The need for upgrading a large number of understrength and functionally obsolete bridges in the United States has been well documented. The purpose of this project was to investigate two schemes for strengthening the negative-moment regions of continuous composite bridges: (1) Postcompression of stringers; and (2) superimposed trusses attached to stringers. Both strengthening schemes were designed to resist negative moments and the resulting stresses from service loads. A series of tests was conducted on a full-scale mockup with each strengthening scheme in place. The postcompression strengthening scheme was effective in reducing the bottom-flange stresses (36%); however, top-flange stresses were slightly increased (18%), because of the tension applied to the section. The superimposed truss-strengthening scheme was effective in reducing both the top (46%) and bottom (48%) flange stresses because it applied only reverse moment to the section. In addition to reducing stresses, since the superimposed trusses provide additional support to the bridge beams, the trusses also reduce live-load bridge deflections. The superimposed trusses system is a very practical strengthening system that with appropriate connection modifications can be used to strengthen the negative-moment regions of essentially any type of bridge. INTRODUCTION General Background Nearly half of the approximately 600,000 highway bridges in the United States were built before 1940. The majority of those bridges were designed for lower traffic volumes, smaller vehicles, slower speeds, and lighter loads than they experience today. In addition, maintenance has not been adequate on many of these older bridges. According to the Federal Highway Administration (FHWA), almost 40% of the nation's highway bridges are classified as deficient and thus in need of rehabilitation or replacement. The deficiency in some of these bridges is their inability to carry current legal live loads. Rather than posting these bridges for reduced loads or replacing them, strengthening has been found to be a cost-effective alternative in many cases. Many different methods exist for increasing the live load-carrying capacity of various types of bridges. Through numerous research projects sponsored by the Iowa Department of Transportation, (Iowa DOT) the second and third writers have developed posttensioning to strengthen steel beam, concrete-deck, simple span bridges (Klaiber et al. 1982; Dunker et al. 1986) as well as continuous span bridges (Dunker et al. 1990; Klaiber et al. 1990a, b). In two instances, where posttensioning is required in the negative-moment regions of continuous span bridges, it is necessary to remove a portion of the bridge deck to install the strengthening system. Since this action is usually undesirable (extra cost, bridge closing, etc.), alternative methods of reducing stress in the negative-moment regions of continuous, composite bridges is obviously desired. This paper describes the investigation of two strengthening schemes for use in such situations. Objectives From numerous potential strengthening schemes conceived for use in negative-moment regions of continuous beams, two were selected for additional investigation in this study. Therefore, the primary objective of this study was to determine the feasibility of strengthening the negative-moment region of composite bridges by two new methods: (1) Postcompression of stringers; and (2) superimposed truss attached to stringers. More detailed objectives were undertaken to evaluate the feasibility of these strengthening techniques: * Determining the best design for applying and maintaining post-compression in negative-moment regions of composite bridges * Determining the effectiveness of postcompression in reducing flexural stress in the negative-moment region * Determining the best configuration and design for a superimposed truss * Determining the effectiveness of the superimposed truss in reducing flexural stress in the negative-moment regions These objectives were pursued by reviewing engineering literature, testing a full-scale mockup of a composite bridge beam in the laboratory, and conducting a finite-element analysis of the mockup with each of the previously described strengthening schemes in place. Literature Review The research completed in this project can be viewed as work in two separate areas: postcompression and applied strengthening mechanisms. Postcompressing a structure is analogous to posttensioning a given structure. With postcompression, however, the member is subjected to axial tension rather than axial compression. Although strengthening of structures by posttensioning is much more common, the writers did locate literature on postcompression. The use of slender compressed elements in concrete structures was suggested by Billig in the early 1950s. In 1953 a West German patent was