T he M ain S treet B ridge (N o. 4 overpass), located in W innipeg, M anitoba, C an ad a w as constructed in 1963. T he brid g e consists o f four-span prestressed concrete girders as show n in Fig. 8.1. T he bridge is 56.2 m long (4 @ 14.05 m) and 26.8 m w id e (6 traffic lanes) supported by tw enty tw o C -shaped prestressed concrete girders (Fig. 8.2) p restressed w ith N o. 13 steel strands (A p = 99 m m 2 each). T he brid g e is skew ed at an angle o f 5° 4 5 '. T he P erim eter H ighw ay, u n d er the M ain Street B ridge, encircles the C ity 231 Y ail J. K im , P .E ng., P h.D . T hesis
hence considerable, including intensive o p eration o f heav y trucks.
The exterio r bridge gird er w as frequently struck by heavy trucks. T he im pact loads caused spalling o f the concrete and com plete ru pture o f the steel strands, but no apparent visual cracks w ere fu rth er o b serv ed in the girder, as show n in Fig. 8.3. T he upw ard cam ber o f the exterio r gird er w as visu ally o b served to be around 30 m m to 50 m m . T he bridge looked to be in re lativ ely sound co n d itio n except for the d am aged locations. T he M an ito b a D epartm ent o f T ran sp o rtatio n and G o vernm ent S ervices ap proved the repair plan o f the dam aged bridge em ploying an innovative repair m ethod w ith prestressed C FR P sheets. T he re p air d esign and co rresponding site application w ere perfo rm ed by Q u e e n ’s U n iversity and V e cto r C on stru ctio n G roup. T his re p air w ork using prestressed C FR P sheets w as the first N o rth A m erican site application o f such technology.
8.3.2. Design o f the repair
A fter the field inspection, the re p air d esign w as conducted. B ecause the o w ner o f the bridge did n o t allow coring to obtain o n-site strengths o f the m aterials from the bridge girder, basic design p ro perties such as the specified strength o f concrete ( f ’c = 25 M P a) and steel strands (fpi, = 1,725 M P a) w ere assum ed in accordance w ith the C anadian H ighw ay B ridge D esign C ode (C H B D C 2000) w h ich included a specific ch ap ter for bridge evaluation. T he follow ing levels o f prestress in the steel strands w ere used for the repair design: f pi = 1,135 M P a (66 % o f f ptl), f pe = 906 M P a (53 % o f f pi) , and f py= 1,553 M P a (90 % o ff pi) for the initial, effective, and yield strengths, respectively;
w here f pu is the ultim ate strength o f the strands. T he long-term loss o f the prestress w as 232 Y ail J. K im , P .E ng., Ph.D . T hesis
C hapter 8: R ep air o f Im pact-dam aged B ridge w ith P restressed C F R P Sheets
adequately included in f pe in accordance w ith the A m erican A sso ciatio n o f S tate H ighw ay and T ra n sp o rtatio n O fficials L oad R esistance F acto r D esign (A A S H T O L R F D 1994).
N om inal areas o f the steel strands w ere used for design, alth o u g h there m ight be a p ossibility o f reduced strand areas due to the long-term exposure to the environm ent.
B ecause the P rovince o f M an ito b a o fficially accepts the A A S H T O L R F D B ridge D esig n S pecification, the rep air design w as co nducted based on A A S H T O L R F D (1994). N ote that this re p air design w as re-ch eck ed after site applications w ith the new ly pu b lish ed A A S H T O L R F D (2003). T he C ode recom m ends that the bend in g m om en t in a bridge girder be reduced i f the bridge is skew ed less than 10° (i.e., the skew correctio n factor);
how ever, this requirem ent w as no t included in the design. The reason w as that acceptable reduction factors for the current type o f b rid g e had not been d eveloped in the C ode yet.
T he ow ner suggested a design vehicle to be a m odified A A S H T O H SS 25, show n in Fig.
