Evaluation of a High Performance Box

Andreas Greuel T. Michael Baseheart, Ph. D. Graduate Research Assistant Associate Professor of Civil University of Cincinnati Engineering Cincinnati, Ohio University of Cincinnati Cincinnati, Ohio

Bradley T. Rogers Engineer LJB, Inc. As part of the FHWA (Federal Highway Admin- Dayton, Ohio istration) High Performance Concrete Bridge Program, two full-scale truckload tests of Bridge GUE-22-6.57 were carried out. The main ob- jectives of these tests were to investigate the static and dynamic response of the high perfor- Richard A. Miller, Ph. D. mance concrete (HPC) structure. A secondary Associate Professor of Civil Engineering objective was to investigate the load transfer University of Cincinnati between the box through experimental Cincinnati, Ohio middepth shear keys. The structure was loaded using standard Ohio Department of Transporta- tion (ODOT) dump trucks. A model test of the bridge was conducted as well. It was found that the bridge behavior is well predicted using sim- ple models. The bridge behaves as a single unit and all girders share the load almost equally. Bahram M. Shahrooz, Ph. D. The dynamic behavior of the bridge is typical Associate Professor of Civil for comparable structures. Engineering University of Cincinnati Cincinnati, Ohio

60 PCI JOURNAL he use of high performance con- located on US Route 22, a heavily in that the Ohio has only a crete (HPC) can lead to more traveled two-lane highway near Cam- 5 in. (127 mm ) thick bottom flange Teconomical bridge designs be- bridge, Ohio. rather than the 5.5 in. (140 mm) flange cause the designer can often eliminate The new bridge replaced a 70 ft used in the AASHTO box. As a result, girder lines, use shallower sections or (21.4 m) single span, steel stringer the Ohio box can accommodate only extend the span of a section and elimi- bridge with a concrete deck. The hy- a single, full layer of 23 strands in the nate the need for intermediate supports. draulic requirements required the span bottom flange and a partial layer of While the use of HPC can lead to lower of the new bridge to be increased to four strands and several layers of two initial costs through elimination of piers 115.5 ft (35.2 m ). Note that the bridge strands in the webs. or girder lines, the high durability of was originally designed as a three- If the bridge were designed using HPC can also lead to lower long-term span, noncomposite, adjacent box the largest Ohio box (B42-48, see costs because of reduced maintenance girder system using normal strength Fig. 3), standard 5500 psi (39 MPa) and a longer bridge life. concrete [5500 psi (35 MPa)] and 0.5 concrete and 0.5 in. (13 mm) diam- Despite the above advantages, as de- in. (13 mm) diameter strands [see Fig. eter strands, the maximum span of signers use longer and shallower spans 1(b)]. the girder would be 110 ft (33.5 m). with fewer girders, the structures become Normally, a span of 115.5 ft (35.2 m) If the steel area is increased by using more flexible. This leads one to ques- would not be too long for a box girder. 0.6 in. (15 mm) diameter strand, the tion whether the structures will deflect However, the Ohio box girder (see Fig. girder could easily span the required or vibrate excessively or if the greater 3) differs from the AASHTO standard length. However, the larger strand flexibility will affect the load distribution between adjacent box girders. In 1998, the Ohio Department of Fig. 1(a). Transportation (ODOT) constructed HPC single-span design. a HPC, adjacent box . In this bridge, a preliminary design for a three-span bridge was converted to a single-span design [see Figs. 1(a) and 1(b)]. Prior to construction, research- ers at the University of Cincinnati (UC) formulated the high performance concrete mixes which would be used and conducted several tests on proto- type girders. The prototype test results showed the simple span box girders to be suf- Fig. 1(b). ficiently strong and ductile. These tests Original, three-span confirmed that girder behavior was design. predictable using linear elastic theory prior to cracking and a strain compat- ibility approach for the ultimate be- havior. Satisfied with prototype girder performance, ODOT let the contract for the bridge construction, yet, there were still questions as to how the ac- tual bridge would behave. To try to answer these questions, the UC research team tested the bridge during and after construction. This paper summarizes the result of destruc- tive and nondestructive testing of pro- totype girders and of nondestructive testing the completed bridge structure.

