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1 Structural Faults and Repair 2001 previous home section next SEISMIC REHABILITATION OF STATE STREET BRIDGE CP Pantelides, Y Okahashi, D Moran University o Utah Dept o Civil & Environmental Engineering Salt Lake City Utah 84112, USA F Alameddine Caliornia Dept o Transportation Oice o Earthquake Engineering Sacramento Caliornia USA T Sardo Iowa Dept o Transportation Ames Iowa USA KEYWORDS: Bridge, Concrete, Earthquakes, Fiber Reinorced Polymer Composites, Rehabilitation ABSTRACT The State Street Bridge in Salt Lake City, was designed and built in 1965 according to the 1961 AASHO Speciications but was not designed to resist any earthquake-induced orces or displacements. The bridge consists o our multicolumn reinorced concrete bents supporting composite welded girders; the bents are supported on cast-in-place concrete piles and pile caps. The bridge had deiciencies in: (a) lap splices at the bottom o the columns, (b) insuicient lateral coninement o columns, (c) shear capacity o columns, bent cap, and bent cap-column oints, (d) small ooting ootprint, (e) no top mat o reinorcement in the ootings, and () pile ootings ounded on very sot soil. A seismic retroit design using carbon iber reinorced polymer (CFRP) composites, and a design using steel ackets were perormed or three design spectra, including a service level earthquake, a ten percent probability o exceedance in 50 years earthquake, and a ten percent probability o exceedance in 250 years earthquake. The CFRP composite design was selected or implementation; application o the composite was carried out in the summer o 2000 while the bridge was in service. The paper describes the design o the CFRP composite, which in addition to column ackets, implemented an ankle wrap or improving oint shear strength, and a U-strap or improving anchorage o column bars; other rehabilitation measures are also described, such as bumper brackets and deck slab rehabilitation. INTRODUCTION Seismic rehabilitation techniques or concrete bridges involving steel acketing, concrete acketing, and FRP composite ackets or columns have been implemented in the last ew years (Priestley et al. 1996). Various circular and rectangular columns with carbon FRP composite retroits have been tested in the laboratory (Seible at al. 1997); in addition, in-situ tests have been perormed on FRP retroitted columns (Gamble and Hawkins 1996) and bridge bents (Pantelides et al. 1999, 2001). All o the tests showed that the carbon FRP composite acket is as eective as a comparable steel acket system. The purpose o the present study was to design and implement a seismic rehabilitation method or the State Street Bridge. The State Street Bridge at Interstate 80 in Salt Lake City was designed in 1965 according to the State o Utah Standard Speciications or Road and Bridge Construction, 1960 Edition and Supplements, and the AASHO Speciications o 1961 and Interim Speciications. As such, the bridge was not designed to resist any earthquake-induced orces or displacements. The 55m-long bridge consists o two 11m end spans and a 34m middle span. The substructure consists o cast in place concrete piles, concrete pile caps, concrete columns and concrete bent cap. The dimensions and reinorcement o one o the our bents o the bridge is shown in Fig. 1. The girders, which are not shown, are composite welded steel beams. Design parameters or the analysis required or the seismic retroit design were established as ollows: (1) concrete compressive strength = 29 MPa, (2) maximum strain = 0.004, (3) steel yield stress: 300 MPa; oundation springs were also developed including (4) oundation lateral stiness in horizontal direction = 28.3 kn/mm, (5) axial stiness in vertical direction = kn/mm, and (6) rotational stiness = 45,685 kn-m/rad. Design Spectra or three design earthquakes were determined as ollows: (1) 0.2 g service level earthquake, (2) 10% probability o exceedance in 50 years, (3) 10% probability o exceedance in 250 years.

