Rapport sur les causes techniques de l effondrement du viaduc de la Concorde

Transcription

1 Rapport sur les causes techniques de l effondrement du viaduc de la Concorde Annexe A8 Essais de chargement en laboratoire réalisés à l Université McGill Rédigé par : Denis Mitchell, ing., Ph.D. William D. Cook, Ph.D. Mai 2007

2 Annexe A8 EXPERIMENTAL SIMULATION OF CANTILEVER - DE LA CONCORDE BRIDGE Report Prepared for: Report Prepared by: Denis Mitchell, Ph.D., Eng. William D. Cook, Ph.D. May 2007

3 Annexe A8 page 2 Table of Contents 1. Introduction Details and Construction of the Test Specimens Geometry and Reinforcing Details Construction of the Cantilevers Material Properties Instrumentation Loading Load Stages Replacement of Expansion Joint Traffic study Details of expansion joint replacement Properties of concrete in new expansion joint Behaviour of South Cantilever with As-Designed Details Load deflection response of south end Crack development Failure of south end with as-designed details Measured strains External clamping of south end after failure of south end Behaviour of North Cantilever with As-Built Details Load deflection response of north end Crack development Failure of north end with as-built details Measured strains Conclusions... 50

4 Annexe A8 page 3 EXPERIMENTAL SIMULATION OF CANTILEVER - DE LA CONCORDE BRIDGE 1. Introduction The purpose of this report is to summarize the results of testing two cantilever spans to study the differences in the reinforcing details on the responses. One span was constructed with the as-designed details and the other end with the as-built details. The experimental study also enabled a study of the influence of the replacement of the expansion joint. 2. Details and Construction of the Test Specimens 2.1 Geometry and Reinforcing Details The geometry of the test specimens was chosen to correspond to the design drawings by Desjardins & Sauriol dated July 31, 1969 (CEVC to ) (see Fig. 2.1). One of the cantilevers was constructed with the details shown on the drawings and is referred to as-designed. Figure 2.1 Details of cantilever from drawings by Desjardins & Sauriol dated July 31, 1969 (COM-19, p. )

5 Annexe A8 page 4 Figure 2.2 shows the test specimen that was constructed and tested in the Structures Laboratory at McGill University. The test specimen is 48 in. wide and has two cantilever spans of 13 ft. The dapped ends represent the beam seats at each end of the specimen. N S Figure 2.2 Test specimen The reinforcing details for the two different ends are shown in Fig. 2.3 and the bar list is given in Table 2.1. The details at the south end of the specimen simulate the details from the structural drawings by Desjardins & Sauriol dated July 31, 1969 (COM-19) and are referred to as the as-designed conditions. The reinforcing details at the north end simulate the details of the reinforcing bars as-built determined from investigations on the actual failure blocks at the evidence site.

6 Annexe A8 page N A 8-#14 (@ 6") MK " MK beam thickness #7 MK #8 (@ 10") MK #6 5-#6 (@10") 10-#10 (@ 5") MK10 01 MK06 05 MK #6 MK #6 (@10") MK #5 (@12") MK #6 MK #6 MK06 01 (support bars) 5-#8 (@10") MK08 01 MK a) North end 12" MK #14 (@ 6") MK14 01 B S 4-#5 (@12") MK #6 (@10") MK #6 (@10") MK #10 (@ 5") MK #8 (@ 10") MK #7 MK07 01 MK #8 (@10") MK #6 MK06 01 b) South end 2 clear concrete cover Section A Section B Figure 2.3 Reinforcing details for south end ( as-designed ) and north end ( as-built )

7 Annexe A8 page 6 Table 2.1 Reinforcing bar list for cantilever test specimen Reinforcing bar list Mark Size Length Number Type A B C 1401 # # # # # # # # # # # # M M B A C B C A C A C B 146 o A 34 o 37 o C B 135 o B A C The details of the reinforcement for the as-designed conditions were determined from the structural drawings. It was assumed that the #8 U-shaped hanger bars had their hooks around the #7 transverse bars which were in turn placed in contact with and immediately below the #14 longitudinal bars. In addition, measurements were taken on site to confirm the geometry of the bent bars, including overall height of the #8 U-shaped hanger bars, the length of hooks and the concrete cover used in construction. These measurements were taken close to the failure location (see Fig. 2.4 and 2.5). Figure 2.4 Investigating details of reinforcement