BRIDGE DESCRIPTION Bridge GUE-22-6.57 is a 115.5 ft (35.2 m) long prestressed, noncompos- ite, adjacent box girder bridge [see Fig. 1(a)]. The cross section of the bridge is shown in Fig. 2. The structure is Fig. 2. Cross section of HPC bridge.

November-December 2000 61 This bridge utilizes an experimental shear key at middepth of the cross section. This shear key configuration was found to be less susceptible to cracking.1 After tying adjacent girders together with non-prestressed threaded rods located transversely through dia- phragms at the ends and quarter points of the bridge (see Fig. 4), the shear keys are grouted. Note that in this configuration, only the shear key itself was to be grouted. The area above the middepth shear key was filled with sand and a sealant was applied to the top of the joint to further guard against leakage (see Fig. 5). Fig. 3. Ohio B42-48 section. MATERIAL PROPERTIES The first task for the UC research generates a greater prestressing force water from the bridge surface to leak team was developing a mix design and, therefore, high strength concrete between the sides of adjacent box which would produce extremely du- is required. girders. rable concrete that had high release Another reason for using HPC is Leakage can cause serious damage and ultimate strengths. To make the that it has a high durability. Adja- to the tendons and reinforcement when girder span the required 115.5 ft (35.2 cent box girders have shear keys cast water and deicing chemicals penetrate m), it was determined that the concrete between the girders to transfer shear the concrete. Leakage is a major prob- would need a minimum compressive between adjacent girders. Shear keys lem with adjacent box girder , strength of 10 ksi (70 MPa ). For dura- of adjacent box girder bridges tend so HPC with its high durability is an bility, a rapid chloride permeability of to crack, and subsequently, allow ideal choice for this type of structure. less than 1000 coulombs was desired. The mix was designed using the ma- terials which the precaster had avail- able. Normally, Type I cement is used for HPC, but the precaster usually used Type III and it was not economically desirable to change to Type I. To improve durability and strength, a water to cementitious material ratio (w/c+p) of less than 0.3 was chosen and microsilica was added to the mix. The precaster did not have a silo avail- able to store the microsilica, so it was batched from bags. To avoid having to weigh the microsilica separately, the mix was designed using single bag increments of 25 lb (11.3 kg). This is why there is an unusual per- centage (11.8 percent) for the micro- silica. Because of the low w/c+p ratio and presence of microsilica, a water reducer was required both to provide enough workability and to defloculate the cement particles so the microsilica would be able to fit in between them and densify the mix. The fine aggregate was natural river sand. A No. 8, partially crushed, river gravel (3/8 in. or 10 mm max.) was used as the coarse aggregate. Be- Fig. 4. Installing tie rods. cause the aggregate was only partially

62 PCI JOURNAL Fig. 5. strain gauges were placed between the Middepth shear key strands at approximately 1 ft (3 m) and tie rods. intervals from each end of the girder prior to casting. After the girder had cured and the strands were cut, the measured strain was used to determine that somewhere between 35 in. and 48 in. (0.89 and 1.2 m) the transfer was complete. This means that the transfer length was between 60D and 80D (where D is the strand diameter). The AASHTO crushed, the aggregate/paste bond did the research team could verify the be- Standard Specifications2 use a transfer not appear to be particularly good and havior of the girders. An in-depth dis- length of 50D, the AASHTO LRFD this appeared to limit the concrete cussion of the prototype fabrication Specifications5 use a transfer length of strength. Making the specified strength and destructive testing can be found in 60D while a transfer length of 80D has required a high cement content. The a previous work.4 For completeness, a been suggested in the literature.6 specified strength could have been summary is presented here. The prototype girders were subjected obtained with lower cement contents One area explored was the trans- to destructive testing. Each girder was and/or higher w/c+p ratios with the fer length for 0.6 in. (15 mm) diam- supported on neoprene pads such that use of a better aggregate (e.g., crushed eter strand when used with HPC. To the test span was 115.5 ft (35.2 m) and limestone). measure transfer length, vibrating wire loaded with two-point loads placed Thirteen trial mixes were investi- gated.2 The final mix proportions are shown in Table 1. During fabrication Table 1. Concrete mix proportions. of the prototype and final bridge gird- Material Lb per cu yd kg/m3 ers, test cylinders and modulus of rup- ture beams were cast and later tested Cement 846 497 to determine the concrete properties. Table 2 provides a list of the measured Microsilica 100 58.7 concrete properties. Water 262 145

