CHAPTER 2 LITERATURE REVIEW

Transcription

1 12 CHAPTER 2 LITERATURE REVIEW 2.1 GENERAL For understanding the seismic behavior of precast concrete structures, the study of behavior of joints is of great importance as the connections form the weakest link in the structure. Experimental studies are necessary as it gives the realistic response of the structure. But Finite Element Modeling as gained importance as experimental investigations though accurate can be time consuming and costly. The use of Finite Element packages to model the structural elements is faster and cost effective. Hence, many parameters can be studied by modeling the structural elements using Finite Element packages. Several researchers worldwide have investigated the behaviour of precast beam-column connections under earthquake loading both experimentally and analytically. A detailed review of the literature has been carried out to understand the behaviour of precast beam column connections under cyclic loading. Among these the most significant literatures are briefly summarized in this chapter. The finite element modeling related to precast beam-column modeling related work are also reviewed.

2 OVERVIEW OF LITERATURE Studies on Experimental Investigations of Precast Beam Column joints under Seismic Loading Wet Connections Bull and Park (1986) investigated the performance of cast-in-place reinforced concrete moment resisting frames incorporating precast prestressed concrete U- beam shells subjected to seismic loading. The precast beams acted as permanent formwork and were not connected by steel to the cast-in-place concrete of the beam or column. Three full scale exterior beam column subassemblies were tested. It was concluded that the two specimens that were designed for seismic loading was satisfactory and can be used in ductile seismic resisting frame. The third specimen that was designed without special provisions for seismic loading was suitable for non seismic resisting frames where the seismic loads are carried by walls and other structural systems. Cheok and Lew (1991) attempted to develop moment resisting precast concrete connections in seismically active regions by testing four onethird scale monolithic concrete beam-to-column connections. Two were designed according to the 1985 Uniform Building Code (UBC) Seismic zone 2 criteria and two according to UBC zone 4 criteria. In addition, two precast post-tensioned concrete beam-to-column connection similar in design to the monolithic zone 4 specimens were tested. It was concluded that posttensioned precast concrete beam-column connections are strong and as ductile as the monolithic connections, for high seismic regions. However, the per cycle and cumulative energy dissipation characteristics, of the precast beamcolumn connections could be improved.

3 14 Cheok and Lew (1993) tested eight 1/3-scale model precast beam to column interior connections under cyclic loading. In general, the precast concrete specimens had higher storey drifts at failure and higher initial stiffness than monolithic specimens. The measured maximum concrete strengths exceeded the calculated values and performed as well as monolithic specimens in most cases. The cumulative energy dissipated to failure by precast specimens was greater than that of monolithic specimens. Castro et al (1994) investigated the seismic performance of a newly developed precast system with the concrete members at the ends and the bar connections are located at the middle of the precast members where the stresses are small. Tests were conducted on nine two-thirds scale interior beam-column joints including a monolithic specimen. The behaviour with respect to bending strength of the beams, the shear strength and bond deterioration at the beam column joint core and energy dissipation were studied. It was concluded that precast concrete specimens can sustain inelastic deformations under cyclic loading and can be ductile as cast-in-situ specimens. Loo and Yao (1995) conducted experimental investigations on eighteen half scale interior connection models to evaluate their strength and ductility properties under static and repeated loading. Eighteen half scale models were fabricated, making six groups of two precast specimens (Type A and Type B) and one monolithic specimen. The perspective view of the connection Type A and Type B are shown in Figures 2.1 and 2.2 respectively. All models had the same dimensions but different concrete strengths and / or steel ratios. It was concluded that under both static and repeated loading, the precast connections attained a higher flexural strength than monolithic connections. The precast connection types under repeated loading, possessed larger energy absorbing capacities than monolithic models.

4 15 Figure 2.1 Perspective View of Type A Connection (Loo and Yao, 1995) Figure 2.2 Perspective View of Type B Connection (Loo and Yao, 1995)

5 16 Stone et al (1995) developed a hybrid precast system, which was designed to have the same flexural strength as a conventionally reinforced system with the same beam size. The hybrid system was self-centering and displayed essentially no residual drift. The hybrid system had a very large drift capacity. The hybrid system dissipated more energy per cycle than the conventional system for upto 1.5 percent drift. The concrete in the hybrid suffered negligible damage, even at drifts up to 6 percent. Restrepo et al (1995) conducted tests on six assemblages of perimeter frames under quasi static cyclic reversed loading. Four units were connected at the beam midspan and two units were connected at the beam-tocolumn joint region. Units 1, 2 and 3 had connections between precast concrete elements at mid span of beams consisting of overlapping hooks or straight splices in cast-in-situ concrete showed excellent performance under cyclic loading. Unit 4 had strong regions at the ends of the precast beams and a diagonally reinforced cast-in-place connection region at the beam midspan, with diagonal bars connected by bolted steel plates welded to the bars. The test of this unit showed limited ductility response due to the bursting forces that had not been considered in the initial design. It showed full ductility after the damaged region was repaired by adding transverse reinforcement and bearing rods at the bend of the diagonal reinforcement. Unit 5 had the precast concrete placed between the columns and a cast-in-place concrete joint core between the ends. Unit 6 had precast concrete beam element passing the column and the longitudinal column bars grouted in vertical corrugated ducts in beam-to-column joint region. Both Unit 4 and 5 showed excellent performance in terms of strength and ductility. Preistly and MacRae (1996) tested two ungrouted post-tensioned precast concrete beam to column sub-assemblages under cyclic reversals of inelastic displacements. One sub-assemblage represented an exterior joint

