Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954
Seismic Behaviour of Eccentrically Braced Frame with Vertical Link 1
Vahid Osat1, Ehsan Darvishan2,a, Morteza Ashoori3 1 – M. Sc., Department of Civil Engineering, College of engineering, Roudehen Branch, Islamic Azad university, Roudehen, Iran 2 – Assistant Professor, Department of Civil Engineering, College of engineering, Roudehen Branch, Islamic Azad university, Roudehen, Iran 3 – M. Sc., Department of Civil Engineering, Sharif University of Technology, Tehran, Iran a – Darvishan@riau.ac.ir DOI 10.2412/mmse.25.78.451 provided by Seo4U.link
Keywords: steel, eccentrically braced frame, vertical link, time history, seismic performance, nonlinear analysis.
ABSTRACT. The design of an eccentrically braced frame is based on providing a weak section in frame which will remain essentially elastic outside a well define link. Eccentrically braced frames combine stiffness of centrically braced frame with ductility and capability to dissipate seismic energy of moment resistant frame. In this paper the seismic behavior of eccentrically braced frame with vertical link is presented. Three regular frame with vertical link which its length is correlated to the capability to dissipate seismic energy are considered. The analytical models used to simulate the test through inelastic time history analyses that is performed in OpenSees software and simulation results were obtained. The results indicated that by reducing the vertical link length in EBFs, the maximum lateral displacements of roofs have been reduced for all frames due to 5 real earthquake records. Reducing the vertical link length decreases the maximum base shear of the structure. Therefore, it can be said that vertical link length improved the seismic performance of the EBFs.
Introduction. Eccentrically braced frames (EBF) which are used in seismic design and seismic rehabilitation of structure in seismic areas constitute a suitable compromise between seismic resistant moment resistant frames and concentrically braced frames. In EBF frames, a horizontal or vertical eccentricity (e) forms at the end of brace members that is called link or fuse. There are two type eccentrically braced frame, eccentrically braced frame with vertical and horizontal link that are shown in Fig. 1. One of the advantages of vertical links over their horizontal counterparts is the exclusion of plastic deformation from the main structure result on no damage in the roof of the structures under severe earthquake; easy and simple rehabilitation; and the replacement of link after earthquake. using the vertical links for seismic rehabilitation of the existing buildings is possible with minor changes in the main structure; however, in large or tall building and also in strengthening of the existing structures, due to limitation of dimensions of the existing components of the structures, the application of the single vertical link has lots of obstacles. The transferred shear from the vertical links, especially in concrete structures, can limit the application of big vertical links. In such case, using double vertical links is recommended[1].
1
© 2017 The Authors. Published by Magnolithe GmbH. This is an open access article under the CC BY-NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/
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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954
Fig. 1. Geometrical configuration of eccentrically braced frames [2]. The length and section property of vertical links and the bracing configuration affect the linear and nonlinear behaviour of EBFs. If the length of vertical link is long enough, flexural yielding will occur prior to shear yielding. AISC-2005 recommends Eq.1 to ensure formation of shear hinges prior to flexural hinges [3].
e
MP , VP
(1)
where e, M P and V P are link length, nominal plastic moment and shear capacities, respectively. Kasai et al. suggested using of the factor 1.4 in lieu of 1.6 in order to ensure shear behavior. As mentioned in the test setup for vertical links, the end moments of the link will not be equal, Figure. 5. Thus, Eq. 1 will be modified to Eq. 2 for such links [4], [5]:
e 0.8(1 )
MP VP
(2)
where
M2 M1
,
(3)
where M 1 and M 2 are internal bending moments along the link. In 1998, Vetr applied the Eq.4 to design maximum length of vertical shear links of specimens based on reports of Kasai et al. about weld rupture at connection of horizontal links to column[6]:
e 0.35(1 )
MP VP
(4)
Description of case study frames and analysis. Four regular EBFs with four bays are considered which their configurations are shown in Figure 2. The frames are designed based on Iranian code of MMSE Journal. Open Access www.mmse.xyz
Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954
practice for seismic resistant design of building (Standard 2800) [7]. Once the frames are designed, to evaluate their seismic response of frame, they are modelled with program OpenSees. Section property of structural member are listed in Table 1.
