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  THE
  ANATOMY OF BRIDGEWORK




  THE
  ANATOMY OF BRIDGEWORK

  BY

  WILLIAM HENRY THORPE
  ASSOC. M. INST. C. E.

  WITH 103 ILLUSTRATIONS

  [Illustration]

  London
  E. & F. N. SPON, LIMITED, 57 HAYMARKET
  New York
  SPON & CHAMBERLAIN, 123 LIBERTY STREET

  1906




PREFACE


In offering this little book to the reader interested in Bridgework, the
author desires to express his acknowledgments to the proprietors of
"Engineering," in which journal the papers first appeared, for their
courtesy in facilitating the production in book form.

It may possibly happen that the scanning of these pages will induce
others to observe and collect information extending our knowledge of
this subject--information which, while familiar to maintenance engineers
of experience, has not been so readily available as is desirable.

No theory which fails to stand the test of practical working can
maintain its claims to regard; the study of the behaviour of old work
has, therefore, a high educational value, and tends to the occasional
correction of views which might otherwise be complacently retained.

  60 WINSHAM STREET,
  CLAPHAM COMMON, LONDON, S.W.
  _October_, 1906.




CONTENTS


  CHAPTER I.

  INTRODUCTION--GIRDER BEARINGS.

                                                                    PAGE

  Pressure distribution--Square and skew bearings--Fixed bearings--
  Knuckles--Rollers--Yield of supports                                 1


  CHAPTER II.

  MAIN GIRDERS.

  _Plate webs_: Improper loading of flanges--Twisting of girders--
  Remedial measures--Cracks in webs--Stiffening of webs--[T]
  stiffeners                                                           9

  _Open webs_: Common faults--Top booms--Buckling of bottom booms--
  Counterbracing--Flat members                                        17


  CHAPTER III.

  BRIDGE FLOORS.

  Liability to defects--Impact--Ends of cross and longitudinal
  girders--Awkward riveting--Fixed ends to cross girders--Plated
  floor--Liberal depths desirable--Type connections--Effect of "skew"
  on floor--Water-tightness--Drainage--Timber floors--Jack arches--
  Corrugated sheeting--Ballast--Rail joints--Effect of main girders
  on floors                                                           20


  CHAPTER IV.

  BRACING.

  Effect of bracing on girders--Influence of skew on bracing--Flat
  bars--Overhead girders--Main girders stiffened from floor--
  Stiffening of light girders--Incomplete bracing--Tall piers--Sea
  piers                                                               34


  CHAPTER V.

  RIVETED CONNECTIONS.

  Latitude in practice--Laboratory experiments--Care in considering
  practical instances--Main girder web rivets--Lattice girders
  investigated--Rivets in small girders--Faulty bridge floor--
  Stresses in rivets--Cross girder connections--Tension in rivets--
  Defective rivets--Loose rivets--Table of actual rivet stresses--
  Bearing pressure--Permissible stresses--Proposed table--Immunity of
  road bridges from loose rivets--Rivet spacing                       45


  CHAPTER VI.

  HIGH STRESS.

  Elastic limit--Care in calculation--Impact--Examples of high stress
  --Early examples of high stress in steel girders--Tabulated
  examples--General remarks                                           61


  CHAPTER VII.

  DEFORMATIONS.

  Various kinds--Flexing of girder flanges--Examples--Settlement
  deformations--Creeping--Temperature changes--Local distortions--
  Imperfect workmanship--Deformation of cast-iron arches              73


  CHAPTER VIII.

  DEFLECTIONS.

  Differences as between new work and old--Influence of booms and web
  structure on deflection--Yield of rivets and stiffness of
  connections--Working formulae--Set--Effect of floor system--
  Deflection diagrams--Loads quickly applied--"Drop" loads--Flexible
  girders--Measuring deflections--New method of observing deflections
  --Effect of running load                                            85


  CHAPTER IX.

  DECAY AND PAINTING.

  Examples of rusting of wrought-iron girders--Girder over sea-water
  --Rate of rusting--Steelwork--Precautions--Red-lead--Repainting--
  Scraping--Girders built into masonry--Cast iron--Effect of sea-
  water on cast iron--Examples--Tabulated observations--Percentage of
  submersion--Quality of metal                                        96


  CHAPTER X.