6 17 while the other was an interior joint of a one-way prestressed concrete frame. The test specimens were designed with gradually reduced beam and joint shear reinforcement compared with equivalent monolithic joints, but with special spiral confinement of the beam plastic hinge regions. Both subassemblages performed well, with only minor damage upto drift ratios of 3 percent. It was concluded that satisfactory seismic performance can be expected from well designed ungrouted precast, post-tensioned concrete frames. Stanton et al (1997) studied a precast framing system with precast elements connected by unbounded post tensioning steel and bonded reinforcing bars. The behavior was compared with a pair of conventional monolithic; cast-in-place frames. It was concluded that a hybrid system can be designed to have the same flexural strength as a conventionally reinforced system with members of the same size. The shear resistance of the hybrid system was superior to that of a conventionally reinforced frame. The hybrid system was self centering and displayed no residual drift and had very large drift capacity. It dissipated more energy per cycle than conventionally reinforced frame up to a drift of 1.5 percent. It was also observed that damage in the hybrid system was minimal. Elliot et al (1998) studied the behavior of structural beam to column connections in precast concrete skeletal and portal structures. The types of connections adopted for beam to column testing is shown in Figure 2.3. In most of the connections, ductile modes of failure were observed. The authors concluded that the frame stability can be enhanced by utilizing the strength and stiffness of precast concrete beam to column connections in a semi rigid frame analysis. This method was found for internal connections but not for edge connections.

7 18 Figure 2.3 Types of Connections Used in Precast Beam to Column Tests (a) Billet (b) Welded Plate ( Elliot et al, 1998) Vasconez et al (1998) developed a high energy absorbing joint for precast concrete structures in seismic zones. The material of the joint was a High Performance Fibre Reinforced Cement Composite (HPFRC) matrix. The FRC based connection design was successful in making the connection act as a plastic hinge by spreading the yielding from centre to the interfaces. The steel fibres used in cast-in-place connection lead to an increase in strength, energy capacity, stiffness, displacement and rotation capacities. There was a decrease in damage and shear deformations. Steel fibres were found to be more effective in improving the response of the joint than polyvinyl alcohol fibres. It was also observed that reducing the confinement provision of the ACI by 50 percent in steel fibre reinforced connections resulted in improved behaviour compared to normal RC specimens with full confinement.

8 19 Alcocer et al (2002) conducted experiments on two full scale beamcolumn precast concrete joints under uni-directional and bi-directional loading that simulated earthquake type loadings. The most relevant feature of the connection is that conventional mild steel reinforcing bars or prestressing strands, rather than welding or special bolts, were used to achieve beam continuity. Specimen design followed the strong-column weak-beam concept. Beam reinforcement was purposely designed and detailed to develop hinges at the joint faces and to impose large inelastic shear force demands into the joint. As expected, the joint controlled the specimen failure. In general, the performance of both beams-to-column connections was satisfactory. Joint strength was 80 percent of that expected for monolithic reinforced concrete construction. Specimen behavior was ductile due to hoop yielding and bar pullout, while strength was nearly constant up to drifts of 3.5 percent. Khaloo and Parastesh (2003) studied four types 2/5 scale model precast connection and one monolithic concrete beam-column connections. In the precast specimen, the load was transferred in the spliced reinforcement by a combination of lap splicing and end anchorage of bars. The end portions of the beams were designed in the form of a channel that sat on the column bearing area and carried the shear stresses due to the slab. Then the connection length region is grouted to form the monolithic connection. The main variables of this study were the level of axial load of the column, spacing of beam stirrup in the connection length region, gravity load on the beam and use of steel fibre in grout of the connection region. The authors concluded that all the specimens were capable of providing strength, ductility, and storey drift and energy dissipation comparable with that of reference specimen. A reduction in the axial load, the use of steel fibre region in grout of the connection length region significantly increased the ductility, storey drift, strength and energy absorption of the precast connection. The presence

9 20 of concentrated gravity loads on beams increased the strength, ductility and storey drift as compared with reference specimen. Khaloo and Parastesh (2003) carried out an experimental study to investigate a simple moment-resisting precast concrete beam-column connection under cyclic inelastic loading. Four precast beam-column connections and one monolithic connection were tested. The variables examined were the connection length of reinforcements and presence of transverse bars at mid height of connection. It was concluded that the reduction in connection length reduced strength, ductility and energy absorption. The failure mode changed toward partial separation and slippage of bond between the precast concrete beam and the cast-in-place grout. The presence of transverse bars in the connection length enhanced the seismic behavior of the precast connection system. Blandon and Rodriguez (2005) conducted experimental study of a half-scale two storey precast concrete structure built with a dual structural system (combination of structural walls and frames). A typical feature in some of the beam to column connections in the test structure was that the beam bottom longitudinal bars in the joint region were poorly anchored. The test structure was subjected to simulated seismic loading until the structure reached failure. The responses of the precast structural elements and their connections during testing, including beam to column connections, column to foundation connections and the diaphragms were observed. A pull-out was observed in the beam bottom bars. Due to this failure pattern, the use of dual structural systems is a promising solution for the construction of seismic resisting precast concrete building. In a dual system, the deformation demands in the beam to column connections of the frame subsystem can be significantly reduced when compared with the case the case of a building built

10 21 with frames only system for resisting lateral loads. As a result, the frame system could be designed for limited ductility. Joshi et al (2005) performed experiments on two precast and an equivalent monolithic exterior beam-column joint sub-assemblage specimen. The schemes for the anchorage of beam bars were different in the two set of specimens. In the first type of detailing, a single U-bar is used as top and bottom beam reinforcements as shown in Figure 2.4. The other type of detailing conforms to the Indian Standard Code for ductile detailing of reinforced concrete sections as shown in Figure 2.5. In precast specimens, the connectivity of reinforcement bars between beam and column was achieved by welding the exposed bars of the components in the point region. Under displacement controlled pseudo-static loading, the monolithic specimen with beam bars anchored into the column performed better than the monolithic specimen with continuous U-bars as beam reinforcement. The cumulative energy dissipation for the monolithic specimen with continuous U-bar reinforcement was more than the other monolithic specimen. The precast specimens with beam bars anchored into the column performed better than the corresponding monolithic beam. The precast specimen with continuous U- bars as beam reinforcement performed worse than the corresponding monolithic specimen, due to high average strength and stiffness deterioration. Of the two precast specimens, the one with the beam bars anchored into the column with the welding of the lap splices performed better than the one with continuous U-bars as beam reinforcement.

