Structural Bearings and Expansion Joints. for Bridges. Structural Engineering Documents. Gunter Ramberger IABSE AIPC IVBH

Transcription

1 Structural Engineering Documents Gunter Ramberger Structural Bearings and Expansion Joints I for Bridges International Association for Bridge and Structural Engineering Association lnternationale des Ponts et Charpentes lnternationale Vereinigung fur Bruckenbau und Hochbau IABSE AIPC IVBH

2 Copyright by International Association for Bridge and Structural Engineering All rights reserved. No part of this book may be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without permission in writing from the publisher. ISBN Printed in Switzerland Publisher: IABSE-AIPC-IVBH ETH Honggerberg CH-8093 Zurich, Switzerland Phone: Int Fax: Int secretariat@iabse.ethz.ch Web:

3 Table of Contents 1. Bearings 1.1 Introduction 1.2 The role of bearings 1.3 General types of bearings and their movements 1.4 The layout of bearings 1.5 Calculation of bearing reactions and bearing movements 1.6 Construction of bearings 1.7 Materials for bearings 1.8 Analysis and design of bearings 1.9 Installation of bearings 1.10 Inspection and maintenance 1. I 1 Replacement of bearings 1. I 2 Codes and standards 1.13 References 2. Expansion Joints 2.1 Introduction 2.2 The role of expansion joints 2.3 Calculation of movements of expansion joints 2.4 Construction of expansion joints 2.5 Materials for expansion joints 2.6 Analysis and design of expansion joints 2.7 Installation of expansion joints 2.8 Inspection and maintenance 2.9 Replacement of expansion joints 2.10 References

4 Dedicated to the commemoration of the late Prof. Dr. techn. Ferdinand Tschemmernegg, University of Innsbruck. Preface It is my hope that this treatise will serve as a textbook for students and as information for civil engineers involved in bridge construction. My intent was to give a short guideline on bearings and expansion joints for bridge designers and not to mention all the requirements for the manufacturers of such products. These requirements are usually covered by product guidelines, which vary between different countries. Not all the references are related to the content of this document. They are more or less a collection of relevant papers sometimes dealing with special problems. I express many thanks to Prof. Dr.-Ing. Ulrike Kuhlmann, University of Stuttgart, chairperson of Working Commission 2 of IABSE, who gave the impetus for this work; to her predecessor of the IABSE Commission, Prof. Dr. David A. Nethercot, Imperial College of Science, Technology and Medicine, London, for reviewing the manuscript, and Prof. Dr. Manfred Hirt, Swiss Federal Institute of Technology, Lausanne, for his contributions and comments. I wish to thank J. S. Leendertz, Rijkswaterstaat, Zoetermeer; Eugen Briihwiler, Swiss Federal Institute of Technology, Lausanne; Prof. R. J. Dexter, University of Minnesota; G. Wolff, Reissner & Wolff, Wels; 0. Schimetta t, Amt der 00 Landesregierung, Linz; Prof. B. Johannsson, LuleA Tekniska Universitet, for amendments, corrections, remarks and comments. I thank also my assistant Dip1.-Ing. Jorgen Robra for his valuable contributions to the paper, especially for the sketches and drawings, and my secretaries Ulla Samm and Barbara Bastian for their expert typing of the manuscript. Finally, I would like to thank the IABSE for the publication of this Structural Engineering Document. Vienna, April 2002 Gunter Ramberger

5 1 Bearings Introduction All bridges are subjected to movements due to temperature expansion and elastic strains induced by various forces, especially due to traffic loads. In former times our bridges were built of stones, bricks or timber. Obviously, elongation and shortening occurred in those bridges, but the temperature gradients were small due to the high mass of the stone bridges. Timber bridges were small or had natural joints, so that the full elongation values were subdivided into the elongation of each part. On the other hand, the elongation and shortening of timber bridges due to change of moisture is often higher than that due to thermal actions. With the use of constructional steel and, later on, of reinforced and prestressed concrete, bridge bearings had to be used. The first bearings were rocker and roller bearings made of steel. Numerous rocker and roller bearings have operated effectively for more than a century. With the development of ageing-, ozone- and UV-radiation-resistant elastomers and plastics, new materials for bearings became available. Various types of bearings were developed with the advantage of an area load transmission in contrast to steel bearings with linear or point load transmission, where elastic analysis leads theoretically to infinite compression stresses. For the bearings the problems of motion in every direction and of load transmission were solved, but the problem of insufficient durability still exists. Whilst it is reasonable to assume the life of steel bearings to be the same as that of the bridge, the life of a bearing with elastomer or plastic parts can be shorter. 1.2 The role of bearings The role of bearings is to transfer the bearing reaction from the superstructure to the substructure, fulfilling the design requirements concerning forces, displacements and rotations. The bearings should allow the displacements and rotations as required by the structural analysis with very low resistance during the whole lifetime. Thus, the bearings should withstand all external forces, thermal actions, air moisture changes and weather conditions of the region. 1.3 General types of bearings and their movements Normally, reaction forces and the corresponding movements follow a dual principle - a non zero bearing force corresponds to a zero movement and vice versa. An exception is given only by friction forces which are nearly constant during the movement, and by elastic restraint forces which are generally proportional to the displacement. Usually, the bearing forces are divided into vertical and horizontal components. Bearings for vertical forces normally allow rotations in one direction, some types in all directions. If they also transmit horizontal forces, usually vertical forces are combined.