filed, and in 1956 a separate patent was filed in Austria, both for the use of a postcompression system. Further work was done on postcompression by Reiffenstuhl (1982a, b, 1983), during the early 1970s. Reiffenstuhl developed a method of applying postcompression in concrete structures to cancel the axial effects associated with posttensioning. By posttensioning and postcompressing a member simultaneously, the bending moment due to eccentric forces could be effectively doubled, while tension and compression axial forces cancel. Applied strengthening mechanisms are independent structuers that when added to an existing structure provide redundancy to the original structure, impost desired forces and displacements to the original structrure, or both. Strengthening mechanisms are independent except for the lateral stability provided in some cases by the original structure. Failure of either the original structure or the applied strengthening mechanism does not necessarily cause collapse of the entire structure. Several examples of applied strengthening mechanisms were found in the literature. Strengthening of a railway bridge in Ostrava, Czechoslovakia, was accomplished with an applied strengthening mechanism (Ferjencik and Tochacek 1975). To remove dead-load stress from the existing girders, the entire bridge was jacked upward before a strengthening mechanism was attached to the outside of each existing plate girder. The bridge was then lowered; each of the girders was strengthened by the addition of the attached mechanisms. In Switzerland, a wrought-iron truss bridge was strengthened with a mechanism similar to that proposed by Kandall (1968). Each of the bridge's two 47.85 m (157 ft) truss spans was strengthened with the addition of posttensioning tendons and independent compression members. In the United States, Kim et al. (1984) proposed and applied strengthing mechanisms in through-truss bridges. To provide redundancy and to strengthen this type of bridge, they developed an arch that is constructed within a truss. The superimposed arch is then stressed against the existing truss. Except for lateral stability, however, the arch is independent of the original structure. Laboratory Testing The research program consisted of the distinct parts as previously noted; however, a strong emphasis was placed on laboratory testing. As part of a previous research project, a full-size mockup was constructed representing the negative-moment region of a typical bridge stringer in a continuous composite bridge (Wiley et al. 1990). The postcompression strengthening scheme (henceforth referred to as ST1) and the superimposed trusses (henceforth referred to as ST2) were tested on this mockup. Postcompression tests consisted of a series of vertical-load cycles applied to the mockup, to which several different magnitudes of postcompression force had initially been applied. Variations of the superimposed trusses were developed and tested on the full-scale mockup by using a similar testing procedure. Thus, a comparison of the effectiveness of the various strengthening schemes could be made. During all tests, deflections as well as strains at various locations in both the mockup and the strengthening system were monitored. DESCRIPTION OF TEST SPECIMENS As was previously noted, the full-scale mockup of the negative-moment region, in a stringer of a continuous bridge (henceforth simply referred to as the mockup), was initially fabricated to test negative-moment-region posttensioning schemes (Dunker et al. 1990; Wiley et al. 1990). The theoretical development and fabrication of the mockup are reported in detail in the aforementioned references and thus are only briefly discussed here. The mockup was designed to simulate the negative-moment region over an interior support of a continuous, composite bridge, as shown in Fig. 1. General dimensions of the mockup, which are representative of stringers in bridges found in the field, are given in Fig. 2. The American Association of State and Highway Transportation Officials' (AASHTO) effective-width requirements for interior stringers were used to determine the concrete slab width |1.91 m (6 ft 3 in.)