Fig. 2. Frames configuration. Table 1. Section properties of structural members for 4-story frame. Story
Columns
Beams
Bracing
Fuse
1
IPB360
IPE330
2UNP120
IPE280
2
IPB360
IPE330
2UNP120
IPE280
3
IPB260
IPE330
2UNP120
IPE300
4
IPB260
IPE330
2UNP120
IPE180
Table 2. Section properties of structural members for 8-story frame. Story
Columns
Beams
Bracing
Fuse
1
IPB500
IPE330
2UNP120
IPE180
2
IPB500
IPE330
2UNP120
IPE280
3
IPB450
IPE330
2UNP120
IPE300
4
IPB450
IPE330
2UNP120
IPE330
5
IPB400
IPE330
2UNP120
IPE330
6
IPB360
IPE330
2UNP120
IPE330
7
IPB300
IPE330
2UNP120
IPE330
8
IPB300
IPE300
2UNP120
IPE330
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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954
Table 3. Section properties of structural members for 12-story frame. Story
Columns
Beams
Bracing
Fuse
1
IPB700
IPE330
2UNP120
IPE180
2
IPB700
IPE330
2UNP120
IPE180
3
IPB600
IPE330
2UNP120
IPE300
4
IPB600
IPE330
2UNP120
IPE330
5
IPB500
IPE330
2UNP120
IPE360
6
IPB500
IPE330
2UNP120
IPE360
7
IPB450
IPE330
2UNP120
IPE360
8
IPB450
IPE330
2UNP120
IPE360
9
IPB400
IPE330
2UNP120
IPE360
10
IPB360
IPE330
2UNP120
IPE360
11
IPB300
IPE330
2UNP120
IPE360
12
IPB300
IPE330
2UNP120
IPE360
Time history analysis. 5 real earthquake records were registered in Soil type II according to Iranian Code (Standard 2800) that is listed in Table 2. The spectral acceleration of ground motion records is scaled and matched to the target spectrum that is obtained Standard 2800. Comparison earthquake records and target spectrum is indicated in Fig. 2 [8]. Table 4. Specification of earthquake records Earthquake
Recording Station
ID NO.
M
Year
Name
Name
Owner
1
6.5
1979
Imperial Valley
Elcentro Array
USGS
2
6.9
1995
Kobe, Japan
Nishi Akashi
CUE
3
7.4
1990
Mnjil, Iran
Abbar
BHRC
4
7.3
1992
Tabas, Iran
Tabas
USGS
5
7.3
1952
Tast, USA
California
alluvium
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Sa [g]
Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954
1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0
1
2
3
4
5
Time [sec]
Fig. 3. Comparison mean spectrum and target spectrum. Result and discussion. The nonlinear dynamic analyses were conducted on EBFs subjected to the earthquake excitations by using OpenSees program. Fig. 4 indicates the displacement response of the EBFs. As it is seen from Fig. 4, by reducing the vertical link length in EBFs, the maximum lateral displacements of roofs have been reduced for all frames due to 5 real earthquake records. For example, the maximum lateral displacements of roofs for 4-story EBFs by vertical link length equal to 60 cm due to Imperial Valley earthquake is 11.7 cm, while it was observed the maximum lateral displacements of roofs for 4-story EBFs by vertical link length equal to 40 cm due to Imperial Valley earthquake is 7.0 cm.
Displacement [cm]
15
Vertical link length= 60cm Vertical link length= 40cm
10 5 0 -5 -10 -15
0
5
10
15
20
25
30
35
40
45
Time [sec]
Fig. 4. Comparison lateral displacement of roofs time histories for 4-story EB frames due to Imperial Valley earthquake.
Displacement [cm]
30 20 10 0 -10
Vertical Link lenngth=40cm Vertical Link lenngth=60cm
-20 -30
0
5
10
15
20
25
30
35
40
45
Time [sec]
Fig. 5. Comparison lateral displacement of roofs time histories for 8-story EB frames due to Imperial Valley earthquake. MMSE Journal. Open Access www.mmse.xyz
Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954
Displacement [cm]
40
Vertical Link length= 40cm Vertical Link length= 60cm
20 0 -20 -40 -60 -80
0
5
10
15
20
25
30
35
40
45
Time [sec]
Fig. 6. Comparison lateral displacement of roofs time histories for 12-story EB frames due to Imperial Valley earthquake. The maximum lateral displacement of roofs for frames subjected to five different earthquakes in two cases with different vertical link length, i.e., 40cm and 60 cm, are shown in Tables 5. Table 5. The maximum lateral displacement of roofs for four, eight and twelve stories frames. The maximum lateral displacement (cm) Records
4-Story
8-Story
12-Story
El centro
Kobe
Manjil
Tabas
Tast
Vertical link length=40 cm
7.0
4.9
1.8
4.9
1.9
Vertical link length=60 cm
11.7
4.9
1.9
5.0
2.5
Vertical link length=40 cm
28.4
9.3
10.1
8.3
8.1
29.2
10.5
11.6
8.4
9.2
Vertical link length=40 cm
20.5
19.0
25.8
14.2
14.2
Vertical link length=60 cm
24.1
25.1
25.9
14.8
20.6
Vertical link length=60 cm
The time histories of seismic base shear of four, eight and twelve-story frames due to Imperial Valley earthquake are given in Fig. 7, 8 and 9.