6 ~ 8 1. Bearings A special type of bearing transmits only horizontal forces, while allowing vertical displacements. The following table (Table 1.3-1) shows the common types of bearings, including the possible bearing forces and displacements. Friction and elastic restraint forces are not considered. - Symbol Function All translation fixed Rotation all round Horizontal movement in one direction Rotation all around Construction Point rocker bearing Pot bearing; Fixed elastomeric bearing; Spherical bearing Constr. point rocker sliding bearing; Constr. pot sliding bearing; Const. elastomeric bearing; Constr. spherical sliding bearing Horizontal movement in all directions Rotation all round Free point rocker bearing; Free pot sliding bearing; Free elastomeric bearing; Free spherical sliding bearing; Link bearing with universal joints (tension and compression) + All translation fixed Rotation about one axis Horizontal movement in one direction Rotation about one axis Horizontal movement in all direction Rotation about one axis All horizontal tranal. fixed Rotation all round Horizontal movement in one direction Rotation all round Line rocker bearing Leaf bearing (tension and compression) Roller bearing; Link bearing (tension and compression); Constant line rocker sliding bearing Free rocker sliding bearing; Free roller bearing; Free link bearing HoriLontal force bearing Guide bearing

7 I 1.4 The layout of bearings 9 Tuble The layout of bearings General Bearings can be arranged at abutments and piers (fig ; fig ) under the webs of the main girders, under diaphragms (fig ), and under the nodes of truss bracings. The webs and the diaphragms of concrete bridges have to be properly reinforced against tensile splitting; steel bridges need stiffeners in the direction of the bearing reactions to transfer the concentrated bearing loads to the superstructure and the substructure. Abutments and piers also have to be properly reinforced under the bearings against tensile splitting. -77 Fig. I.4. I - I: Bearings at an abutment I - ~- ~ Fig : Bearings at u pier, I7 Fig : Bearing at a single pier

8 10 1. Bearings The layout of the bearings should correspond to the structural analysis of the whole structure (super- and substructure together!). If the settlement and the deflection of the substructure can be neglected the structural analysis of the superstructure, including the bearings, can be separated from that of the substructure. Sometimes the model for the analysis, especially of the superstructure, will be simplified by assuming the following: bearings are situated directly on the neutral axis of the girder (fig ), the motion of the bearings occurs without restraint, bearings have no clearance, etc. In this case we must consider the correct system (fig ) at least for the design of the bearings and take into account the influence of the simplifications on the structure. & Fig. I.4.1-4: Reality A Fig. I.4.1-5: Correct system On the abutments or separating piers it is normal to use at least two vertical bearings to avoid torsional rotations. At intermediate piers one or more vertical bearings may be used. If more than one bearing is used the rotational displacement at the pier is restrained. More than three vertical supports of the superstructure lead to statically indeterminate bearing conditions, but even the simplest bridge has at least four vertical bearings. If the torsional stiffness of the superstructure is low (e.g. open cross sections) it may be neglected and the layout with four bearings becomes isostatic. If the torsional stiffness is not negligible (e.g. box girders) we have to take it into account for the structural analysis, especially for skewed and curved bridges. On a bridge with n > 3 vertical supports, n - 3 bearing reactions can be chosen freely within a reasonable bandwidth. This possibility can be used to prestress the superstructure and to distribute the bearing reactions as desired. If the bearings are situated (nearly) in a plane we need at least one horizontally fixed and one horizontally moveable bearing. The moving direction must not be orthogonal

