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Bhavior of Bolted Column Beams Connection

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Behavior of Unbonded, PostTensioned, Precast Concrete Connections with Different Percentages of Mild Steel Reinforcement This paper presents the results of tests performed on post-tensioned, precast concrete moment-resisting, beam-column connections containing different mild steel reinforcement contents. In the experimental program, five hybrid connections were tested under displacementcontrolled reversed cyclic loading. The main variable was the mild steel’s percentage of contribution to the flex
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  32 PCI JOURNAL Behavior of Unbonded, Post-Tensioned, Precast ConcreteConnections with DifferentPercentages of Mild SteelReinforcement This paper presents the results of tests performed on post-tensioned, precast concrete moment-resisting, beam-column connections con-taining different mild steel reinforcement contents. In the experimental  program, five hybrid connections were tested under displacement-controlled reversed cyclic loading. The main variable was the mild steel’s percentage of contribution to the flexural capacity of the con-nection, ranging from 0% to 65% of the connection’s moment capac-ity. Each hybrid connection was compared with the test result of thereference monolithic subassembly in terms of connection strength,stiffness degradation, energy dissipation, and permanent displace-ment. The objective of this study was to determine the effect of mild steel reinforcement content on the behavior and performance of post-tensioned, precast concrete hybrid connections. The response of post-tensioned, precast concrete hybrid connections approached that of the monolithic subassembly as the mild steel reinforcement contentincreased. Connection capacities were well predicted by the joint gap-opening approach. The design assumptions of hybrid connections arebest satisfied with a 30% mild steel reinforcement contribution to theconnection’s flexural capacity . A lthough precast concrete con-struction provides high-qualitystructural members, the perfor-mance of the overall structural systemis mostly governed by the capacity of the connections. A properly designedand detailed beam-to-column connec-tion should be able to transfer forcesbetween precast concrete elements,even when subjected to occasionaloverloads. According to Englekirk, 1 theconcept of properly designed connec-tion was expected to do the following: ã Avoid extensive welding; ã Incorporate adequate tolerancesfor assembly; Onur Ertas Research AssistantDepartment of Civil EngineeringBogazici UniversityBebek, Istanbul, Turkey Sevket Ozden, Ph.D. Assistant ProfessorDepartment of Civil EngineeringKocaeli UniversityKocaeli, Turkey 04-061_Ozden.indd 32   04-061_Ozden.indd 32 2/13/07 4:26:01 PM 2/13/07 4:26:01 PM  March–April 2007 33 which passed through the joint, wasshown to be effective in contributing tothe joint’s core shear strength. More-over, the analytical study revealedthat the introduction of a prestressingtendon at mid-depth of a concrete sec-tion that was reinforced equally on topand bottom with non-prestressing steelincreased the cracking and flexuralstrengths of the section without signifi-cantly reducing its ductility. 5 Different types of precast concreteconnections, of which three werepost-tensioned or composite connec-tions with post-tensioning, were testedby French et al. 6,7 It was reported thatfailures were observed either inside oroutside of the connection region forthe post-tensioned concrete specimens.The strength and energy dissipationcharacteristics of the connections wereadequate with respect to monolithicspecimens regardless of the location of failure.In the research of Palmieri et al., 8  two types of connections were de-signed based on nonlinear elastic post-tensioning concepts. The main differ-ence between specimens (UMn-PTSand UMn-PTB) was the member sizeand post-tensioning type. Furthermore,a third connection (UT-PTS) was de-signed with pre-tensioning instead of post-tensioning. It was reported thatthe prestressed concrete specimensexhibited the desired nonlinear elasticbehavior through the 3% drift level.In the development of a moment-resisting precast concrete connectionwith post-tensioning, a multiyear re-search program was developed by theNational Institute of Standards andTechnology (NIST). In phase I of theexperimental program, monolithicand post-tensioned precast concretesubassemblies were tested. The post-tensioning bars were fully grouted inthis phase of the study. 