2 A C m φ m 51x305x305 KEY 4.877m A 4.877m B B C 2.019m 5.029m φ x305x305 KEY m 3.048m 1.829m 3.048m 1.829m 3.048m 1.829m 152φ mm 29mm 36mm 51mm 13mm 16mm@305mm 16mm@610mm 36mm 1.067m 16mm 36mm 1.067m 29mm 12-29mm 13mm@305mm 457mm R 51mm SECTION A-A SECTION B-B 36mm 51mm 13mm 16mm@305mm 16mm@610mm 16mm 1.067m SECTION C-C VARIES 29mm Figure 1. Dimensions and Reinorcement o Typical Bent o State Street Bridge. BRIDGE IN AS-IS CONDITION Static pushover nonlinear analyses o the bridge bent were perormed in the longitudinal and in the transverse direction. A two-dimensional model was used in the transverse direction, as shown in Fig. 2. The structure was modeled using the sotware DRAIN-2DX (Prakash et al. 1992), or both the as-is and rehabilitated bents. Gravity loads applied to the bent cap rom the superstructure were used in developing

3 the pushover curve o the structure in the as-is condition, and or the design o the carbon iber reinorced polymer (CFRP) composite retroit. The pushover curve o the bent in the as-is condition in the transverse direction is shown in Fig. 3. The global displacement ductility o the as-is bent was calculated as µ = 2.9. A capacity evaluation was perormed and the ollowing deiciencies were ound: (a) inadequate coninement o the column lap splice region, (b) inadequate coninement o the lexural plastic hinges in the columns, and (c) inadequate shear capacity in the columns, bent cap, and the bent capcolumn oint region. Table 1 shows a summary o displacement demands, and column lexure and shear demand/capacity ratios, which were obtained using the X-Section and W-rame programs developed by Caltrans (Mahan 1998). These results reveal lexural deiciency o the column or the 10% in 250 years hazard level. Furthermore, the results o shear demands in Table 1 show shear orce deiciency o the columns under the same hazard level = Beam-Column = Spring Figure 2. Structural Model o State Street Bridge Bent or Analysis in Transverse Direction. Figure 3. Pushover Curve in Transverse Direction or Bent in the As-is Condition.

4 Design EQ 0.2g (Service Level) 10% in 50 yrs (No Collapse) 10% in 250 yrs (No Collapse) PSA 0.33 g 0.79 g g demand 39 mm (1.54") 58 mm (2.30") 199 mm (7.83") y 37 mm (1.47") 37 mm (1.47") 37 mm (1.47") ultimate 60 mm (2.36") 60 mm (2.36") 60 mm (2.36") µdemand µcapacity Flexure d/c Ratio Shear d/c Ratio 0.65 Essentially Elastic <1.0 elastic 0.97 Marginal 0.57 Acceptable Table 1. Displacement Demands and Demand/Capacity Ratios N.G. or Flexure 5.07 N.G. or Shear The pushover curve o the bent in the as-is condition in the longitudinal direction is shown in Fig. 4. As seen in Fig. 4, the load delection diagram or the longitudinal direction shows the eect o a very lexible ooting associated with a weak rotational spring. This eect allows the column to ride the earthquake with very little plastic action, thus engaging the abutment lateral stiness. This behavior requires the addition o a bumper bracket at the bottom o each o the steel girders to ensure positive engagement o the abutment stiness in the longitudinal direction. Bearings and end diaphragms were checked and ound to be adequate or the load path examined. The largest slenderness actor kl/r was ound to be equal to 58. This ratio is adequate or the seismic action and is considered relatively strong or the same vintage o structures where the kl/r actor is typically substantially larger. Top o Column Delection (in.) Figure 4. Pushover Curve in Longitudinal Direction or Bent in the As-is Condition. SEISMIC REHABILITATION METHODOLOGY Several strategies exist or the seismic rehabilitation o older RC bridges, such as the State Street Bridge. The irst strategy would be to perorm steel encasing on all columns or both shear and lexure. This would be accompanied by a bent cap retroit using conventional methods such as concrete acketing accompanied by thru bolts to increase coninement, ater the columns had been acketed with steel

