Keywords

1 Introduction

As there is expansion of cities and value of land continues to high, buildings are rising tall and slim. More investment on land and the need to protect farming area has contributed to navigate residential buildings toward higher levels. To make possible for slimmer structures, various structural systems for high-rise building have been widely used. The lateral load resisting systems in common in high-rise structures are framed system, structural walls, dual systems, outriggers, and tube systems. Among them frames with braced-outriggers proved to be one of the efficient procedures to increase the lateral stiffness of tall building, reducing the storey drifts.

The outrigger member is a cantilever-shape horizontal member and is connected to the outer column and inner core of the structure. Under the action of lateral loads, core undergoes rotation and the outrigger connecting core and the outer column induces tension and compression alternatively in the same members. Besides the outer columns, it is common to locate other outer columns to serve in restricting the rotation of outriggers. Introduction of a deep spandrel girder at the levels of the outrigger around the structure shows the control of tilting of outrigger. Some examples of outrigger system used are US Bank Center (Milwaukee), New York Times Tower (New York), China World Tower (Beijing, China), Taipei 101 (Taiwan), Water Front Place (Brisbane), and Exchange Square (Hong Kong).

2 Literature Review

Parametric study of tall building structures braced with outriggers is carried out by some authors [1] considering the flexibility of the outriggers. Mathematical expressions for optimum location of the outriggers, moments in the core and the storey drift were developed. Some researchers, Kamath, and others [2] studied the behavior and response of the frames with outriggers for high-rise structures for static and dynamic loads. In their study, the 3D structural models with flexural rigidity ratio range of (0.25–2.0) EI0/EI optimum outrigger location are obtained from the range of 0.975–0.40 (Hs/H), from the top of the building. Studies on different number for steel frame outrigger-belt truss systems were conducted on structures with 20, 30, 40, and 50 storeys to propose the optimal number and location of the outrigger. Babaei [3] studied optimal solutions in the design domain for storey drift and weight of the structure. The degree of stiffening and the level of drift control on the number of the outrigger trusse are studied by Gerasimidis et al. [4].

Location optimization of outrigger for high-rise RCC structure was investigated by Krunal Z Mistry and others. In their study tall building with ten different arrangements of outriggers was considered. Best outrigger location and the performance efficiency of each outrigger were arrived at when pair of outriggers were used [5]. Hoenderkamp presented a preliminary design for outrigger braced high-rise shear walls subjected to lateral forces. One of the outrigger locations is fixed, while the second one location is investigated for optimum position [7]. Shivacharan and others [10] studied the optimum position of outriggers for tall structures with vertical irregularity. Authors concluded that there is 12.78 and 11.5% reduction of lateral displacement and storey drift [10]. Some researchers analyzed tall buildings and found the optimum location of outriggers system for lateral loads and concluded that the topmost deflection was at the top storey when flat slab with core was engaged and is 625.7 mm, which was reduced up to 411.18 mm when first outrigger was provided at mid-height of structure and 335.15 mm when provided with second outrigger at 3/4th height of the structure [11]. Alex and Otto conducted the study on structures braced with multiple outriggers based on the continuum approach. Authors concluded that continuum analysis yields good results for any number of outriggers. Design curves were established by some researchers for estimating the structural responses for any kind of structural configuration [13]. However, in the present work, authors tried to arrive at the optimum location of the braced outrigger in a two-outrigger system with one fixed outrigger location (at top storey).

3 Numerical Study

In the present study, moment resisting building frames with belt trusses in multiple numbers and at different locations are considered for analysis. Linear static and response spectrum analysis are performed. Analysis is done using ETABS and as per the latest codal provisions. Locations of outriggers are taken according to CTBUH guidelines. The selected topology is with two outriggers, one is fixed at top storey, and the location of the second outrigger is optimized for least lateral displacements and storey drifts.

Figure 1 shows the plan area and dimensions of the models as 6 m as bay length with 5 × 5 grid and with central core as opening. Figs.  2 and 3 show the position of outrigger with central core, one outrigger fixed at top storey, and other at mid-height along with top storey, respectively. The height of each storey is 3.5 m. Moment Resisting Frames (MRF) are considered as base structural systems.

Fig. 1
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Plan of the model

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Fig. 2
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Outrigger at top storey

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Fig. 3
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Outrigger at top storey and mid-height

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Structural steel with yield strength of 250 N/mm2 is used for all columns, beams, and belt trusses. A value of 2 × 105 N/mm2 is considered as Young’s modulus. A dead weight of 5 kN/m2 and imposed load of 3 kN/m2 were considered. Based on IS 1893:2016 earthquake loads are calculated and using the ETABS software options. The seismic zone is considered as Vijayawada city. Basic wind speed is considered as 50 m/sec and the wind pressure is determined according to IS 875:1987 part-3.

4 Results and Discussions

Table 1 displays the results of earthquake and wind analysis of maximum displacement for various outrigger positions.

Table 1 Maximum displacements (mm) at top storey
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It is observed from Table 1, for the case of cap truss +1/2nd case the displacements are less showing maximum reduction of 39.44 and 36.68% for seismic and wind effects, respectively. This is because of increase in the lateral stiffness of the structure.

Table 2 shows the result of maximum storey drift for different position of outriggers.

Table 2 Maximum storey drifts (mm)
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It is observed from Table 2, for the case of cap truss +1/2nd case the displacements are less showing maximum reduction of 29.46 and 33.90% for seismic and wind effects, respectively. From the above results, the maximum reduction of displacement as well as storey drift is seen when the outrigger is placed at middle of the storey height along the cap truss.

From Fig. 4, maximum percentage reduction in displacement is 36.10% which is at when the outrigger is placed at 0.5 times the height of the building along with cap truss.

Fig. 4
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Maximum displacement of response spectrum analysis

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Figure 5 shows the reduction of storey drift of 29.46% when the outrigger is placed at a height of 0.5 times height of the building along with cap truss.

Fig. 5
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Maximum storey drifts

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5 Conclusions

As discussed in Sect. 4, the following conclusions are drawn

The structural system with two outriggers considered, yielded better structural responses. The results show maximum reduction of 39.44% in lateral displacements when the outriggers are placed at top storey and at 0.5 times the height of the building. Maximum reduction of 33.96% is found in the case of storey drift reduction for the same case. However, from the present study, it can be concluded that the outrigger structural system proves to be an efficient way to reduce inter-storey drifts and controls top displacements when compared with the frame with only central core. Inverted “V” shape bracing shows good results in all cases considered.