GIRDER PROPERTIES Fine aggregate 927 544

The girders are standard ODOT Coarse aggregate 1774 1042 B42-48 box girders (see Fig. 3) and were designed using the provisions Air 6 percent 6 percent of the AASHTO Standard Specifica- 3 tions. The ODOT require bridges to Admixtures Ounces per cu yd l/m3 be designed for an HS-20 loading, but this bridge was designed using Air entrainer 21 0.81 the HS-25 loading to allow for future Retarder 28 1.08 increases in traffic loads. Because of the experimental aspect High range water reducer 203 7.84 of the bridge, the engineer added a few extra strands for additional safety. Thirty 0.6 in. (15 mm) diameter Table 2. Average concrete properties. strands were used in the bottom flange with two additional strands placed in Compressive strength at release* 8 ksi (56 MPa) the top flange to control tension at Compressive strength at 56 days‡ 11.8 ksi (80 MPa) release. Modulus of elasticity at 56 days‡ 5800 ksi (40000 MPa) Modulus of rupture at 56 days§ 1250 psi (8.6 MPa) Split strength at 56 days† 620 psi (4.3 MPa) DESTRUCTIVE TESTING OF Shrinkage after one year§ 0.1 percent PROTOTYPE GIRDERS Creep coefficient after one year** 2.1 Prior to the fabrication of the actual Rapid chloride permeability at 56 days 360 coulumbs bridge girders, two prototype box gird- * 6 x 12 in. cylinders - bed cured. ers were cast and tested. This was done † 6 x 12 in. cylinders - moist cured. ‡ 6 x 6 x 14 in. beams - moist cured. so that the precaster could gain more § 3 x 3 x 11.25 in. prisms - 28 days moist cure experience in placing HPC and so that ** 6 x 12 in. cylinders - test started at 1 day (i.e., at release of prestressing force)

November-December 2000 63 Fig. 6. Testing the prototype girder.

47.25 ft (14.4 m) from each support higher modulus of rupture. If the mea- The test was stopped at this point for (see Fig. 6). Loads were applied such sured modulus of rupture of 1250 psi safety reasons. that the deflection of the girder under (8.6 MPa) is used, the calculated Fig. 7 shows a load versus midspan the load points increased in 0.05 in. cracking moment becomes 3525 kip-ft deflection graph for one of the pro- (1.3 mm) increments. (4780 kN-m), which is very close to totype girders. Also plotted is the be- The first cracks in both girders ap- the measured value. havior predicted using a computer pro- peared when the applied load at each The use of the higher modulus of gram called RESPONSE.7 For a given point was approximately 42 kips (187 rupture is important since the provi- moment, RESPONSE generates cur- kN). Counting self weight, the total sions of the AASHTO Standard Speci- vature and strain values for the cross cracking moment was 3570 kip-ft fications require that φ Mn > 1.2 Mcr, section by strain compatibility. (4840 kN-m). This value was larger where Mn is the nominal moment, Mcr Deflections are found by first choos- than the 3155 kip-ft (4280 kN-m) is the cracking moment and φ is the ing an applied load level, plotting the cracking moment predicted using the capacity reduction factor. If Mcr is un- moment diagram for the total moment provisions of the AASHTO Standard derestimated, the design may not be (dead load plus applied load), and then Specifications. conservative. using the result of this analysis to plot Note that in calculating the crack- The girders were loaded to approxi- a curvature diagram. The curvature ing moment, a loss of prestressing mately 80 kips (356 kN) at each load can then be double integrated to find force of 20 percent was assumed. This point. Again, accounting for self weight, the deflection. loss value was calculated from the the applied moment was 5320 kip-ft Load versus strain behavior is avail- provisions of the AASHTO Standard (7220 kN-m). This exceeded the ulti- able directly from the RESPONSE Specifications. mate moment of 5130 kip-ft (6960 kN- results. Clearly, the girder deflection One reason for the difference be- m), calculated using the provisions of behavior is well predicted. A graph of tween the calculated and measured the AASHTO Standard Specifications. load versus extreme fiber compressive values of the cracking moment is that Under this load, the girder had a strain (see Fig. 8) also shows good the modulus of rupture was taken as very large deflection of 24 in. (61 m) agreement between the analysis and 7.5 (in psi units), or 750 psi (5.3 at the midspan. This corresponds to a experiments up to a strain of 0.002 MPa). However, HPC has a deflection of L/58, where L is the span. (where the test was stopped).

