Experimental and analytical study of replaceable Buckling-Restrained Fuse-type (BRF) mild steel dissipaters

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1 Experimental and analtical stud of replaceable Buckling-Restrained Fuse-tpe (BRF) mild steel dissipaters F. Sarti, T. Smith, A. Palermo, S. Pampanin Universit of Canterbur, Christchurch, New Zealand. D. Bonardi Technical Universit of Milan, Ital D.M. Carradine Building Research Association of New Zealand, Judgeford, Porirua, New Zealand NZSEE Conference ABSTRACT: High-performance post-tensioned technologies for seismic resistance were first developed in the late 1990s as the main outcome of the PREcast Seismic Structural Sstems programme (PRESSS). Of these technologies the hbrid connection, composed of the combination post-tensioning strands/bars (re-centring) and some form of mild steel element (hsteretic dissipation), was the most promising. This paper focuses on the experimental stud of a particular tpe of mild steel dissipation device: the fuse-tpe mild steel replaceable dissipater. This device is made up of a milled down plain mild steel bar and a confining tube filled with either epox or grout. The use of a milled area concentrates ielding over a defined length while the tube prevents buckling, similar to the Buckling Restrained Braces (BRB) used in steel frame buildings. The main purpose of the experimental campaign was to provide a comprehensive parametric stud and preliminar guidelines for the design of the replaceable device. Several quasi-static push-pull tests were performed with varing geometric parameters such as the fuse diameter and length; two different filler materials were also used. Based on the experimental data collected, a simple analtical model was developed which is able to predict the force-displacement behaviour of the dissipater. 1 INTRODUCTION High-performance post-tensioning sstems were first proposed in the late 1990s as a main outcome of the PREcast Seismic Structural Sstems (PRESSS) programme, coordinated b the Universit of California, San Diego. Different technologies were developed during the programme and tested on a five-store building in the third and last phase of the experimental campaign (Priestle 1991). One of the outcomes of this bod of research was the hbrid connection which combined re-centering (posttensioning elements) and dissipative contributions (mild steel dissipaters) (Priestle et al. 1999). As part of a series of experimental test campaigns performed at the Universit of Canterbur, a new external dissipation device, previousl conceived b (Christopoulos et al. 2002) has been further developed, detailed and tested for both concrete and timber structural sstems (Amaris et al. 2006; Palermo et al. 2006; Marriott et al. 2008; Smith et al. 2008). The dissipater is made of a mild steel bar machined down to a reduced diameter in order to concentrate ielding. To avoid buckling, the fuse area is encased in a steel tube which is filled with either grout or epox (Figure 1). During two of these experimental campaigns (see Figure 2) characterisation testing has been performed on the stand-alone dissipative elements. Testing performed b Amaris Mesa (Amaris et al. 2006; Amaris Mesa 2010) included three different fuse diameters (7mm, 10mm, 13mm) with an unbonded length of 90mm. A 34mm diameter tube (2mm thick) filled with epox was used as antibuckling restraint. The dissipaters showed stable hsteresis in tension, however in compression an increased stiffness was observed as well as buckling of the dissipater at relativel low displacements. 1

2 Figure 1. Buckling-restrained fuse-tpe (BRF) dissipater schematic and smbols During the testing of Marriott (Marriott 2009), the dissipater elements were fabricated out of 20mm mild steel (Grade300) plain bars, with a reduced diameter of 10mm. The buckling restraining tube was 34mm in diameter (2mm thick) and filled with epox. Test results showed a ver stable hsteresis and high energ dissipation capabilities (Marriott 2009). Figure 2. External dissipaters applications to beam-column joint (top left)(amaris et al. 2006) and wallfoundation joint (top right)(marriott et al. 2007). Detail of dissipater behaviour (bottom) The fuse-tpe dissipation device has also been adopted for use in several buildings around New Zealand which have used post-tensioned technolog. The first of these was the Learning and Research Building at Victoria Universit in Wellington (Figure 3 left). The building comprises of six three-ba concrete frames, spanning 8.4m, carring double-tee flooring units spanning 9.9m. Energ dissipation was provided b external devices which were bolted to the pre-cast beams and columns after erection (Cattanach et al. 2008). Figure 3. Beam column Joint real applications. Learning and Research building (left), Victoria Universit, Wellington (Cattanach et al. 2008), Merritt building (right), Christchurch More recentl a PRESSS multi-store timber building, the Merritt building in Christchurch (Figure 3 2