~. Reinforcement in the concrete slab consisted of eight #5s for top longitudinal reinforcement, two #5s for bottom longitudinal reinforcement, and #6s on 46 cm (18 in.) centers top and bottom for transverse reinforcement. The 12 shear connectors were the angle-plus-bar type. Both the slab reinforcement and shear connectors duplicated size and spacings found in actual stringers. Postcompression Strengthening Technique (ST1) The ST1 scheme is only briefly discussed because it was found to be ineffective. ST1 was designed to produce positive-moment bending in the negative-moment region of the mockup. A tension force below the beam's neutral axia was applied to the mockup within the negative-moment region in order to create this bending. The required tension was applied by post-compressing a member located in this region. The compressive force required to reduce the service had stresses in the mockup by the desired amount was calculated to be approximately 889.6 kN (200 kips). Potential buckling of the required compression members introduced problems that obviously are not encountered with posttensioned systems; also a method for locking the compression into the compression members had to be designed. The locking mechanism required great precision since the compressive forces created only small deformations in the compressive members. Thus, small seating losses would cause large decreases in the compressive force in the compression members. The final design for ST1 is shown in Fig. 3. The system consists of two brackets and one compression tube mounted on each side of the web of the mockup. One bracket (designated the live bracket) was designed to transfer the required load to the compression member. The bracket at the opposite end of the compressive member (designated the dead bracket) simply resisted the applied loading. The live bracket was designed to accommodate a 533.8-kN (60 ton) hydraulic cylinder and to transfer the compressive force in the structural tubes to the beam web. Transfer of compressive force to the structural tubes was accomplished by using a sliding cartridge that housed the hydraulic cylinder. To lock a compression force in the compression tube, four 0.025 m (1 in.) diameter high-strength threaded rods were tightened against the back of the sliding cartridge. Each threaded rod transferred the compressive force to a 0.051 m (2 in.) thick bearing plate at the rear of the live bracket. Once the threaded rods were tighted, the hydraulic cylinder could be removed. The compression members were designed to resist approximately 533.8 kN (120 kips) each--the original design value plus an expected increase in load due to vertical loading of the mockup. Locating a lateral restraint near the center of the members increased the buckling strength of the compressive tubes, and thus reduced the unbraced length to slightly less than 2.7 m (9 ft). Only symmetric sections were considered, because of the potential for bending about either axis. On the basis of the design requirements, 6 in. X 6 in. X 1/4 in. (0.152 m X 0.152 m X 0.635 cm) A500 structural tubes were selected for the compression members. Superimposed Truss Strengthening Techniques (ST2) The superimposed truss was designed with two configurations for applying the required upward forces to the mockup. Although each design had unique end conditions, ST2 applied force to the upper flange, while ST3 applied force to the lower flange--the tendons and compression struts in both systems were the same and thus the effectiveness and behavior of both configurations were essentially the same. Only one of the configurations (ST2) is presented here; for information on the second configuration the reader is referred to Klaiber et al. (1989). ST2 was also designed to produce positive-moment bending in the negative-moment region of the mockup. Tensioning the top member of the truss shown in Fig. 4 created upward forces on the mockup. The trusses, one on each side of the beam web, were tensioned simultaneously. The upward force required in each span to reduce the service load stresses in the mockup by the desired amount was calculated to be approximately 111.2 kN (25 kips). Since one truss was located on each side of the web of the mockup, approximately 55.6 kN (12.5 kips) of upward force was required at each end of the two trusses. The pin bracket for the truss acted as a true pin-ended condition for the compression struts because a 0.063 m (2 1/2 in.) diameter pin passed through the webs of the compression struts and into the bracket on each side of the beam. Since the brackets reacted against each other (horizontal) and into the bottom flange (vertical), the connection to the mockup only had to resist lateral forces. On the basis of the resulting forces, researchers selected 6 in. X 6 in. X 1/4 in. (0.152 m X 0.152 m X 0.635 cm) A500 structural tubes as compression struts for ST2. The tubes had stiffeners added at the pin-bracket end and bearing plates added to the jacking end. The top surface of each tube was also slotted, to permit the 0.032 m (1 1/4 in.) diameter tendons to pass through. An upward force to the lower surface of the top flange of the mockup was applied through a plate--roller bearing combination that was attached to the compression struts. As the 1 1/4-in.