11 22 Figure 2.4 Details of the Reinforcement of the First Specimen (Joshi et al 2005) Figure 2.5 Details of the Reinforcement of the Second Specimen (Joshi et al 2005) Korkmaz and Tankut (2005) investigated the seismic behaviour of the connection detail proposed by an industrial partner and the specimens with improved details, in order to develop a moment resisting precast concrete beam to beam connection. In this study, six beam beam connection

12 23 subassemblies were tested under reversed cyclic loading simulating severe earthquake action. The first specimen was a monolithic specimen used as a reference specimen and tested to define the reference behaviour. The second specimen was a precast specimen, which was detailed by a company specializing in precast concrete production. The remaining specimens were modified according to the results of the formerly tested specimens. All of the specimens were identical in dimensions. All test specimens were 1/2.5 scaled models of the improved connection details used in the highly critical earthquake zones. The behavior of the precast members was compared with that of the reference one and with the others. Though the original connection did not perform well, the modified precast connection showed satisfactory performance and was recommended for use in seismic zones. Rodriguez and Blandon (2005) tested a half-scale two-storey precast concrete building incorporating a dual system representing a parking structure in Mexico City under simulated cyclic loading. The observed global response showed that showed the importance of the reinforced concrete wall participation in the response. This participation led to an important reduction in the deformation demands in the critical section of the precast frame members. The displacement ductility demand was found to be higher in the wall sub-system than in the frame sub-system of the dual system. Some of the beam to column details that had substandard reinforcing details had poor deformation capacity. But the observed and calculated deformation demands in these connections were not critical since they were significantly reduced by the wall interaction. Khoo et al (2006) tested two full scale precast concrete sub-frames in which the connection are constructed on the beam span and kept away from the column faces so as to avoid coinciding with the plastic hinge regions during seismic excitations. The variable examined was the connection detail.

13 24 One connection was composed of overlapping 90 hooks. All the beams longitudinal bars were spliced using such hooks and the overlaps started at about 1.8d from the column face where d is the effective beam depth. Two sets of stirrups spaced at 120mm were installed at the overlapping hooks. The other connection consisted of overlapping 180 hooks starting at 1.75 d from the column face. The stirrups were similar to that of the first connection. It was concluded that the precast concrete frames were capable of matching the overall performance of the monolithic connections and thereby providing moment resisting behavior. Chun et al (2007) assessed the effectiveness of headed bars terminating in exterior beam-column joints. Nine inter storey and five rooflevel joint specimens were tested under reversed cyclic loading. The primary test parameters were the anchorage type, size and arrangement of the beam bars and the heads and the detailing provided for roof joints. The test results indicated that hysteretic behaviour of exterior joints constructed with headed bars was similar or superior to joints constructed and tested with hooked bars. Head size with a net area of three to four times the bar area was sufficient to anchor the beam reinforcement effectively within the exterior beam column joint. It was also concluded that in addition to providing vertical U-bars at roof joints, heads on column bars should extend beyond the beam top bars to provide improved behavior. Nishiyama and Wei (2007) conducted cyclic load tests on seven precast, prestressed concrete beam to column joint assemblages. The experimental parameters studied were location of tendon anchorage, prestressing steel content in the beam section, concrete compressive strength and to investigate the shear strength of the beam to column joint. It was concluded that maximum load capacities for test units with inside anchorages were 9% to 13% less than specimens with anchorage outside the joint core.

14 25 Joint shear deformation was less in the test units with outside anchorage than in test units with inside anchorage. Damage to the beam to column joint assemblages and the decay of the maximum capacities of the test units were due not only to joint shear failure but also to anchorage deterioration of the prestressing steel. Xue and Yang (2010) studied the behavior of precast concrete connections in a moment resisting frame under cyclic loading. The connections studied were exterior connection, interior connection, T connection and knee connection. It was observed that Knee connections were less effective when compared to other connections. All the connections exhibited strong column-weak beam failure mechanism. It was concluded that all the connections performed satisfactorily in seismic conditions with respect to strength, ductility and energy dissipation capacity Dry Connections Dolan and Pessiki (1984) demonstrated that the behaviour characteristics of a welded monotonically loaded precast concrete connection can be simulated using models. Tests of one-quarter scale models of a single beam to column connection were conducted. Good agreement was found between the strength and the normalized moment rotation response of the model and the prototype. The effects of weld quality and design eccentricities had similar consequences in both model and prototype. Ochs and Ehsani (1993) tested five precast beam to column subassemblies under simulated earthquake type loading. The columns included steel plates or angles embedded in the columns and beams which facilitated field erection. Various connection details were studied. One connection was a monolithic specimen and four were precast specimens. Specimen P1 and PR1 consisted of two fabricated T-sections embedded in the

