- Email: [email protected]
Seismic performance of high-rise buildings with energy-dissipation outriggers
Journal of Constructional Steel Research 134 (2017) 80–91
Contents lists available at ScienceDirect
Journal of Constructional Steel Research
Seismic performance of high-rise buildings with energy-dissipation outriggers Huanjun Jiang a,b, Shurong Li a,b,⁎, Yulong Zhu c a b c
State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China Research Institute of Structural Engineering and Disaster Reduction, Tongji University, Shanghai 200092, China East China Electric Power Design Institute Co., Ltd., Shanghai 200063, China
a r t i c l e
i n f o
Article history: Received 7 September 2016 Received in revised form 5 March 2017 Accepted 18 March 2017 Available online xxxx Keywords: Outrigger Buckling restrained brace Numerical simulation Performance-based seismic design
a b s t r a c t To improve the seismic performance of high-rise building structures with outriggers, a new structure with energy-dissipation outriggers installed using buckling restrained braces (BRBs), which replace the ordinary diagonal bracing, is proposed in this study. To verify the seismic performance of the new structure, a case study was carried out. Two high-rise structures, one with conventional outriggers, the other with energy-dissipation outriggers, were designed. The numerical models for the two structures were established with the aid of a commercial software, Perform-3D. The responses of the two structures under frequent earthquakes, basic earthquakes, and rare earthquakes were analyzed and compared. The results show that compared to the ordinary structure, the seismic performance of the new structure is improved significantly. Under frequent earthquakes, both structures remain basically elastic, and the responses of the two structures are almost identical. Under basic earthquakes and rare earthquakes, the BRBs in the new structure yield initially and dissipate a large amount of the input energy to provide adequate protection to the main structural members. Furthermore, the influences of the strength and layout of BRBs on the seismic performance of the new structure were investigated, providing reference for engineering applications. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction With the socio-economic development and rapid urbanization in mainland China, many supertall buildings have been constructed or are under construction. A large proportion of these tall buildings apply steel-concrete hybrid structure systems such as steel frame-reinforced concrete (RC) core tubes owing to their unique advantage of reducing construction cost and saving construction time. To effectively reduce the interstory drift of high-rise buildings under earthquakes and winds, outriggers connecting the outer frame and core tube are usually adopted to form a strengthened story. However, because of the effect of the outriggers, the lateral stiffness of the strengthened story is much larger than that of the adjacent stories, resulting in the abrupt change of internal forces in the structural members of such stories and the possible formation of weak stories under strong earthquakes [1]. To improve the performance of the structure with outriggers under earthquakes and winds, Smith and Willford [2] developed a new structure with energy-dissipation outriggers, in which viscous dampers are installed between peripheral frame columns and outriggers, as the ⁎ Corresponding author at: State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China. E-mail address: [email protected] (S. Li).
http://dx.doi.org/10.1016/j.jcsr.2017.03.013 0143-974X/© 2017 Elsevier Ltd. All rights reserved.
relative vertical deformation between the core tube and column is large. The new energy-dissipating outrigger system was successfully applied to the Saint Francis Shangri-la Place in Manila. Zhou and Li [3] analyzed the earthquake responses of two high-rise steel structures, one with conventional outriggers and the other with similar viscous damped outriggers. It was found that under severe earthquakes, the responses of the structure with viscous damped outriggers, which contributes an additional damping ratio of 4%, are much lesser than those of conventional structures. However, compared to the conventional structure with a rigid connection between the outrigger and the peripheral column, the lateral stiffness of the structure with a viscous damped outrigger is significantly reduced owing to the weakened damping connection between the outrigger and the peripheral column. Thus, under frequent earthquakes, the lateral displacement and interstory drifts of the structure with damped outriggers are larger than those of the conventional structure. The function of the outriggers in reducing the lateral displacement and interstory drifts of a structure under frequent earthquakes could not be performed effectively. To further improve the seismic performance of tall building structures with outriggers, a new structure with energy-dissipation outriggers installed with buckling restrained braces (BRBs), replacing the ordinary diagonal bracing, is proposed in this study. Compared with the temperature-sensitive viscous dampers, BRBs are usually cheaper
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
81
Table 2 Dimensions of components in outrigger truss (unit:mm). Member number
Width
Height
Web thickness
Flange thickness
Component length
OTB-1 OTB-2 OTX-1 OTX-2
350 350 350 350
700 700 700 700
16 12 40 12
36 20 60 20
10,500 10,500 6723 10,500
with conventional outriggers. In addition, the strength of BRBs is selected so that under frequent earthquakes, the BRBs as well as the main structure remain in the elastic stage. Furthermore, under basic earthquakes and rare earthquakes, the BRBs yield initially and dissipate most of the input energy so that the main structural members can be adequately protected. To verify the seismic performance of the new structure, a case study was carried out in this research. 2. Project profile
Fig. 1. Structural plan layout of typical floor.
Fig. 2. Schematic diagram of floor with outrigger trusses.
Table 1 Strength grade of structural material.
A typical high-rise steel and concrete composite frame-RC core tube structure with 55 stories and a total height of 229.8 m was designed for the case study. The seismic intensity is 7. The peak ground acceleration (PGA) values of the three earthquake levels are 55, 150, and 310 gal, respectively. The site soil class is III, the design group is 2, and the characteristic period of the ground motion is 0.55 s. The height of each story is 4.2 m excluding the bottom floor (4.8 m) and mechanical floor (3.6 m). The structural plan layout of the typical floor is shown in Fig. 1. The peripheral frame consists of steel-reinforced concrete (SRC) columns and steel beams. The outrigger trusses are arranged at the 32nd floor in two directions, as shown in Fig. 2. The strength grade of the structural material is given in Table 1. The outrigger truss consists of the top chord members, bottom chord members, and diagonal bracings, as shown in Fig. 3. All of them are steel I-section members. The dimensions of the components in the outrigger truss are listed in Table 2. Accordingly, another structure with the previously mentioned energy-dissipation outriggers was designed. In this new type of structure, the diagonal braces (named OTX-1) in the side span located outside the core tube in the conventional structure are replaced by BRBs. The dimensions of the BRBs are determined by the equal-rigidity principle. All structural details except for the outrigger trusses are identical for the two structures. 3. Numerical models
Concrete Steel Steel bar
Column
Beam
Shear wall
Slab
Outrigger truss
C50 Q345 HRB400
– Q345 –
C50 – HRB400
C35 – HRB400
– Q460 –
and more reliable in terms of durability [4]. Previous studies have shown that BRBs have a large energy-dissipating ability, and can be used not only as ordinary bearing bracings, but also as displacement dampers [5,6]. To limit the lateral displacement of a new structure under frequent earthquakes, the axial stiffness of the BRBs is determined using the principle of equal-rigidity substitution. This ensures that the new structure has the same lateral stiffness as the structure
3.1. Finite element models The finite element models of the two structures were established with the aid of a commercial program, Perform-3D [7]. Beams were simulated using a plastic hinge model, assuming that flexural plastic hinges occurred only at the two ends of the members. For beams with a relatively large span-to-depth ratio, flexural deformation is the controlling factor, and the shearing behavior is regarded as elastic. With the same forming mechanism involving plastic hinges for beams, columns were simulated using the plastic hinge model as well. The force–displacement relationship of the cross-section was derived from the fiber model. The truss members were simulated by inelastic axial bars, in
Fig. 3. Schematic diagram of outrigger truss.