8 Annexe A8 page 7 Figure 2.5 Determining details of reinforcement and concrete cover Care was taken to match as closely as possible the details in the actual bridge. The dimensions were confirmed by detailed measurements taken at the evidence site. Figure 2.6 shows the close match between the actual #8 U-shaped hanger bar and the #8 hanger bar used in the test specimen. The horizontal leg missing on the actual hanger bar was used to obtain the yield stress of the reinforcement. The yellow marker indicates the location where this horizontal leg ended before the leg was removed. The overall height and the overall width of the #8 U-shaped hanger reinforcement measured 40 in. and 18 in. from outside-to-outside, respectively. The top hooks had free end extensions of 18 inches and the bend diameter was 6 in. (6 bar diameters). Figure 2.6 Comparison of #8 U-shaped hanger bar from actual cantilever (foreground) with #8 hanger bar used in constructing the test specimen

9 Annexe A8 page 8 3. Construction of the Cantilevers In order to simulate the details of the reinforcement in the actual construction, #14, #10, #8, #7 and #6 reinforcing bars were imported from the U.S. Figure 3.1 shows the details of the reinforcement near the beam seat of the as-designed end and Fig. 3.2 shows the details at the as-built end. Figure 3.1 Reinforcement details at beam seat of as-designed end Figure 3.2 Reinforcement details at beam seat of as-built end

10 Annexe A8 page 9 Figures 3.3 and 3.4 show the reinforcement details of the as-designed end after placement of the #14 top bars and the expansion joint reinforcement. Figure 3.3 Top #14 bars and expansion joint reinforcement at as-designed end Figure 3.4 View of top #14 longitudinal bars, #7 transverse bars and hooks of #8 hanger bars and #6 diagonal bars at as-designed end Figures 3.5 and 3.6 show the reinforcement details of the as-built end after placement of the #14 top bars and the expansion joint reinforcement. Figures 11 and 13 illustrate the

11 Annexe A8 page 10 differences in the hook locations of the #8 U-shaped hanger bars and the #6 diagonal bars. The hooks at the as-built end are placed lower down in the cross section and are inclined. Figure 3.5 Top #14 bars and expansion joint reinforcement at as-built end Figure 3.6 View of top #14 longitudinal bars, #7 transverse bars and hooks of #8 hanger bars and #6 diagonal bars at as-built end

12 Annexe A8 page 11 Investigations at the evidence site indicated that some additional vertical #6 bars were placed in the cantilever, as shown in Fig These bars were not shown on the structural drawings and were probably used to provide supports for the top reinforcement. They were spaced at approximately 4 ft in the transverse direction of the bridge. Figure 3.7 shows one of the #6 bars located at 110 in. from the expansion joint. Figure 3.8 shows another vertical #6 bar located at a distance of 43 in. from the expansion joint. Figure 3.7 Vertical # 6 bar in as-built end Figure 3.8 Vertical #6 bar in as-built end near hooks

13 Annexe A8 page 12 Figure 3.9 shows the formwork for the double-cantilever test specimen and Fig shows the concrete pumping operation. Figure 3.9 Formwork for test specimen centred in testing machine Figure 3.10 Pumping of concrete into formwork

14 Annexe A8 page Material Properties Ready mix concrete suitable for pumping was delivered to the laboratory. The concrete had a specified compressive strength of 25 MPa. A maximum aggregate size of 20 mm and a water/cement ratio of 0.54 were used. Type 10 cement and fly ash were used in the proprietary mix. The concrete was supplied in 2 batches, with Batch 1 having a slump of 135 mm and a measured air content of 6.5%. Batch 2 (upper portion of test specimen) had a slump of 180 mm and an air content of 7%. Table 4.1 summarizes the measured concrete properties ( f c is the cylinder compressive strength, f r is the modulus of rupture and f sp is the split-cylinder strength). It is noted that the failures of the south end and north end occurred at a concrete age of 84 and 88 days, respectively. The concrete used in the expansion joint had an age of 25 days when failure occurred at the north end. The following specifications were followed in determining the concrete properties shown in Table 4.1: CSA A23.2-9C-04 Compressive Strength of Cylindrical Concrete Specimens CSA A C-04 Splitting Tensile Strength of Cylindrical Concrete Specimens ASTM C78-02 Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Third Point Loading Table 4.1 Mechanical properties of the concrete (averages of 3 specimens tested) Age f c f r f sp days (std. dev.) (std. dev.) (std. dev.) MPa MPa MPa Cantilever (0.70) (0.96) (1.05) 4.54 (0.41) 2.73 (0.20) Joint Replacement (0.50) (0.04) (1.40) Table 4.2 summarizes the mechanical properties ( stress, ε sh bars (0.27) f y is the yield stress, fu is the ultimate is the strain hardening strain and ε u is the rupture strain) of the reinforcing

