IMPROVED PERFORMANCE OF BOLT-CONNECTED LINK DUE TO CYCLIC LOAD

Transcription

1 IMPROVED PERFORMANCE OF BOLT-CONNECTED LINK DUE TO CYCLIC LOAD Muslinang Moestopo 1, Dyah Kusumastuti 1 and Andre Novan 2 1 Associate Professor, Structural Engineering Research Division, Faculty of Civil and Environmental Engineering, Institut Teknologi Bandung, Indonesia. 2 Graduate Student, Faculty of Civil and Environmental Engineering, Institut Teknologi Bandung, Indonesia. moestopo@si.itb.ac.id ABSTRACT: Recent development in seismic resistant steel frames was directed to the development of an effective and efficient energy dissipater such as a link in eccentrically braced frames. This research was carried out to study the performance of the shear-type link due to cyclic load, especially to improve the energy dissipation as well as the ductility and the strength of the link itself. Experimental work has been carried out to two specimens of bolt-connected link. Additional side-extended plates were welded at top and bottom flanges of each end of the link to prevent early failure of the link. A cyclic load was applied under displacement control. The results show that the side-extended plate has significantly improved the performance of the link, both in the strength, ductility and energy dissipation. 1. INTRODUCTION The recent development of seismic resistance structure considers the performance of the structure as the design base. By this approach, the strength of the structure is no longer the only design criteria. The new Indonesian Structural Steel Design Code (Badan Standar Nasional, 22) as well as the Seismic Provision by American Institute of Steel Construction (AISC, 25) provide the design code base on the performance of the structure in dissipating energy due to seismic load. A number of seismic resisting structural systems as well as the structural requirement of each structural system are included in the code as result of very intensive and extensive researches following the Northridge earthquake (1994). Eccentrically braced frames has been developed and widely used as a seismic resisting steel structure. Its advantages are supported by both the diagonal braces that provide lateral structure stiffness, and the link that mobilize the energy dissipation by its yielding mechanism. Previous studied by Kasai and Popov (1986) showed that the shear link or short link provided better energy dissipation than the flexural link or long link. The good performance of the shear link depends on the cyclic yielding mechanism on the web that should ensure a stable and fat hysteretic curve. The compactness of the link web contributes to this performance. The connections between the link end and adjacent beam or column are commonly provided by welded-connection due to its advantage over the bolted-connection. So far, the welded-connection provides more rigidity or stiffness and more capacity, i.e. strength, plastic rotation and energy dissipation. Study by the first author (Moestopo and Mirza, 26) showed that the lack of stiffness and strength of the bolts as well as the lack of stiffness of end-plate contributed to the deficiencies of the bolt-connected link. As shown by Gobarah (Ramadan and Ghobarah, 1995) and Stratan- Dubina (22), the strong bolt-connected link could provide better energy dissipation due to cyclic load. Although the strong bolt-connected link could not exceed the rigidity and capacity of the weld-connected link, its advantage over the weld-connected link are obvious, i.e. easy to assembly and easy to replace after the earthquake occurrence. This paper presents the experimental work by the authors on the bolt-connected link to study the improved performance of the shear-link in providing more energy dissipation by the yielding of the web of entire length of the shear-link. Back to Table of Contents 652