  EXAMINATION, REPAIR, AND STRENGTHENING OF RIVETED BRIDGES.

  Purpose--Methods of examination--Calculations--Stress in old work--
  Methods of reducing stress--Repair--Loose rivets--Replacing wasted
  flange plates--Adding new to old sections--Principles governing
  additions--Example--Strengthening lattice girder bracings--Bracing
  between girders--Strengthening floors--Distributing girders        107


  CHAPTER XI.

  STRENGTHENING OF RIVETED BRIDGES BY CENTRE GIRDERS.

  Principal methods in use--Method of calculation--Adjustments--
  Connections--Method of execution--Checks--Effect of skew on method
  considered--Results of calculation for a typical case--Probable
  error--Practical examples--Special case--Method of determining
  flexure curves                                                     122


  CHAPTER XII.

  CAST-IRON BRIDGES.

  Limitations of cast iron--Stress examples--Advantages and
  disadvantages--Foundry stresses--Examples--Want of ductility of
  cast iron--Repairs--Restricted possibilities                       141


  CHAPTER XIII.

  TIMBER BRIDGES.

  Perishable nature--Causes of decay--Sag--Lateral bracing--Piles--
  Uncertainty respecting decay--Examples--Conditions and practice
  favourable to durability--Bracing--Protection--Repair--Piles--Cost 149


  CHAPTER XIV.

  MASONRY BRIDGES.

  Definition--Cause of defects or failure--Spreading of abutments--
  Closing in--Example--Stop piers--Example of failure--Strength of
  rubble arch--Equilibrium of arches--Effect of vibration on masonry
  --Safety centring--Methods of repair--Pointing--Rough dressed
  stonework                                                          157


  CHAPTER XV.

  LIFE OF BRIDGES--RELATIVE MERITS.

  Previous history--Causes of limited life--Tabulated examples of
  short-lived metallic bridges--Timber and masonry bridges--
  Durability--Maintenance charges--First cost--Comparative merits--
  Choice of material                                                 165


  CHAPTER XVI.

  RECONSTRUCTION AND WIDENING OF BRIDGES.

  CONCLUSION.

  Measuring up--Railway under-bridges--Methods of reconstruction in
  common use--Reconstruction of bridges of many openings--Timber
  staging--Traffic arrangements--Sunday work--Railway over-bridges--
  Widenings--Junction of new and old work--Concluding remarks--Study
  of old bridgework                                                  172

  INDEX                                                              187




  THE
  ANATOMY OF BRIDGEWORK.




CHAPTER I.

INTRODUCTION.


No book has, so far as the author is aware, been written upon that
aspect of bridgework to be treated in the following pages. No excuse
need, therefore, be given for adding to the already large amount of
published matter dealing with bridges. Indeed, as it too often happens
that the designing of such constructions, and their after-maintenance,
are in this country entirely separated, it cannot but be useful to give
such results of the behaviour of bridges, whether new or old, as have
come under observation.

In the early days of metallic bridges there was of necessity no
experience available to guide the engineer in his endeavour to avoid
objectionable features in design, and he was, as a result, compelled to
rely upon his own foresight and judgment in any attempt to anticipate
the effects of those influences to which his work might later be
subject. How heavily handicapped he must have been under these
conditions is evident from the mass of information since acquired by the
experimental study of the behaviour of metals under stress, and the
growth of the literature of bridgework during the last forty years. That
many mistakes were made is little occasion for surprise; rather is it a
cause for admiration that some very fine bridges, still in use, were
the product of that time. Much may be learned from the study of defects
and failures, even though they be of such a character that no
experienced designer would now furnish like examples.

Modern instances may, none the less, be found, with faults repeated,
which should long since have disappeared from all bridgework, and are
only to be accounted for by the unnatural divorce of design and
maintenance already referred to. As the reader proceeds, it may appear
that details are occasionally touched upon of a character altogether too
crude and objectionable to need comment; but the consideration of these
cases is none the