15 Annexe A8 page 14 These properties were determined in accordance with the following specification: ASTM A Standard Test Methods and Definitions for Mechanical Testing of Steel Products Table 4.2 Mechanical properties of the reinforcing bars (Averages and standard deviations in brackets) Bar size f y f u ε sh ε u MPa MPa mm/m mm/m #6 441 (2.5) 696 (3.6) 8.73 (0.48) 170 (12.7) #7 417 (6.2) 669 (17.5) 6.48 (0.54) 192 (4.2) #8 428 (0.3) 731 (1.3) 3.09 (0.17) 176 (8.3) # (2.1) 758 (1.0) 7.18 (0.08) 173 (22.5) # (1.7) 745 (0.2) (18.9) #5 wall bars 478 (12.8) 589 (11.2) 20.7 (1.3) 168 (6.1)

16 Annexe A8 page Instrumentation Figure 5.1 shows the locations of the strain gauges on the reinforcing bars N SG-N-U3 SG-N-U4 24 SG-N-D1 SG-N-D2 SG-N-OV1, SG-N-OV2 SG-N-IV1, SG-N-IV2 SG-N-H1, SG-N-H2 SG-N-H3, SG-N-H SG-N-U1 SG-N-U2 a) North end 16 SG-S-H1, SG-S-H2 SG-S-H3, SG-S-H4 SG-S-OV1, SG-S-OV2 SG-S-IV1, SG-S-IV2 S SG-S-D1 SG-S-D2 SG-S-U3 SG-S-U4 24 SG-S-U1 SG-S-U b) South end Figure 5.1 Locations of the strain gauges Figure 5.2 shows the locations of the linear voltage differential transformers (LVDT s) on the west and east faces of the test specimen. These LVDT s measure the change in length between the pins that are attached to the concrete and enable the average strains to be determined. The longer vertical LVDT s are connected to pins drilled into the concrete at locations 2 in. from the concrete surfaces. The shorter LVDT s have a gauge length of 300 mm.

17 Annexe A8 page 16 N NW-V7 NW-V3 2 from edge (typical) SW-V3 SW-V7 S LVDT s on west face S N S-V4 S-V3 S-V2 S-V1 N-V1 N-V2 N-V3 N-V4 S-V7 S-V6 S-V5 S-H2 S-H1 N-H1 N-H2 N-V5 N-V6 N-V7 S-D N-D S-Tip S-Support N-Support N-Tip LVDT s on east face Figure 5.2 Locations of the LVDT s. Figure 5.3 shows the locations of the LVDT s on the east face of the north cantilever. Figure 5.3 LVDT s attached to east face of specimen

18 Annexe A8 page Loading Figure 6.1 shows the test setup. A central load was applied to a steel loading beam by a computerized testing machine with a capacity of 11,450 kn. The loading produced equal loads at the two bearing pads. This permitted testing both cantilevers at the same time and resulted in each cantilever being subjected to identical loading histories. Figure 6.1 Testing setup showing steel loading beams in MTS testing machine Table 6.1 gives details of the loading of the two cantilevers. The numbers of cycles of live load are given along with the values of dead load and live load for each level of loading. The loading was first increased to the desired dead load level on each pad and then cyclic live loading was applied. Table 6.1 Different loading regimes Cycle # # of Dead Load Live Load Total Load Comments Cycles (kn) (kn) (kn) 1-18, Joint Replacement on north end only Expansion joint replacement 18,701-31, ,201-34, ,201-45, ,201-46, ,701-49, ,701-51, ,201-52,

19 Annexe A8 page 18 Cycle # # of Dead Load Live Load Total Load Comments Cycles (kn) (kn) (kn) 52,701-54, ,201-55, ,701-57, ,201-58, , South End failure 58,702-60, ,200-62, ,201-63, ,201-63, ,701-64, ,201-64, , North End failure