Fig. 7. Load versus deflection for prototype girder. Fig. 8. Load versus top compressive strain for prototype girder.

64 PCI JOURNAL Measured losses of prestressing Table 3. Loss of prestressing force – prototype girder. force were determined experimentally Girder 1 Girder 2 PCI AASHTO on the prototype girders. The girders were loaded to cracking and then un- Crack 1 Crack 2 Crack 1 loaded. When unloaded, the prestress- ing force causes the cracks to close. 17 percent 18 percent 17.5 percent 18 percent 20 percent Clip gauges were placed across the closed cracks so that the crack open- ing could be measured when the girder lems occurred during the fabrication initial prestressing force. The calcu- was reloaded. of the girders and only minor adjust- lated loss due to elastic shortening was When the girder is reloaded, the ten- ments to the usual fabrication process approximately 6 percent. sile stress from the applied moment were needed to accommodate the use relieves the compressive stress caused of HPC. Most of these accommoda- INSTRUMENTATION by the prestressing force and eventu- tions centered around the fact that ally overcomes it, causing the crack to HPC tends to be sticky, does not move OF THE GIRDERS open. Theoretically, the load at which well under vibration and has a high During the fabrication of the actual the crack just begins to open (as mea- rate of slump loss. The workers sim- box girders for the bridge, a total of sured by the clip gauges) corresponds ply needed to use greater care in the 69 internal sensors were embedded. A to a zero stress state. Knowing the method and rate of placement to pre- total of 57 vibrating wire strain gauges applied moment which causes a state vent cold joints or voids. and 12 bonded foil strain gauges were of zero stress at a known point on the Girder camber was measured at re- embedded inside the girders. All the girder, it is possible to back-calculate lease and at various times while the internal sensors were installed at mid- the effective prestressing force. girders were in storage. At release, span. The measured values are shown in the girders showed a camber of about Vibrating wire strain gauges were Table 3. Calculated values of the loss 1/8 in. (3 mm), but growth in storage used to measure static truckload strains of prestressing force, from two dif- began immediately. Before the girders but are not capable of measuring dy- ferent methods, are also presented. were shipped to the construction site, namic events. In addition, temperature One method of calculating the loss they had cambered up an average of changes during curing and afterwards of prestress is that given in the PCI 1.5 in. (38 mm). This behavior is in could be measured by thermistors in Design Handbook8 and the other is the reasonable agreement with a predicted the vibrating wire gauges. The foil method given in the AASHTO Stan- value of 1.66 in. (42 mm), which was strain gauges measure instantaneous dard Specifications.2 calculated using elastic theory and ac- strains under traffic loading and can The calculated values of loss of pre- counting for time-dependent effects by capture dynamic responses from mov- stressing force are final values, which using the multipliers given in the PCI ing trucks. assume a large amount of time has Design Handbook. Depending on the girder location, the passed and all of the creep, shrink- Embedded strain gauges (see next number and layout of the embedded age and relaxation of the strands has section on Instrumentation) were read strain gauges were varied. This scheme occurred. The prototype girders were before and after detensioning and sev- allowed a reasonable and adequate only 6 to 9 months old at the time eral times during girder storage. Using number of sensors. All the girders had a of testing, so an exact comparison these readings, the strain change in single foil strain gauge near the middle cannot be made. However, at 6 to 9 concrete at strand level was calculated of the bottom flange. As a minimum, months, approximately 80 percent of to be 580 microstrain at transfer. Thus, two vibrating wire strain gauges were the time-dependent deformations have the loss at detensioning due to elastic used in each girder – one in the top occurred. With this in mind, the data shortening becomes 8.8 percent of the flange and one in the bottom flange. in Table 3 suggest that the calculated values of loss of prestressing force are reasonable. The results of the prototype testing clearly showed that the girders were sufficiently strong and predictable. With the prototype girder testing com- plete, the girders for the actual bridge were fabricated.