15 26 column. Each T-section had three holes to allow for the passing of the column longitudinal bars. Four No.7 standard 90 degree hooks were welded to the T- sections to provide adequate anchorage of the plate within the joint. The beam end included two large single angles to which longitudinal reinforcement was welded. The beam angles were welded to column section with fillet welds over full width of the beam. Specimen PR1 differed from P1, as additional intermediate reinforcement in the form of U-shaped No.6 bars was used. For specimens P2 and PR2 the connection was similar to that of specimen P1 and PR1. The only change made was that on the lower side a straight plate which extended from the column face was utilized instead of a T-section. It was concluded that precast concrete specimens performed similarly to that of monolithically cast concrete connection. The precast column was strong enough to force a plastic hinge away from the column face. The critical part of the precast connections was the welded beam bars as they initiated the failure of specimens. It was also observed that the intermediate reinforcing bars in the precast concrete specimens had less effect on the capacity of the specimen early in the test, however as the test progressed, these bars contributed to the specimen capacity. Ersoy and Tankut (1993) tested precast concrete beams with dry joints designed for multistory buildings located in a seismic area under reversed cyclic loading. The original beam consisted of two steel plates one at top, the other at the bottom, welded to the anchored steel plates in the column bracket and the beam. The design was later revised by adding side plates. The main variables were presence of side plates and joint width. The authors concluded that the joint width is an important parameter and therefore tolerances should be checked carefully during erection. The strength, stiffness and energy dissipation of the member with side plates were comparable to those of monolithic member.

16 27 Englekirk (1995) developed an energy absorbing ductile connector that can be to construct a seismic moment resisting frame of precast concrete components. The ductile connector was a ductile rod which was the yielding element. The function of the ductile rod was to accommodate post yield system deformations. Two types of ductile rods were used (i) milled and (ii) cast. It was suggested that the rod bearing transfer mechanism could be improved by increasing the bearing area and adding a confining plate at the face of the column. It was observed that the strain hardening characteristics of the material used in the casting were better than that used in milled rod. Priestley et al (1999) tested a large-scale five-storey precast concrete building constructed to 60 percent scale under simulated seismic loading. It was concluded that behaviour of the structure was extremely satisfactory, with only minimal damage in the shear wall direction, and no significant strength loss in the frame direction, though it was tested to drift levels upto 4.5 %, more than 100 percent higher than the design drift level. The different precast connections adopted are shown in Figure 2.6(a) to (d). Spieth et al (2004) presented the results of an experimental study together with companion of analytical modeling of two distinctly different precast concrete beam to column connections. The first consists of precast concrete beams with armored end connections connected directly to the column, while the second is a connection offset away from the column at about the 1/8 point within the span. In both cases, the beams were connected via un-bonded post-tensioned high strength prestressing thread bars to a prestressed concrete column. Lateral loading tests were conducted up to ±4% drift with and without supplementary mechanical energy dissipators. The results show that the non-linear moment-rotation performance can be accurately modeled. From this study it was concluded, that with appropriate

17 28 armouring of the precast members, damage can be avoided to the connection, while the entire structure is self-centered following an earthquake. a) Hybrid Post Tensioned Connection b) Pretensioned Connection Figure 2.6 Different Precast Connections Adopted (Priestley et al 1999)

18 29 c) TCY-Gap Connection d) TCY Connection Figure 2.6 (Continued)

19 30 Ousalem et al (2009) studied the seismic performance of an assembled precast high strength concrete beam with a simple and innovative lap splice connection in high rise buildings. The flexibility variation along the lap splice connection of the beam, which involves a reduced profile, was also investigated. The lap splice connection, located at beam mid span was connected by transverse bolts. The authors concluded that the beams under reversed cyclic loading proved to be ductile and failure occurred outside the lap splice connection similar to monolithic ordinary reinforced concrete beams. The flexural stiffness varied along the lap splice connection of the assembled beam and declined at the transition section of the reduced profile under large loading. The reduction in the effective flexural stiffness at the location of the transition section did not jeopardize the performance of the assembled precast beam within the design limits. Ousalem et al (2009) investigated the seismic performance of two precast high strength reinforced concrete exterior beam-column joints subjected to varying high axial levels. High grade steel bars were used as reinforcement. Splice grout-sleeves and mechanical anchors were used in columns and beams respectively. The maximum axial tension level in the columns was 90% of the yield strength of the main bars. It was concluded that the tested specimens under high axial tension loads performed well and showed stable response with the lateral storey drift angle of 3%, exhibited appropriate response characteristics, lateral force resistance and energy absorption capacity. The bond deterioration of the beam main bars in joints subjected to varying axial load was higher under axial tension load than under axial compression load. The mechanical anchors were very effective and no sign of concrete crushing was observed within lateral storey drift of 2%. Kaplan et al (2009) tested a typical pin connected precast concrete frames strengthened with external shear walls under reversed cyclic imposed

20 31 drift at a constant rate. The experiments showed that the structure with shear walls showed increased lateral stiffness and lateral load resisting capacity and provided an effective diaphragm for the structure Hybrid Connections Dolan et al (1987) tested a two bay by two storey moment resisting frame which included several moment resisting connections. The various types of connections adopted were (i) beam to column connection using welded plates for the positive and negative connections (ii) beam to column connections using continuous reinforcing through the column and cast-inplace topping and positive moment connection using welded plates (iii) connection using bolts (iv) a precast beam constructed into a cast-in-place column (v) a precast beam post-tensioned to a column (vi) a precast beam installed on a grouted dowel (vii) a precast beam made continuous using DYWIDAG threaded bars screwed into couplers cast in the column. It was concluded that all the connections developed strengths and were considered strong enough for their intended use. The bolted connection was found to exhibit energy dissipation similar to monolithic connections. Ertas et al (2006) presented the test results of four types of ductile, moment-resisting precast concrete frame connections and one monolithic concrete connection, all designed for use in high seismic zones. The performances of the precast concrete connections subject to displacement controlled reversed cyclic loading were compared with that of the monolithic connection. The precast concrete connections tested were subdivided into three groups namely cast-in-place, composite with welding, and bolted. The cast-in-place connections were located in either the beam or the column of the precast concrete subassemblies. The composite connection is a common detail used in the Turkish precast concrete industry. Two bolted specimens without corbels were also tested. Through these tests, the responses of different