5 casings. This strategy was not preerred because o concern or the long time required or construction o the concrete acketing o the bent cap. A second strategy would be the use o in-ill walls between each column at both bents. However, this strategy was also reected, since it does not take care o the longitudinal inadequacies present in the columns. As an alternative to steel casings or the columns, the use o several dierent types Fiber Reinorced Polymer (FRP) casings or wraps, one o which is CFRP composite ackets was considered. Currently, Caltrans policy does not permit the use o FRP wraps to seismically retroit a bent cap (Caltrans 1997). Based on extensive in-situ testing results carried out by the University o Utah (Pantelides et al. 1999, 2001), the CFRP composite retroit design, including retroit o the bent cap and bent cap-column oints was adopted since it caused minimum disruption and could be completed in the shortest time. Other rehabilitation measures were also implemented. In order to reduce uture maintenance requirements, the deck was made continuous over the expansion oints. The abutments o the bridge were o the seat type; in order to engage the abutment lateral stiness, bumper brackets at the bottom o each o the steel girders were used to ensure positive engagement o the abutment stiness in the longitudinal direction, as shown in Fig. 5. Figure 5. Bumper Bracket Details. SEISMIC REHABILITATION OF COLUMNS The goal o the seismic rehabilitation was to improve the displacement ductility o the bridge bents and thus reduce the earthquake-induced orces on the bridge. To address the inadequacies ound in the existing columns, the seismic rehabilitation design o the columns using CFRP composites considered the ollowing elements: (a) lexural plastic hinge coninement o the columns, (b) lap splice clamping at the bottom o the columns, and (c) shear strengthening o the columns; these are described briely in what ollows. A more detailed description can be ound in the related report (Pantelides et al. 1999). Flexural Plastic Hinge Coninement or Columns To conine the plastic hinge region, the composite layout was designed as a circular acket using the appropriate CFRP acket thickness. The thickness o the composite layers was calculated by the expression proposed by Seible et al. (1997) as:

6 t 0.09D ( ε cu 0.004) = φ uε u ' cc [1] where ε cu = required ultimate concrete strain which was taken as based on the required ductility increase; this was targeted to a displacement ductility o 5, rom a displacement ductility o approximately 2.9 or the as-is bent; cc =compressive strength o conined concrete assumed as 1.5 times the compressive strength o the concrete; u =ultimate composite strength evaluated according to ASTM D-3039 speciications as 630 MPa; ε u =ultimate composite strain evaluated using ASTM D-3039 as 0.01; and φ = lexural capacity reduction actor taken as 0.9. It should be noted that the material assumed in this application was 48,000 ibers per tow unidirectional carbon iber. The required CFRP composite thickness calculated rom equation [1] is 3.6 mm. Column Lap Splice Clamping The thickness o the composite required or clamping the lap splice region is determined rom the dierence between the lateral clamping pressure required to maintain the bond, and the contribution o the steel hoops. The lateral clamping pressure can be deined as: l = p 2n As sy + 2( d b + cc) L s where A s =area o one longitudinal column reinorcing bar (645mm 2 ); sy =yield strength o the longitudinal column bars; p=inside crack perimeter along the longitudinal lap-spliced bars (2.294m); n=number o longitudinal bars (i.e. 12); d b =diameter o longitudinal bars (29mm); cc=concrete cover to the longitudinal bars (63.5mm); and L s =lap splice length (914mm). Equation [2] gives l =765 kpa. The contribution o the reinorcing steel ties to the clamping orce was calculated as: [2] h 0.002Ah E = Ds h [3] where A h =area o the transverse ties (129mm 2 ); E h =elastic modulus o ties (200 GPa); D=column diameter (914mm); and s=spacing o ties (305mm). The resulting stress was ound as h =186 kpa. Based on the values calculated using equation [2] and [3], the thickness o the composite to clamp the lap splice region is calculated by the expression proposed by Seible et al. (1997) as: t 500D ( = E l h ) [4] where the only unknown is E, the modulus o the composite acket, which was determined experimentally as 65 GPa. The required CFRP composite thickness was ound to be 4.1 mm. Outside the plastic hinge region, assuming a minimum coninement pressure o 1,034 KPa yields a CFRP composite thickness o 1.8 mm. Shear Strengthening o the Columns In order to design the composite acket or shear, irst, each o the shear resisting components were evaluated and then subtracted rom the design shear. The design shear was estimated as 1.5 times the column shear at yielding. No shear strengthening was necessary outside the plastic hinge region. The thickness o the composite acket inside the plastic hinge region was calculated as (Seible et al. 1997):