9 I.4 The layout of bearings 11 to the polar line from the fixed to the moveable bearing. If more than two bearings in the horizontal direction are necessary, the basic principle should be that an overall uniform extension, caused by temperature or shrinkage, shall be possible without restraint. In general, there are two possibilities for the arrangement of the bearings: a) arrangement in a horizontal position (fig ) b) arrangement in a position parallel to the road or rail surface (fig ). I 1 ---_---,--a Fig : Horizontal arrangement of the bearings (case a) -(I f=-- I,,displaced bridge ( Fig : Inclined arrangement ofthe bearings (case b) Case a) has the advantage that only vertical bearing reactions and no permanent horizontal reactions result from vertical loads, but it has the disadvantage that bridges with inclined gradients require a step at the expansion joint due to movements in the superstructure. The greater the elongation or shortening, the greater the step required. Case b) has the advantage that the slope of the expansion joint is independent of the movement of the bridge. The inclination of the surface of support gives the direction of the normal force. Besides vertical reaction forces, also horizontal reaction forces result from vertical loads. Permanent horizontal actions can lead to a displacement by creep of the concrete and the soil and, thus, to crooked piers.

10 12 1. Bearings The layout for different types of bridges For single span girders the layout of the bearings is straightforward. One fixed and one moveable bearing is provided on each abutment, all other bearings are just vertical supports, moveable in any horizontal direction. For wide bridges the horizontally fixed bearings are located in or near the bridge axis. Formerly, the classical arrangement of the bearings for a bridge with two main girders consisted of one fixed and one lengthwise moveable bearing at one abutment and one lengthwise moveable and one free bearing at the other abutment (fig ). This layout has the advantage that longitudinal horizontal forces (braking and traction forces) can be distributed into the two bearings at the abutment, but it has the disadvantage that horizontal forces in the cross direction (wind) and temperature differences cause horizontal restraint forces, provided that bearings have no clearance on the abutments. The author prefers the statically determinate system with only one lengthwise restrained bearing at the abutment concerned because the actual clearance of a bearing is not determinable in reality (fig ). ++- LA- 11, %I, I Fig : Classical layout ;c _ :.. Fig : Horizontally statically determinate system (better than classical layout) -_ Fig : System with separated vertical and horizontal bearings (statically determinate system)

11 1.4 The layout of bearings 13 For skewed or horizontally curved single span bridges we have to decide whether the horizontal force should be combined with the higher or with the lower vertical reaction force. For all bearing constructions it is easier to transfer horizontal forces in combination with a high vertical force. In this case the resultant force stays nearer to the centre, its angle to the vertical is smaller and leads to smaller bending moments in suband superstructure (fig ).! I I H I Fig : Inclination of the resultant force Thus, the horizontally constrained bearings for skewed bridges should be placed at the obtuse corners of the bridge, for curved bridges at the outer side (fig ). Fig : Skewed bridge Fig : Layoutfor continuous girders

12 14 1. Bearings For straight continuous girders normally two bearings are used at every abutment and pier. If the torsional stiffness is high (box girder) the intermediate piers can be reduced to a round column with one bearing on the axis under the diaphragm. Constrained bearings in the cross direction are the rule at all piers. If the horizontal bending stiffness is very high we can transfer the horizontal forces only at the abutments. The same considerations are suitable also for skewed and curved bridges (fig ). Bearings for horizontal forces and guide bearings which transfer only horizontal forces may be used in combination with leaf or link bearings which cannot transmit horizontal forces. The movement of an expansion joint must be linked by a guide like a constraint bearing. The main movement of an expansion joint should be in the axis of the traffic way. Generally, this direction does not coincide with the direction of the polar line from the fixed bearing to the moveable bearing at the abutment (fig ). If all other bearings have the same angle between the polar line and the moving direction there results a layout of the bearings with no restraints on uniform elongation or shortening (e.g. caused by thermal actions or shrinkage), as shown below (fig ). Fig : Layout for curved bridges Fig : Layout for curved continuous girders (no constraint under overall tempe ra tu re) Fig : Geometrical situation

13 I.4 The layout of bearings 15 The elongation is A,, = k. r, << r, A- = k. r << r k proportional elongation The rotation is A;tana A;tana k.r.tana - - = k.tana r+a, r r For Cp, = Cp, the bridge simply rotates as a rigid body without constraint. One special case of this general rule is well known: the bearings are moveable in the direction of the polar lines with a = 0 (fig ). However, this layout has the disadvantage that generally the main movement of the joint does not coincide with the movement of the bearing. Fig : Special case with a = Special bearing conditions, advice etc. It is important to note that the layout of the bearings has a great influence on the structural system. The above mentioned arrangements of bearings are typical for average bridges. The following examples show some special effects which have to be considered for the design of bridges and bearings. These examples do not lay claim to completeness. a) The already mentioned bearing layout, consisting of one bearing fixed in all sliding directions and one fixed lengthwise at one abutment, leads to high constraint forces not only under horizontal but also under eccentric vertical loading (fig.].4.3-1). It is interesting that this eccentric loading has no prying effect if the bearings are situated directly on the neutral axis of the girder. This effect results only from the (small) eccentricity of the bearing under the lower flange.