21 32 connection types under the same loading pattern and test configuration were compared. Comparisons of performance parameters, such as energy dissipation and ease of fabrication, revealed that the modified bolted connections may be suitable for use in high seismic zones. Ozden and Ertas (2007) presented the test results 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 displacement controlled reversed cyclic loading. Each hybrid connection was compared with the test result of the reference monolithic subassembly in terms of connection strength, stiffness degradation, energy dissipation, and permanent displacement. The response of post tensioned, precast concrete hybrid connections approached that of the monolithic subassembly as the mild steel reinforcement content increased. Connection capacities were well predicted by the joint gap opening approach. The design assumptions of hybrid connections are best satisfied with a 30% mild steel reinforcement contribution to the connection s flexural capacity. Kulkarni and Li (2009) conducted experimental and finite element method investigation of hybrid steel-concrete beam-column joints subjected to seismic loading. Four prototype specimens of beam-column joints with slabs were tested under reversed cyclic loading. Two were cast-in-place concrete specimens, and two were precast concrete specimens constructed with hybrid connections. The rectangular column simulated two different structural combinations, one was strong column-weak beam and another was weak columns-strong beam. Both were tested to evaluate how the connection details of the different systems influenced the strength of the joints. It was observed that the precast concrete achieve consistent hysteretic loops throughout the cyclic loading and behaved well compared with cast-in-place connections. The top reinforcement of precast concrete specimens was stressed to a higher level than cast-in-place concrete top reinforcement during

22 33 seismic loading, but a lower state of stress level can be achieved in precast concrete construction with an increase in plate thickness of the hybrid connection. The hybrid connection with a strong column-weak beam system, an axial load between zero and 0.2f c A g enhances performance. For the connections with a weak column-strong beam system, the axial load ranging from zero to 0.1 f c A g, where f c and A g are cylinder compressive strength and gross area of column respectively. The increase in plate thickness, the hybrid connection was capable of carrying the required storey shears and the energy dissipation of the joint increases. The Finite Element analysis and experimental results were found to be good agreement. Li et al (2009) conducted experimental and analytical investigations of hybrid-steel concrete connections. Four full scale specimens, included one cast-in-place and three precast specimen were tested under cyclic load reversals. The critical parameters influencing the joint s behavior such as continuation of beam bottom reinforcement, column axial load, the size and embedded length of the angle sections are varied and their effects including possible implications on code specifications are discussed. Experimental observations showed that precast specimens under cyclic loading experienced no abrupt damage within the joint core region and therefore, the final failure was not controlled by the capacity of the joint core. The precast specimen s performance was good at exhibiting adequate ductile behavior under seismic loading and it also agreed well with cast-in-place specimen. Embedment of the steel sections in the joint greatly enhanced the strength of the joint core with the specimens carrying storey shears up to a ductility factor of 3.5. Beam to column connection of precast specimens was sufficiently stiff and ductile and effectively resisted both shear forces and bending moments. Joint core regions of the precast specimens were adequately confined by the incorporated steel sections, providing significantly high degree of restraint

23 34 and reducing the joint core deformation under reversed cyclic loading. The Finite Element analysis results compared well with the experimental results. Thinh et al (2009) tested a new type of precast unbounded posttensioned exterior beam-column joint of a long span frame under the simultaneous action of gravity load and cyclic load. Four specimens were tested. The first specimen was designed with shear bracket that resisted the shear force induced by designed gravity load. The second specimen was designed without shear bracket. The third specimen was designed with shear bracket to resist shear force induced by gravity load which is 1.5 times of that of the first specimen. The fourth specimen was designed similar to the first specimen, but had the slab and spandrel beam. It was concluded that specimen with shear bracket exhibited good hysteretic behavior with small residual deformation. Specimen without shear bracket experienced large beam slip. Excessive crushing of the slab together with fracture of slab reinforcement caused deterioration of strength with large residual deformation. It was proposed that the design of shear bracket and inverted U-shaped steel box should be modified to prevent the deformation of these parts under the action of very large gravity load and cyclic load Studies on Analytical Investigations of Precast Elements Beam Elements Faherty (1972) studied a simply supported reinforced and prestressed concrete beam loaded with two symmetrically placed concentrated transverse loads using the finite element method of analysis. The nonlinear analysis considered the concrete nonlinear properties, the linear bond slip relation with a destruction of the bond between the steel and concrete, and bilinear steel properties. The transverse loading was incrementally applied whereas the dead load, release of the prestressing force, the elastic prestress