2 2. PERFORMANCE OF SHEAR LINK As a seismic resisting component that is expected to dissipate the energy by the yielding mechanism, the link of the eccentrically brace frame behaves differently according to its length. Link with the length of e, and its plastic moment and plastic shear are M p and V p respectively, will behave as follows: a. e 1.6 M p /V p : pure shear link, yielding is dominated by shear b. 1.6 M p /V p e 2.6 M p /V p : yielding by more dominant shear and flexure c. 2.6 M p /V p < e 5 M p /V p : yielding by shear and more dominant flexure d. e 5 M p /V p : pure flexure link, yielding is dominated by flexure Performance of the shear link is specified the codes (Badan Standar Nasional, 22). It focuses on the plastic rotation angle of the link at the condition when the inelastic drift ratio of the structure is at the limit, i.e. 2% for structure with fundamental period of T >.7 sec and 2,5% for structure with T <.7 sec. Figure 1 shows the definition of the plastic rotation angle of the link and the drift ratio. To ensure the expected performance of the link due to seismic load, a number of requirement are specified in the code, e.q. the web slenderness ratio, the dimension and the spacing of the link stiffener, and the design force for the link connection. Figure 1 Plastic rotation angle, Split K-Brace D-Brace. Study on intermediate link by Richards and Uang (26) showed that the severe inelastic deformation occurred at end part of the link due to higher bending moment. The fact that higher bending moment locates at link-ends also occurs at the shear-link. The damaged bolt-connected link in the study by Stratan and Dubina (22) showed that the energy dissipation on the link-web mostly took place at the end part of the link. This was clearly shown by the plastic deformed shape (skewed) of the web at the link-end while the web at the middle part of the link showed much less significant plastic deformation. For some extent this showed that the link has not been optimally mobilized to provide maximum energy dissipation through the web yielding of the entire beam length. In this study, an improvement was proposed to increase the energy dissipation of the shear link by preventing early fracture at the weld connection of the tensile flange. Side extended plates (SEP) are welded to each flange ends to increase the flange area and thus to reduce the tensile stress due to flexural moment on the link. This is illustrated in Figure 2 where the average tensile stress σ 2 < σ 1, since area of the side-extended flanges is larger than of the original flanges, L 2 t > L 1 t. It is expected that strengthening the flange would not significantly affect the behavior of the link prior to yielding. However, since the yielding of the shear link is concentrated at the web, the delayed fracture at the flange weld is expected to mobilize more energy dissipation through higher level of load and larger plastic deformation of the link-web. More over it is expected that the delay would mobilize more yielding of the web toward the middle part of the link. Back to Table of Contents 653

3 P σ1 = L t 1 σ 2 P = L t 2 L 1 P L 2 P Figure 2 Average tensile stress at the welded link end Ordinary link SEP link 3. EXPERIMENTAL WORK Two specimens of shear link were determined as component of a three-storey eccentrically braced frame structure type split K-brace of an office-residential building designed in seismic region I according to Indonesian Seismic Code. Each link is a mm link of Wide Flange 2x1x5.5x8, grade BJ-41 (or A36), as shown in Figure 3. The link web is divided into 4 equal segments by 1-mm thick vertical intermediate stiffeners at both side of web. Flush-end type of connection is designed for both link-ends using the actual value of yield-stress and six highstrength bolts (A49) with 25-mm diameter. Figure 3 Link specimen Ordinary link SEP link. Specimen 1 and specimen 2 are identical, except that tapered-extended plates are welded at the sides of top and bottom flange on each end of the specimen as shown in Figure 3. The specimen 2 is then identified as Side-Extended Plate (SEP) specimen. The shear-link behavior under seismic loading is modeled by supporting one of the link-end to a fixed strong frame, while the other end is connected to the vertically moving actuator by moment resisting connection (Figure 4). The cyclic displacement load was applied with a rate of.2 mm/sec by the actuator mounted to a 1, kn loading frame. The displacement-controlled loading was applied according to the loading pattern shown in Figure 5. Back to Table of Contents 654

4 Actuator Link FRAME II FRAME I Figure 4 Testing set up Loading frame, Position of LVDT (red) and post yield strain gauge (blue) Displacement ( δy ) Loading Cycle Figure 5 Loading pattern/loading protocol. A number of LVDT is used to monitor the vertical deflection of the link at the end link, relative vertical movement of end plate and support and bolt elongation. A number of rosette strain gauges is mounted at the link web and other strain gauges are placed at the top and bottom flange (Figure 4). For monitoring the yielding part of the link, post-yield type of strain-gauges are used. Some limitations of instrumentation are considered. Back to Table of Contents 655

