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5 Construction Sequence ship between the maximum force and embedment length of the untensioned prestressing strand in a stress field similar to Twenty-eight respondents (80%) indicated that the dia- that in the diaphragm of a highway bridge was developed. In phragm and deck were cast at the same time. Fourteen respon- all, 69 pull-out specimens were tested with three strand con- dents (40%) cast the diaphragm, or some part of the dia- figurations: straight, frayed, and a 90° bent. From the inves- phragm, first. Again, the total responses exceed 100% because tigation, the modulus of slip for each of the strand configu- some respondents did both. Only 12 respondents indicated rations was determined, and equations relating steel stress to specifying a minimum age for the girders before the dia- the embedment length were developed. It was found that bent phragm and deck are cast. The minimum ages varied from strand provided the best anchorage, having half the slip of the 28 to 90 days with no two respondents having the same age straight or frayed strand. Straight strand was found to per- requirement. Some respondents indicated multiple age require- form marginally better than frayed strand. The other vari- ments. The initial survey also asked questions about con- ables included in the testing program were strand diameter, structability and connection performance. More detailed ques- concrete strength, and containment reinforcing, but they were tions were asked in a second survey. The results of this second found to be of little consequence. survey showed that the most common concerns were conges- The second phase was to verify and apply the expressions tion in the diaphragm area and the possibility that the concrete developed in the first phase to an actual I-beam end connec- could not be adequately consolidated in this area. tion (9). Six full-scale bent-strand connections were tested: Fabrication problems included a difficulty in installing three specimens consisting of two short, 6-ft 3-in. stub beams, bars pre-bent (due to form interference), difficulties in bend- a 30-in. diaphragm, and a 6.5-in. slab; and three specimens ing bars or strands after the girders were fabricated, and that made only of the beams and diaphragms (no slab). As with an extended bar or strand were sometimes cut off or broken the PCA tests (4,6), the specimens were tested for positive off during fabrication or while being transported. If a bar or moment as simple spans, but only monotonic static loads were strand was accidentally cut or broken, the solution was to drill applied. The stub girders were embedded 17.5 in. into the dia- and epoxy a new bar or strand in place. phragm and 3/4-in.-diameter coil tie rods were used to trans- The respondents were asked to rate the significance of mit the force from the end of the beams to the diaphragm. identified problems in terms of increased cost, increased con- From the results of Phases I and II, a design method was struction time, decreased quality, or a combination thereof. proposed for positive moment connections using bent strand. In the overwhelming majority of responses, the problems were The required area of extended, 90o bent strand (Aps req'd) is rated as being of only minor significance. given by: The respondents were also asked to provide costs for pro- viding the connections. Many did not respond, and the responses that were received were difficult to interpret. In M - As f y ( jd ps + d - d ps ) Aps ( req'd ) = some cases, the respondents indicated that the cost of the con- f ps jd ps nection could not be easily separated. In other cases, the cost depended largely on the detail used and the fabricator's meth- where ods. However, it appeared that the maximum cost of provid- M = positive moment; ing the positive moment connection was $200 per girder. This As fy = area and yield stress of the diaphragm coil tie rod; was insignificant compared with the overall girder cost. d = depth from extreme compression fiber to centroid of diaphragm coil tie; dps = depth from extreme compression fiber to centroid of LITERATURE REVIEW strand; A search was made for literature that had been published jdps = internal moment arm; since the publication of NCHRP Report 322 in 1989 (11) or fps = (Le - 8.25 in.)/0.228 < 150 ksi; and that had not been included in that report. The most signifi- Le = embedment length (in.). cant findings were a series of reports done for the Missouri Highway and Transportation Department (now the Missouri After determining the required area of steel and embedment DOT) (810). These reports cover experimental work done length, the section is checked for ultimate strength by: on bent-strand types of connections. fpu = (Le - 8.25 in.)/0.163; In the early 1970s, the Missouri Cooperative Highway Research Program commissioned a study on the feasibility of a = (Aps fpu - As fy )/0.85fc b; using extensions of the prestressing strand to develop posi- tive moment continuity of prestressed I-beam members. The Mu = N[Aps fpu(dps - a/2) + As fy(d - a/2)]; first phase of the research dealt with an investigation of the bonding characteristics of untensioned prestressing strand (8), b = width of compression face (rectangular section which could be used in the continuity connection. A relation- assumed);