7 t V0 159 φv = ( V E c + V D s + V p ) [5] where V 0 =design shear estimated at 472 kn; V c =shear contribution o concrete equal to 116 kn; V s =shear contribution o ties equal to 138 kn; V p =eect o axial load taken as 18 kn; D=width o the column (914mm); and φ v = shear strength reduction actor equal to The required thickness was calculated as 1.0mm inside the plastic hinge region. The pushover analysis or the rehabilitated bent with CFRP ackets applied on the columns only, is shown in Fig. 6. The global displacement ductility was ound as µ = 5, corresponding to a displacement o 305mm. Using the above thickness requirements, the CFRP acket design or the columns is shown in Fig. 7, where n = number o layers; one layer is 1.32 mm thick, with the assumed CFRP composite properties used in the design. Figure 6. Pushover Curve in Transverse Direction o Rehabilitated Bent. REHABILITATION OF BENT CAP Visual inspection o the bridge had revealed that there was delamination o the concrete cover at the bent cap. For a CFRP composite retroit design to be successul, it is very important that the delaminated concrete be removed, and be replaced by shotcrete to achieve a good bond. The design o the CFRP composite or the bent cap was based on the analysis o the bent with the CFRP composite applied at the columns, as calculated above. Flexural Strengthening o the Bent Cap At a lateral displacement o the bent o 305mm, which corresponds to a displacement ductility o 5, the bent cap yields in the positive region, in elements 3 and 8 (see Fig. 2). The positive moment capacity o the reinorced concrete bent cap is less than the demand by an amount equal to 418 kn-m at a lateral displacement o 305mm. Thereore, lexural strengthening o the bent cap using CFRP reinorcement is required. This reinorcement is to be applied at the bottom o the bent cap, and the carbon ibers are parallel to the axis o the beam. Assuming a width o CFRP composite w =914 mm, the ollowing thickness is required:

8 t T = [6] ε Ε w where T=required tension orce in the composite (783 kn), E =65 GPa, and ε =tensile strain that can be developed in the composite which is assumed conservatively as or one-hal o the ultimate tensile strain o the CFRP composite. The required CFRP composite thickness is 2.6 mm. n=3 n=4 n=4 n=3 n=3 n=4 n=4 n= m 914 n=4 n=7 n=5 914 n=4 n=7 n=5 51 Figure 7. CFRP Composite Design or Columns, Bent Cap, and Joint Ankle Wrap. Shear Strengthening o the Bent Cap The concrete shear capacity in the bent cap is calculated as V c =300 kn. The contribution o the stirrups comes rom the 2 interior legs and ½ o the exterior leg on each side, or a total o 3 legs. The reason or not counting or the total area o the exterior stirrups is electrochemical corrosion, which was evident rom inspection o the bridge. The shear capacity due to the stirrups is V s =560 kn, or a total shear capacity o 860 kn. The demand is taken as 1.5 the shear at yield or the shear at the ultimate displacement o 305mm. The latter is equal to 1,125 kn at node 10 o element number 9 (see Fig. 2.). The thickness o the composite is calculated similar to equation [5] ignoring the axial component. The resulting CFRP acket or both the haunches and the region within the column supports is 0.8 mm thick. Flexural Plastic Hinge Coninement or the Bent Cap Since yielding occurred in the bent cap, plastic hinge coninement is considered. Equation [1] is applied, with an ultimate concrete strain=0.0078, which was calculated at a lateral displacement o 305mm. In addition, since the bent cap has a rectangular cross-section, an equivalent diameter o 1.509m is used. The resulting CFRP composite thickness o the 90-degree hoops is calculated as 4.1 mm. It was recommended that the above thickness be provided in the positive moment zone or a width o at least 914 mm, as well as in the negative moment zone or a width o 914 mm. Using the above thickness requirements, the design shown in Fig. 7 was obtained or the hoop CFRP reinorcement in the 90-degree direction. For lexure, only the bottom o the beam should be reinorced with the 914mm-wide CFRP composite sheets, at a thickness o 2.6mm, or two layers. The negative moment regions do not require lexural strengthening.