9 As a result of the low energy dissipation observedin the precast concrete specimens of phase I, several methods for increas-ing the energy dissipation capacity of the precast concrete connections wereexplored in phase II. These meth-ods include changing the location of the post-tensioning steel and usingprestressing strands instead of post-tensioning bars. 10 Zero slope hysteresis loops were ob-served upon load reversal in both thephase I and phase II tests of the NISTresearch. The subassemblies sufferedexcessive stiffness degradation andslip at low displacements. It was con-cluded that such stiffness degradationwas mainly caused by a reduction inthe effective clamping force developedby the prestressing bars. 10 Because theprestressing bars of the phase I andphase II specimens were fully grouted,the plastic deformation of the bars wasforced to short bar lengths, resultingin strains beyond the bar proportionallimits and even beyond the ultimatelimit.The required ultimate displacementof the subassembly could be achievedwithout exceeding the proportionallimit of the prestressing steel by choos-ing correct debonded bar lengths, anoption that also solved the problem of the zero slope hysteresis loops. 11 Thisconcept of correct debonded bar lengthwas applied in phase III of the NISTresearch, and the specimens did notexhibit zero stiffness upon load rever-sals. 10 In phase IV of the NIST research,post-tensioned, precast concrete con-nections with mild steel reinforcementwere studied. In these connections, themild steel reinforcement and the pre-stressing tendons contributed to theconnection moment capacity. It wasconcluded that the use of bonded mildsteel reinforcement at the top and bot-tom of the cross section, along with theuse of unbonded prestressing tendons,resulted in the most practical combina-tion in the response of the subassem-bly. 12 Phase IV tests also demonstratedthat hybrid connections were self-centering and displayed essentially noresidual drift.Two ungrouted, post-tensioned, pre-cast concrete beam-to-column jointsubassemblies were tested by Priest-ley and MacRae. 13 One of the subas-semblies represented an exterior joint,while the other represented an interior joint. It was reported that the structuralresponse of both specimens was satis-factory, despite the very low levels of reinforcement provided in the beams,columns, and joints.The objective of the Precast SeismicStructural Systems (PRESSS) researchprogram was to develop an effective ã Avoid large, formed wet joints;and ã Minimize crane time with proper joint detailing.Post-earthquake field investigationsof precast concrete structures after the1999 Kocaeli and Duzce earthquakesin Turkey revealed that the damageand performance of the buildings wereclosely related to the performance of the connections. As a result, a two-phase research program on the perfor-mance of ductile beam-column connec-tions of precast concrete elements wasdeveloped at the Bogazici and Kocaeliuniversities following the 1999 earth-quakes.This research program is funded bythe Scientific and Technical ResearchCouncil of Turkey (TUBITAK-Proj-ect No. ICTAG I589) and the TurkishPrecast Concrete Association. PhaseI of the study focused on the per-formance of composite, bolted, andcast-in-place concrete connections 2  while phase II mainly dealt with theperformance of post-tensioned, pre-cast concrete connections containingvarious levels of mild steel reinforce-ment. This paper presents the experi-mental observations from phase II of the research program. All connectionsin this research program were detailedaccording to ACI T1.2-03. 