14 ~ ~ 16 I. Bearings I A+ Fig : Prying effect due to a eccentric loading b) A similar situation occurs for a continuous girder with chequer pattern loading. Fig : Prying effect due to chequer pattern loading c) It is not generally known that a skewed bridge with horizontally fixed bearings only in one line exhibits the same effect under vertical loading, as the following figure shows: Fig : Prying forces for a skewed bridge with vertical loading Similar effects can occur for curved bridges. For the correct analysis of the bearing reactions it is always necessary to model the bearings at the very point where they are actually situated, and in combination with the substructure. The deflection of the substructure can influence the constraint bearing reactions significantly. 1.5 Calculation of bearing reactions and bearing movements Actions According to Eurocode 1 (ENV 1991) the actions can be subdivided into: - permanent actions, - variable actions, - extraordinary actions.

15 1.5 Calculation of bearing reactions and bearing movements 17 The bridge should take up the desired shape under all permanent loads, at the average temperature (+lo" C in most of the European countries) and, if time-dependant displacements occur, at the time t = 00, at which time all moveable bearings should be in the zero adjustment (null position). Variable actions and extraordinary actions lead to deviation from this form. Variable actions to consider are: - traffic loads, considering the applicable dynamic coefficients - loads due to traffic loads, i.e. nosing forces centrifugal forces braking forces traction forces - wind loads wind on construction wind on traffic loads - settlements of abutments and piers - thermal actions ' uniform temperature vertical temperature gradient horizontal temperature gradient temperature differences between individual parts of the bridge (e.g. stay cables, pylon and stiffening girder) - creep and shrinkage of concrete Extraordinary actions to consider are: - earthquake actions - vehicle impact - derailment - rupture of the conductor line others Bearing reactions For permanent actions such as self-weight of the construction, dead load and prestressing, the bearing reactions can be calculated as one load case. For the analysis of the bearings it is necessary to consider different combinations of the bearing reactions: - maximum vertical force and the adjacent horizontal force, - minimum vertical force and the adjacent maximum horizontal force, - maximum horizontal force and the adjacent maximum vertical force, - maximum horizontal force and the adjacent minimum vertical force. The simplest way to obtain these combinations is to calculate the variable actions, especially the traffic load, according to the influence line. One should bear in mind that horizontal actions such as centrifugal forces or braking forces are proportional to the vertical traffic load, but other loads, such as wind or traffic or traction forces for railways, are not.

16 18 1. Bearings To obtain the extreme bearing reaction it is necessary to consider that all bridges are three-dimensional and not merely plane systems. The influence lines (influence surfaces) of the bearing reactions can be found as the displacement curves (displacement surfaces) of the system, due to unit displacements F = 1 or cp = 1, acting at the position and in the direction of the required force. If these analyses are performed on a three dimensional model, the definitive influence area will result directly (fig.1 S.2-1; fig.1 S.2-2). If plane models are used for the analyses, special care is necessary, particularly with continuous girders with open or box sections. The following examples demonstrate the difference: Fig : Influence area for the verticul bearing reaction A, box section. Fig : ZnJuence area for the vertical bearing reaction A, open section Bearing displacements As already mentioned, the zero adjustment (null position) of every bearing has to be defined. The displacements are measured from that position. Thus, for concrete and composite bridges it is usual to consider displacements under time-dependent actions such as creep and shrinkage from the time of installation of the bearing to the time defined for the null position (normally t = w), from which position the displacements due to variable actions are measured. To obtain the maximum displacements and rotations, again we can use influence lines. The influence line of a displacement can be calculated as the displacement curve due to the corresponding unit force P = I. To take into account the imperfections due to installation, the temperature difference for the calculation of bearing displacements should be assumed higher than for the structural analysis of the bridge, or some additional displacement should be considered.

17 I.6 Construction of bearings Construction of bearings Fig gives un overview for the most common bearings.