9 REHABILITATION OF BENT CAP-COLUMN JOINTS The design o the CFRP composite or the beam-column oints was based on the analysis o the bent with the CFRP composite applied at the columns, as well as the bent cap as calculated above. Shear Strengthening o the Bent Cap and Column Joint In order to design the thickness o the composite in the oint region, the oint shear orces had to be evaluated. This was achieved by modeling the rehabilitated bent with CFRP composites, using DRAIN- 2DX. The conined properties o the concrete were determined using the procedure developed by the irst author and his co-workers (Gergely et al. 1998, Moran and Pantelides 2001). The peak lateral load was increased by 38% compared to the as-built condition, which resulted in higher oint shear orces as ollows: (a) the horizontal oint shear is 1,760 kn, and the horizontal oint shear stress is 1,903 kpa; (b) the vertical oint shear is 1,530 kn, and the vertical oint shear stress is 1,730 kpa; and (c) the oint compression stress resulting rom the axial orce is not included since it renders the design less conservative. The resulting principal stresses are: (a) tension σ 2 =1,820 kpa, and (b) compression σ 1 =1,820 kpa. To maximize the contribution o the composite layers, the orientation o the ibers is ± 45 0 rom the longitudinal axis o the bent cap. The demand or the oint principal tensile stress was increased by σ i =440 kpa, rom 1,380 kpa (value or the as-built bent calculated in a similar manner) to 1,820 kpa or the retroitted bent. The cracking capacity o the concrete is a tensile stress o 1,565 kpa. Thereore, in this case the incremental stress controls since it is only σ ii =255 kpa. To ind the number o composite layers inclined at 45 0 required to provide the higher shear capacity, a diagonal tension crack in the oint region is analyzed. The orce acting normal to the crack is the orce resisted by one composite layer stressed in tension, in the iber direction. The magnitude o this orce (F 2 ) can be calculated as: F 2 = tε E d e cosθ p where θ p =angle between the longitudinal axis o the member and the optimal iber direction (45 ); t=thickness o CFRP sheets; ε =average axial strain in the iber direction at peak horizontal load (0.002 as veriied by strain gage measurements on the composite during in-situ testing o a rehabilitated bridge bent (Pantelides et al. 2001); E =elastic modulus o CFRP; and d e =eective oint depth which is the height o the oint minus twice the eective bond length o the composite sheets to the concrete; rom previous studies it is approximately 76 mm. The oint eective depth is equal to 914 mm, and the resulting orce F 2 was ound as 222 kn. To ind the tensile stress in one composite layer, the value o the orce F 2 is divided by the width o the oint (b=1,067 mm) and by the inclined length (along the crack) bordered by the eective depth as: [7] σ = F 2 cosθ bd e p [8] This calculation yields σ =158 kpa. Enough layers, each o a tensile stress capacity equal to σ, have to be used in order to resist the σ ii =255 kpa stress increase rom the as built to the rehabilitated bent. Thereore, the total number o layers required is given by: σ n = σ [9] which yields 1.6 layers o 1.32 mm per layer thickness, or a total thickness o 2.2 mm. In order to have a symmetric composite acket around the oint, 2 layers o unidirectional abric material must be applied. These layers must be provided in both directions to take into account the cyclic nature o the earthquake

10 loads; this orms the ankle-wrap scheme shown in Fig. 7. A 51 mm gap is let between the column and the pile cap, and the column and the bent cap, to avoid any strength and stiness increase rom the retroit. In addition, the corners o the bent cap must be rounded to 51 mm to provide better anchorage. The sequence o application o the composite in the bent cap and beam-column oints is as ollows: (1) apply the CFRP composite in the diagonal direction at the oint at angle o ± 45 0 ; (2) apply the lexural CFRP reinorcement at the bottom o the bent cap in the positive moment regions (within the columns); and (c) apply the CFRP composite in the hoop direction at 90 deg with respect to the bent cap axis. U-strap In order to improve the tensile bond o the longitudinal column bars ending in the beam cap, a U-strap CFRP composite scheme is implemented. The thickness o the CFRP composite is determined using equation (6). The tensile orce in the column o the bent rehabilitated with CFRP composites, was T=1,230 kn. The eective width o the composite strap on each side o the semi-circular shape was assumed as 710 mm. The eective strain was assumed as 50% o the CFRP ultimate strain, i.e to avoid premature tensile ailure. This leads to a thickness o two layers as shown in Fig. 8. The straps were brought down 305 mm rom the bottom o the bent cap to avoid stress concentration eects; in addition, one CFRP composite layer around the column is needed or clamping the strap on the column. The gap let between the strap, the bent cap and the column must be illed with oam. The U-strap is the inal step in the CFRP composite retroit and is to be applied last; the inal design is shown in Fig A m BEAM CAP m 51 mm 635 n=1 A φ =0.914 m 3.963m COLUMN m "NEW" CONCRETE m BEAM CAP m FOAM n=1 φ=0.914 m COLUMN SECTION A-A Figure 8. CFRP Composite Design or Bent Cap U-Strap.

