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03.13 - General Design - Bracing Systems & Use of U-Frames in Steel Bridges | Beam (Structure) | Applied And Interdisciplinary Physics
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03.13 - General Design - Bracing Systems &amp; Use of ...
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PART 13 BA 53/94 BRACING SYSTEM AND THE USE OF UFRAMES IN STEEL HIGHWAY BRIDGES
SUMMARY This Advice Note is intended to provide guidance on the design of bracing systems and the use of U-frames in steel composite highway bridges and structures. INSTRUCTIONS FOR USE This is a new document to be incorporated into the Manual. 1. 2. 3. Insert BA 53/94 into Volume 1, Section 3. Remove contents page dated May 1994 and insert page dated December 1994. Archive this sheet as appropriate
Note: The new contents page (dated December 1994) replaces the page which was not to be made available with BD 57/94. BD 57/94, BA 57/94 and BA 42/94 will not now be available until February/March 1995.
.THE HIGHWAYS AGENCY BA 53/94 THE SCOTTISH OFFICE INDUSTRY DEPARTMENT THE WELSH OFFICE Y SWYDDFA GYMREIG THE DEPARTMENT OF THE ENVIRONMENT FOR NORTHERN IRELAND Bracing Systems and the Use of U-Frames in Steel Highway Bridges Summary: This Advice Note is intended to provide guidance on the design of bracing systems and the use of U-frames in steel composite highway bridges and structures.
NOT FOR USE OUTSIDE THE AGENCY December 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED .Volume 1 Section 3 Part 13 BA 53/94 REGISTRATION OF AMENDMENTS Amend No Page No Signature & Date of incorporation of amendments Amend No Page No Registration of Amendments Signature & Date of incorporation of amendments ELECTRONIC COPY .
Registration of Amendments REGISTRATION OF AMENDMENTS Amend No Page No Signature & Date of incorporation of amendments Amend No Page No Volume 1 Section 3 Part 13 BA 53/94 Signature & Date of incorporation of amendments ELECTRONIC COPY .NOT FOR USE OUTSIDE THE AGENCY PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED December 1994 .
6. 8. 3. 4. 10. 7. Introduction Definitions Braced I-Beam Bridges The Use of BS 5400 Part 3 Continuous Composite Deck Type Bridges Skew Bridges Bracing System During Construction Fatigue References and Bibliographies Enquiries Figures Annex A: December 1994 . 9.DESIGN MANUAL FOR ROADS AND BRIDGES VOLUME 1 SECTION 3 HIGHWAY STRUCTURES: APPROVAL PROCEDURES AND GENERAL DESIGN GENERAL DESIGN PART 13 BA 53/94 BRACING SYSTEM AND THE USE OF U-FRAMES IN STEEL HIGHWAY BRIDGES Contents Chapter 1. 5. 2.
Implementation 1. this would not result in significant additional expense or delay progress.2 This Advice Note is applicable to the design and assessment of steel only and steel-concrete composite bridges.1 This Advice Note gives guidelines on the design of bracing systems and U-frames for steel and composite highway bridges and footbridges. improvement and maintenance and all assessments of steel and composite bridges currently being prepared provided that.Volume 1 Section 3 Part 13 BA 53/94 Chapter 1 Introduction 1. in the opinion of the Overseeing Organisation. Scope 1. ELECTRONIC COPY .NOT FOR USE OUTSIDE THE AGENCY December 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 1/1 .4 This Advice Note should be used forthwith on all schemes for the construction. Its application to particular schemes and assessments should be confirmed with the Overseeing Organisation.3 Any reference in this Advice Note to a British Standard is to that standard as implemented by the appropriate DMRB Standard. It is based on the findings and recommendations of a desk study on the topic carried out for the Department of Transport. 1. INTRODUCTION General 1.
2.1 Bracing system.Volume 1 Section 3 Part 13 BA 53/94 Chapter 2 Definitions 2. This is the lateral restraint at discrete points with sufficient stiffness such that the effective length can be taken as the length between the points. An effective bracing system should provide sufficient stiffness to the member so that buckling or lateral instability does not occur at the ultimate limit state.4 Flexible restraints.5 Plan bracing. 2. 2.NOT FOR USE OUTSIDE THE AGENCY December 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 2/1 .3 Effective discrete lateral restraint. A bracing system is the structural means to restrain members from buckling and lateral instability. DEFINITIONS 2. The strength of compression members and I beams may be limited by buckling. ELECTRONIC COPY . 2. These do not have sufficient individual stiffness to provide effective discrete intermediate lateral restraints at brace locations. This is defined as a horizontal triangular framework or a decking system which is capable of transferring the restraint forces to the support.2 Effective bracing system.