-diameter tendon (top member of the truss) was tensioned, the ends of the compression struts deflected upward into contact with the lower surface of the mockup. TESTS AND TEST PROCEDURES Loading Mechanism The loading mechanism used to create the negative moment is shown in Fig. 5. Fig. 5 indicates that the left and right 'inflection points' were located 3.86 m (12 ft 8 in.) and 3.78 m (12 ft 5 in.), respectively, from the 'interior support.' The left end of the beam was preloaded to resist the loading applied at the free end of the beam. As previously noted, Fig. 1 illustrates how this loading mechanism produces a negative-moment region that simulates negative-moment regions of a prototype bridge. Applied forces were accurately measured by using load cells. The loading mechanism produced the desired negative moment |maximum moment = 724 kN|center dot~m (534 ft-kips)~ when a 191.3 kN (43 kip) vertical load was applied at the load point. Instrumentation The instrumentation for all tests consisted of electrical-resistance strain gages (strain gages), direct-current displacement transducers (DCDTs), and load cells. Strain gages were temperature compensated and were attached to the specimens with recommended surface preparation and adhesive. Each strain gage was positioned with its axis parallel to the axis of the beam, tube, or tendon. All instrumentation was read and recorded with a computerized data-acquisition system. The locations of strain gages used on the beam and cover plates are shown in Fig. 5. Strain gages were offset from the support centerline because of the sole plate. They were also offset from the cover plate cutoff points to avoid the high stress gradients at these locations. A total of 28 strain gages were placed on the beam and cover plates--two each on the top and bottom surfaces of the beam at each of the numbered sections in Fig. 5. Strain gages were placed on the postcompression tubes of ST1 at a section approximately midway between the lateral restraint and each end of the tube to detect bending. At each section, six strain gages were arranged around the tube to avoid stress concentrations of the seam and the corners of the tubes. A total of 38 strain gages were used on ST2. Strain gages on the compression struts were similar to those on the postcompression tubes--sections of six strain gages at the midpoint of each strut. Strain gages were also positioned near the pin bracket on one tube to determine the amount of bending. Each of the 1 1/4 in. diameter (0.032 m) tendons were also strain gaged to determine the tension being applied to the truss. Vertical-Load Tests For each of the strengthening schemes, a nominal strengthening force was established. This force was the amount of compression in the tubes |ST1: 266.9 kN (60 kips) compression per tube~ or tension in the tendons |ST2: 444.8 kN (100 kips) tension per tendon~ determined to produce the TABULAR DATA OMITTED desired stress change in the mockup. For the majority of tests performed on the mockup, the strengthening schemes were loaded to these levels. However, in order to determine the behavior of the mockup with each strengthening scheme in place, tests were also performed with strengthening loads both above and below these nominal design values. For ST1, compression loads of 177.9 kN (40 kips) and 333.6 kN (75 kips) per tube were also investigated. For ST2, tension forces of 222.4 kN (50 kips) and 578.24 kN (130 kips) per tendon were examined. A maximum vertical load of 191.3 kN (43 kips) was initially applied to the mockup to limit stress in the steel beam and cover plates to 124.1 MPa (18 ksi) compression or tension under various test conditions. For the later tests of the mockup with ST2 in place, this value was increased to approximately 378.1 kN (85 kips) to investigate the behavior of the mockup and strengthening scheme at higher stress levels. The mockup was also strengthened while a partial vertical load was present to simulate the replacement of a portion of the bridge deck. A summary of all tests performed on the mockup is presented in Table 1. Note that values given in the table are nominal values. Forces actually applied are slightly different and are given in the various figures that follow. ANALYSIS AND TEST RESULTS To illustrate the effectiveness of the strengthening systems on the mockup, deflections and strain distributions at the various sections shown in Fig. 5 were recorded. In the following, only deflections at the load point (the location of maximum deflection) and strains at section 4 (location of maximum strains) are presented. Test 1 established the deflection and strain characteristics of the unstrengthened mockup. At a vertical load of 191.3 kN (43 kips), the deflection at the load point was 1.87 cm 0.735 in.) downward. The finite-element analysis of the mockup predicted a downward deflection of 1.35 cm (0.531 in.) for a 191.3 kN (43 kip) load, indicating that the finite-element model was stiffer than the mockup under negative-moment bending. In general, the theoretical and experimental strains of the bottom flange were in excellent agreement. However, for certain load