82
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91 Table 3 Characteristic parameters and deformation limit of components. Component type
SRC column Steel beam Coupling beam Flexure-dominated shear wall Shear-dominated shear wall Bracing in compression Bracing in tension
Characteristic parameters
Deformation limit
a(d)
b(e)
c
IO
LS
CP
0.015 4θy 0.02 0.009 0.0075 0.5Δc 11ΔT
0.025 6θy 0.04 0.012 0.02 8Δc 14ΔT
0.2 0.2 0.5 0.6 0.4 0.2 0.8
0.003 1.25θy 0.005 0.003 0.004 1.25Δc 1.25ΔT
0.012 3θy 0.01 0.006 0.006 6Δc 8ΔT
0.015 4θy 0.02 0.009 0.0075 8Δc 10ΔT
Notes: θy is the yield rotation angle, Δc is the yield deformation of the bracing in compression, and ΔT is the yield deformation of the bracing in tension. Fig. 4. Elastoplastic model for steel.
which the buckling behavior can be considered. The BRB element specified in the program was adopted to simulate the BRB, which only bears the axial force. The core tube was simulated by a shear wall element, which consists of vertical fibers and a concrete shear layer. The fiber layers were used to model the bending and axial behaviors, and the concrete shear layer takes into account the contribution of concrete to the shear strength. The elastoplastic analytic model employs a rigid floor assumption. Meanwhile, the P-Δ effect was also considered in the nonlinear time history analysis. 3.2. Constitutive relationships for steel and concrete The elastoplastic model with no strain hardening is adopted for steel, as shown in Fig. 4. Owing to the limitation of the Perform-3D program, the constitutive relationship curves for concrete have to be first
converted into piecewise linear lines based on the principle of energy conservation to obtain the parameters of corresponding key points required in the procedure. The simplified Mander model [8] was employed for the core concrete and cover concrete, respectively, as shown in Fig. 5.
3.3. Performance levels of structural components According to FEMA 356 (Prestandard and Commentary for the Seismic Rehabilitation of Buildings) [9], the seismic performance of structural components is divided into three levels: immediate occupancy (IO), life safety (LS), and collapse prevention (CP), as shown in Fig. 6. There are two types of idealized force–deformation curves, one in terms of deformation, and the other in terms of deformation ratio. Table 3 presents the characteristic parameters in the force–deformation curves and the deformation limit of components corresponding to
Fig. 5. Constitutive model for concrete: (a) unconfined concrete; (b) confined concrete.
Fig. 6. Force versus deformation curves and performance levels: (a) force–deformation relationships; (b) force–deformation ratio relationships; (c) performance levels.
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91 Table 4 Characteristics of the first six natural vibration modes. Mode number
Period (s)
Vibration form
1 2 3 4 5 6
4.355 4.355 3.400 1.327 1.327 1.233
Translation in X direction Translation in Y direction Torsion Translation in X direction Translation in Y direction Torsion
the individual performance level determined in accordance with FEMA 356 [9] and ASCE 41 [10]. The plastic hinge rotation was selected as the performance index for columns and beams. The performance index for
83
the shear wall controlled by flexure and shear was the plastic hinge rotation and chord rotation, respectively. For truss members, the axial deformation was used as the performance index. 3.4. Dynamic characteristics The characteristics of the first six natural vibration modes are listed in Table 4. The dynamic characteristics of the two structures, the new structure with BRBs, and the conventional structure are identical, because of the design principle of equal rigidity. The period ratio between the first torsional mode and the first translational mode is 0.78, which meets the requirement that the period ratio should not be larger than 0.85, to prevent excessive structural torsion [11].
Fig. 7. Normalized acceleration time history curves: (a) GM-749-3-E03230 (X direction); (b) GM-749-3-E03140 (Y direction); (c) GM-2657-3-BRS090 (X direction); (d) GM-2657-3BRS000 (Y direction); (e) USER-1.
84
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
same PGA of 55 gal with the design spectrum specified in the Chinese code [11] is shown in Fig. 8. The PGAs for frequent earthquakes, basic earthquakes, and rare earthquakes are 55 gal, 150 gal, and 310 gal, respectively. The structures were subjected to earthquakes in the X and Y directions simultaneously. The damping ratio was taken as 0.04, 0.04, and 0.05 under frequent earthquakes, basic earthquakes, and rare earthquakes, respectively.
4. Earthquake responses of structures 4.1. Displacement responses
Fig. 8. Comparison of acceleration spectrum.
3.5. Earthquake ground motions Three sets of earthquake ground motions as well as two natural waves, GM-749-3 and GM-2657-3, and one artificial wave USER-1 were selected as seismic inputs based on the requirements of the Chinese seismic design code [11]. The normalized acceleration time history curves are shown in Fig. 7. The comparison of the acceleration spectrum for the primary component of individual set of ground motions with the
The interstory drift ratio is a significant and commonly used index in evaluating the seismic performance of structures. Thus, we present the maximum interstory drift ratios of the structure under different levels of ground motions in Table 5. The envelopes of the interstory drift ratio of the two structures in the X direction under basic earthquakes and rare earthquakes are shown in Figs. 9 and 10, respectively. For simplicity, the normal structure is named NOR, and the new structure is named BRB. Under frequent earthquakes, the displacement responses of the two structures are almost the same. Under basic earthquakes and rare earthquakes, compared to the normal structure, the distribution of the interstory drift ratio for the new structure is more uniform. In particular, the abrupt change at the strengthened story with outrigger trusses in the new structure is significantly relieved, and the maximum interstory drift of the new structure is slightly reduced.
Table 5 Maximum interstory drift ratio. Seismic wave
GM-749-3 GM-2657-3 USER-1
Structure number
NOR BRB NOR BRB NOR BRB
Frequent earthquakes
Basic earthquakes
Rare earthquakes
X direction
Y direction
X direction
Y direction
X direction
Y direction
1/883 1/883 1/959 1/957 1/691 1/692
1/1038 1/1038 1/1124 1/1133 1/810 1/814
1/334 1/340 1/425 1/445 1/256 1/263
1/395 1/402 1/498 1/519 1/298 1/307
1/174 1/175 1/243 1/245 1/144 1/147
1/196 1/198 1/290 1/288 1/160 1/165
Fig. 9. Envelopes of interstory drift ratio in X direction under basic earthquakes: (a) GM-749-3; (b) GM-2657-3; (c) USER-1.
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
85
Fig. 10. Envelopes of interstory drift ratio in X direction under rare earthquakes: (a) GM-749-3; (b) GM-2657-3; (c) USER-1.
4.2. Energy dissipation Under frequent earthquakes, the dissipated energy in the two structures is similar, mainly consisting of the strain energy and damping energy, which indicates that both structures are basically in the elastic state. Under basic earthquakes and rare earthquakes, the compositions of the dissipated energy at the end of the input for each set of ground motions are presented in Tables 6 and 7. From the tables, it can be observed that the normal structure dissipates energy mainly through the plastic deformation of coupling beams and shear walls; in the new structure, the outrigger trusses dissipate a large part of the total energy. The energy dissipated in the coupling beams and shear walls reduced significantly; thus, the damage in the core tube could be mitigated effectively.