20 Annexe A8 page Load Stages Table 7.1 gives the details of the load stages and some of the measured crack widths at the end of the cycle indicated. The pad load includes the dead load and the maximum live load during cycling. Throughout the testing detailed information on the cracking was recorded. This included photographs of the west side at each load stage with the cracks highlighted with lines drawn with felt markers and the crack widths indicated with labels beside each significant crack. The crack widths were measured using a crack comparator. Hairline cracks (less than 0.05 mm in width) were marked but no crack width was indicated. Hairline cracks are indicated in Table 5.1 with a width of 0.02 mm. Table 7.1 reports on the development of flexural crack widths (both at the level of the #14 bars and in regions below the top steel) and on the width of inclined cracks at the re-entrant corner at the beam seat. These cracks formed at early load stages. In addition, in the Comments column, the details of other important cracks are indicated, along with the measured crack widths. Load Stage # Pad Peak Load # of Cycles Table 7.1 Loading stages and crack widths North End Max Crack - Top Steel Level, mm North End Max Crack - Below top Steel, mm South End Max Crack - Top Steel Level, mm South End Max Crack - Below top Steel, mm North End Max shear crack in nib, mm South End Max shear crack in nib, mm Comments

21 Annexe A8 page 20 Load Stage # Pad Peak Load # of Cycles North End Max Crack - Top Steel Level, mm North End Max Crack - Below top Steel, mm South End Max Crack - Top Steel Level, mm South End Max Crack - Below top Steel, mm North End Max shear crack in nib, mm South End Max shear crack in nib, mm Comments Joint repair Joint repair

22 Annexe A8 page 21 Load Stage # Pad Peak Load # of Cycles North End Max Crack - Top Steel Level, mm North End Max Crack - Below top Steel, mm South End Max Crack - Top Steel Level, mm South End Max Crack - Below top Steel, mm North End Max shear crack in nib, mm South End Max shear crack in nib, mm Comments Hairline cracks at joint N end Inclined crack in joint N end Failure of S end 0.15 mm shear crack into joint corner 0.2 mm shear crack into corner mm horiz crack at back face of joint horizontal crack across full width on back face 0.15 mm horizontal crack across full width on back face 0.20 mm 0.25 mm shear crack into joint mm horiz crack at back face of joint 0.30 mm horiz crack at back

23 Annexe A8 page 22 Load Stage # Pad Peak Load # of Cycles North End Max Crack - Top Steel Level, mm North End Max Crack - Below top Steel, mm South End Max Crack - Top Steel Level, mm South End Max Crack - Below top Steel, mm North End Max shear crack in nib, mm South End Max shear crack in nib, mm Comments face of joint 0.30 mm shear crack into joint, inclined joint crack 0.15 mm crack on joint back face 0.35, shear crack into joint 0.4, extension of major shear crack into joint 0.50 mm shear crack into joint, 0.40 crack in joint, 0.50 mm crack at level of hooks growing rapidly 0.80 mm crack at level of hooks - Failure N end

24 Annexe A8 page Replacement of Expansion Joint The expansion joint on the north end ( as-built details) was replaced after 18,700 cycles (see Tables 6.1 and 7.1). 8.1 Traffic study To simulate what happened on the actual structure the expansion joint was replaced after some load cycling. It is noted that the bridge was constructed in 1970 and the expansion joint was replaced in A traffic study titled Expertise de Circulation et de Chargement Viaducs de la Concorde et De Blois dated March 2007 by Jean Hamaoui concluded the following: 1. For heavy trucks having a mass ranging from 30,000 kg to 55,000 kg, a total of about 18,700 cycles are predicted during the period for the South lane (eastbound traffic). This is the critical region for loading on the cantilever. 2. For heavy trucks having a mass ranging from 30,000 kg to 55,000 kg, a total of about 12,500 cycles are predicted during the period for the South lane (eastbound traffic). It is assumed that trucks having masses ranging from 30,000 kg to 55,000 kg (from transportation study of vehicle load distributions) may be considered as heavy trucks. If it is assumed that this range of trucks with variable number of axles can be approximated by an H20-S16 truck with only 3 axles, then an approximate truck load reaction on one neoprene pad can be estimated. It is noted that as a benchmark, the live load on an interior pad is 69 kn using 3D finite element analysis and H20-S16 truck loading and ignoring impact effects. The weight of an H20-S16 truck is 72 kips (320 kn). The average weight heavy truck weight is about 38,000 kg (372 kn). Hence the estimated live load reaction, assuming a dynamic amplification factor of 1.1 is about: = 88 kn 320 For the initial cycling the loads on the first interior pads were taken as 350 kn (dead load) and 90 kn (live load), giving a maximum load during cycling of 440 kn. Thus theses loads were used for the initial cycling for 18,700 cycles, at which time the expansion joint was replaced. It is noted that the edge pad gives higher dead loads. 8.2 Details of expansion joint replacement During the entire operation of replacing the expansion joint, a dead load of 350 kn was kept constant on both the north and south end pads to simulate the dead load of the actual structure. Figure 8.1 shows the separate jacking system, using two 100 ton jacks and two