cases, top-flange strains (theoretical versus experimental) varied by as much as 50% (indicating the concrete slab of the mockup was much less effective in tension, because of cracking) from the finite-element model, which modeled an intact, elastic concrete deck. When the deck was in tension, top-flange strains approximated those computed for the steel beam with composite deck reinforcing. cracking in the deck obviously reduced the composite action of the mockup. Effects of ST1 Fig. 6 illustrates the eflects of vertical loading on the strengthened mockup with ST1 in place. The relationship between vcrtical load and deflection at the vertical load point for compressive forces of 177.9 kN, 266.9 kN, or 333.6 kN (40, 60, 75 kips) per tube is illustrated. Also shown in Fig. 6 is the deflection of the unstrengthened mockup. The initial positive deflections indicate the effect of the postcompression that was applied first. A review of the various curves in Fig. 6 reveals that: (1) The deflection at the load point decreased with increasing postcompression force; (2) the reduction in deflection remained essentially constant throughout the application of vertical loading; and (3) the compression tubes added no significant stiffness to the mockup (note the essentially parallel lines). Fig. 7 gives the strain distributions for the mockup at sections 4. Only strains for a postcompression load of 266.9 kN (60 kips) per tube have been presented. The solid line represents the strains for an unstrengthened beam with a vertical load of 191.36 kN (43 kips). The dashed line indicates the effect of postcompression on the mockup. As expected, the tension strains increased with the amount of postcompression applied. This figure also presents a line representing strains due to ST1 and a vertical load of 191.3 kN (43 kips). By comparing the strengthened and unstrengthened beam results, one can determine the effectiveness of ST1. As indicated in Fig. 7, ST1 decreased the bottom-flange compression strains and increased the top-flange tension strains. At section 4, ST1 reduced the bottom-flange compressive strains 36%, while increasing the top-flange tensile strains 18%. Although the percentage varied, similar results occurred when post-compression forces of 177.9 kN (40 kips) and 333.6 kN (75 kips) were applied. Eliminating this problem (the increase in top-flange strains) would require ST1 to be positioned below the bottom flange, which is not practical, because of the presence of the pier. Effects of ST2 Fig. 8 illustrates the effects of vertical loading when ST2 was attached to the mockup. The graphs plot vertical load versus deflection for three truss loads: 222.4 kN, 444.8 kN, and 578.2 kN (50, 100, and 130 kips) per tendon as well as for the unstrengthened mockup. The graphs display behavior similar to that found for ST1 in that deflection of the load point decreased with increasing tension in the tendon and the relationship between applied loading and deflection was essentially linear. As with ST1, the ST2-strengthened beam curves are essentially parallel to the unstrengthened curves; thus, ST2 did not add appreciable stiffness to the mockup. Shown in Fig. 9 are the strain distributions for the mockup with ST2 in place; the stains at section 4 for tendon loads are 222.4 kN, 444.8 kN, and 578.2 kN (50, 100, and 130 kips) per tendon. Data in Fig. 9 indicate that ST2 essentially caused only positive-moment bending on the mockup--compression strains in the top flange and tension strains in the bottom flange. The positive moment increased as the force in the tendons increased. Comparing the strengthened and unstrengthened beam strains indicates that the technique is very effective in reducing the strains in the loaded mockup. Since ST2 causes essentially only positive-moment bending in the mockup, both the top- and bottom-flange strains were reduced. For example, at section 4, with 444.8 kN (100 kips) per tendon and 191.3 kN (43 kips) vertical load, the top-flange tension strain was reduced 46%, while the bottom-flange compression strain was reduced 48%. The final test of the investigation involved load testing to failure of the mockup with ST3 in place--force applied to the lower flange. The deflection of the mockup was essentially linear up to the previous load of 191.3 kN (43 kips). Yielding in the bottom flange at section 4 first occurred at a vertical load of 467.1 kN (105 kips). At a vertical load of 556.0 kN (125 kips), yield stress was exceeded at section 2, 4, and 6. Buckling of the flange occurred exactly at section 2, 0.152 m (6 in.) past the end of the cover plates. As expected, the test to failure established that the mockup would fail before the strengthening system. For the vertical load of 556.0 kN (125 kips), yielding was not detected at any point in the strengthening system. The final compression strut load was 756.2 