A typical example is a series of parallel bracing members jointing two or more I-beams at the compression flange level forming a Vierendeel truss on plan (see Figure 4). it should be capable of transferring the restraint forces to the supports. (b) The compression flange is restrained by flexible restraints in the form of intermediate U-frames with a plan triangulated framework or a decking system at the level of the cross members of the U-frame. (b) and (c) may be used. For continuous beams. This includes simply supported deck type bridges with concrete slab on top of the beams (see Figures 1a and 1b). They are commonly used in the hogging moment regions of long span deck type bridges and bridges at construction stages (see Figure 3).4 Where a system of plan bracing as described in (a) and (b) above is provided. 3. consideration should be given in the design of the bracing to facilitate such operation either by removing the bracing or by designing the bracing to take the additional forces induced by the operation. while the provision of (b) or (c) through out the whole length of the beams is sufficient. This type of bracing provides flexible restraints and is outside the scope of BS 5400 Part 3 unless it is connected to another effective bracing system or braced structure.2 Various types of bracing systems can be provided to restrain steel I-beams and in general they fall into the following categories: (a) The compression flange is restrained directly by the deck or by a plan triangulated framework.Volume 1 Section 3 Part 13 BA 53/94 Chapter 3 Braced I-Beam Bridges 3. ELECTRONIC COPY . This displacement causes additional lateral bending and torsional stresses in the beam and forces in the bracing members. The beam bracing therefore needs to be designed to provide sufficient stiffness by restricting lateral and torsional deformation to prevent overstress in the beam and the bracing.NOT FOR USE OUTSIDE THE AGENCY December 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 3/1 .6 Where it is anticipated that the bridge beams will be jacked up during the life of the bridge. any one of the systems described in (a). 3. 3. (c) The beam is restrained by effective discrete lateral restraints or intermediate stiff bracing which provides effective lateral torsional restraint by restricting the twisting of the beam or the lateral movement of the compression flange at the bracing locations. BRACED I-BEAM BRIDGES 3. 3. The presence of initial imperfections and the flexibility of the braced structure give rise to displacement of the whole system when the beams are subjected to bending.3 For simply supported beams. (d) The beam is restrained by intermediate flexible bracing which does not have sufficient stiffness to provide effective lateral torsional restraint at the brace locations as described in (c) or which does not form intermediate U-frames as in (b). This includes intermediate vertical triangulated frames or diaphragms which are also known as stiff torsional bracing. eg. This includes half through type bridges and the hogging moment regions of deck type bridges where inverted Uframes are provided (see Figures 2a and 2b). The procedure for such jack up operation should be detailed in the Maintenance Manual. (a) is commonly supplemented by (b) or (c) at hogging moment regions.5 The provision of stiff torsional bracing as described in (c) above should be considered where the change of direction of compression flanges occurs such as in haunched beams and in curved bridges where straight beam elements are used.1 Steel I-beams in bridges are often intermediately braced within the span to provide a braced structure. for the replacement of bearings. 3.
The Fc force may be reduced if the interaction between the cross member.1.1 to 9. longitudinal braking force. 4.12.12. U-frames with flexible end posts 4.3 In calculating the effective length for beams restrained by U-frames using Clause 9.5.6.4.NOT FOR USE OUTSIDE THE AGENCY December 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 4/1 . (c) Calculate the various restraint forces acting at the intermediate lateral restraints as given in Clauses 9.2 Detailed background information and the derivations of some of the formulae may be found in reference 3. acting at the tip of the U-frame at the compression flange level. Their uses.3. BS 5400 Part 3 assumes that the supports have sufficient stiffness to prevent the lateral movement of the compression flange so that the ends of the beam remain almost vertical.2 are considered satisfying the assumptions. these forces should be added to the other load effects which may result from wind.1(c) may also be reduced.5 When vertical loading is applied to the cross member of a U-frame.5 may be calculated from first principle using beam on elastic foundation approach. centrifugal force and other relevant forces. 4.2.4.12. However.12.6 and design the beam using the appropriate Clauses in Section 9. The support restraints should also satisfy the stiffness requirements as given in Clause 9. in some cases.12. the value of Fc in Clause 12. difficulties encountered and alternative solutions are given in the following paragraphs.4. the vertical stiffeners and the compression flanges are taken into account. the number and the locations of lateral restraints. (b) Calculate the effective length of the beam using Clause 9. temperature.2 and ELECTRONIC COPY .12. the coefficient of 2. The equation for Fc in Clause 9.2. Add these forces to the effects of wind and other applied forces and design the support restraints. the cross member deflects and generates a force.3 is known to be unduly conservative and.3 as appropriate. THE USE OF BS5400 PART 3 General 4. Fc. When such stiffeners are used in simply supported bridges. This is also known as beams with rigid end posts. (d) Calculate the various restraint forces acting at the support as given in Clause 9. When designing the restraints.6 Similarly. therefore less rigid stiffeners (or so called flexible end posts) may need to be considered.12. the stiffeners still will need to comply with the force requirements in Clause 9.6.4. difficult to cater for.4.12.5 in the equation for le in Clause 9. 4. U-frames with cross members subjected to vertical loading 4.5.1 The procedure for the design of bracing systems in BS 5400 Part 3 can be summarized as follows: (a) Determine the form of the bracing system. it can be difficult to provide stiffeners which comply with Clause 9.Volume 1 Section 3 Part 13 BA 53/94 Chapter 4 The Use of BS5400 Part 3 4. Stiffeners which comply with the stiffness requirements in Clause 9.4 In some cases.1.