25 Annexe A8 page kip load cells. The jacks reacted against the loading frame of the MTS testing machine. Figure 8.1 Jacking system reacting against the testing machine to provide constant dead load during the replacement of the expansion joint Figure 8.2 shows the stages and details of the demolition of the original expansion joint and the construction of the new expansion joint.

26 Annexe A8 page 25 a) Details of original expansion joint 450 mm b) Removal of concrete 3-20M bars 450 mm 82.5 mm 4 pairs 10M hooked bars c) Details of new expansion joint Figure 8.2 Details of original expansion joint and construction of new joint A 20 mm saw cut was made across the specimen to delineate the region of concrete chipping (see Fig. 5.3)

27 Annexe A8 page 26 Figure 8.3 Saw cutting of limit of concrete chipping Care was taken in the chipping of the concrete during the removal of the original joint. A 30 kg pneumatic hammer was used to remove the cover concrete and a smaller 7 kg hammer was used to remove concrete close to and around the reinforcing bars (see Fig. 8.4). Figure 8.4 Chipping concrete with a 7 kg pneumatic hammer Figure 8.5 gives the details of the steel plate devices that were used as reinforcement in the new expansion joint.

28 Annexe A8 page 27 Figure 8.5 Drawing of expansion joint reinforcement from Z-Tech (CEVC002191)

29 Annexe A8 page 28 Figure 8.6 shows the details of the plate assembly with two 20M bars passing through holes in the plates. Also shown are the 10M hooked bars that hook around the 20M bars passing through the plate assembly and hook around an additional 20M bar below the plate assembly. Care was taken to simulate the actual construction details used for the expansion joint replacement. The plate assembly and reinforcing details matched the details measured at the evidence site. Figure 8.6 Placement of expansion joint reinforcement in region of new joint. Figure 8.7 shows the placement of the low-slump concrete in the formwork for the new joint. Figure 8.7 Placement of concrete for new expansion joint

30 Annexe A8 page 29 Figure 8.8 shows the new expansion joint after the formwork was removed. Figure 8.8 New concrete in expansion joint 8.3 Properties of concrete in new expansion joint The mix proportions of the concrete used for the replacement of the expansion joint are given in Table 8.1. The measured slump was 65 mm. Table 8.1 Concrete mix proportions for replacement of expansion joint Component Quantity Cement (Type HE) 445 kg/m 3 Fine aggregate 900 kg/m 3 Coarse aggregate ( max aggregate size 12.5 mm) 945 kg/m 3 Water 200 kg/m 3 Air entraining admixture 260 ml/m 3 Two shrinkage specimens were cast with the joint concrete and were subjected to the same curing regime as the joint concrete (see Fig. 8.9). The standard shrinkage measurements were made in accordance with the following specification: ASTM A Standard Test Method for Length Change of Hardened Cement Mortar and Concrete. Strain readings on the top surface of the new joint concrete were also taken with a mechanical gauge to determine the shrinkage of the restrained joint concrete (see Fig. 8.9).

31 Annexe A8 page 30 Figure 8.9 Two shrinkage specimens on top of new joint used to determine free shrinkage strains and mechanical gauge used to determine restrained shrinkage strains on top surface of joint Figure 8.10 shows the development of the free shrinkage strains and the restrained shrinkage strains with time in days. These shrinkage values were obtained by averaging two shrinkage strain measurements. 800 Shrinkage (microstrain) Free shrinkage Surface shrinkage on joint concrete Time (days) Figure 8.10 Development of free shrinkage and restrained shrinkage strains with time