FABRICATION OF THE BRIDGE GIRDERS The actual bridge girders were fab- ricated at Prestress Services, Mel- bourne, Kentucky. No major prob- Fig. 9. Girder numbering for Phase I and Phase II.

November-December 2000 65 Additional sensors were installed gauges to a data acquisition system at removed and the channel in this area in some of the girders to measure the the bridge site was coiled and placed was widened. One-half of each new distribution of strains and temperature inside electrical boxes, which had been abutment, consisting of concrete filled gradient through the depth and across secured to the reinforcing cage. The tube piles, a pile cap and a beam seat, the width of the girders. Considering end of each electrical wire was then was constructed at each end. that Girders 4 through 9 (see Fig. 9) taken out of the formwork. These elec- The first seven girders (see Figs. will resist most of the traffic loads, trical wire ends were then connected 2 and 9) were delivered and set on these girders were instrumented more to a data acquisition system. neoprene bearing pads. After the heavily than the other girders. The instruments were checked dur- threaded rods were installed, the shear The vibrating wire strain gauges in ing fabrication to ensure no damage keys were grouted and the longitudinal the bottom flange were placed between had occurred. Later, instruments were joints were sealed. The top surface of the strands and gauges in the web were used to measure the heat of hydra- the Phase I girders was covered with placed between the stirrups. To ensure tion during curing and monitor the a waterproofing membrane and then adequate protection and long-term du- response of the girders during deten- a layer of asphalt. Traffic was then rability, the foil strain gauges were sioning. Prior to shipping the girders, routed to the completed half of the mounted onto auxiliary No. 4 mild the extra electrical cable was cut and new bridge. steel reinforcement with adequate de- the area around the electrical cable on In Phase II, the other half of the old velopment length. These auxiliary bars the top flange was sealed. After the bridge was removed and the channel were gauged in the laboratory. Du- girders were placed, the instruments was widened. After constructing the rable adhesives, requiring oven curing, could be attached to the data acquisi- remaining halves of the two abut- were used. tion system by accessing the wires ments, Girders 8 to 12 (see Figs. 2 Following the strain gauge manufac- through electrical boxes embedded in and 9) were sequentially put in place, turer's recommended procedures, each the bottom of the girders. installing and tightening the threaded gauge was protected from moisture. rods along the way. However, only The strain-gauged bars were tied onto ERECTION OF the shear keys between Girders 8 to the bottom transverse reinforcement. THE BRIDGE 12 were grouted. The shear key at the The electrical wires to the internal construction joint between Girder 7 in sensors were numbered and routed The bridge was erected in two Phase I and Girder 8 in Phase II was to what would be the east end of the phases. In Phase I, the old bridge was not grouted because of the constant girder when the bridge was completed. cut in two sections and traffic was movement of Girder 7 due to the traf- At this location, a sufficient length routed onto one-half of the structure. fic on Phase I half of the bridge. of the electrical wire to connect the The other half of the old bridge was Without the shear key in place, the vertical displacements of Girder 7 due to the traffic load caused spalling on Table 4. Truck dimensions. the bottom edge of Girder 8 where Length between axles (L) Width between tires (W) it contacted Girder 7. This occurred Truck number [center-to-center axles] [center-to-center front tires] because there is a small amount of tension in the transverse rods which is ft m ft m used to pull the girders together during

527 14.58 4.44 6.58 2.00 erection (but is not intended as a trans- verse post-tensioning). 585 14.58 4.44 6.58 2.00 To arrest the spalling until the shear key could be placed, the tension in 624 14.58 4.44 6.58 2.00 threaded rods between Girders 7 and