Table 8 Maximum ratio of deformation demand to capacity for each type of structural member under frequent earthquakes. Earthquake wave
Structure Coupling number beam
GM-749-3
NOR BRB GM-2657-3 NOR BRB USER-1 NOR BRB
Shear wall Flexural Shear deformation deformation
Column Frame beam
IO
LS IO
LS
IO
IO
IO
0.162 0.160 0.158 0.157 0.353 0.351
– – – – – –
– – – – – –
0.046 0.045 0.043 0.041 0.062 0.059
0.167 0.168 0.160 0.161 0.180 0.186
0.077 0.076 0.074 0.074 0.079 0.078
0.305 0.305 0.267 0.262 0.498 0.489
Table 6 Compositions of dissipated energy under basic earthquakes. Earthquake wave
Structure number
Total energy (kN·m)
Inelastic energy (kN·m)
Proportion of inelastic energy (%)
Compositions of inelastic energy (%) Coupling beam
Wall
Outrigger truss
Column
Frame beam
GM-749-3
NOR BRB NOR BRB NOR BRB
82,150 80,560 79,360 79,850 109,200 106,500
8610 12,520 5788 8803 14,890 19,590
10.5 15.5 7.3 11.0 13.6 18.4
27.6 14.3 34.5 20.9 37.2 21.6
65.9 37.9 58.6 32.9 57.5 34.5
0 43.6 0 41.3 0 40.1
6.3 4.1 6.7 4.9 4.8 3.6
0.2 0.1 0.2 0 0.5 0.2
GM-2657-3 USER-1
Table 7 Compositions of dissipated energy under rare earthquakes. Earthquake wave
GM-749-3 GM-2657-3 USER-1
Structure number
NOR BRB NOR BRB NOR BRB
Total energy (kN·m)
Total inelastic energy (kN·m)
Proportion of inelastic energy (%)
189,300 184,500 167,300 172,500 293,100 283,500
66,510 69,350 44,290 50,640 117,500 118,500
35.1 37.6 26.5 29.4 40.1 41.8
Compositions of inelastic energy (%) Coupling beam
Wall
Outrigger truss
Column
Frame beam
57.4 39.5 66.5 48.3 63.8 49.4
34.2 28.0 29.7 24.2 28.2 23.9
5.9 30.4 1.6 25.8 6.3 25.2
2.4 2.1 2.1 1.7 1.7 1.5
0.1 0 0.1 0 0 0
86
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
Table 9 Maximum ratio of deformation demand to capacity for each type of structural member under basic earthquakes. Earthquake wave
Structure number
Coupling beam
Shear wall Flexural deformation
GM-749-3 GM-2657-3 USER-1
NOR BRB NOR BRB NOR BRB
Column
Frame beam
Shear deformation
IO
IO
LS
CP
IO
IO
IO
0.517 0.467 0.417 0.383 0.591 0.516
– 0.681 – 0.658 – 0.852
0.812 – 0.572 – – –
– – – – 0.798 –
0.231 0.063 0.154 0.063 0.357 0.073
0.262 0.254 0.226 0.228 0.306 0.322
0.117 0.117 0.104 0.101 0.134 0.139
Table 10 Maximum ratio of deformation demand to capacity for each type of structural member under rare earthquakes. Earthquake wave
Structure number
Coupling beam
Shear wall Flexural deformation
GM-749-3 GM-2657-3 USER-1
NOR BRB NOR BRB NOR BRB
Column
Frame beam
Shear deformation
LS
CP
LS
CP
IO
IO
IO
0.812 0.732 0.583 0.560 – 0.960
– – – – 0.850 –
– 0.573 – 0.717 – 0.781
0.847 – 0.681 – 1.270 –
0.581 0.226 0.317 0.204 0.677 0.433
0.457 0.538 0.304 0.341 0.604 0.678
0.191 0.200 0.154 0.159 0.233 0.272
4.3. Damage state Table 8 presents the maximum ratio of deformation demand to the capacity for each type of structural member under frequent earthquakes. The damage state of the two structures is similar. All structural members are at the performance level of IO. The maximum ratios are much lower than 1, which indicates that there is very slight damage in
the two structures, and in general, the two structures are basically in the elastic state. Table 9 presents the maximum ratio of deformation demand to the capacity for each type of structural member under basic earthquakes. In the normal structure, some shear walls are at the performance level of LS and some are at the CP level. In the new structure, all shear walls are at the performance level of IO. As for the coupling beams, although
Fig. 11. Plastic damage of coupling beam (elevation at Y = 42 m, LS): (a) NOR; (b) BRB.
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
Fig. 13. Plastic flexural damage of shear wall (LS): (a) NOR; (b) BRB. Fig. 12. Plastic flexural damage of shear wall (IO) (a) NOR; (b) BRB.
the performance levels of the two structures are all at IO, the maximum ratio of the new structure is slightly lower than that of the normal structure. Compared with the normal structure, the damage in the core tube of the new structure is significantly reduced. For the peripheral frame beams and columns of the two structures, the performance levels are at IO, the maximum ratios are similar, i.e., much lower than 1, and the damage is very slight. Table 10 presents the maximum ratio of deformation demand to capacity for each type of structural member under rare earthquakes. Figs. 11 to 15 show the plastic damage distribution of different types of structural members of the two structures subjected to the earthquake wave GM-749-3. In the normal structure, all shear walls are at the performance level of CP, while in the new structure, all shear walls are at the performance level of LS. As for the coupling beams, in the normal structure, some are at the performance level of LS and some are at CP, while in the new structure, all are at LS. Compared with the normal structure, the damage in the core tube of the new structure is significantly reduced. Furthermore, the damages at the lower stories and the stories adjacent to the strengthened story are much larger than those of other stories. For the peripheral frame beams and columns of the two structures, the performance levels are at IO, the maximum ratios are similar, and the damage is slight. 4.4. Performance of diagonal bracing The axial force–deformation hysteretic curves of the diagonal bracing in the outrigger truss of the two structures subjected to the earthquake wave GM-749-3 at the basic earthquake level and rare earthquake level are shown in Figs. 16 and 17, respectively. Under basic earthquakes, the diagonal bracing in the normal structure remains elastic while the diagonal bracing in the new structure yields and dissipates considerable energy to protect the main structure. Under rare
Fig. 14. Plastic flexural damage of shear wall (CP) (a) NOR; (b) BRB.