32 Annexe A8 page Behaviour of South Cantilever with As-Designed Details 9.1 Load deflection response of south end Figure 9.1 gives the applied pad load versus the beam seat deflection for the south ( asdesigned ) cantilever. A summary of the crack widths is given in Table 7.1. The specimen was first loaded up to a dead load of 350 kn on the neoprene bearing pad. The maximum flexural crack width was 0.10 mm at this stage. The specimen was then loaded to 440 kn and cycled for 18,700 cycles. After this cycling, at load stage 24, the expansion joint was replaced at the north end and the load was held constant at 350 kn. The increased deflection during this operation is visible in the plot Pad Load, kn Tip Deflection, mm South End Figure 9.1 Applied load on the neoprene pad versus deflection at the beam seat south end

33 Annexe A8 page Crack development Figure 9.2 and Fig. 9.3 show the crack patterns and crack widths at load stage 24. The south end had displayed some shrinkage cracking at the level of the top reinforcement and this horizontal crack is visible in Fig mm crack 0.20 mm crack Figure 9.2 Flexural cracking at load stage 24 at south end 0.05 mm crack Figure 9.3 Crack pattern and crack widths at load stage 24 at south end

34 Annexe A8 page 33 After the replacement of the expansion joint at the north end, the testing was resumed and the additional 12,500 cycles of live load were carried out, giving a total of 31,200 cycles by load stage 40. The level of dead load was eventually increased and additional live load cycling was carried out as shown in Table 6.1 and Table 7.1. Figure 9.4 and Figure 9.5 show the crack patterns and crack widths at load stage 50. By this stage the specimen had undergone a total of 39,200 cycles. Load stage 50 had a total pad reaction of 548 kn (495 kn of dead load and 53 kn of live load). Horizontal crack 0.50 mm crack 0.30 mm crack Figure 9.4 Flexural cracking at load stage 50 at south end (maximum flexural cracks of 0.25 mm at the level of the top #14 bars and 0.50 mm below this level)

35 Annexe A8 page mm crack Figure 9.5 Crack pattern and crack widths at load stage 50 at south end (width of inclined crack was 0.10 mm at re-entrant corner) Figure 9.6 shows the overall cracking pattern of the south end at load stage 74 (dead load of 566 kn and live load of 106 kn) and Figure 9.7 shows the region near the beam seat at this load stage mm crack 0.20 mm crack Figure 9.6 Development of cracks at load stage 74 (maximum flexural cracks of 0.35 mm at the level of the top #14 bars and 0.50 mm below this level)

36 Annexe A8 page mm crack Figure 9.7 Crack pattern and crack widths at load stage 74 at south end (width of inclined crack was 0.20 mm at re-entrant corner) Figure 9.8 shows the overall cracking on the south end of the specimen at load stage 82, just before failure occurred. There was very little distress in the region of the beam seat, with a crack width at the re-entrant corner of 0.25 mm. The maximum flexural crack widths had not increased since load stage 74. An inclined crack had progressed from the top of the specimen, at about the quarter point of the span, and joined with a major flexural crack. This flexural crack had already developed an inclined diagonal tension crack near the bottom of the beam and this became the critical shear crack for the failure of the specimen.

37 Annexe A8 page 36 flexural crack Inclined crack Inclined crack 0.25 mm crack Figure 9.8 Inclined crack joining with flexural crack to form critical shear crack. 9.3 Failure of south end with as-designed details The south side failed in a brittle shear mode at a pad reaction of 810 kn. A shear-bond failure progressed at the top of the beam along the #14 bars and the hooks of the #8 U- shaped hanger bars and the #6 inclined bars. The south end of the beam was prevented from totally collapsing by blocking under the beam seat. This blocking was added before the failure occurred because it was necessary to reinforce this failed end before progressing with additional loads on the north end. The cantilever failed at a deflection at the beam seat of about 12 mm which corresponds to a very small deflection of about L/330, where L is the cantilever span. This small deflection at failure indicates a brittle and very sudden failure mode, consistent with a shear failure.