11 CAPACITY VS. DEMAND The rehabilitated and as-built bents are evaluated with respect to the ollowing design earthquake levels: (1) the 0.2g service level earthquake, (2) the 10% probability o exceedance in 50 years earthquake, and (3) the 10% probability o exceedance in 250 years earthquake. As-Is Bent For the as-is bent, the period was calculated as T= 0.86 sec. The displacement ductility was calculated as µ=2.9, as shown in Fig. 3; the orce reduction actor (FHWA 1995) is R F =2.27; assuming an equal energy approach, the inelastic displacement can be obtained by multiplying the elastic spectral displacement by a ductility related expression leading to an ampliication actor, R=1.32. For the three design earthquake levels, the ollowing applies: (1) 0.2g service level earthquake: The peak spectral acceleration can be ound rom the design spectra as 0.27g. The corresponding lateral orce demand is F d =1,272 kn. The reduced orce demand is F r =560 kn. The capacity is larger than the demand, and this design level is satisied. As ar as displacement, the elastic demand displacement can be approximated with the irst mode spectral displacement as 50mm, which when ampliied is 66mm, which is less than the capacity o the as-is bent o 103mm. (2) 10% in 50 years earthquake: The peak spectral acceleration can be ound rom the design spectra as 0.41g. The corresponding lateral orce demand is F d =1,935kN. The reduced orce demand is F r =852 kn. Since the capacity is higher than the demand, this design level is satisied. The elastic demand displacement is 75mm in this case, and when ampliied it is 99mm, which is less than the capacity o the as-is bent o 103mm. (3) 10% in 250 years earthquake: The peak spectral acceleration can be ound rom the design spectra as 1.42g. The corresponding lateral orce demand is F d =6,694 kn. The reduced orce demand is F r =2,949 kn. Since the capacity rom Fig. 3 is 1,352 kn, this design level is not satisied. The elastic demand displacement is 261mm, which when ampliied or inelastic eects becomes 344mm, which is more than the capacity o the as-is bent o 103mm. Thereore, it is expected that the as-is bent can perorm with minor damage or the 0.2g event, it will sustain severe damage but will carry the gravity load and be unctional or the 10% in 50 years event, but will be severely damaged and probably not be able to carry the gravity load in the 10% in 250 years event. Rehabilitated Bent For the bent rehabilitated with the FRP composite, the period was calculated as T=0.91 sec. The usable ductility was calculated rom Fig. 6 as µ=5, R F =3.68, and R=1.67. For the three design earthquake levels, the ollowing applies: (1) 0.2g service level earthquake: The peak spectral acceleration is 0.26 g. The corresponding lateral orce demand is F d =1,228 kn, and the reduced orce demand is F r =334 kn. The capacity rom Fig. 6 is larger, and this design level is satisied. The elastic demand displacement is 53mm, which when ampliied is 89mm, which is less than the capacity o the rehabilitated bent o 305mm. (2) 10% in 50 years earthquake: The peak spectral acceleration is 0.39g. The corresponding lateral orce demand is F d =1,837 kn, and the reduced orce demand is F r =499 kn. Since the capacity is greater than 419 kn, this design level is satisied. The elastic demand displacement is 80mm, which when ampliied is 134mm, which is less than 305mm. (3) 10% in 250 years earthquake: The peak spectral acceleration can be ound rom the design spectra as 1.35g. The corresponding lateral orce demand is F d =6,365 kn. The reduced orce demand is F r =1,730 kn. Since the capacity rom Fig. 6 is 1,864 kn, this design level is satisied. The elastic demand displacement is 278mm, when ampliied or inelastic eects it is 464mm, which is more than 305mm. Thereore, it is expected that the rehabilitated bent can perorm with minor damage or the 0.2g event, it will sustain moderate but repairable damage and will carry the gravity load and be unctional or the 10% in 50 years event, and will also sustain severe damage but be unctional and carry the gravity load or the 10% in 250 years event.