707 11.50 3.51 7.33 2.24 8 was relieved. Shortly afterwards, while the traffic was rerouted during the night, the threaded rods between Girders 7 and 8 were retightened and Table 5. Truck weights. the shear key grouted with a fast set- Total weight Front axle Rear axle ting magnesium phosphate grout. Truck Once the shear key between the two number lb kN lb kN lb kN phases of the bridge was in place, the

527 29780 132.5 8040 35.8 21740 97.7 spalling stopped. After sealing the construction joint, the remaining half 585 27120 120.6 7120 31.7 20000 89.0 of the bridge was covered with the waterproofing and asphalt. The entire 624 31680 140.9 8300 36.9 23380 104.0 bridge was then opened to traffic.

707 30960 137.8 9300 41.4 21660 96.4 TRUCK LOAD TESTING 66 PCI JOURNAL Two truck load tests were conducted The relative displacement between center distances between the wheels) on the bridge. The first occurred when adjacent girders was measured using were measured for each truck prior to Phase I was complete but before Phase linear variable differential transformer the test (see Tables 4 and 5). I was opened to traffic. The second (LVDT) displacement transducers To examine various responses, dif- test was done after the entire bridge placed at various locations. The rela- ferent truck positions were imple- was complete and had been opened to tive displacement was obtained by at- mented. For each test, the trucks were traffic for about 2 weeks. taching a displacement transducer on placed as closely as possible to pre- To measure the vertical deflection one girder and targeting it to the adja- determined locations marked on the of each girder, 21 wire potentiometer cent girder (see Fig. 9). bridge. For the Phase I test, seven displacement transducers were used. During Phase I, the relative dis- different load cases with one to four During Phase I [with Girders 1 to 7 placements between Girders 2 and 3, trucks placed on the bridge were used. in place (see Fig. 9)], the deflection at 4 and 5 and 6 and 7 at their midspan The different load cases for Phase I are the quarter points and midspans were were measured. During Phase II, the shown in Fig. 10. measured. For the second test with the relative displacement at the construc- Load Cases 1, 2 and 6 were used to bridge completed, only the midspan tion joint between Girders 7 and 8 induce the maximum moments and de- deflections for each girder were re- was measured at the quarter points and flection at midspan. These cases were corded. midspan. also used to investigate the linearity Each sensor was mounted onto a of the structure. In Load Cases 3 and 4, one truck was placed as far as pos- small wooden plate, which had been LOADING CASES screwed to the bottom face of each sible to the edge of the structure to girder. The wires from the wire po- Up to four ODOT trucks filled with investigate the load transfer across the tentiometers were attached to concrete gravel were used to load the bridge. width of the bridge. Load Case 5 was blocks anchored firmly in the creek The weight and dimensions (longitu- intended to look for the load distribu- bed directly under each sensor. dinal as well as transverse center-to- tion between the girders. Load Case 7

Fig. 10. Truck locations for Phase I test.

November-December 2000 67 represents the maximum lane load for bridge with a speed of about 50 miles Although the bridge was designed the structure. per hour (80 km/h) and the deflections for an HS-25 loading (which is 25 per- The test program for the completed of the structure were recorded at time cent larger than the HS-20), the ODOT bridge (Phase II) consisted of six load intervals of 0.05 seconds. standard is HS-20 and this was used as cases (see Fig. 11). Load Cases 1 and the maximum test load. The measured 6 were used to induce the maximum TEST RESULTS deflection is about half of the L/1000 moments and deflections at midspan. (where L is the span length) permitted Load Cases 2 and 3 were maximum Discussed below are the major re- by AASHTO Standard Specifications. lane load in the westbound lane (Case sults regarding deflections, strain dis- It is still clear that the girders have 2) and examined the shear transfer tribution and moving load. adequate stiffness and, therefore, de- across the shear key that joined Phase flection is not a concern regardless of I and Phase II construction. Load Case Deflections whether the HS-20 or HS-25 design 4 represents the maximum possible The maximum deflections for both loading is used. lane load for the eastbound lane. Load the Phase I and Phase II tests, as a A similar argument can be made for Case 5 was intended to look into the fraction of span length, L, are summa- the completed bridge (Phase II). In this distribution of the reactions along the rized in Table 6. For the Phase I tests, case, the maximum deflection occurs west abutment. the maximum deflection occurs for under Load Case 1 (which is similar to In addition, a moving load test was Load Case 6. Using an elastic analy- Load Case 6 in the Phase I test). The done for the completed bridge (no such sis, it was determined that this loading maximum deflection is about 35 per- test was done in Phase I as the bridge would provide stresses and deflections cent of the allowable, so the bridge is was still under construction and a mov- roughly equivalent to having an HS-20 adequate under either loading. ing load test was deemed to be too dan- design truck in each lane with no im- The deflection profiles in the trans- gerous). The trucks traveled over the pact applied. verse direction show that all the gird-