38 Annexe A8 page 37 Figure 9.9 Failure of as-designed cantilever (maximum load of 810 kn) Figure 9.10 shows a close-up of the failure plane along the level of the #14 bars and the hooks. Figure 9.10 Horizontal failure plane along level of #14 bars and hooks

39 Annexe A8 page Measured strains The strain gauges on the #14 top bars (see Fig. 5.1) indicated a maximum tensile strain of 1700 microstrain. It is noted that this flexural reinforcement had a maximum strain below the yield strain of 2645 microstarin, indicating that the flexural steel did not yield during the testing. The #10 U-shaped reinforcing bars in the nib had a maximum measured tensile strain near the re-entrant corner of 550 microstrain, which is well below the yield level of 2570 microstrain. The maximum measured strain in the #8 U-shaped hanger bars near the beam seat was 350 microstrain which indicates that this reinforcement was considerably below the yield level of 2140 microstrain. The #6 inclined bar had a maximum strain of 1150 microstrain which below one-half of the yield strain of 2205 microstrain. It is concluded that the south end failed in shear without any yielding of the principal reinforcement. 9.5 External clamping of south end after failure of south end Figure 9.10 shows the heavy external reinforcement that was added to the south end to permit further testing of the north end. The beam seat was blocked fro moving downwards, 5 sets of external threaded rod externally applied stirrups were clamped to the cantilever and two heavy clamping beams strapped the cantilever to the testing strong floor. Figure 9.10 External stirrups and clamping devices added to the south end to permit further testing of the north end

40 Annexe A8 page Behaviour of North Cantilever with As-Built Details 10.1 Load deflection response of north end Figure 10.1 gives the applied pad load versus the beam seat deflection for the north ( asbuilt ) cantilever. A summary of the loading regime is given in Table 6.1 and the crack widths are given in Table 7.1. The specimen was first loaded up to a dead load of 350 kn on the neoprene bearing pad. The maximum flexural crack width was 0.10 mm at this stage. The specimen was then loaded to 440 kn and cycled for 18,700 cycles. After this cycling, at load stage 24, the expansion joint was replaced at the north end and the load was held constant at 350 kn. The increased deflection during this operation is visible in the plot Pad Load, kn Tip Deflection, mm North End Figure 10.1 Applied load on the neoprene pad versus deflection at the beam seat north end

41 Annexe A8 page Crack development Figure 10.2 and Fig show the crack patterns and crack widths at load stage 24, just before the expansion joint was replaced. Figure 10.2 Crack pattern at load stage 24 (maximum flexural cracks of 0.20 mm at the level of the top #14 bars and 0.30 mm below this level) Figure 10.3 and Figure 10.4 show the crack pattern at load stage 50 (dead load of 495 kn and a live load of 53 kn)

42 Annexe A8 page mm crack Horizontal crack Figure 10.3 Crack pattern at load stage 50 after joint replacement (inclined crack width of 0.10 mm at re-entrant corner and hairline horizontal crack near beam seat) 0.40 mm cracks Figure 10.4 Crack pattern at load stage 50 after joint replacement (maximum flexural cracks of 0.30 mm at the level of the top #14 bars and 0.50 mm below this level)

43 Annexe A8 page 42 Figure 10.5 shows the cracking observed at load stage 74 (dead load of 566 kn and live load of 106 kn) Horizontal crack 0.20 mm crack Figure 10.5 Crack pattern at load stage 74 (inclined crack width of 0.20 mm at reentrant corner and hairline horizontal crack near beam seat, cracking in joint area) 0.10 mm crack 0.30 mm crack 0.20 mm crack Figure 10.6 Crack pattern at load stage 86 (inclined crack width of 0.30 mm at reentrant corner, 0.20 mm inclined crack into joint and horizontal crack of 0.10 mm along back face of joint)

44 Annexe A8 page mm crack in joint 0.40 mm crack 0.40 mm crack Figure 10.7 Cracking details at load stage 101 (inclined crack width of 0.40 mm at reentrant corner, 0.40 mm inclined crack into joint and horizontal crack of 0.35 mm along back face of joint) 0.35 mm horizontal crack Figure 10.8 Cracking details at load stage 101 showing horizontal crack of 0.35 mm along back face of joint

45 Annexe A8 page mm crack at hooks 0.50 mm inclined crack 0.40 mm crack Figure 10.9 Cracking details at load stage 104 (inclined crack width of 0.50 mm at reentrant corner, 0.40 mm inclined crack into joint, 0.40 mm inclined crack in joint, 0.50 mm flat crack in region of hooks of #6 inclined bars) Figure shows the significant cracking at load stage 105 (dead load of 885 and live load of 190 kn). Figure shows a close-up of region close to the joint with a rapidly increasing flat crack located at the level of the hooks of the #6 inclined bars. This crack increased in width from 0.50 mm to 0.80 mm at this load stage.