87
88
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
intensities. Three sets of earthquake ground motions used in the said case study were adopted here. The principle of equal-rigidity substitution was adopted for the design of the energy-dissipation outrigger truss. 5.1. Layout of BRBs The energy-dissipation outrigger truss consists of the top chord members, bottom chord members, and diagonal BRBs. Three types of BRB layouts were considered here, as shown in Fig. 18. The strength grade of Q225LY was adopted for the BRB. By using the principle of equal-rigidity substitution, the properties of the BRB for different layouts were determined as listed in Table 11. Under frequent earthquakes, the BRBs in all structures do not yield, and all the structural components remain basically elastic. Under basic earthquakes and rare earthquakes, the BRBs in all structures yield. The inelastic energy dissipated by the BRBs and the ratio of the energy dissipated by the BRBs to the total inelastic energy dissipated by the entire structure for each structure under basic earthquakes and rare earthquakes are shown in Figs. 19 and 20, respectively. In the TRUSS1 structure, both the energy dissipated by the BRBs and the energy ratio are larger compared to the others; thus, the main structural components could be better protected from damage. Compared to the difference under basic earthquakes, the difference between the structures with different truss types under rare earthquakes is less. Therefore, the TRUSS1 layout is the best choice. 5.2. Strength of BRBs
Fig. 15. Plastic shear damage of shear wall (IO): (a) NOR; (b) BRB.
earthquakes, although the diagonal bracings in the two structures yield, the energy dissipated by the BRB in the new structure is much larger than that in the diagonal bracing of the normal structure. 5. Parametric analysis The strength and layout of the BRB are the main design parameters for the energy-dissipation outrigger truss mentioned above. The effects of the two parameters on the seismic performance of the new structure with the energy-dissipation outrigger truss were investigated here, based on the structure designed in the abovementioned case study through time history analysis under earthquakes with different
In different energy-dissipation outrigger trusses, the cross-sectional area of the BRBs is identical to keep the stiffness of the BRBs and the stiffness of trusses invariant. The strength of the BRBs was altered by adopting the steel core element with different strength grades. Four strength grades, i.e., Q100LY, Q160LY, Q225LY, and Q235, with yield strengths of 100 MPa, 160 MPa, 225 MPa, and 235 MPa, respectively were considered. The TRUSS1 layout was adopted for all trusses. The responses of the new structure with different outrigger trusses under earthquakes with different levels of intensity were predicted. Under frequent earthquakes, the structures with BRB strength grade of Q100LY and Q160LY yield, while the other structural components in the structure remain basically elastic. As for the structures with BRB strength grade of Q225LY and Q235, no BRBs yield, and all the structural components remain basically elastic. The maximum interstory drift ratio of the structures with BRB strength grade of Q100LY and Q160LY is larger than that of the structures with BRB strength grade of Q225LY and Q235. Under basic earthquakes and rare earthquakes, the BRBs in all structures yield. The inelastic energy dissipated by the BRBs and the ratio of the
Fig. 16. Axial force–deformation hysteretic curves of diagonal bracing under basic earthquake: (a) NOR; (b) BRB.
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
89
Fig. 17. Axial force–deformation hysteretic curves of diagonal bracing under rare earthquake: (a) NOR; (b) BRB.
Fig. 18. Layout of BRB: (a) TRUSS1; (b) TRUSS2; (c) TRUSS3.
protected from damage. Under rare earthquakes, the effect of the strength of the BRBs is not noticeable.
Table 11 Mechanical properties of BRBs. Layout type
Cross-sectional area (mm2)
Yield force (kN)
TRUSS1 TRUSS2 TRUSS3
5.8680 × 104 1.2722 × 105 2.5880 × 105
1.3232 × 104 2.8687 × 104 4.6688 × 104
energy dissipated by the BRBs to the total inelastic energy dissipated by the entire structure for each structure under basic earthquakes and rare earthquakes are shown in Figs. 21 and 22, respectively. Under basic earthquakes, with the decrease in the strength of the BRBs, both the energy dissipated by the BRBs and the energy ratio increase, and the main structural components could be better
6. Conclusions A new structure with energy-dissipation outrigger trusses installed with BRBs is proposed in this study. The seismic performance of the new structure was verified by a case study involving elastoplastic time history analysis, comparing the seismic responses of the new structure with the normal structure under three levels of earthquakes. Under frequent earthquakes, both structures remain basically elastic, and the responses of the two structures are almost identical. Under basic earthquakes and rare earthquakes, the BRBs in the new structure yield initially and dissipate a large amount of energy so that the main
Fig. 19. Energy dissipated by BRBs for different layouts under basic earthquakes: (a) inelastic energy; (b) inelastic energy ratio.
90
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
Fig. 20. Energy dissipated by BRBs for different layouts under rare earthquakes: (a) inelastic energy; (b) inelastic energy ratio.
Fig. 21. Energy dissipated by BRBs for different strengths under basic earthquakes: (a) inelastic energy; (b) inelastic energy ratio.
Fig. 22. Energy dissipated by BRBs for different strengths under rare earthquakes: (a) inelastic energy; (b) inelastic energy ratio.
structural members, particularly the core tube, can be adequately protected. Compared to the normal structure, the distribution of the interstory drift ratio in the new structure is more uniform, and the maximum interstory drift in the new structure is slightly decreased. The
parametric analysis shows that the effects of the strength and layout of BRBs on the seismic performance of the new structure are significant. An appropriate strength should be selected so that the outrigger truss will not yield prematurely under frequent earthquakes, but dissipates
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
a large portion of the inelastic energy under basic earthquakes and rare earthquakes. The outrigger truss with the TRUSS1 layout is recommended for application in the new structure. Further studies are necessary to optimize the design parameters of BRBs and obtain a better performance from new structures under earthquakes with different levels of intensity. Acknowledgements The authors are grateful for the support from the Ministry of Science and Technology of China through Grant No. SLDRCE14-B-2 and Key Research and Development Program of Hainan Province through Grant No. SQ2016SHFZ0062. References [1] K. Kamath, N. Divya, A.U. Rao, A study on static and dynamic behavior of outrigger structural system for tall buildings, Bonfring Int. J. Ind. Eng. Manag. Sci. 2 (4) (2012) 15–20.
91
[2] R.J. Smith, M.R. Willford, The damped outrigger concept for tall buildings, Struct. Des. Tall Special Build. 16 (4) (2007) 501–517. [3] Y. Zhou, H. Li, Analysis of a high-rise steel structure with viscous damped outriggers, Struct. Design Tall Spec. Build. 23 (13) (2014) 963–979. [4] M.E. Eryaşar, C. Topkaya, An experimental study on steel–encased buckling–restrained brace hysteretic dampers, Earthq. Eng. Struct. Dyn. 39 (5) (2010) 561–581. [5] Y.K. Ju, M.H. Kim, J. Kim, et al., Component tests of buckling-restrained braces with unconstrained length, Eng. Struct. 31 (2) (2009) 507–516. [6] D.H. Kim, C.H. Lee, Y.K. Ju, et al., Subassemblage test of buckling-restrained braces with H-shaped steel core, Struct. Design Tall Spec. Build. 24 (4) (2015) 243–256. [7] Computer and Structures, Inc. Perform-3D, Nonlinear Analysis and Performance Assessment for 3D Structure User Guide, Version 4, Computers and Structures, Inc., Berkeley, CA., 2006 [8] J.B. Mander, M.J.N. Priestley, R. Park, Theoretical stress-strain model for confined concrete, J. Struct. Eng. ASCE 114 (8) (1988) 1804–1826. [9] Council, BSS, Prestandard and Commentary for the Seismic Rehabilitation of Buildings, FEMA-356, Federal Emergency Management Agency, Washington, DC, 2000. [10] ASCE/SEI Seismic Rehabilitation Standards Committee, Seismic Rehabilitation of Existing Buildings (ASCE/SEI 41-06), American Society of Civil Engineers, Reston, VA, 2007. [11] China Ministry of Construction (CMC), Code for Seismic Design of Buildings (GB 50011-2010), China Architecture & Building Press, Beijing, 2010 (in Chinese).