Fig. 11. Truck locations for Phase II test.

68 PCI JOURNAL Table 6. Maximum deflection. Maximum deflection

Load case Phase I test Phase II test

1 L/3310 L/2790

2 L/63409 L/3270

3 L/6610 L/3410

4 L/6550 L/2910

5 L/3410 L/11570

6 L/1760 L/2975

7 L/2070 – ers are working together, and the Fig. 12. Transverse vertical deflection profiles – Phase II test. transverse deflection profiles are very similar to that which would be ex- pected in a slab bridge (see Fig. 12). value which would be computed using girders are essentially behaving linear More uniform load cases show almost a lumped girder model; i.e., treating elastically. uniform deflection while load cases the bridge as a single girder with the Based on the measured strains, the where the trucks are closer to one side moment of inertia equal to the mo- live load tensile strain is about 90 mi- show larger deflections to that side. ment of inertia of a single girder times crostrain. Using the average measured the number of girders. modulus of elasticity, the correspond- Strain Distribution The distribution of the strains for ing live load stress is about 520 psi Fig. 13 shows the strain distribution the girders also supports the previ- (3.6 MPa). For Phase II, the maximum over the depth of the girders for Load ous observation that the girders are stress (again for Load Case 6) was Case 6, Phase I. As shown, the strain working together as a system. The about 60 microstrain and the calcu- is almost identical in all the girders. strains in the top and bottom flanges lated live load stress would be approx- The strain is also very close to the are almost equal in values; hence, the imately 350 psi (2.4 MPa).

Fig. 13. Strain distribution through height of girders, Load Case 6, Phase I test.

November-December 2000 69 The measured relative displacement research team were walking across 2. Loss of prestressing force was between adjacent girders was found the bridge, they could not feel any measured in the prototype girders and to be less than the specified accuracy vibration even when large trucks were found to be about 18 percent at 6 to 9 of the transducers. Clearly, no appre- crossing the bridge. To feel the bridge months. The AASHTO Standard Spec- ciable movement between the girders vibrate, the observer had to stand per- ifications predict a loss of 20 percent was measured. The shear keys and the fectly still at midspan, and even then (calculated at time equals infinity). threaded rods were adequate to ensure the vibration did not seem excessive. 3. The bridge was constructed in two that the girders work as a system. Fig. 15 shows the completed bridge. phases – Phase I where Girders 1 to 7 were erected and Phase II where the Moving Load CONCLUSIONS remaining five girders were added. A To measure the dynamic response, In this project, the design for an ad- truck load test was conducted after the deflections at the midspan of Girder jacent box girder bridge was changed completion of each phase. These tests 7 were measured with ODOT trucks from three spans to a single span. This consisted of using up to four dump traveling across the bridge at about was done by extending the maximum trucks, each weighing approximately 50 miles per hour (80 km/h). The re- length of ODOT’s largest box section 30 kips (133 kN), placed on the bridge sponse is shown in Fig. 14. Based on through the use of HPC and 0.6 in. in different configurations. The test the measured attenuation of the vibra- (15 mm) diameter strands. The per- results showed: tional wave, the damping of the bridge formance of the bridge was verified (a) The bridge did not deflect exces- can be calculated to be between 1.5 through testing of prototype girders sively under load. Maximum static and 2 percent using the logarithmic and truck load testing of the actual deflections were 35 to 50 percent decrement. This is a reasonable value bridge. of that allowed by the AASHTO for this type of structure. The results of the investigation lead Standard Specifications. The dynamic deflection under one to the following conclusions: (b) The maximum live load stress truck is approximately equal to the 1. Testing of the prototype girders (computed from measured strain) value when the bridge was loaded showed that the girders were suffi- under static loading was about statically with one truck. Truck 4 ciently strong and ductile. Cracking 500 psi (3.5 MPa). From elastic followed approximately 20 ft (0.61 strengths were accurately predicted analysis, the stress caused by an m) directly behind Trucks 3 when only if the higher tensile strength of HS-20 truck (without impact) they passed over the bridge. The the HPC was taken into account. Ul- would be 550 psi (3.8 MPa), so deflection is almost doubled, as ex- timate strength was adequately pre- the measured value is in reason- pected. There is no apparent dynamic dicted using the provisions of the able agreement with design cal- amplification. AASHTO Standard Specifications. culations. There was one final interesting Common analytical methods could (c) Under dynamic load, the bridge observation. When members of the predict overall girder behavior. did not vibrate excessively. The

