Plumb, Mahlon J

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Bridge Repair vs. ReplacementM a h l o n  J . P l u m b  PresidentP lum b, T uckett and  H u b ba rd , Inc. C onsulting Engineers Gary, Ind ianaIN T R O D U C T IO NT h e subtitle for this talk  could be “Please ask questions.’’Most o f you here today are representatives of the O w ner, and  you have a very heavy ob ligation  to pro tect the best interests of the O w ner. T h e  O w ner, of course, is the general public, or m ore particu larly  the taxpayers. You are often u n der considerable pressure from  citizens, politicians, im provem ent associations, and  o ther pressure groups to replace certa in  bridges. Some of these groups m ay talk  as though  tax  revenues, and  particu larly  federal funds, are alm ost un lim ited . O ne of your responsibilities is to m ake the best possible use of tax revenue.In  my case the O w ner is usually a ra ilroad , which is usually only m arg inally  pro fitab le , or even close to bankruptcy . In your case, the O w ner is also often having a h a rd  tim e raising enough m oney to do all the things he has to do.W hen it comes to bridges, the best way you can be a good rep resen­tative of the O w ner is to ask questions. I will try to ind icate  some of the questions you can ask.IN S PE C T IO NAll bridges should be looked over at least once a year. This is p a r ­ticularly  tru e  of larger bridges and  those which are no ted  to have p ro ­blem s. T ake along a copy of any previous inspection reports and  look closely for any changes in condition . T h e  princ ipal tool you’ll need is a ch ipp ing  ham m er. You have to clim b around  on the struc tu re , use a ladder if necessary and  physically touch the bridge, even pound  on it w ith the ch ipp ing  ham m er. You d o n ’t have to be experienced if you have an  inspection report by an  experienced engineer.I have two safety rules you m ay w ant to include. Never go inspect­ing alone and  never clim b anywhere if you d o n ’t feel secure in doing it. Never hesitate to adm it you d o n ’t like to clim b. C lim bing is a ta len t tha t not everyone has to the sam e extent.T h en  com pare your notes with those from  previous inspections. If there are differences, ask questions. If the previous inspection was13prep ared  by a consultan t, ask h im  abou t his findings a lthough  if it has been some tim e since his repo rt, you m ay have to pay a fee to review it. If  there  are discrepancies in the notes, ask w hether it is som ething you missed, som ething the previous repo rt missed, or some change in field conditions. T he la tte r m ay wave a red  flag at you.R A T IN G .A fter a bridge has been inspected, it is ra ted  to de term ine its load carrying capacity . F requently  bridges are over-designed, which is a p ractice  I ap p laud . Ask w hether the  corrosion loss has really h u rt the struc tu re .W e are curren tly  advising a ra ilroad  on the rep a ir of a series of su b ­ways in Chicago. These involve over 10,000 steel beam s, m ost o f which have lost 10% to 80%  of the ir section. W e have determ ined  th a t m any of them  can stand  a loss of 50, 60 or 70% and  still carry  the requ ired  loads.R eduction  of section on a beam  flange n ear an end  is no t as serious as the  sam e loss n ear the cen ter of a span. A bend  in a tension m em ber caused by a blow from  an e rra n t vehicle or ice is no t as serious as the sam e bend  in a com pression m em ber.If an  engineer has recom m ended  the repa ir or rep lacem ent of a b ridge, ask to see his ra tin g  com putations. Ask if the com putations take into  account corrosion losses and  any possible over-design. Check to see if w ind forces are taken  into  account and  ask why. It is m y belief th a t very few bridges are ever in danger of being  destroyed by w ind.R E H A B IL IT A T IO NW hen you are convinced th a t a b ridge needs a tten tion , always ask w hether it m igh t be b e tte r to rep a ir it, ra th e r  th a n  rep lace it. R epair, even with extensive strengthen ing , is usually m uch  less expensive th a n  rep lacem ent.O n m asonry structures, or the m asonry portion  of o th er structures, we have h ad  very good results w ith pressure g rou ting  and  shot-Crete p atch ing . Pressure g rou ting  is a techn ique w here holes are drilled  into m asonry piers or abu tm en ts th a t have voids or cracks, and  then  a ce­m en t g rou t is forced in u n de r pressure to fill the voids an d  seal the cracks.Shot-crete is a m ethod  of restoring  the surface of m asonry w here it has spalled off or deterio ra ted . It can  also be used to provide an  entirely  new surface all over. If  poor m asonry is your prob lem , ask w hether one of these rep a ir m ethods m ay be useful.Some bridges are ad eq u ate  except for w idth so check if it is possible to w iden the s truc tu re . Some bridges have ad eq u ate  substructure, bu t14the  superstructu re  is deterio ra ted . See if new beam s can  be p laced on the old foundations.R E PLA C EM EN T.W hen it has been determ ined  th a t no portion  of a b ridge can be salvaged, there  are still a n u m b er of questions th a t need to be asked. Does the am oun t of traffic w arran t the expend itu re, or w ould it be possible to aban d o n  the site? Should the new struc tu re  be on the present alignm ent, or w ould there  be significant savings if the new bridge were bu ilt in a new location?D uring  the design of a new bridge there are several questions which still need to be answ ered. O ne p rincip le to keep in m ind  is th a t sim ple, c lean lines usually m ean  a struc tu re  tha t is less expensive to bu ild  and  m a in ta in . For instance, if you can m ake a web p la te  th icker and  thereby elim inate  stiffeners, you m ay have a heavier to tal weight b u t a reduced  to tal cost, since stiffeners m ay cost th ree  tim es as m uch  as web pla te . W henever you can  elim inate  fancy work on handrails  or o ther so-called g inger-bread , you have not only saved m oney on original construction, b u t you have also reduced  m ain tenance problem s.A nother question which should always be asked, bu t one which m any com peten t engineers often fail to ask, is w hether you can save m oney by overdesign. For steel structures, the benefits o f using heavier steel can  be surprisingly good.For instance, take a typical s truc tu re  w here the design specifica­tions say the m eta l should be one-half inch thick. Assume th a t you, as representative of the  O w ner, arb itrarily  increase th a t thickness to 5 /8  inch. You have increased the steel weight by 25%  b u t you haven’t changed  the fab rica tion  or erection  costs, so you have only increased the cost of the steel by abou t 12% . For our typical s truc tu re , the cost of the steel is generally  abou t one-fourth  o f the to tal cost of the im provem ent. T h e  rest o f the budget goes for substructure, deck, approaches, detours, an d  such, so heavier steel has only increased the cost o f the im prove­m ent by 3% .Now let us assume the steel is subjected  to severe corrosion p ro ­blem s, such th a t after 30 years the steel has lost one-sixteenth inch from  each surface. T h e  steel as originally specified would be reduced  to 75% o f its original thickness, and  the O w ner w ould be facing a decision to rep a ir or rep lace the struc tu re . But since you arb itrarily  m ade the steel heavier, it w ould still be the requ ired  one-half inch thick  and  would have m any years o f useful life rem aining .Observations du ring  the inspection of hundreds of corroded bridges lead  m e to the inescapable conclusion th a t the use of an ex tra15one-eighth  inch thickness for all steel th a t m ay be subject to corrosion would ex tend  the life of an average s truc tu re  at least 50% . In  sum m ary, you can often  ex tend  the useful life of a steel s truc tu re  50%  by increas­ing the budget only 3% .16

Bridge Repair vs. Replacement

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Bridge Repair vs. Replacement

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