Contents lists available at ScienceDirect
Journal of Constructional Steel Research
Seismic performance of high-rise buildings with energy-dissipation outriggers Huanjun Jiang a,b, Shurong Li a,b,⁎, Yulong Zhu c a b c
State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China Research Institute of Structural Engineering and Disaster Reduction, Tongji University, Shanghai 200092, China East China Electric Power Design Institute Co., Ltd., Shanghai 200063, China
a r t i c l e
i n f o
Article history: Received 7 September 2016 Received in revised form 5 March 2017 Accepted 18 March 2017 Available online xxxx Keywords: Outrigger Buckling restrained brace Numerical simulation Performance-based seismic design
a b s t r a c t To improve the seismic performance of high-rise building structures with outriggers, a new structure with energy-dissipation outriggers installed using buckling restrained braces (BRBs), which replace the ordinary diagonal bracing, is proposed in this study. To verify the seismic performance of the new structure, a case study was carried out. Two high-rise structures, one with conventional outriggers, the other with energy-dissipation outriggers, were designed. The numerical models for the two structures were established with the aid of a commercial software, Perform-3D. The responses of the two structures under frequent earthquakes, basic earthquakes, and rare earthquakes were analyzed and compared. The results show that compared to the ordinary structure, the seismic performance of the new structure is improved significantly. Under frequent earthquakes, both structures remain basically elastic, and the responses of the two structures are almost identical. Under basic earthquakes and rare earthquakes, the BRBs in the new structure yield initially and dissipate a large amount of the input energy to provide adequate protection to the main structural members. Furthermore, the influences of the strength and layout of BRBs on the seismic performance of the new structure were investigated, providing reference for engineering applications. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction With the socio-economic development and rapid urbanization in mainland China, many supertall buildings have been constructed or are under construction. A large proportion of these tall buildings apply steel-concrete hybrid structure systems such as steel frame-reinforced concrete (RC) core tubes owing to their unique advantage of reducing construction cost and saving construction time. To effectively reduce the interstory drift of high-rise buildings under earthquakes and winds, outriggers connecting the outer frame and core tube are usually adopted to form a strengthened story. However, because of the effect of the outriggers, the lateral stiffness of the strengthened story is much larger than that of the adjacent stories, resulting in the abrupt change of internal forces in the structural members of such stories and the possible formation of weak stories under strong earthquakes [1]. To improve the performance of the structure with outriggers under earthquakes and winds, Smith and Willford [2] developed a new structure with energy-dissipation outriggers, in which viscous dampers are installed between peripheral frame columns and outriggers, as the ⁎ Corresponding author at: State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China. E-mail address: [email protected] (S. Li).
http://dx.doi.org/10.1016/j.jcsr.2017.03.013 0143-974X/© 2017 Elsevier Ltd. All rights reserved.
relative vertical deformation between the core tube and column is large. The new energy-dissipating outrigger system was successfully applied to the Saint Francis Shangri-la Place in Manila. Zhou and Li [3] analyzed the earthquake responses of two high-rise steel structures, one with conventional outriggers and the other with similar viscous damped outriggers. It was found that under severe earthquakes, the responses of the structure with viscous damped outriggers, which contributes an additional damping ratio of 4%, are much lesser than those of conventional structures. However, compared to the conventional structure with a rigid connection between the outrigger and the peripheral column, the lateral stiffness of the structure with a viscous damped outrigger is significantly reduced owing to the weakened damping connection between the outrigger and the peripheral column. Thus, under frequent earthquakes, the lateral displacement and interstory drifts of the structure with damped outriggers are larger than those of the conventional structure. The function of the outriggers in reducing the lateral displacement and interstory drifts of a structure under frequent earthquakes could not be performed effectively. To further improve the seismic performance of tall building structures with outriggers, a new structure with energy-dissipation outriggers installed with buckling restrained braces (BRBs), replacing the ordinary diagonal bracing, is proposed in this study. Compared with the temperature-sensitive viscous dampers, BRBs are usually cheaper
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
81
Table 2 Dimensions of components in outrigger truss (unit:mm). Member number
Width
Height
Web thickness
Flange thickness
Component length
OTB-1 OTB-2 OTX-1 OTX-2
350 350 350 350
700 700 700 700
16 12 40 12
36 20 60 20
10,500 10,500 6723 10,500
with conventional outriggers. In addition, the strength of BRBs is selected so that under frequent earthquakes, the BRBs as well as the main structure remain in the elastic stage. Furthermore, under basic earthquakes and rare earthquakes, the BRBs yield initially and dissipate most of the input energy so that the main structural members can be adequately protected. To verify the seismic performance of the new structure, a case study was carried out in this research. 2. Project profile
Fig. 1. Structural plan layout of typical floor.
Fig. 2. Schematic diagram of floor with outrigger trusses.
Table 1 Strength grade of structural material.
A typical high-rise steel and concrete composite frame-RC core tube structure with 55 stories and a total height of 229.8 m was designed for the case study. The seismic intensity is 7. The peak ground acceleration (PGA) values of the three earthquake levels are 55, 150, and 310 gal, respectively. The site soil class is III, the design group is 2, and the characteristic period of the ground motion is 0.55 s. The height of each story is 4.2 m excluding the bottom floor (4.8 m) and mechanical floor (3.6 m). The structural plan layout of the typical floor is shown in Fig. 1. The peripheral frame consists of steel-reinforced concrete (SRC) columns and steel beams. The outrigger trusses are arranged at the 32nd floor in two directions, as shown in Fig. 2. The strength grade of the structural material is given in Table 1. The outrigger truss consists of the top chord members, bottom chord members, and diagonal bracings, as shown in Fig. 3. All of them are steel I-section members. The dimensions of the components in the outrigger truss are listed in Table 2. Accordingly, another structure with the previously mentioned energy-dissipation outriggers was designed. In this new type of structure, the diagonal braces (named OTX-1) in the side span located outside the core tube in the conventional structure are replaced by BRBs. The dimensions of the BRBs are determined by the equal-rigidity principle. All structural details except for the outrigger trusses are identical for the two structures. 3. Numerical models
Concrete Steel Steel bar
Column
Beam
Shear wall
Slab
Outrigger truss
C50 Q345 HRB400
– Q345 –
C50 – HRB400
C35 – HRB400
– Q460 –
and more reliable in terms of durability [4]. Previous studies have shown that BRBs have a large energy-dissipating ability, and can be used not only as ordinary bearing bracings, but also as displacement dampers [5,6]. To limit the lateral displacement of a new structure under frequent earthquakes, the axial stiffness of the BRBs is determined using the principle of equal-rigidity substitution. This ensures that the new structure has the same lateral stiffness as the structure
3.1. Finite element models The finite element models of the two structures were established with the aid of a commercial program, Perform-3D [7]. Beams were simulated using a plastic hinge model, assuming that flexural plastic hinges occurred only at the two ends of the members. For beams with a relatively large span-to-depth ratio, flexural deformation is the controlling factor, and the shearing behavior is regarded as elastic. With the same forming mechanism involving plastic hinges for beams, columns were simulated using the plastic hinge model as well. The force–displacement relationship of the cross-section was derived from the fiber model. The truss members were simulated by inelastic axial bars, in
Fig. 3. Schematic diagram of outrigger truss.