24 35 loss, the time dependent prestress loss, and the loss of tensile stress in the concrete as a result of concrete rupture were applied as single loading increments. The results for the reinforced and prestressed beam showed that deflections computed using the finite element model compared well with the experimental results. Barbosa and Ribeiro (1998) analysed a simply supported reinforced concrete beam subjected to uniformly distributed load using finite element package ANSYS. Due to transversal and longitudinal symmetry, a quarter of the beam was modeled. Reinforcement was modeled as discrete reinforcement and smeared reinforcement. Each type had been analyzed four times with four different material models. Linear elastic behavior for both concrete and steel was adopted for the first model, the former capable of cracking in tension and crushing in compression. In the second model, crushing of compressed concrete was disabled and an elastic perfectly plastic model based on Drucker-Prager yield criterion had been used instead. A multilinear uniaxial stress-strain relation, simulating a parabolic curve represented concrete compressive behavior in the third model. Finally, crushing had been associated to the multilinear stress-strain curve in order to compose the fourth compression model for concrete. It was concluded that satisfactory prediction of the response of reinforced concrete structures were obtained for all the models. Fanning (2001) conducted nonlinear analysis of reinforced and post-tensioned concrete T-beams using finite element package ANSYS. Quarter and half models were modeled for reinforced and post-tensioned concrete T-beams respectively. SOLID65 element and LINK8 element were used to model concrete and internal reinforcement. Discrete reinforcement was favoured over the alternative smeared stiffness capability as it allowed the reinforcement to be precisely located whilst remaining a relatively coarse

25 36 mesh for the surrounding concrete medium. For formulating the model for post-tensioned beam LINK8 element was used for the post-tensioning cables, with the remaining internal reinforcing bar modeled using distributed smeared stiffness approach. The numerical model predicted well the nonlinear loaddeflection response of the beams upto failure. The finite element model predicted the crack pattern similar to the test beam. It was concluded that for capturing the flexural modes of failure of reinforced concrete systems, the smeared crack model was an approximate numerical model. Kachlakev et al (2001) studied the behavior of four concrete beam members with externally bonded Carbon Fiber Reinforced Polymer (CFRP) fabric using ANSYS. SOLID65 element, LINK8 element, SOLID46 element and SOLID45 element were used to model concrete, steel reinforcement, FRP composites and steel plates respectively. Symmetry allowed one quarter of the beam to be modeled. It was concluded that in the load strain plots, the strain in the linear stage from the FE analysis correlated well with those from the experimental data. The yield load of steel from FE analysis was 14% lower than that of the test results. In the linear range, the load deflection plot was stiffer when compared to the experimental results. The first cracking loads obtained form ANSYS was higher than the test data. ANSYS underestimated the ultimate load of the beams by 5% to 24%. Hu et al (2004) conducted numerical analysis using ABAQUS finite element program to predict the ultimate load carrying capacity of rectangular reinforced concrete beams strengthened by fibre reinforced plastics applied at the bottom or on both sides of the beams. The steel reinforcing bars, plain concrete and fibre reinforced plastics was simulated using appropriate constitutive models. The influences of fibre orientation, beam length and reinforcement ratios on the ultimate strength of the beams were investigated. The behaviors of the beams with high and low

26 37 reinforcement ratios and strengthened with FRP at the bottom are not influenced by the length of the beam significantly. The beams with high reinforcement ratios and strengthened with FRP at the bottom had more cracks at the central region than those with low reinforcement ratios. With the same FRP layers, the ultimate strengths and the numbers of cracks of the beams strengthened by FRP on both sides were much less than those strengthened by FRP at the bottom. Santhakumar et al (2004) conducted numerical study to simulate the behavior of retrofitted reinforced concrete shear beams. The study was carried out on the unretrofitted RC beam designated as control beam and RC retrofitted using carbon fibre reinforced plastic (CFRP) composites with ±45º and 90º fibre orientation. The finite elements adopted by ANSYS were used for this study. A quarter of the full beam was modeled by taking advantage of the symmetry of the beam and loadings. When compared with the experimental models showed 8% increase in the ultimate load for control beam and uncracked retrofitted beam and 8% decrease in the ultimate load for precracked retrofitted beam. At the ultimate stage all the numerical models show less deflection especially the precracked retrofitted beam showed 31% less deflection. Wolanski (2004) studied the flexural behavior of reinforced and prestressed concrete beams using finite element analysis ANSYS. SOLID65 element and LINK8 were used to model the concrete and whereas SOLID45 was used to simulate the steel plate for loading area and supports. The SOLID65 element required both linear isotropic and multilinear isotropic material properties to properly model concrete. The multilinear isotropic material used the Von Mises failure criteria along with William Warnke model to define the failure of concrete. Deflections and stresses at the centerline along with initial and progressive cracking of the finite element

27 38 model compared well with the experimental data. The failure mechanism of the reinforced and prestressed concrete beam was modeled well and the failure load was close to the experimental results. Ibrahim and Mubarak (2009) studied the behavior of externally prestressed continuous concrete beams subjected to symmetrical static loading. A numerical model based on the finite element method using ANSYS. The elements SOLID65 and LINK8 were used to model concrete and steel reinforcement. The prestress in the finite element was given as an initial strain in the link element. SOLID45 element was used for steel plates at the support and loading location to avoid stress concentration problems. The anchorage zone was modeled as steel plate which was connected to the tendon element. The finite element analysis showed good agreement with the experimental results throughout the entire range of behavior and failure mode. Ibrahim and Mahmood (2009) presented an analysis model for reinforced concrete beams externally reinforced with fibre reinforced polymer (FRP) laminates using finite elements method adopted by ANSYS. The finite element models are developed using a smeared cracking approach for concrete and three dimensional layered elements for the FRP composites. The results obtained from the ANSYS finite element analysis were compared with the experimental data. The comparisons were made for load-deflection curves at mid-span; and failure load. The results from finite element analysis were calculated at the same location as the experimental test of the beams. The accuracy of the finite element models is assessed by comparison with the experimental results, which are to be in good agreement. The load-deflection curves from the finite element analysis agree well with the experimental results in the linear range, but the finite elements results are slightly stiffer than that from the experimental results. The failure load obtained from the numerical solution for all beams is slightly smaller than experimental load.