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6 fc = compressive strength of concrete; and 2. The girders were loaded to produce negative moment over the support: the debonded girders exhibited N = understrength factor. flexural shear cracking where the fully bonded gird- ers did not. Deflection behavior was the same for both The results of the two phases are given in two interim reports bonded and debonded girders until the flexural shear (8, 9) and a summary appears in the final report (10). cracks occurred, at which point the debonded girders Another study was conducted in 1980 (13). In this study, showed more deflection. the fatigue resistance of the untensioned, bonded prestress- 3. The negative moment behavior was evaluated by two ing strand, consistent with the bent-strand configuration for models: the PCA model and the CTL model. The a precast I-beam bridge, was examined. Eighteen single-type PCA model considers center supports to be a single sup- specimens, consisting of overlapping, opposing bent strand port and the connection to be zero width and infinitely cast into a single block of concrete, and twenty-three double- rigid (i.e., the beam is modeled as two spans). In the CTL type specimens, consisting of U-shaped strand with a con- model, the center support is modeled as two supports crete block cast on each end, were tested in cyclic tension. and the connection is considered to be of finite width and This study recommended that the stress in the strand be lim- flexible (i.e., the beam is modeled as three spans with ited to 15% of the ultimate strength of the strand to prevent the center span being the finite length connection/ fatigue failure. It was further recommended that the dia- diaphragm). The results show the CTL method pro- phragm be cast before the slab. vides reasonable and conservative estimates of conti- In addition to the Missouri studies, five other studies on nuity moments and negative moment capacity and that continuous span bridges were found. Abdalla, Ramirez, and the PCA method was not conservative. Lee (14) tested three continuous Type I girders and a 27-in. 4. Flexural shear cracks in the I girder specimens box girder, all with debonded strands. In each case, the gird- occurred earlier than predicted by the PCA and ers were assembled into a single beam line of a two-span CTL methods. For the CTL method, analysis was done bridge with 24-ft 4-in. spans and an 18-in. diaphragm (except using both uncracked and cracked transformed sec- for the first I girder specimen, which had 24-ft spans and a tions. The PCA method and the CTL method using the 30-in. diaphragm). The ends of the girders were embedded uncracked sections overpredicted the flexural shear 6-in. into the diaphragm (except for the first Type I specimen, cracking load by 35% for the specimens with 50% which had an 8.5-in. embedment). A total of 12 strands were debonding, by 52% for specimens with 67% debond- used in each I girder, while the box girders had 20 strands. ing, and by 82% percent for specimens with 83% The number of debonded strands was 0%, 50%, 67%, or 83% debonding. When evaluated with the CTL method, but for the I girders and was 50% for the boxes. Note that the using the cracked section, the overprediction dropped to AASHTO Standard Specifications for Highway Bridges (15) 19%, 33%, and 49%, respectively. The cracks opened does not limit the number of debonded strands, but this level prematurely in the debonded regions, and moment redis- of debonding would not be permitted under the AASHTO tribution was noted. LRFD Specifications (12). Continuity for negative moment 5. The equations for web shear cracking given in the was achieved by using eight No. 6 bars in the 4-ft × 4-in. deck AASHTO Standard Specifications for Bridge Design slab. For positive moment continuity, four strands were bent (15), coupled with the PCA and CTL methods, pro- at 90o angles and embedded in the diaphragm. vided conservative estimates of shear strength for The significant conclusions of these studies were as follows. the I girders. For the debonded girders, the measured load was 25 to 60% greater than predicted. The mea- 1. Time-dependent moments were measured at the sured load was 10 to 30% greater than predicted for the connection by evaluating the change in center sup- fully bonded girders. The web shear cracking for the port reactions. These moments were compared with box girders was slightly underpredicted, with the mea- the predictions from the PCA method (7 ) and the sured load being 80% of the predicted load for the method suggested in NCHRP Report 322 (11) (called debonded girders and 93% of the predicted load for the the "CTL method" [Construction Technology Labora- fully bonded girders. tories method] in Reference 14 ). When using the CTL method, the time-dependent moments were evaluated In another series of studies, Ramirez and Peterman (16, 17 ) both with and without the effect of restraint from the studied continuous, topped deck panels. However, since these deck reinforcing steel. It was found that the method studies were on panels, not girder and deck bridges, they given in NCHRP Report 322 did a better job of pre- were not seen as germane to this research. dicting time-dependent moments if the effect of the Tadros et al. (18) and Ma et al. (19) explored continuity in deck steel was accounted for; however, there were still Nebraska NU-type girders. The first phase of the research some significant differences between measured and pre- was a parametric study to examine the effect of construction dicted values. sequence on the development of time-dependent moments. It