82
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91 Table 3 Characteristic parameters and deformation limit of components. Component type
SRC column Steel beam Coupling beam Flexure-dominated shear wall Shear-dominated shear wall Bracing in compression Bracing in tension
Characteristic parameters
Deformation limit
a(d)
b(e)
c
IO
LS
CP
0.015 4θy 0.02 0.009 0.0075 0.5Δc 11ΔT
0.025 6θy 0.04 0.012 0.02 8Δc 14ΔT
0.2 0.2 0.5 0.6 0.4 0.2 0.8
0.003 1.25θy 0.005 0.003 0.004 1.25Δc 1.25ΔT
0.012 3θy 0.01 0.006 0.006 6Δc 8ΔT
0.015 4θy 0.02 0.009 0.0075 8Δc 10ΔT
Notes: θy is the yield rotation angle, Δc is the yield deformation of the bracing in compression, and ΔT is the yield deformation of the bracing in tension. Fig. 4. Elastoplastic model for steel.
which the buckling behavior can be considered. The BRB element specified in the program was adopted to simulate the BRB, which only bears the axial force. The core tube was simulated by a shear wall element, which consists of vertical fibers and a concrete shear layer. The fiber layers were used to model the bending and axial behaviors, and the concrete shear layer takes into account the contribution of concrete to the shear strength. The elastoplastic analytic model employs a rigid floor assumption. Meanwhile, the P-Δ effect was also considered in the nonlinear time history analysis. 3.2. Constitutive relationships for steel and concrete The elastoplastic model with no strain hardening is adopted for steel, as shown in Fig. 4. Owing to the limitation of the Perform-3D program, the constitutive relationship curves for concrete have to be first
converted into piecewise linear lines based on the principle of energy conservation to obtain the parameters of corresponding key points required in the procedure. The simplified Mander model [8] was employed for the core concrete and cover concrete, respectively, as shown in Fig. 5.
3.3. Performance levels of structural components According to FEMA 356 (Prestandard and Commentary for the Seismic Rehabilitation of Buildings) [9], the seismic performance of structural components is divided into three levels: immediate occupancy (IO), life safety (LS), and collapse prevention (CP), as shown in Fig. 6. There are two types of idealized force–deformation curves, one in terms of deformation, and the other in terms of deformation ratio. Table 3 presents the characteristic parameters in the force–deformation curves and the deformation limit of components corresponding to
Fig. 5. Constitutive model for concrete: (a) unconfined concrete; (b) confined concrete.
Fig. 6. Force versus deformation curves and performance levels: (a) force–deformation relationships; (b) force–deformation ratio relationships; (c) performance levels.
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91 Table 4 Characteristics of the first six natural vibration modes. Mode number
Period (s)
Vibration form
1 2 3 4 5 6
4.355 4.355 3.400 1.327 1.327 1.233
Translation in X direction Translation in Y direction Torsion Translation in X direction Translation in Y direction Torsion
the individual performance level determined in accordance with FEMA 356 [9] and ASCE 41 [10]. The plastic hinge rotation was selected as the performance index for columns and beams. The performance index for
83
the shear wall controlled by flexure and shear was the plastic hinge rotation and chord rotation, respectively. For truss members, the axial deformation was used as the performance index. 3.4. Dynamic characteristics The characteristics of the first six natural vibration modes are listed in Table 4. The dynamic characteristics of the two structures, the new structure with BRBs, and the conventional structure are identical, because of the design principle of equal rigidity. The period ratio between the first torsional mode and the first translational mode is 0.78, which meets the requirement that the period ratio should not be larger than 0.85, to prevent excessive structural torsion [11].
Fig. 7. Normalized acceleration time history curves: (a) GM-749-3-E03230 (X direction); (b) GM-749-3-E03140 (Y direction); (c) GM-2657-3-BRS090 (X direction); (d) GM-2657-3BRS000 (Y direction); (e) USER-1.
84
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
same PGA of 55 gal with the design spectrum specified in the Chinese code [11] is shown in Fig. 8. The PGAs for frequent earthquakes, basic earthquakes, and rare earthquakes are 55 gal, 150 gal, and 310 gal, respectively. The structures were subjected to earthquakes in the X and Y directions simultaneously. The damping ratio was taken as 0.04, 0.04, and 0.05 under frequent earthquakes, basic earthquakes, and rare earthquakes, respectively.
4. Earthquake responses of structures 4.1. Displacement responses
Fig. 8. Comparison of acceleration spectrum.
3.5. Earthquake ground motions Three sets of earthquake ground motions as well as two natural waves, GM-749-3 and GM-2657-3, and one artificial wave USER-1 were selected as seismic inputs based on the requirements of the Chinese seismic design code [11]. The normalized acceleration time history curves are shown in Fig. 7. The comparison of the acceleration spectrum for the primary component of individual set of ground motions with the
The interstory drift ratio is a significant and commonly used index in evaluating the seismic performance of structures. Thus, we present the maximum interstory drift ratios of the structure under different levels of ground motions in Table 5. The envelopes of the interstory drift ratio of the two structures in the X direction under basic earthquakes and rare earthquakes are shown in Figs. 9 and 10, respectively. For simplicity, the normal structure is named NOR, and the new structure is named BRB. Under frequent earthquakes, the displacement responses of the two structures are almost the same. Under basic earthquakes and rare earthquakes, compared to the normal structure, the distribution of the interstory drift ratio for the new structure is more uniform. In particular, the abrupt change at the strengthened story with outrigger trusses in the new structure is significantly relieved, and the maximum interstory drift of the new structure is slightly reduced.
Table 5 Maximum interstory drift ratio. Seismic wave
GM-749-3 GM-2657-3 USER-1
Structure number
NOR BRB NOR BRB NOR BRB
Frequent earthquakes
Basic earthquakes
Rare earthquakes
X direction
Y direction
X direction
Y direction
X direction
Y direction
1/883 1/883 1/959 1/957 1/691 1/692
1/1038 1/1038 1/1124 1/1133 1/810 1/814
1/334 1/340 1/425 1/445 1/256 1/263
1/395 1/402 1/498 1/519 1/298 1/307
1/174 1/175 1/243 1/245 1/144 1/147
1/196 1/198 1/290 1/288 1/160 1/165
Fig. 9. Envelopes of interstory drift ratio in X direction under basic earthquakes: (a) GM-749-3; (b) GM-2657-3; (c) USER-1.
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
85
Fig. 10. Envelopes of interstory drift ratio in X direction under rare earthquakes: (a) GM-749-3; (b) GM-2657-3; (c) USER-1.
4.2. Energy dissipation Under frequent earthquakes, the dissipated energy in the two structures is similar, mainly consisting of the strain energy and damping energy, which indicates that both structures are basically in the elastic state. Under basic earthquakes and rare earthquakes, the compositions of the dissipated energy at the end of the input for each set of ground motions are presented in Tables 6 and 7. From the tables, it can be observed that the normal structure dissipates energy mainly through the plastic deformation of coupling beams and shear walls; in the new structure, the outrigger trusses dissipate a large part of the total energy. The energy dissipated in the coupling beams and shear walls reduced significantly; thus, the damage in the core tube could be mitigated effectively.