12 CONCLUSIONS The design o the seismic retroit o State Street Bridge has been presented. The elements o the design include the columns, bent cap and oints o the bridge rehabilitated with CFRP composites. Additional elements included the addition o bumper brackets at the ends o the steel girders, and making the deck continuous over the expansion oints. An evaluation or the design earthquakes o the as-is and rehabilitated bridge bents has shown that the rehabilitated bent can be assumed unctional in all three design earthquakes, whereas the as-is bent will not survive the 10% in 250 years event. In addition, the as-is bent will be unctional ater the 10% in 50 years event, but will sustain severe damage and may have to be replaced. The rehabilitated bent is expected to survive the 10% in 50 years event with minor damage. Both the as-is and the rehabilitated are expected to survive the 0.2g service level earthquake event with minor damage. ACKNOWLEDGEMENTS This study was unded by the Federal Highway Administration and the Utah Department o Transportation. The authors would like to thank the ollowing individuals or their support and encouragement throughout this proect: Mr. Doug Anderson and Mr. Sam Musser, o the Research Division o the Utah Department o Transportation, Proessor Steve Bartlett o the University o Utah, and Mr. William Gedris o the Federal Highway Administration. REFERENCES Caliornia Department o Transportation, (1997), Memos to Designers 20-4, Attachment B, Sacramento, Caliornia. Federal Highway Administration, (1995), Seismic Retroitting Manual or Highway Bridges, U.S. Dept. o Transportation, Publ. No. FHWA-RD , McLean, Virginia. Gamble, W.L. & Hawkins, N.M., (1996), Seismic retroitting o bridge pier columns, Proc. Struct. Congress XIV, Chicago, Illinois, ASCE, Vol. 1, Gergely, I., Pantelides, C.P., Nuismer, R.J. & Reaveley, L.D., (1998), Bridge Pier Retroit Using Fiber- Reinorced Composites, J. o Composites or Construction, ASCE, 2(4), Gergely, I., Pantelides, C.P. & Reaveley, L.D., (2000), Shear Strengthening o RC T-Joints using CFRP Composites, J. o Composites or Construction, ASCE, 4(2), Mahan, M., (1998), X-Section and W-Frame Programs, Version 2.30, Caliornia Department o Transportation, Sacramento, Caliornia. Moran, D.A. & Pantelides, C.P., (2001), An analytical stress-strain model or FRP conined concrete, J. o Structural Engineering, ASCE, Under Review. Pantelides, C.P., Okahashi, Y. & Moran, D.A., (1999), Seismic rehabilitation o State Street Bridge, Technical Report UUCVEEN 99-02, Dept. o Civil & Env. Eng., Univ. o Utah, 55 pg. Pantelides, C.P., Gergely, J., Reaveley, L.D. & Volnyy, V.A., (1999), Retroit o RC bridge pier with CFRP advanced composites, J. o Structural Engineering, ASCE, 125(10), Pantelides, C.P., Gergely, J., & Reaveley, L.D., (2001), In-situ veriication o rehabilitation and repair o reinorced concrete bridge bents under simulated seismic loads, Earthquake Spectra, (Accepted). Prakash, V., Powell, G.H. & Filippou, F.C., (1992), DRAIN-2DX Base Program User Guide, Rep. No. UCB/SEMM-92/29, Univ. o Caliornia, Berkeley, Caliornia. Priestley, M.J.N., Seible, F. & Calvi, G.M., (1996), Seismic Design and Retroit o Bridges, John Wiley & Sons, Inc., New York. Seible, F., Priestley, M.J.N., Hegemier, G. & Innamorato, D., (1997), Seismic retroitting o RC columns with continuous carbon iber ackets, J. o Composites or Construction, ASCE, 1(2),