8.1, w eig h in g a total axle load o f 59,660 kg (or 585 kN ). T his loading w as thus consistently u sed for the entire design process. To reflect the w orst loading condition in the d am aged exterio r girder, the extrem e loading case w as consid ered as show n in Fig. 8.1 (b). T he dynam ic load allow ance (D L A ) w as adequately included by m ultiplying the D L A to the design tru ck load in the re p air design. T he d esign loading w as som ew hat conservative since the truck on the p ed estrian w alk w ay should no t in clu d e the dynam ic load allow ance as stated in A A S H T O L R F D (1994, 2003). T he current design m ethod (A A SH T O L R FD ) m ay no t separate the tru ck load w ith and w ith o u t the d ynam ic load allow ance due to its unique live load distributions. B ased on the intensive analysis o f the dam aged ex terio r girder, the follow ing conditions did no t satisfy the C ode requirem ents:
233 Y ail J. K im , P.E ng., P h.D . T hesis
serviceability in term s o f crack control, as show n in T ables 8.1 and 8.2.
A n innovative strengthening m ethod u sin g three layers o f p restressed C F R P sheets (Afrp = 173 m m 2) w as sugg ested fo r strengthening the dam aged girder. T he C F R P sheets w ere to be prestressed up to 21 % o f the u ltim ate fibre strain (i.e., ap p roxim ately 800 M P a in the sheets) and b o n d ed to the tensile soffit o f the dam aged gird er u sin g epoxy adhesive (Wabo® M B race Saturant). A cco rd in g to the m anufacturer, the C FR P sheet (Wabo® M B race C F -160) has a u n id irec tio n al tensile strength o f 3,800 M P a w ith a tensile m odulus o f 227 G Pa, and an ultim ate ru pture strain o f 1.67 %. T he d esig n p roperties before and after rep air are sum m arized in T able 8.1. T he repaired girder w as exp ected to fully reco v er its flexural capacity w ith resp ect to the u n d am aged condition, to re co v er the cracking m o m en t by 85 % o f the u n d am ag e d condition, and to satisfy the lim it o f the crack control p aram eter as required by the C ode (i.e., less than 30,000 N /m m ). T he live load in T able 8.2 w as obtained from the govern in g live load distribution factor calculated based on the L e v e r R u le and the R ig id M ethod, including adequate m ultiple presence factors as p e r the A A S H T O L R F D (1994). D etailed calculations are p ro v id ed in A ppendix C. T he factored m om en t (T able 8.2) w as m uch greater than the load-carrying capacity o f the gird er (T able 8.1); how ever, the level o f external strengthening w as focused on reco v erin g the un d am ag ed state based on the finite elem ent analysis (F E A ), as show n in T able 8.3, and the A A S H T O ratin g factors (discussed later). N ote that the obtained m om ent u n d e r live loads (M wi,h u) from th e F E A , in T able 8.3, w as m ultip lied by the m ultiple presen ce factor for convenience, resu ltin g in conservative predictions up to 4
% w hen com p ared to the m om ent excluding the dead load effect. T he live loads in the 234 Y ail J. K im , P .E ng., Ph.D . T hesis
C hapter 8: R epair o f Im pact-dam aged B ridge w ith P restressed C F R P Sheets
dam aged and repaired states w ere assum ed to be at the sam e level o f the u n d am ag e d state since the C ode did not provide any m eth o d to obtain live loads u n d er d am aged or repaired conditions. T he insignificant reduction o f the dead load m om ent, in duced by the spalling o f concrete, as show n in Fig. 8.3, w as not tak en into account in T able 8.2.
To successfully apply the desired level o f p restress to the C FR P sheets, an adequate anchor system w as required. A steel plate-ty p e an ch o r system w as designed based on the H andbook o f Steel C onstruction (C ISC 2000). T he anchor system , fabricated from 350W structural steel, consisted o f tw o re ctan g u lar plates (or sh ea r p la te s) including w eld ed L- angles and ja c k in g plates, as show n in Fig. 8.4. T he C FR P sheets w ere bon d ed onto the tapered ja c k in g anchor plates, w h ich w o u ld be m o u n ted into the sh ear anchor plates. To p revent stress concentrations at the end o f the ja c k in g plates, the plates w ere fabricated to include a bevel w ith a length o f 100 m m and a slope o f 8 %.