Table 8 Maximum ratio of deformation demand to capacity for each type of structural member under frequent earthquakes. Earthquake wave
Structure Coupling number beam
GM-749-3
NOR BRB GM-2657-3 NOR BRB USER-1 NOR BRB
Shear wall Flexural Shear deformation deformation
Column Frame beam
IO
LS IO
LS
IO
IO
IO
0.162 0.160 0.158 0.157 0.353 0.351
– – – – – –
– – – – – –
0.046 0.045 0.043 0.041 0.062 0.059
0.167 0.168 0.160 0.161 0.180 0.186
0.077 0.076 0.074 0.074 0.079 0.078
0.305 0.305 0.267 0.262 0.498 0.489
Table 6 Compositions of dissipated energy under basic earthquakes. Earthquake wave
Structure number
Total energy (kN·m)
Inelastic energy (kN·m)
Proportion of inelastic energy (%)
Compositions of inelastic energy (%) Coupling beam
Wall
Outrigger truss
Column
Frame beam
GM-749-3
NOR BRB NOR BRB NOR BRB
82,150 80,560 79,360 79,850 109,200 106,500
8610 12,520 5788 8803 14,890 19,590
10.5 15.5 7.3 11.0 13.6 18.4
27.6 14.3 34.5 20.9 37.2 21.6
65.9 37.9 58.6 32.9 57.5 34.5
0 43.6 0 41.3 0 40.1
6.3 4.1 6.7 4.9 4.8 3.6
0.2 0.1 0.2 0 0.5 0.2
GM-2657-3 USER-1
Table 7 Compositions of dissipated energy under rare earthquakes. Earthquake wave
GM-749-3 GM-2657-3 USER-1
Structure number
NOR BRB NOR BRB NOR BRB
Total energy (kN·m)
Total inelastic energy (kN·m)
Proportion of inelastic energy (%)
189,300 184,500 167,300 172,500 293,100 283,500
66,510 69,350 44,290 50,640 117,500 118,500
35.1 37.6 26.5 29.4 40.1 41.8
Compositions of inelastic energy (%) Coupling beam
Wall
Outrigger truss
Column
Frame beam
57.4 39.5 66.5 48.3 63.8 49.4
34.2 28.0 29.7 24.2 28.2 23.9
5.9 30.4 1.6 25.8 6.3 25.2
2.4 2.1 2.1 1.7 1.7 1.5
0.1 0 0.1 0 0 0
86
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
Table 9 Maximum ratio of deformation demand to capacity for each type of structural member under basic earthquakes. Earthquake wave
Structure number
Coupling beam
Shear wall Flexural deformation
GM-749-3 GM-2657-3 USER-1
NOR BRB NOR BRB NOR BRB
Column
Frame beam
Shear deformation
IO
IO
LS
CP
IO
IO
IO
0.517 0.467 0.417 0.383 0.591 0.516
– 0.681 – 0.658 – 0.852
0.812 – 0.572 – – –
– – – – 0.798 –
0.231 0.063 0.154 0.063 0.357 0.073
0.262 0.254 0.226 0.228 0.306 0.322
0.117 0.117 0.104 0.101 0.134 0.139
Table 10 Maximum ratio of deformation demand to capacity for each type of structural member under rare earthquakes. Earthquake wave
Structure number
Coupling beam
Shear wall Flexural deformation
GM-749-3 GM-2657-3 USER-1
NOR BRB NOR BRB NOR BRB
Column
Frame beam
Shear deformation
LS
CP
LS
CP
IO
IO
IO
0.812 0.732 0.583 0.560 – 0.960
– – – – 0.850 –
– 0.573 – 0.717 – 0.781
0.847 – 0.681 – 1.270 –
0.581 0.226 0.317 0.204 0.677 0.433
0.457 0.538 0.304 0.341 0.604 0.678
0.191 0.200 0.154 0.159 0.233 0.272
4.3. Damage state Table 8 presents the maximum ratio of deformation demand to the capacity for each type of structural member under frequent earthquakes. The damage state of the two structures is similar. All structural members are at the performance level of IO. The maximum ratios are much lower than 1, which indicates that there is very slight damage in
the two structures, and in general, the two structures are basically in the elastic state. Table 9 presents the maximum ratio of deformation demand to the capacity for each type of structural member under basic earthquakes. In the normal structure, some shear walls are at the performance level of LS and some are at the CP level. In the new structure, all shear walls are at the performance level of IO. As for the coupling beams, although
Fig. 11. Plastic damage of coupling beam (elevation at Y = 42 m, LS): (a) NOR; (b) BRB.
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
Fig. 13. Plastic flexural damage of shear wall (LS): (a) NOR; (b) BRB. Fig. 12. Plastic flexural damage of shear wall (IO) (a) NOR; (b) BRB.
the performance levels of the two structures are all at IO, the maximum ratio of the new structure is slightly lower than that of the normal structure. Compared with the normal structure, the damage in the core tube of the new structure is significantly reduced. For the peripheral frame beams and columns of the two structures, the performance levels are at IO, the maximum ratios are similar, i.e., much lower than 1, and the damage is very slight. Table 10 presents the maximum ratio of deformation demand to capacity for each type of structural member under rare earthquakes. Figs. 11 to 15 show the plastic damage distribution of different types of structural members of the two structures subjected to the earthquake wave GM-749-3. In the normal structure, all shear walls are at the performance level of CP, while in the new structure, all shear walls are at the performance level of LS. As for the coupling beams, in the normal structure, some are at the performance level of LS and some are at CP, while in the new structure, all are at LS. Compared with the normal structure, the damage in the core tube of the new structure is significantly reduced. Furthermore, the damages at the lower stories and the stories adjacent to the strengthened story are much larger than those of other stories. For the peripheral frame beams and columns of the two structures, the performance levels are at IO, the maximum ratios are similar, and the damage is slight. 4.4. Performance of diagonal bracing The axial force–deformation hysteretic curves of the diagonal bracing in the outrigger truss of the two structures subjected to the earthquake wave GM-749-3 at the basic earthquake level and rare earthquake level are shown in Figs. 16 and 17, respectively. Under basic earthquakes, the diagonal bracing in the normal structure remains elastic while the diagonal bracing in the new structure yields and dissipates considerable energy to protect the main structure. Under rare
Fig. 14. Plastic flexural damage of shear wall (CP) (a) NOR; (b) BRB.