3 LITERATURE SURVEY The performance of post-tensioned,precast concrete connections has beenthe subject of considerable research inthe past two decades. The location of prestressing tendons, the level of post-tensioning force, and the use of bondedor unbonded tendons were the mainvariables in the available literature.In the study of Park and Thomson, 4  10 nearly full-scale beam-to-interior-column precast concrete subassemblieswere tested with different proportionsof prestressing tendon area to non-prestressed steel area. These testsshowed that the ductility of the pre-stressed concrete beams could beenhanced with the presence of non-prestressed reinforcement in the com-pression zones. A central prestress-ing tendon at mid-depth of the beam, 04-061_Ozden.indd 33 04-061_Ozden.indd 33 2/13/07 4:26:03 PM 2/13/07 4:26:03 PM  34 PCI JOURNAL TEST SPECIMEN ANDCONNECTION DETAILS Phase II test specimens of the currentstudy were modeled as exterior jointsof a multistory building. The mono-lithic reinforced concrete specimenwas designed according to the strong-column weak-beam design philosophyfor high seismic regions, while thedesign of post-tensioned, precast con-crete connections was based on boththe ACI T1.2-03 design guidelines andthe recommendations and conclusionsfrom previously published research.All test specimens were scaled to ap-proximately half of the prototype struc-ture in geometry.As a result, the cross-sectional di-mensions of the beam were 11.8 in. × 19.7 in. (300 mm × 500 mm) andthe clear span of beam was 5.25 ft(1.6 m). Hence, the shear-span-to-height ratio ( a/h ) was about 3.2. Theheight of the column was 6.3 ft (1.9 m),and it had a square cross section with15.75 in. (400 mm) dimensions. Thecover thickness in the precast con-crete beam and column was 0.8 in.(20 mm). The dimensional and rein-forcement details of the subassemblyare given in Fig. 1 . Monolithic Specimen The monolithic reinforced concretespecimen (M) was designed accord-ing to the requirements for high seis-mic zones. The longitudinal reinforce-ment ratio in the columns for specimenM and also for the post-tensioned,precast concrete specimens was 2%.Along the column height, includingthe joint region, the spacing of closedstirrups was approximately 4 in. (100mm). The beam’s flexural reinforce-ment consists of four 0.8-in.-diameter(20 mm) and three 0.8-in.-diameterreinforcing bars placed at the top andthe bottom of the beam, respectively(Fig. 1). The bottom reinforcement of the beam was less than the top rein-forcement due to the effects of gravityfor a continuous beam.For all test specimens, monolithicand precast concrete, the same gradeof steel was used for the longitudinaland lateral reinforcement. The nomi-nal diameter of the reinforcing steelfor the longitudinal and transverse re-inforcement was 0.8 in. (20 mm) and0.4 in. (10 mm), respectively. Yieldand ultimate strengths of the 0.8-in.-diameter reinforcing bars were68.5 ksi (472 MPa) and 83.3 ksi(574 MPa), respectively, while thesevalues for the 0.4-in.-diameter trans-verse reinforcement were 72.5 ksi (500MPa) and 81.2 ksi (560 MPa), respec-tively. Elongation of the reinforcingsteel at ultimate strength was 14%for the 0.8-in.-diameter bar and 13%for the 0.4 in. bar. The compressivestrength of the concrete for specimenM was 5801 psi (40 MPa). Post-Tensioned, Hybrid PrecastConcrete Connections All of the precast concrete beamsand columns were produced in a pre-cast concrete manufacturing facility.The main variable investigated in thepost-tensioned, precast concrete speci-mens was the mild steel reinforcementcontent in the connection region. Inthe first precast concrete specimen, nomild steel reinforcement was used inthe connection and the flexural momentseismic structural system for precastconcrete buildings in order to reducebuilding costs through rapid site erec-tion and low labor requirements. 14 Aspart of this program, a 60% scale five-story precast concrete building wastested under pseudo-dynamic test pro-cedures at the University of Californiaat San Diego (UCSD). The major ob- jective of the test was to develop de-sign guidelines for precast/prestressedconcrete seismic systems. 15,16 The building comprised four differ-ent ductile structural frame systems inone direction of response and a jointedstructural wall system in the orthogonaldirection. The seismic frame connectiontypes were hybrid post-tensioned, pre-tensioned, tension-compression yield(TCY)–gap, and TCY connections. Itwas reported that the behavior of thestructure was extremely satisfactory.Damage to the building in the frame di-rection of response was much less thanthe expected damage for an equivalentreinforced cast-in-place concrete struc-ture. On the other hand, the beams of the hybrid frames experienced sometorsion about the longitudinal axes dur-ing testing. 16 Fig. 1. Dimensions and reinforcement detail of monolithic specimen M. Note:1 in. = 25.4 mm; 1 ft = 0.3048 m.    6 .   3   f   t   8 .   2   f   t 15.75in.    1   5 .   7   5   i  n .   1   9 .   