3 Avg. Strain (%) right), has been designed and is currentl under construction. The building is a three store timber structure with post-tensioned frames spanning 8.8m in one direction. Fuse-tpe dissipaters are bolted at the beam-column and column-foundation interface to provide both dissipation and additional moment capacit. The experimental campaign presented for this paper was carried out with the main objective of providing a comprehensive stud into the performance of replaceable buckling-restrained dissipaters, and to develop and validate a simple analtical model capable of predicting dissipater behaviour. 2 EXPERIMENAL TESTING 2.1 Materials Grade300 mild steel was used for both the fuse bars and the confining tubes. The filling material between the fuse bar and the tube was either cement grout or epox. Table 1 presents the characteristic properties of the materials used (provided b manufacturers). 2.2 Displacement protocol The displacement protocol used during the testing was calculated in accordance with ITG (ACI Innovation Task Group ). Each specimen was subjected to a sequence of positive quasi static (50mm/min) displacement-controlled ccles to pre-determined values as shown in Figure 4. The compressive range of displacements was not accounted for because generall in all applications the compressive displacement is not significant in comparison to the tension displacement Ccles Figure 4. Loading protocol according to ITG Specimens and testing schedule Table 1. Material properties Mild steel Epox Strength (MPa) Young s Modulus (GPa) Different geometric parameters, (i.e. the plain/fuse diameters, the slenderness of the fuse bar) and the confining materials have been varied in order to provide a comprehensive set of experimental results from which design guidelines can be developed. For each configuration three specimens were tested (two grouted and one epoxied). A summar of the geometric properties is provided b Table 2. Table 2. Testing schedule for buckling-restrained fuse-tpe (BRF) dissipaters (dimensions in mm) ID No. Plain diam. (D ex1 ) Fuse diam. (D fuse ) Thread Plain length (L ex ) Fuse length (L fuse ) Slender. ratio Tube int. Diam. Tube ext. Diam. Tube length D12L M D20L M D24L M D26L M D26L M D26L M

4 Five different diameters have been chosen and slenderness ratio (λ = 4L fuse /d fuse ) was taken into account as well; the majorit of the specimens have been designed with a slenderness of 60, but for some a higher value has been considered (λ = 75 and λ = 90). 2.4 Test setup Tests were performed using a Dartec displacement-controlled testing rig. Steel plates were fabricated in order to obtain the flexibilit of test setup necessar due to the large range of dissipater size. Two large steel plates were bolted on the top and bottom of the testing machine to which smaller plates were bolted, the specimen was then screwed and bolted into this smaller plate. This setup allowed to the specimen to be easil replaced following testing. The test setup is shown in Figure 5. Figure 5. 3D exploded view (left) and picture (right) of test setup 2.5 Results Results from tests performed are summarized in Figure 6, where the behaviour is reported in terms of force and displacement normalized b the specimen fuse length. A summar of the main parameters resulted from testing is reported in Table 3. ID No. D12L180 D20L300 D24L360 D26L390 D26L488 D26L585 Filler material Epox Epox Epox Epox Epox Table 3. Summar of test results Slender. ratio Max. Aver. Onset Avg. Ult. Avg. Yield Strain strain * strain Force Fuse Dissipater (%) (%) (%) (kn) (kn) * maximum strain reached before the buckling mechanism of the dissipater occurred Max. Compr. Force