46 Annexe A8 page 45 Figure Significant cracking associated with expansion joint and hook details 0.40 mm crack 0.20 mm crack 0.50 mm crack 0.80 mm crack at hooks 0.40 mm crack Figure Close-up of cracks near joint region, showing 0.80 mm crack at level of hooks of #6 inclined bars.

47 Annexe A8 page Failure of north end with as-built details Figure shows the failure of the north end with the as-built details. This cantilever failed in a brittle shear-bond mode with the inclined shear crack emanating from the cracks at the level of the #6 hooks. The failure took place at a maximum load of 1075 kn. Figure Failure of north end with as-built details (maximum load of 1075 kn)

48 Annexe A8 page 47 Figure Failure of north end with as-built details (maximum load of 1075 kn) Figure shows a close-up of the failure plane that occurred between the top #14 bars and the #8 hooks of the U-shaped hanger reinforcement. The top of one of the #8 hooks is exposed. The small 10M hooks for the extra reinforcement in the expansion joint are also evident in the photograph. Figure Close-up of horizontal failure plane

49 Annexe A8 page 48 Figure shows a close-up of the east side of the cantilever showing that the inclined shear crack passed through the #6 inclined bar at the start of the hook. Figure Inclined portion of failure crack on west face of cantilever One key feature of the difference of behaviour of the north end compared to the south end is the presence of the two #6 vertical reinforcing bars that were placed during construction. These bars arrested the growth of inclined shear cracks that had formed. Figure shows the pullout of this #6 bar which crossed the shear failure plane near the wall support. The cantilever failed at a deflection at the beam seat of about 24 mm which corresponds to a small deflection of about L/165, where L is the cantilever span. This small deflection at failure indicates a brittle and very sudden failure mode, consistent with a shear-bond failure.

50 Annexe A8 page 49 Figure Pullout of vertical #6 bar that crossed the shear failure plane Measured strains The strain gauges on the #14 top bars (see Fig. 5.1) indicated a maximum tensile strain of 2075 microstrain, compared to a yield strain of 2645 microstrain. It is noted that this flexural reinforcement did not yield during the testing. The #10 U-shaped reinforcing bars in the nib had a maximum measured tensile strain near the re-entrant corner of 700 microstrain, which is well below the yield level of 2570 microstrain. The maximum measured strain in the #8 hanger reinforcement near the beam seat was 900 microstrain which indicates that this reinforcement was considerably below the yield level of 2140 microstrain. The #6 inclined bar had a maximum strain of 1500 microstrain which below one-half of the yield strain of 2205 microstrain. It is concluded that the north end failed in shear without any yielding of the principal reinforcement. It is also noted that the reinforcement in the north end experienced higher strains than the reinforcement in the south end. The north end failed at a load of 1075 kn applied to the bearing pad compared with a failure load of 810 kn at the south end.

51 Annexe A8 page Conclusions The as-designed cantilever test specimen failed in a brittle shear manner with a large diagonal tension crack and horizontal splitting along the plane of the #14 top bars and the hooks of the #8 hanger bars and the hooks of the #6 diagonal bars. The applied pad load at failure was 810 kn and occurred after 58,700 cycles of loading. The closely spaced reinforcing bars in the region of the hooks of the hanger bars and the hooks of the #6 bars, together with the #14 bars on the same plane caused some splitting cracks to occur at early load stages. Some shrinkage cracks were evident along these bars after the formwork was removed. The as-built cantilever test specimen failed in a brittle shear failure mode at a pad load of 1076 kn after a total of 65,200 cycles of loading. The replacement of the expansion joint was simulated by removal of the original expansion joint and casting a new joint region, while the cantilever test specimen was subjected to the dead load of 350 kn from the girder reactions. Extreme care was taken during the replacement of the expansion joint so as not to damage the concrete during chipping, to get a clean interface for the bonding of the new concrete, and to provide moist curing of the concrete in the joint region. During testing, cracks formed along the expansion joint interface and inclined cracks as well as horizontal cracks formed in the concrete of the new joint. Close to failure a large crack formed along the hooks of the #6 diagonal bars and this crack joined up with a diagonal crack to initiate the shear failure. The diagonal crack from the shear failure passed through the hooks of the #6 bars and resulted in a horizontal splitting failure through the concrete between the #14 bars and the hooks of the #8 hanger bars. The presence of the #6 vertical bar, that was placed during construction but was not shown on the structural drawings, located at a distance of 36 in. from the wall support served to arrest the growth of shear cracking and contributed to the strength of the cantilever in shear.