7   i  n . 11.8in.6.1ft5.25ftFour20 mm diameterThree 20 mm diameter10 mm diameter, 4 in. spacing 04-061_Ozden.indd 34 04-061_Ozden.indd 34 2/13/07 4:26:03 PM 2/13/07 4:26:03 PM  March–April 2007 35 instead of pipes to create more dimen-sional tolerance to compensate for po-tential production errors and to reserveadequate space for multiple mild steelreinforcing bars.The 19.7-in.-long (500 mm) rectangu-lar steel boxes installed at the connectionregion had cross-sectional dimensionsof 4.7 in. × 2.35 in. (120 mm × 60 mm)and were located along the same axeson the beam and the column. To preventthe steel boxes from sliding relative tothe beam concrete, steel rods, whichserved as ribs, were welded around theboxes. In addition, steel anchors passingthrough the box cross section were in-stalled to prevent any possible sliding of the infill grout with respect to the steelbox (Fig. 3).Steel plates were placed at the topand bottom of the beam cross sectionat the connection region to minimizecrushing of the beam concrete. The steelplates were connected to each other bywelding two 0.4-in.-diameter (10 mm)reinforcing bars to the plates. The steelplates were also anchored to the beamconcrete. In this beam end region, closedstirrups were installed with an on-centerspacing of 2.8 in. (70 mm).Initially, in the assembly process,the 0.6 in. (15 mm) gap between theprecast concrete beam and the col-umn was filled with a self-leveling,nonshrink grout with a compressivestrength of approximately 8700 psi (60MPa). After 24 hours of grout curingtime, mild steel reinforcing bars wereplaced through the steel boxes and thethreaded ends were hand tightened.To determine when the mild steel re-inforcement would yield during thetest, strain gauges were attached tothe bars.Steel plates were placed on bothends of the steel boxes (as washers),and the mild steel reinforcing barswere locked with nuts to prevent slip-ping ( Fig. 4 ). The steel boxes were thenfilled with the same self-leveling, non-shrink grout. Finally, 0.5-in.-diameter(13 mm) prestressing strands with an ul-timate strength of 270 ksi (1860 MPa)were placed at mid-depth of the beamwas carried solely by the prestressingstrands. This specimen was designatedPTM0.For the second precast concretespecimen, PTM10, the contribution of mild steel reinforcement to the flexuralmoment capacity was 10%. This 10%ratio is the minimum level of mild steelreinforcement in precast concrete con-nections stated in the Turkish  BuildingCode Requirements for Prestressed Concrete ( TS3233) 17 and Specifications for Structures to Be Built in Disaster  Areas . 18 The mild steel reinforcementcontribution to flexure was increasedto 30% (specimen PTM30) and 50%(specimen PTM50) in the third andfourth precast concrete connection de-signs, respectively.The fourth specimen (PTM50) metthe upper limit for mild steel reinforce-ment contribution to flexural capac-ity according to ACI T1.2-03 designguidelines. In the fifth and last precastconcrete specimen, the mild steel rein-forcement contribution to the flexuralcapacity of the connection was 65%(specimen PTM65), exceeding theupper limit of ACI T1.2-03 design rec-ommendations.The geometry and reinforcementdetails of the precast concrete beams,except for specimen PTM0, were thesame ( Fig. 2) . All precast concretebeams had a blockout channel at thetop and the bottom of the cross sectionto allow installation of mild steel rein-forcement during the assembly process.The length of the channel was 39.4 in.(1000 mm), with cross-sectional di-mension of 5.8 in. × 3.9 in. (150 mm × 100 mm). Also, there was a polyvi-nyl chloride (PVC) pipe with a 3.9 in.(100 mm) inner diameter at the centerof the beam cross section for installa-tion of the prestressing strands. Four0.8-in.-diameter (20 mm) mild steelreinforcing bars were placed at thetop and bottom of the precast concretebeam as main longitudinal reinforce-ment. The detail is shown in Fig. 2.The cross-sectional dimensions of the precast concrete beams in the con-nection region were the same as thoseof the monolithic specimen. For theprecast concrete members (beams andcolumns), rectangular steel boxes wereinstalled in the connection region asillustrated in Fig. 3 . Boxes were used Fig. 2. Details of post-tensioned specimens. Note: 1 in. = 25.4 mm. 04-061_Ozden.indd 35 04-061_Ozden.indd 35 2/13/07 4:26:03 PM 2/13/07 4:26:03 PM
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