28 39 The maximum difference in ultimate loads for all cases is 7.8%. The final loads for the finite element models are the last applied load step before the solution diverges due to numerous cracks and large deflections. Chansawat et al (2009) developed three-dimensional finite element model to simulate the behavior of full scale reinforced concrete beams strengthened with glass and carbon fibre reinforced polymer sheets. It consisted of an unstrengthened control beam, a flexural strengthened beam, shear strengthened beam and shear and flexural strengthened beam. For concrete eight node isoparametric elements with a smeared crack approach was used and FRP composites were modeled as three dimensional layered elements. Analysis results were compared with data obtained from full-scale beam tests through the linear and nonlinear ranges up to failure. It was concluded that FE models could identify qualitatively trends observed in the structural behavior of the full-scale beams. The predicted crack initiation patterns resembled the failure modes observed for the full-scale tests. Buyukkaragoz (2010) studied the strengthening of the beam by bonding with prefabricated plate and a control beam. ANSYS finite element program was used for modeling. SOLID65 element was used for the concrete model in the reinforced concrete beam model. In this study Hognestad concrete was used due to lack of confinement for the concretes. The stressstrain obtained from the model was used in the definition of the multi-linear isotropic material. In addition, the William Warnke failure model was used in the definition of concrete. The steel was defined as bilinear isotropic based on Von Mises yielding criteria. LINK8 element was used to define reinforcement in ANSYS. In the model, epoxy was used to bond the prefabricated plate to the beam. SOLID46 element was used for epoxy in the program. SOLID46 is layered version of the 8-node structural solid (SOLID45) designed to model layered thick shells or solids. Reinforcement and stirrups were modeled with

29 40 discrete method by constituting element definition from mesh nodes constructing the concrete. The results obtained from ANSYS finite element program were similar to the experimental behavior of the beams. Obaidat et al (2010) presented a finite element analysis of eight RC beams retrofitted with Carbon Fibre Reinforced Polymer (CFRP). The commercial numerical analysis tool ABAQUS was used and different material models were evaluated. Linear elastic isotropic models were used for CFRP and a perfect bond model and a cohesive bond model was used for the concrete-cfrp interface. A plastic damage model was used for the concrete. The finite element analysis results showed good agreement with the experimental data regarding load-displacement response, crack pattern and debonding failure mode when cohesive bond model was used. The perfect bond model failed to capture the softening behavior of beams. There was no significant difference between the elastic isotropic and orthotropic models for the CFRP Beam Column Joints Marcakis and Mitchell (1980) attempted to develop a rational analytical model capable of predicting the ultimate capacity of a variety of embedded steel member precast connections. The development of this analytical model is based on the results of a series of experiments in which the different variables like effect of column axial load, effect of additional welded reinforcement, effect of shape of embedded member were studied. A series of experiments indicated that the analytical model conservatively predicted the capacity of connections with axial load levels less than 75 percent of the pure axial load capacity of the column. All the specimens tested with low axial loads failed in the concrete and exhibited ductile behaviour. For higher levels of axial load a significant decrease in the ductility was observed. If larger ductility is required, the connection can be designed such that failure takes

30 41 place in the embedded steel member. The analytical model has been used to prepare a series of non-dimensionalized design curves for connections with or without additional welded reinforcement. Camarena (2006) conducted the finite element analysis of interior precast prestressed beam column connection under seismic loading using finite element package DIANA. The behavior of concrete was modeled with total strain based constitutive model. A bilinear stress-strain relationship that consisted of a elastic part, a yield part and a part with hardening was used for ordinary reinforcement. For the reinforcement an elastic plastic model was used both in tension and compression with Von Mises yield criterion. The rubber pad was modeled with a linear elastic stress strain relation with a Poisson s ratio close to 0.5. For the mortar a total strain model was used similar to one for concrete. The author concluded that the structural response of the ductile beam-column connection of jointed systems under imposed lateral loads was satisfactory. Damage to the beams was minimal; most of the cracks were limited to the regions close to the interface and the concrete cover. There was no loss of prestress in the secondary tendons. The structure achieved a drift of 4% which was higher than the drift of 2 to 3% that is normally assumed in the design of structures. Mostofinejad and Talaeitaba (2006) proposed a finite element modeling for nonlinear analysis of an exterior reinforced concrete joint covered with fibre reinforced plastics (FRP) overlays. The model consisted of the effects of anchorage slip and anchorage extension of the steel reinforcement in the connection zone. ANSYS finite element package was used for the nonlinear analysis. For modeling concrete, longitudinal reinforcement and FRP composites, the elements used were SOLID65, LINK8 and SOLID45 were used. The transverse reinforcement was modeled as smeared reinforcement. The anchorage slip and the anchorage extension of

31 42 the reinforcement were modeled using nonlinear spring model. The exterior beam-column joints, the end supports of the top and bottom columns were fixed and monotonic concentrated load was applied to the tip of the beam. Finer meshes were chosen for the connection region due to the probability of stress concentration and more cracking. To perform the nonlinear analysis, the load was applied step by step and the modified Newton Raphson method was used for the solution. The effects of debonding of FRP laminates in FE analysis were eliminated by limiting the maximum strain in FRP laminates. The results of the numerical analysis were found to compare well with the experimental results. Kulkarni et al (2008) carried out a non-linear finite element analysis of hybrid-steel concrete connections. The critical parameters influencing the joint behavior, such as axial load on column, the connection plate thickness and continuation of beam bottom reinforcement were varied and their effects, especially implication on code specifications were studied. In the study, the specimens were analysed using DIANA software. Two dimensional plane stress elements were used to simulate the concrete and steel plates, while reinforcing bars were modeled as truss elements. In material modeling, the concrete models were based on nonlinear fracture mechanics to account for cracking, and plasticity models were used for the concrete in compression and steel reinforcement. Comparison with the experimental results indicated that the finite element models used were suitable. The failure modes, ultimate ductility capacities, deformations and cracking patterns correlated well with experimental results. Pirmoz and Danesh (2009) studied the effect of the seat angle stiffness on moment-rotation response of the bolted top-seat angle connections using finite element method ANSYS. All components of the connection such as the beam, column, angles and bolts head are modelled