87
88
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
intensities. Three sets of earthquake ground motions used in the said case study were adopted here. The principle of equal-rigidity substitution was adopted for the design of the energy-dissipation outrigger truss. 5.1. Layout of BRBs The energy-dissipation outrigger truss consists of the top chord members, bottom chord members, and diagonal BRBs. Three types of BRB layouts were considered here, as shown in Fig. 18. The strength grade of Q225LY was adopted for the BRB. By using the principle of equal-rigidity substitution, the properties of the BRB for different layouts were determined as listed in Table 11. Under frequent earthquakes, the BRBs in all structures do not yield, and all the structural components remain basically elastic. Under basic earthquakes and rare earthquakes, the BRBs in all structures yield. The inelastic energy dissipated by the BRBs and the ratio of the energy dissipated by the BRBs to the total inelastic energy dissipated by the entire structure for each structure under basic earthquakes and rare earthquakes are shown in Figs. 19 and 20, respectively. In the TRUSS1 structure, both the energy dissipated by the BRBs and the energy ratio are larger compared to the others; thus, the main structural components could be better protected from damage. Compared to the difference under basic earthquakes, the difference between the structures with different truss types under rare earthquakes is less. Therefore, the TRUSS1 layout is the best choice. 5.2. Strength of BRBs
Fig. 15. Plastic shear damage of shear wall (IO): (a) NOR; (b) BRB.
earthquakes, although the diagonal bracings in the two structures yield, the energy dissipated by the BRB in the new structure is much larger than that in the diagonal bracing of the normal structure. 5. Parametric analysis The strength and layout of the BRB are the main design parameters for the energy-dissipation outrigger truss mentioned above. The effects of the two parameters on the seismic performance of the new structure with the energy-dissipation outrigger truss were investigated here, based on the structure designed in the abovementioned case study through time history analysis under earthquakes with different
In different energy-dissipation outrigger trusses, the cross-sectional area of the BRBs is identical to keep the stiffness of the BRBs and the stiffness of trusses invariant. The strength of the BRBs was altered by adopting the steel core element with different strength grades. Four strength grades, i.e., Q100LY, Q160LY, Q225LY, and Q235, with yield strengths of 100 MPa, 160 MPa, 225 MPa, and 235 MPa, respectively were considered. The TRUSS1 layout was adopted for all trusses. The responses of the new structure with different outrigger trusses under earthquakes with different levels of intensity were predicted. Under frequent earthquakes, the structures with BRB strength grade of Q100LY and Q160LY yield, while the other structural components in the structure remain basically elastic. As for the structures with BRB strength grade of Q225LY and Q235, no BRBs yield, and all the structural components remain basically elastic. The maximum interstory drift ratio of the structures with BRB strength grade of Q100LY and Q160LY is larger than that of the structures with BRB strength grade of Q225LY and Q235. Under basic earthquakes and rare earthquakes, the BRBs in all structures yield. The inelastic energy dissipated by the BRBs and the ratio of the
Fig. 16. Axial force–deformation hysteretic curves of diagonal bracing under basic earthquake: (a) NOR; (b) BRB.
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
89
Fig. 17. Axial force–deformation hysteretic curves of diagonal bracing under rare earthquake: (a) NOR; (b) BRB.
Fig. 18. Layout of BRB: (a) TRUSS1; (b) TRUSS2; (c) TRUSS3.
protected from damage. Under rare earthquakes, the effect of the strength of the BRBs is not noticeable.
Table 11 Mechanical properties of BRBs. Layout type
Cross-sectional area (mm2)
Yield force (kN)
TRUSS1 TRUSS2 TRUSS3
5.8680 × 104 1.2722 × 105 2.5880 × 105
1.3232 × 104 2.8687 × 104 4.6688 × 104
energy dissipated by the BRBs to the total inelastic energy dissipated by the entire structure for each structure under basic earthquakes and rare earthquakes are shown in Figs. 21 and 22, respectively. Under basic earthquakes, with the decrease in the strength of the BRBs, both the energy dissipated by the BRBs and the energy ratio increase, and the main structural components could be better
6. Conclusions A new structure with energy-dissipation outrigger trusses installed with BRBs is proposed in this study. The seismic performance of the new structure was verified by a case study involving elastoplastic time history analysis, comparing the seismic responses of the new structure with the normal structure under three levels of earthquakes. Under frequent earthquakes, both structures remain basically elastic, and the responses of the two structures are almost identical. Under basic earthquakes and rare earthquakes, the BRBs in the new structure yield initially and dissipate a large amount of energy so that the main
Fig. 19. Energy dissipated by BRBs for different layouts under basic earthquakes: (a) inelastic energy; (b) inelastic energy ratio.
90
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
Fig. 20. Energy dissipated by BRBs for different layouts under rare earthquakes: (a) inelastic energy; (b) inelastic energy ratio.
Fig. 21. Energy dissipated by BRBs for different strengths under basic earthquakes: (a) inelastic energy; (b) inelastic energy ratio.
Fig. 22. Energy dissipated by BRBs for different strengths under rare earthquakes: (a) inelastic energy; (b) inelastic energy ratio.
structural members, particularly the core tube, can be adequately protected. Compared to the normal structure, the distribution of the interstory drift ratio in the new structure is more uniform, and the maximum interstory drift in the new structure is slightly decreased. The
parametric analysis shows that the effects of the strength and layout of BRBs on the seismic performance of the new structure are significant. An appropriate strength should be selected so that the outrigger truss will not yield prematurely under frequent earthquakes, but dissipates
H. Jiang et al. / Journal of Constructional Steel Research 134 (2017) 80–91
a large portion of the inelastic energy under basic earthquakes and rare earthquakes. The outrigger truss with the TRUSS1 layout is recommended for application in the new structure. Further studies are necessary to optimize the design parameters of BRBs and obtain a better performance from new structures under earthquakes with different levels of intensity. Acknowledgements The authors are grateful for the support from the Ministry of Science and Technology of China through Grant No. SLDRCE14-B-2 and Key Research and Development Program of Hainan Province through Grant No. SQ2016SHFZ0062. References [1] K. Kamath, N. Divya, A.U. Rao, A study on static and dynamic behavior of outrigger structural system for tall buildings, Bonfring Int. J. Ind. Eng. Manag. Sci. 2 (4) (2012) 15–20.
91
[2] R.J. Smith, M.R. Willford, The damped outrigger concept for tall buildings, Struct. Des. Tall Special Build. 16 (4) (2007) 501–517. [3] Y. Zhou, H. Li, Analysis of a high-rise steel structure with viscous damped outriggers, Struct. Design Tall Spec. Build. 23 (13) (2014) 963–979. [4] M.E. Eryaşar, C. Topkaya, An experimental study on steel–encased buckling–restrained brace hysteretic dampers, Earthq. Eng. Struct. Dyn. 39 (5) (2010) 561–581. [5] Y.K. Ju, M.H. Kim, J. Kim, et al., Component tests of buckling-restrained braces with unconstrained length, Eng. Struct. 31 (2) (2009) 507–516. [6] D.H. Kim, C.H. Lee, Y.K. Ju, et al., Subassemblage test of buckling-restrained braces with H-shaped steel core, Struct. Design Tall Spec. Build. 24 (4) (2015) 243–256. [7] Computer and Structures, Inc. Perform-3D, Nonlinear Analysis and Performance Assessment for 3D Structure User Guide, Version 4, Computers and Structures, Inc., Berkeley, CA., 2006 [8] J.B. Mander, M.J.N. Priestley, R. Park, Theoretical stress-strain model for confined concrete, J. Struct. Eng. ASCE 114 (8) (1988) 1804–1826. [9] Council, BSS, Prestandard and Commentary for the Seismic Rehabilitation of Buildings, FEMA-356, Federal Emergency Management Agency, Washington, DC, 2000. [10] ASCE/SEI Seismic Rehabilitation Standards Committee, Seismic Rehabilitation of Existing Buildings (ASCE/SEI 41-06), American Society of Civil Engineers, Reston, VA, 2007. [11] China Ministry of Construction (CMC), Code for Seismic Design of Buildings (GB 50011-2010), China Architecture & Building Press, Beijing, 2010 (in Chinese).