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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954 3000
Vertical length=40 cm Vertical length=60 cm
Base Shear [kN]
2000 1000 0 -1000 -2000 -3000 -4000
0
5
10
15
20
25
30
35
40
45
Time [sec]
Fig. 7. Comparison base shear time histories for 4-story EB frames due to Imperial Valley earthquake.
Base Shear [kN]
4000
Vertical link length= 40cm Vertical link length= 60cm
2000
0
-2000
-4000
0
5
10
15
20
25
30
35
40
45
Time [sec]
Fig. 8. Comparison base shear time histories for 8-story EB frames due to Imperial Valley earthquake.
Base Shear [kN]
4000
Vertical link length=40cm Vertical link length=60cm
2000 0 -2000 -4000 -6000
0
5
10
15
20
25
30
35
40
45
Time [sec]
Fig. 9. Comparison base shear time histories for 12-story EB frames due to Imperial Valley earthquake. The maximum base shear for frames subjected to five different earthquakes in two cases with different vertical link length, i.e., 40 cm and 60 cm, are shown in Tables 6.
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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954
Table 6. The maximum base shear for four, eight and twelve stories frames. Base shear (KN) Records
4-Story
8-Story
12-Story
El centro
Kobe
Manjil
Tabas
Tast
Vertical link length=40 cm
2802.5
2303.5
2311.3
2223.6
2387.9
Vertical link length=60 cm
2319.3
1612
1736.9
1633.6
1897.6
Vertical link length=40 cm
3333
2568.9
2701.7
2300.5
2895.9
Vertical link length=60 cm
2783.2
1851.2
1798.5
1772.4
2049.2
Vertical link length=40 cm
3306.3
2934.7
2494
3046.9
2984.2
Vertical link length=60 cm
3150
2892.6
2717.8
3001
2770.2
Summary. The main goal of this study was to evaluate the seismic performance of eccentrically braced frame. Structural model used in this paper, are two dimensional steel eccentrically braced frames with vertical link. The vertical links were located between the top of inverted V-bracing and the beam. Four, eight and twelve-story frames were selected were each having four bays 5m length. The structural members were designed based on Standard 2800 and AISC-2005. IPE cross-section, and Box cross-section were used in designing of frames. Once the frames are designed, to evaluate their seismic response of frame, they are modelled with program OpenSees. In this approach the earthquake ground motion can be applied to the model simultaneously, the time history of displacement can be considered. The results indicated that by reducing the vertical link length in EBFs, the maximum lateral displacements of roofs have been reduced for all frames due to 5 real earthquake records. In all models due to five different earthquakes, maximum base shear were reduced by increasing the vertical link length. References [1] M.-R. Shayanfar and A.-T. Sina, "Assessment of the seismic behavior of eccentrically braced frame with double vertical link (DV-EBF)," The, vol. 14, pp. 12-17, (2008). [2] R. Montuori, E. Nastri, and V. Piluso, "Theory of Plastic Mechanism Control for MRF–EBF dual systems: Closed form solution," Engineering Structures, vol. 118, pp. 287-306, (2016), DOI 10.1016/j.engstruct.2016.03.050. [3] AISC, Seismic Provision for structural steel Building, (2005), http://aec.ihs.com/news/2006/aiscseismicprovisions.htm. [4] K. Kasai and X. Han, "Refined design and analysis of eccentrically braced frames," Journal of Structural Engineering, ASCE, (1997). [5] K. Kasai and E. P. Popov, "General behavior of WF steel shear link beams," Journal of Structural Engineering, vol. 112, pp. 362-382, (1986), DOI 10.1061/(ASCE)0733-9445(1986)112:2(362). [6] M. Vetr, "Seismic behavior, analysis and design of eccentrically braced frames with vertical shear links," Ph.. D. Thesis. University Tech. Darmstadt W. Germany, (1998), http://www. tudarmstadt. de.
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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954
[7] I. S. Code, "Iranian code of practice for seismic resistant design of buildings," ed: Standard, (2005). [8] H. Mostafaei, M. Sohrabi Gilani, and M. Ghaemian, "Stability analysis of arch dam abutments due to seismic loading," Scientia Iranica A, vol. 24, pp. 467-475, (2017).
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