Fig. 14. Dynamic response to moving trucks.

70 PCI JOURNAL Fig. 15. Completed bridge.

damping was calculated to be of Cincinnati Research Team grate- their thoughtful and constructive about 1.5 to 2 percent of critical fully acknowledges the following comments. damping, reasonable for this type people: Terry Halkyard, James Hoblit- The contents of this paper reflect of structure. Dynamic deflections zell, Suneel Vanikar and Susan Lane the views of the authors, who are were about equal to the static de- of FHWA; Brad Fagrell, Vik Dalal, solely responsible for the accuracy flections and there was no appar- Roger Green and Lloyd Welker of of the facts, data and interpretations ent dynamic magnification. ODOT; and Charles Goodspeed of the presented herein. This paper does not University of New Hampshire. necessarily reflect the policies or offi- The authors are grateful for the cial views of the Federal Highway Ad- ACKNOWLEDGMENTS excellent assistance of the fabrica- ministration or the Ohio Department This project was funded by the Fed- tors, Don Bosse, Joe Roche and Jack of Transportation and this paper does eral Highway Administration through McDonald of Prestress Services of not constitute a standard, regulation or the Ohio Department of Transporta- Melbourne, Kentucky. The authors specification. tion as part of the FHWA HPC Bridge also want to express their gratitude Showcase Program. The University to the PCI JOURNAL reviewers for

References

1. Long, Ellen, "High Performance Concrete Mix Design for an 5. AASHTO, LRFD Bridge Design Specifications, Second Edition, Adjacent Box Girder Bridge," A Thesis Submitted in Partial American Association of State Highway and Transportation Fulfillment of the Requirements for the Degree of Master of Officials, Washington, DC, 1998. Science, Department of Civil and Environmental Engineering, 6. Russell, B., and Burns, N., “Measured Transfer Lengths of 0.5 University of Cincinnati, Cincinnati, OH, March 1998. and 0.6 in. Strands in Pretensioned Concrete,” PCI JOUR- 2. AASHTO, Standard Specification for Highway Bridges, 16th NAL, V. 41, No. 5, September-October 1996, pp. 44-63. Edition, American Association of State Highway and Trans- 7. Collins, M., Mitchell, D., Felber, A., and Kuchma, D., RE- portation Officials, Washington, DC, 1998. SPONSE, V. 1, in Structures, Prentice 3. Miller, R. A., Hlavacs, G., Long, T., and Greuel, A., “Full Scale Hall, Englewood Cliffs, NJ, 1991. Testing of Shear Keys for Adjacent Box Girder Bridges,” PCI 8. PCI Design Handbook, Fifth Edition, Precast/Prestressed Con- JOURNAL, V. 44, No. 6, November-December 1999, pp. 80-90. crete Institute, Chicago, IL, 1999. 4. Miller, R., Shahrooz, B., Baseheart, T. M., Eberenz, E., Jones, J., Knarr, R., Sprague, R., "Testing of a High-Performance Concrete, Single-Span Box Girder,” Transportation Research Record, No. 1624, 1998.

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