32 43 using eight noded SOLID45 elements and bolt shanks are modelled using SOLID64 elements, which can apply a thermal gradient on it to pretension the bolts. The effect of interactions between components, such as slippage of bolts and frictional forces, are modelled using surface contact algorithm. ANSYS can model contact problems using contact pair elements CONTA174 and TARGE170, which pair together in such a way that no penetration occurs during the loading process. Thus the effect of adjacent surface interactions, including angle-beam flange, angle/beam flange-bolt head/nut, bolt hole-bolt shank and effect of friction, are modelled using the mentioned contact elements. Bolt heads and nuts were modelled as hexagons, and were similar to their actual shape. To consider the frictional forces, Coulomb s coefficient was assumed to be 0.25, which had better agreement with test results. The FE method cannot model the fracture or cracking of the material because two elements cannot be separated and thus the material fracture is not considered. In the finite element analysis the difference between test data and numerical models grows in nonlinear portion of curves. A major cause is the nonlinear constitutive laws for materials, especially for situations where only uniaxial values of the stress-strain curves were available. Kaya and Arslan (2009) analytically modeled three precast beam to column connections connected as post-tensioning and the cast-in-place beam to column connections using ANSYS finite element program. In the analytical models: model sizes, material properties, the loading program and the boundary conditions were similar to the test specimens. A smeared crack model was selected to define the cracked concrete. Full bond was assumed between the concrete and steel. For this reason additional bond element was defined between the concrete and steel. A discrete model was used for the analytical models. For this study, Hognestad concrete model was used for due to lack of confinement for concrete. William-Warnke failure model was used in the definition of concrete. The results of the experimental tests and

33 44 analytical analysis showed that the performance of the prestressed connections were adequate for load capacity but the analytical models initial and 1.5% storey drift stiffness differed from the test specimens. The reason for this behavior was the difference in the loading programs applied to the analytical and experiment models. Loading was applied as load-controlled steps to the analytical models. However, for the specimens it was applied as load-controlled step at the beginning and then displacement-control steps were used. Some parameters necessary for modeling concrete, reinforced steel and prestressed strands may not be determined sufficiently such as the concrete fracture parameters. For the effect of the concrete on the behavior of the model to be fully reflected, all the concrete properties, including the modules of elasticity, compressive stress, tension stress and poisson ratio must be carefully determined. Figure 2.7 shows the Reinforcement details of the precast beam column specimens. Figure 2.7 Reinforcement Details of the Precast Beam Column Specimens (Kaya and Arslan, 2009)

34 45 Hawileh et al (2010) developed three dimensional nonlinear finite element model to predict the behavior of precast hybrid beam-column connection subjected to cyclic loading. The precast joint was modeled using three dimensional solid elements and surface-to-surface contact elements between the beam/column faces and interface grout in the vicinity of the connection. The solid element SOLID65 was used to model the concrete. The primary reinforcement post-tensioned strands and mild steel reinforcement bars were modeled as solid elements SOLID185 because they were debonded from the adjacent concrete surfaces in the vicinity of the connection. The regular beam and column reinforcement are discretized using the discrete spar elements. Perfect bond was assumed between the reinforcing steel and concrete element. In this structure, the beam and column faces are in contact with the interface grout, and there was also contact between the mild steel bars and grout in the vicinity of the connection. Two element types CONTA174 and TARGE170 were used for the contact and target surfaces since the contact were between the two different surfaces. Surfaces with finer mesh were designated as contact surface while surfaces with coarser meshes were considered target surfaces. Results showed that the response envelope from the finite element analysis correlated fairly well with the experimental results. Good correlation existed in all stages of lateral cyclic loading. Isometric view of the finite element model of the connection is shown in Figure 2.8. Ozden and Ertas (2010) presented an alternative section analysis and hysteretic modeling for the response of precast concrete hybrid connections which had different level of mild steel contributing to the connection flexural capacity. It was suggested that the well known classical reinforced concrete section analysis approach cannot be directly applied to the precast concrete hybrid connections due to the strain compatibility between the concrete sections and the partially bonded mild steel and the unbounded

35 46 prestressing tendons. The authors proposed a section analysis in which initially the moment rotation behavior of the hybrid connection was modeled by providing a new debonding length formulation for mild steel. A hysteretic response was proposed by considering the residual displacement that was measured during the hybrid connection subassemlage tests. It was observed that the moment rotation envelope model and the cyclic response behavior model both exhibited satisfactory agreement with the previously published test results. Figure 2.8 Isometric View of the Finite Element Model of the Connection (Hawileh, 2010) Sen et al (2010) conducted a finite element analysis for studying the effectiveness of retrofitting technique called strip wrapping technique for using carbon fibres (FRP) for strengthening of RC beam-column connections damaged due to various reasons. The emphasis was mainly for material modeling of the composite layered reinforced concrete structure which took into account the stress-strain behavior of concrete tension stiffening and the cracking of concrete. SOLID65, PIPE16 and SHELL63 were the elements used for discretising concrete, reinforcing bars and carbon fibres respectively. The analytical programme confirmed the externally