5 4. DATA AND ANALYSIS 4.1 Load-Displacement The test results showed that at the early stage of loading, the behavior of two specimens is similar as shown by the elastic part of load-displacement history in Figure 7. Specimen 1 and specimen 2 (SEP) indicate very similar yielding load, i.e. 27 kn and 23 kn respectively at the vertical displacement of 8.1 mm. This showed that the elastic stiffness of both specimens is similar. As the loading increases, the yielding of the web of both specimens increased as indicated by the strain-gauge reading. The cyclic loading was continued until failure occurred on each specimen. The complete hysteretic loop for each specimen is shown in Figure 6, while the hysteretic envelope and the back-bone of loading history is presented in Figure Load (kn) -1 Load (kn) Displacement Δ (mm) -4 Displacement Δ (mm) Figure 6 Complete hysteretic curve Ordinary link SEP link Load (kn) Load (kn) Displacement (mm) Spc2 Spc1 Spc2 Spc Displacement Δ (mm) Spc2 Spc1 Figure 7 Back bone curve loading history Hysteretic envelope. The failure of specimen 1 occurs in the 6 th loading while the SEP specimen fails in the 1 th loading. Figures 7 and 7 show that the side extended plate increases the ultimate load as well as the ductility but only slightly improves the stiffness of the shear link. The maximum loads are 351 kn and 392 kn, the maximum displacements are 56.7-mm And 72.9-mm, while the maximum plastic rotation angles of the link are.126 rad and.162 rad respectively for specimen 1 and SEP specimen. This showed that the strength of the shear link is increased by 11% while the ductility increased by 28%. Back to Table of Contents 656

6 4.2 Energy Dissipation Table 1 and Table 2 show the energy dissipation of each of each specimen. Although the energy dissipation of each loading is quite similar until 6 th loading, the cumulative energy dissipation of SEP specimen is 184% higher that the specimen 1. Table 1 Dissipated energy and input energy of specimen 1. Dissipated Energy Input Energy Ratio Rotation (kn.mm) (kn.mm) Dissipated/Input Cycle (rad) Each Each Each Cummulative Cummulative Cummulative ,347 4,347 6,472 6, ,574 14,921 13,393 19, ,224 33,146 21,61 41, ,72 6,218,864 72, ,76 97,295 4, , ,59 143,85 51, , Table 2 Dissipated energy and input energy of specimen 2 (SEP link). Dissipated Energy Input Energy Ratio Rotation (kn.mm) (kn.mm) Dissipated/Input Cycle (rad) Each Each Each Cummulative Cummulative Cummulative ,579 4,579 6,613 6, ,616 15,195 13,478 2, ,265 33,46 21,7 41, ,21 6,661 31,173 72, ,852 97,514 41, , ,37 144,885 52, , ,694 22,58 62, , ,379 27,959 74,29 3, The significant increase in energy dissipation of SEP specimen is obviously due to the more loading s that can be provided by the SEP specimen. Although the back-bone curve of the load-displacement of both specimen is almost similar until the 6 th loading, there is no further loading was mobilized by the specimen 1 after its failure in the 6 th loading due to fracture in the welding connection of its flanges. 4.3 SEP Link The previous section showed that the side extended plate has significantly improved the strength, the ductility, and the energy dissipation of the shear link. By considering the fact that the hysteretic loading history of both specimen is quite similar until the specimen 1 fails, the improvement performance by the SEP is described by the delay of the fracture of flange welding connection, as expected. Figure 8 showed that the specimen 1 fails when the fracture occurs at the welding between linkflange and the end-plates due to high tensile stress. At this condition, the large inelastic shear deformation is obviously shown in the web of the near-end segment of the link, while the middle part of the link is still in good condition without any visual damage. The failure is also shown by the inelastic buckling of the flange at the end-part of the link. This condition is also shown in previous studies (Kasai and Popov, 1986; Ramadan and Ghobarah, 1995). Back to Table of Contents 657