It is possible to design the hogging moment region more economically by consulting specialist literature which takes into account the buckling of beams under hogging moments. the compression flange is continuously restrained by the deck slab. the compression flange is usually restrained by inverted U-frames or intermediate stiff torsional bracing. CONTINUOUS COMPOSITE DECK TYPE BRIDGES 5. 5.2 Designers should be aware that the lateral torsional buckling rules involving U-frames in BS 5400 Part 3 are derived with simply supported beams in mind and are found to be unduly conservative when applied to the hogging moment region of a continuous composite deck type bridge.Volume 1 Section 3 Part 13 BA 53/94 Chapter 5 Continuous Composite Deck Type Bridges 5. December 1994 5/1 . whereas in the hogging moment region.1 In the sagging moment region of a continuous composite deck type bridge.
it is preferable to use pot bearings over the intermediate supports to avoid uneven stresses in the bearing ELECTRONIC COPY .6 For skew continuous deck type bridges.5 At the obtuse corners. the beam is restrained by L-frames.NOT FOR USE OUTSIDE THE AGENCY December 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 6/1 . torsional restraints square to the beams are more effective than those parallel to the supports if the skew angle is large. Two types of bearings are commonly used: rocker bearing and pot bearing.4 At the acute corners. Skew Continuous Deck Type Bridges 6. The choice of bearings over the abutments is the same as for skew half through type bridges.Volume 1 Section 3 Part 13 BA 53/94 Chapter 6 Skew Bridges 6. SKEW BRIDGES General 6. 6. This is because the twisting of the beam due to the cross member deflection may cause uneven stresses across the rocker bearing and the bearing stiffener.2 At the acute corners of a skew half through bridge. or alternatively. Therefore the U-frame rules in BS 5400 Part 3 are not applicable to this type of bridge.7 Over intermediate supports.1 For skew bridges with widely spaced longitudinal beams. At the obtuse corners. by the end trimmer if a pot bearing is used. A few solutions are discussed in the following paragraphs. Other types of bearings may be used provided that the engineer is satisfied that they would be able to provide the necessary restraints. 6. A rocker bearing prevents the beam from twisting and hence provides torsional restraint whereas a pot bearing allows the beam to rotate in all directions and does not provide any torsional restraint. Difficulties often arise with the design of bracing systems for this type of bridges.3 Special consideration should be given to the choice of bearings at supports. the use of a rocker bearing should be avoided. some of the cross-members may be connected to the main beams at one end and to the end trimmer over the abutment at the other end. the differential deflections at the ends of the cross member are much larger than that of a square bridge. 6. stiffener. although the beams are restrained by Uframes. 6. Skew Half Through Type Bridges 6. the torsional restraint may be provided by using a rocker bearing.
7. additional transverse members or vertical triangulated framework may be necessary. BRACING SYSTEM DURING CONSTRUCTION 7. For deck type bridges.1 At all stages of construction. 7. the bracing system should be effective and provide the necessary stability to the structure as a whole. ELECTRONIC COPY . additional bracing which are not necessary for the permanent works may be required during construction. triangulated framework or diaphragms may be used. in the form of plan triangulated framework.g.2 It should be noted that the preservation of stability and tolerances depends on the structure stiffness and not by designing for a particular force alone. temporary propping. In some cases. additional transverse members. Such bracing is the recommended first choice. 7. an effective plan bracing at the level of the cross-members. must be provided and extend to the supports. 7.NOT FOR USE OUTSIDE THE AGENCY December 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 7/1 . This also applies to the hogging moment region of continuous deck type bridges.6 To provide torsional restraint at supports. e.Volume 1 Section 3 Part 13 BA 53/94 Chapter 7 Bracing System During Construction 7. The use of stiff vertical triangulated framework between a pair of I-beams may permit several adjacent beams to be braced from the stiff pair.3 On many steelwork assemblies.4 Special consideration should be given to temporary intermediate bracing during slab concreting.5 For half through type bridges where the stability of the steel beams during slab concreting also relies on the intermediate U-frames. 7. stiff torsional bracing in the form of vertical triangulated framework between beams provides effective intermediate restraints.