5 Avg. Onset Strain (%) Figure 6. Force vs. normalized displacement chart of the tested dissipaters The specimens with a slenderness value of 60 could undergo a normalized displacement of 9% before the dissipater started buckling. The buckling mechanism of the more slender specimens occurred on the reverse ccle at 9% and 6% (average onset strains in Table 3) for slenderness values of 75 and 90 respectivel. As Figure 6 shows, no major difference between the different materials used to fill the anti-buckling tubes was observed, with the dissipaters undergoing the same displacements, and onset at similar loads. The main differences in the use of either epox or grout as a filler material are in the ease of manufacturing and performance under high temperature. Epox is more fluid before hardening and is simple to pump into the cavit created b the anti-buckling tube. Epox does however have the disadvantage of degrading substantiall when heated. This can occur during the tension compression ccles of the dissipater device as ielding of the material creates heat. is less convenient from the constructabilit point of view, but its properties do not change significantl with increasing temperatures. A minimum onset averaged strain of 6% up to a maximum value of 13% was observed. Onset strain is plotted in Figure 7 against the dissipater slenderness (i.e. the slenderness ratio of the composite section, see values in Table 3) and a linear trend was observed and plotted x Figure 7. Onset strain vs. dissipater slenderness Dissipater Slenderness 5

6 3 ANALYTICAL MODEL The hsteretic behaviour of a post-tensioned sstem is reliant on the ielding in tension of the fusetpe dissipation devices. It is therefore crucial that the force-displacement characteristics of these elements are correctl predicted. The analtical model presented is broken down into two main contributions: pre-ield and post-ield displacements. Both contributions are evaluated through the assemblage of three spring elements in series. Yield is limited to the fuse portion of the element. 3.1 Displacement contribution pre-ield The ield displacement (Δ ), considering ield to be isolated in the fuse area (F = f A fuse ) can be written as: f A L f A L f A L fuse ex1 fuse fuse fuse ex 2 (1) EA EA EA ex1 fuse ex 2 Where f is the steel ield stress. In most case A ex1 = A ex2 therefore a single A ex value will be used giving: f A ( L L ) f L fuse ex1 ex 2 fuse (2) EA ex E A single value of the total external length (L ex = L ex1 + L ex2 ) will be used and given that A fuse = (π/4)d 2 fuse and A ex = (π/4)d 2 ex Δ can be rewritten as: 2 f L fuse d fuse L ex 1 E d L ex fuse This means that at ield a factor k a can be defined in order to account for the additional displacement due to stresses in the areas between the anchorage of the dissipater and the fuse area: k f L a fuse (4) k a d d E fuse ex 2 L L ex fuse Displacement contribution post-ield Considering an elastic-plastic stress-strain relationship, the strain of the steel beond the ielding point (Δ) is: f f s L fuse 1 (6) E r Where r is the bi-linear post-ield stiffness factor and f s is the steel stress, and the ield is assumed to be occurring in the fuse part onl. 3.3 Discussion As mentioned above, a complete understanding of the force-displacement relationship of the fuse-tpe dissipater is critical for incorporation within a post-tensioned sstem. Figure 8 shows a comparison between the procedure presented above and experimental results. A third line shows the forcedisplacement response when the fuse length onl is considered (i.e. without the factor k a ) (3) (5) 6

