design, is that ductile failures are considered preferable to brittle failures. The limit state of brittle fracture is therefore disposed of by ensuring that the steel used has sufficient ductility and notch toughness so that brittle fracture does not govern the strength of bridges; this is covered in 6.4 and 6.5. Certain aspects of local buckling are dealt with similarly, by means of the shape limitations in 9.3 and 10.3. It would not, of course, be economic to proportion Licensed copy:fm03, Faber Maunsell, 15/08/2007, Uncontrolled Copy, © SCI steel bridges so that buckling never affected the strength; hence the major part of the Code consists of the calculations required to assess the strength of elements of bridges failing due to the combined effects of yield and buckling; and all elements of a bridge must be checked against these requirements. To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ The requirement given in the Part 1 description of the ultimate limit state is that the structure should be checked for equilibrium as a rigid body, but this is not explicitly repeated in Part 3. Where the forces acting on a structure are such that its own self-weight is insufficient to guarantee that it always remains in overall equilibrium, then devices such as holding-down bolts or bearings that resist tension are required, and the system of partial factors used in the Code ensures that the proper design of these elements will ensure the stability of the bridge as a whole. 4.2.2 Serviceability limit state The design checks specifically listed under the serviceability limit state heading are limited to six locations, as listed in Table 1 of the Code. One of these arises because slip in normal friction-grip bolted connections is defined as a serviceability limit state. The others are all concerned with preventing excessive permanent deformation of bridges under ‘normal’ loading. Although the exact origin of the cause for concern varies from case to case, it tends to arise from the presence of certain forms of structural behaviour, such as shear lag, restraint of warping or stable elastic buckling, which can cause high local stresses to be present in the elastic stress distribution, but which may not significantly affect the ultimate strength of the bridge. In such cases, the process of structural failure will be associated with large amounts of plastic yielding, which may start at relatively low loads, causing parts of the bridge to distort significantly well before failure occurs. These serviceability limit state requirements therefore consist of checks to ensure that yield is not exceeded under normal loading, including all the complications of the elastic stress distribution in the calculations. By comparison, the ultimate limit state may usually be checked after redistributing peak stresses to correspond more closely to the state of stress at failure. In clauses of the Code other than those in Table 1, serviceability limit state checks are not required. This is because the elastic stress distribution and the stress distribution at failure are sufficiently similar for the ultimate limit state calculation 9
ADSENSE

CÓ THỂ BẠN MUỐN DOWNLOAD

LV.15: Bộ Đồ Án Tốt Nghiệp Chuyên Ngành Cơ Khí
65 tài liệu
2431 lượt tải
THÔNG TIN
TRỢ GIÚP
HỖ TRỢ KHÁCH HÀNG
Theo dõi chúng tôi

Chịu trách nhiệm nội dung:

Nguyễn Công Hà - Giám đốc Công ty TNHH TÀI LIỆU TRỰC TUYẾN VI NA

LIÊN HỆ

Địa chỉ: P402, 54A Nơ Trang Long, Phường 14, Q.Bình Thạnh, TP.HCM

Hotline: 093 303 0098

Email: support@tailieu.vn

Giấy phép Mạng Xã Hội số: 670/GP-BTTTT cấp ngày 30/11/2015 Copyright © 2022-2032 TaiLieu.VN. All rights reserved.

 

Đồng bộ tài khoản
2=>2