7 (c) Figure 8 Failure of specimen 1 Fracture at link end upper-weld Fracture at link-end bottom-weld (c) Large inelastic shear deformation only at near end segment. The failure of the SEP specimen occurs similarly at the welding of the flange, but it occurs at almost all of the welding between flanges and the intermediate stiffeners as well as the end-plates. Figure 9 shows that the inelastic shear deformation has been mobilized in the web of all segment of the link, including the middle part of the link. It is worth to note that the buckling of the flange at the end of link is not visually indicated. This fact explains how the further loading s could be sustained by the SEP. (c) Figure 9 Failure of specimen 2 Fracture at link end and SEP weld Fracture at intermediate stiffener and web (c) Large inelastic shear deformation at web of all web segment. The test also confirmed that the design of the bolt and the end-plate based on the actual (not the nominal) strength of the link provides a relatively good performance of bolt and end-plate that could maintain their elastic condition until very severe loading is applied to the link. 5. CONCLUSION The experimental work in this study showed that the performance of the shear link as a seismic resisting component in the eccentrically braced frame has been improved by adding side extended plates at the ends of the link. The improvement includes strength, ductility, and energy dissipation. The extended plate has successfully delayed the early failure of the shear link due to the fracture at the welding of the tensile flange that occurred in previous link model. It also enables the web shear yielding to spread to the entire length of the link. The failure is also spread to all welding of the flange. As the result, the capacity of the shear link has been fully mobilized to provide maximum energy dissipation as demanded. 6. ACKNOWLEDGEMENT This research is funded by Research Grant from Institut Teknologi Bandung Year 27 for which the authors expressed their appreciation. 7. REFERENCES AISC (25). Seismic Provision for Structural Steel Building, Chicago, American Institute of Steel Construction. Arce, G., Okazaki, T., Ryu, H.C. and Engelhardt, M.D. (24). Recent research on link performance in steel eccentrically braced frames, 13 th World Conference on Earthquake Engineering. Paper No 2. Vancouver, Canada. Back to Table of Contents 658

8 International International Conference Conference Earthquake on Earthquake Engineering Engineering and Disaster and Mitigation, Disaster Mitigation Jakarta, April , 28 Arce, G., Okazaki, T. and Engelhardt, M.D. (25). Experimental study of local buckling, overstrength and fracture of links in eccentricallt braced frame, Journal of Structural Engineering, ASCE, October, 25. Badan Standar Nasional (22). Tata Cara Perencanaan Struktur Baja untuk Bangunan Gedung, SNI Broderick, B. and Thomson, A. (21). The response of flush end plate joints under earthquake loading, Journal of Constructional Steel Research, Elsvier, September, 21. Kasai, K. and Popov, E.P. (1986). General behaviour of WF steel shear link beams, Journal of the Structural Division, Vol.112, No. 2: , February, ASCE. Malley, O.M. and Popov, E.P. (1983). Shear links in eccentrically braced frames, Journal of the Structural Division, Vol. 11, No. 9, March, ASCE. Moestopo, M. and Mirza, A. (26). Kinerja sambungan baut pada link struktur rangka baja eksentrik, Seminar & Pameran HAKI, Agustus 26. Moestopo, M. and Khairullah (23). On improved performance of eccentrically braced frames, 9 th East Asia-Pacific Conference on Structural Engineering and Construction, Bali. Popov, E.P. (1981). Recent research on eccentrically braced frame, End. Struct. Vol. 5, January, ASCE. Richard, P.W. and Uang, C.M. (25). Effect of flange width-thickness ratio on eccentrically braced frame link cyclic rotation capacity, Journal of Structural Engineering, ASCE, August 25. Richard, P.W. and Uang, C.M. (26). Testing protocol for short link in eccentrically braced frame, Journal of Structural Engineering, ASCE, August 26. Ramadan, T. and Ghobarah, A. (1995). Behaviour of bolted link-column joints in eccentrically braced frame, Can. Journal of Civ.Eng Shih, H., Khandelwal, K. and El-Tawil, S. (26). Ductile web fracture initiation in steel shear links, Journal of Structural Engineering, ASCE, August 26. Stratan, A. and Dubina, D. (22). Bolted Link for Eccentrically Braced Steel Frames, The Polytecnica of Timisoara, Romania. Back to Table of Contents 659