7 Force (kn) Experimental Analtical Simplified Displacement (mm) 0 2 Figure 8. Experimental dissipater (D20L300) test results, the analtical model and the simplified assumption model comparisons Using the analtical model proposed, the ield displacement of the dissipater D20L300 is evaluated as Δ = 0.62mm. Assuming the strain developing onl in the fuse portion, the ield displacement of the dissipater would be Δ = F L/EA = 0.48mm. Comparison between these two ield displacement values and testing results shows that considering onl fuse length (i.e. no k a factor) significantl underestimates the ield displacement (b 23% in this example). No significant difference is observed in the post-ield behaviour ON GOING AND FUTURE RESEARCH The research project on fuse-tpe dissipaters includes further numerical investigations. Finite element models will be calibrated on experimental data gathered during this stud as well as data from an experimental database including previous tests performed at the Universit of Canterbur (Marriott 2009; Amaris Mesa 2010). The results will allow the calibration of a macro-model capable of capturing the general dissipater behaviour and its failure mechanisms. 5 CONCLUSIONS A comprehensive parametric experimental campaign was presented in the paper. The different dissipaters tested showed stable hsteresis up to high level of displacements (9% average strain for most of the specimens). An experimentall calibrated linear relationship was worked out, relating the onset strain to the total slenderness of the specimen. This can be used for the evaluation of the maximum design displacement of dissipaters of this tpe. Consistent results were observed with different filling materials (epox or cement grout), highlighting that this variable did not influence the behaviour of the devices. An analtical model was developed to predict the behaviour of the dissipater and it was compared with the tests results. The experimental-analtical comparison showed the model could predict the ielding point as well as the post-ielding branch. Moreover, the model highlighted that for this dissipater tpe, the displacement contribution of the external parts cannot be neglected. 7

8 ACKNOWLEDGMENTS The authors would like to acknowledge the Structural Timber Innovation Compan (STIC) for funding this research. REFERENCES ACI Innovation Task Group Acceptance criteria for special unbonded post-tensioned precast structural walls based on validation testing and commentar : an ACI standard. Farmington Hills, Mich., American Concrete Institute. Amaris, A., Pampanin, S., Palermo, A Uni and bi-directional quasi static tests on alternative hbrid precast beam column joint subassemblies. NZSEE Conference, Napier, New Zealand. Amaris Mesa, A.D Developments of Advanced Solutions for Seismic Resisting Precast Concrete Frames. Doctor of Philosoph, Universit of Canterbur. Bonardi, D Analtical Modeling of the Cclic Behaviour of a Replaceable Dissipater Device for Damage-Resisting Controlled Rocking Sstem. Master of Engineering, Technical Universit of Milan. Cattanach, A., Pampanin, S st Centur Precast: the Detailing and Manufacture of NZ's First Multi- Store PRESSS-Building. NZ Concrete Industr Conference. Rotorua. Christopoulos, C., Filiatrault, A., Uang, C.M., Folz, B Post-tensioned Energ Dissipating Connections for Moment Resisting Steel Frames. ASCE Journal of Structural Engineering 128(9): Marriott, D., Pampanin, S., Bull, D.K., Palermo, A Improving the Seismic Performance of Existing Reinforced Concrete Buildings using Advanced Rocking Wall Solutions. NZSEE Conference, Palmerston North, New Zealand. Marriott, D.J The Development of High-Performance Post-Tensioned Rocking Sstems for the Seismic Design of Structures. Doctor of Philosoph, Universti of Canterbur. Marriott, D.J., Pampanin, S., Bull, D., Palermo, A Dnamic Testing of Precast, Post-Tensioned Rocking Wall Sstems with Alternative Dissipating Solutions. Bulletin of the New Zeland National Societ for Eartquake Engineering 41(2): New Zealand Concrete Societ PRESSS Design Handbook. Newcombe, M Seismic design of multistore post-tensioned timber buildings. Master of Engineering, Università degli Studi di Pavia. Palermo, A., Pampanin, S., Fragiacomo, M., Buchanan, A.H., Deam, B Innovative Seismic Solutions for Multi-Store LVL Timber Buildings. 9th World Conference on Timber Engineering. Portland, Oregon, USA. Priestle, M.J.N Overview of PRESSS research program. PCI Journal 36(4): Priestle, M.J.N., Sritharan, S., Conle, J.R., Pampanin, S Preliminar Results and Conclusions from the PRESSS Five-Stor Precast Concrete Test Building. PCI Journal 44. Smith, T., Pampanin, S., Buchanan, A.H., Fragiacomo, M Feasibilit and Detailing of Post-tensioned Timber Buildings for Seismic Areas. New Zealand Societ of Earthquake Engineering, Annual Conference, Wairakei, New Zealand. STIC. 8