kN (170 kips) per tube, which is approximately 30% above the calculated American Institute of Steel Construction (AISC) allowable load for this element. The force in the 0.032 m (1 1/4 in.) diameter tendons increased from 578.2 kN (130 kips) prior to vertical loading to 667.2 kN (150 kips) at 556.0 kN (125 kips) of vertical load. CONCLUSIONS The following conclusions were developed as a result of this study. Postcompression strengthening (ST1), when applied to the negative-moment region, caused positive moment and tension in the section. While there was a reduction in bottom-flange beam stress, an undesirable increase in top-flange-beam and deck stress also resulted. Superimposed truss strengthening (ST2), when applied to the negative-moment region, caused only positive moment in the section. Stress reduction in both the top- and bottom-beam flanges was significant. Research is presently in progress to install and test ST2 on an existing bridge. Neither of the strengthening schemes (ST1 and ST2) caused a significant increase in stiffness of the mockup. ACKNOWLEDGMENTS This paper summarizes a portion of the work conducted as part of a research program investigating alternative methods of bridge strengthening. The program has been conducted by the Bridge Engineering Center under the auspices of the Engineering Research Institute of Iowa State University and funded by the Highway Division, Iowa DOT, and the Iowa Highway Research Board. The writers thank the engineers of the Iowa DOT, especially William A. Lundquist, Bridge Engineer, and John P. Harkin, Chief Structural Engineer, for their cooperation and counsel. Special thanks are accorded to the several civil engineering graduate and undergraduate students at Iowa State University for their assistance during various states of the program. The opinions, findings, and conclusions expressed herein are those of the writers and are not necessarily those of the Iowa DOT. APPENDIX. REFERENCES Dunker, K. F.,Klaiber, F. W., Daoud, F. K., Sanders, W. W., Jr. (1990). 'Strengthening of continuous composite bridges.' J. Struct. Engrg., ASCE, 116(9), 2464-2480. Dunker, K. F., Klaiber, F. W., and Sanders, W. W., Jr. (1986). 'Post-tensioning distribution in composite bridges.' J. Struct. Div., Proc., ASCE, 112(11), 2540-2553. Ferjencik, P., and Tochacek, M. (1975). Die Vorspannung im Stahlbau |prestressing in steel structures~. Wilhelm Ernest & Sohn, Berlin, Germany (in German). Kandall, C. (1968).'Increasing the load-carrying capacity of existing steel structures.' Civ. Engrg., 38(10), 48-51. Kim, J. B., Brungraber, R. J., and Yadlosky, J. M.(1984). 'Truss bridge rehabilitation using steel arches.' J. Struct. Engrg., ASCE, 100(7), 1588-1597. Klaiber, F. W., Dunker, K. F., Planck, S. M., Sanders, W. W., Jr. (1990a). 'Strengthening of an existing continuous-span, steel-beam, concrete-deck bridge by post-tensioning.' ISU-ERI-Ames 90210, Engineering Research Institute, Iowa State University, Ames, Ia. Klaiber, F. W., Dunker, K. F., Sanders, W. W., Jr. (1990b). 'Strengthening of existing bridges (simple and continuous span) by post-tensioning.' External pre-stressing in bridges (SP-120), American Concrete Institute, Detroit, Mich., 207-228. Klaiber, F. W., Dunker, K. F., Sanders, W. W., Jr. (1982). 'Strengthening of single span steel beam bridges.' J. Struct. Engrg. Div., ASCE, 108(12), 2766-2780. Klaiber, F. W., Wipf, T. J., Dunker, K. F., Abu-Kishk, R. B., and Planck, S. M. (1989). 'Alternate methods of bridge strengthening,' ISU-ERI-Ames-89262, Engineering Research Institute, Iowa State University, Ames, Iowa. Reiffenstuhl, H. (1982a). 'Das Vorspannen von Bewehrung auf Druck: Grundsatzliches und Anwendungsmoglichkeiten |prestressing of reinforcing in compression: fundamentals and application possibilities~.' Beton-und Stahlbetonbau, Berlin, 77(3), 69-73 (in German). Reiffenstuhl, H. (1982b). 'Eine Brucke mit Druckspannbewehrung-Konstuktion, Berechnung, Baudurchfuhrung, Messungen |a bridge with compression-stressed reinforcing-system, computation, construction, field measurements~.' Beton-und Stahlbetonbau, 77(11), 273-278 (in German). Reiffenstuhl, H. (1983). 'Verstakung eines Sporthallendaches mit Druckspannbewehrung |strengthening of an athletic building roof with compression stressed reinforcement~.' Beton-und Stahlbetonbau, 78(6), 149-154 (in German). Wiley, W. E., Klaiber, F. W., and Dunker, K. F. (1990). 'Behavior of composite steel bridge betrays subjected to various post-tensioning schemes.' Transp. Res. Record 1123, Transportation Research Board, Washington, D.C., 63-72. S. M. Planck, Engr., HDR Engrg., Inc., 5175 Hillside Circle, El Dorado Hills, CA 95630-5700. F. W. Klaiber, Prof., Dept. of Civ. and Constr. Engrg., and Mgr., Bridge Engrg. Ctr., Iowa State Univ., Ames, IA 50011. K. F. Dunker, Assoc. Prof., Dept. of Civ. and Const. Engrg., Iowa State Univ., Ames, IA.