1 Fatigue in a bracing system requires close and detailed attention since stress ranges can be high due to restraints at the joints. (e) Use welds of proper type and size. (b) Use simple joint details and avoid joint eccentricities.NOT FOR USE OUTSIDE THE AGENCY December 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 8/1 . Designers should consider the following guidelines to improve fatigue life of the structure and reduce maintenance. (a) Avoid unnecessary bracing as it could impose restraint causing stress concentration. (f) Avoid welded attachments to elements which are subject to high stress ranges. ELECTRONIC COPY . FATIGUE 8. (d) Avoid bracing systems which give excessive local stresses or attract large live load stresses.Volume 1 Section 3 Part 13 BA 53/94 Chapter 8 Fatigue 8. (c) Remove temporary bracing as far as possible.
(DMRB 1. Code of Practice for Design of Composite Bridges. 18 September 1990. British Standards Institution.NOT FOR USE OUTSIDE THE AGENCY December 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 9/1 . Steel.3) 2. Miscellaneous U-frame restraint against instability of steel beams in bridges. (DMRB) Volume 1 Section 3 General Design BD 13 Design of Steel Bridges. Concrete and Composite Bridges. 1979. Volume 68. The Structural Engineer. Code of Practice for Design of Steel Bridges. Design Manual for Roads and Bridges. No 18. Use of BS 5400: Part 5: 1979 (with Amendment No 1 dated December 1987). British Standards BS 5400: Part 3: 1982. ELECTRONIC COPY .Volume 1 Section 3 Part 13 BA 53/94 Chapter 9 References and Bibliographies 9. 3. British Standards Institution. Steel. REFERENCES AND BIBLIOGRAPHIES References 1. Use of BS 5400: Part 3: 1982. 1982. Concrete and Composite Bridges.3) BD 16 Design of Composite Bridges. BS 5400: Part 5: 1979. (DMRB 1.
Volume 1 Section 3 Part 13 BA 53/94 Chapter 10 Enquiries 10.NOT FOR USE OUTSIDE THE AGENCY December 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 10/1 . J. Andrew's House Edinburgh EH1 3TG J INNES Deputy Chief Engineer Head of Roads Engineering (Construction) Division Welsh Office Y Swyddfa Gymreig Government Buildings Ty Glas Road Llanishen Cardiff CF4 5PL B H HAWKER Head of Roads Engineering (Construction) Division Assistant Chief Engineer (Works) Department of the Environment for Northern Ireland Road Service Headquarters Clarence Court 10-18 Adelaide Street Belfast BT2 8GB D O'HAGAN Assistant Chief Engineer (Works) ELECTRONIC COPY . PICKETT Head of Bridges Engineering Division The Deputy Chief Engineer Roads Directorate The Scottish Office Industry Department New St. ENQUIRIES All technical enquiries or comments on this Advice Note should be sent in writing as appropriate to: Head of Bridges Engineering Division The Highways Agency St Christopher House Southwark Street London SE1 0TE A.
NOT FOR USE OUTSIDE THE AGENCY December 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED A/1 .Volume 1 Section 3 Part 13 BA 53/94 Annex A FIGURES Figure 1 (a) Compression flange restrained by a plan triangulated framework Figure 1 (b) Compression flange restrained by the deck ELECTRONIC COPY .
Annex A Volume 1 Section 3 Part 13 BA 53/94 Figure 2 (a) Half through type bridge restrained by U-frame (deck or plan bracing not shown for clarity) Sagging Moment Region Hogging Moment Region Figure 2 (b) Inverted U-frames at the hogging moment region of a deck type bridge (deck or plan bracing not shown for clarity) ELECTRONIC COPY .NOT FOR USE OUTSIDE THE AGENCY A/2 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED December 1994 .
NOT FOR USE OUTSIDE THE AGENCY December 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED A/3 .Volume 1 Section 3 Part 13 BA 53/94 Annex A Sagging Moment Region Hogging Moment Region Figure 3 Stiff torsional bracing at the hogging moment region Figure 4 Inneffective bracing ELECTRONIC COPY .
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