Outriggers

The advancements in engineering and technology, development of high strength structural materials, rise in population and lack of land area available due to the raise in land prices and rapid urbanization have led to the development of high rise buildings in the modern era. The tall buildings are the most susceptible to the lateral loadings due to the wind and seismic forces. The trend of taller and slender structures makes it necessary to develop the structural configuration which can resist the lateral loads effectively with minimum cost. The outrigger bracing system provides the solution to control excessive drift and displacement in such buildings. In this study, the conventional structural model having the central core of reinforced cement concrete and the models with outrigger at different heights of the building are modeled for the 20, 40 and 60 storey building heights. The aim of this paper is to suggest a suitable location of outrigger bracing system to control excessive top storey displacement.

در این مطالعه به بررسی عملکرد لرزه ای یکی از سیستم های سازه‌ای برای ساختمانهای بلند پرداخته شده است . نوع سیستم مورد بررسی  قرار گرفته سيستم با هسته برشي و مهار بازويي  است كه جزو سيستم هاي داخلي محسوب مي‌شود ، اين سيستم شكل اصلاح شده قاب مهاربندي و قاب با ديوار برشي مي باشد . به عنوان يك سيستم مؤثر ، سيستم هسته برشي با مهار بازويي ، متشكل از يك هسته مركزي با خرپاهاي بازويي است كه هسته را به ستونهاي خارجي متصل مي‌كنند.
در اين تحقيق هدف بررسي عملكرد لرزه‌اي چنين سيستم اي بوده است به طوريكه  سازه‌ای 42 طبقه بتنی به ارتفاع 150 متر برای بارهای محتمل با روش تحلیل دینامیکی طیفی  آنالیز و طراحی گردیده است سپس با نتایج بدست آمده از تحلیل استاتیکی غیر خطی، به بررسی توالی ايجاد مفاصل پلاستيك  پرداخته شده است بطوریکه، ایجاد مفاصل پلاستیک در مهار‌بازویی را نشان می‌دهد و همچنین، هیچ کدام از مفاصل ایجاد شده در پوش سازه تا تغییر مکان هدف، به ناحیه فرو‌ریزش وارد نشدند‌. در ترازهای دارای مهار بازویی دریفت طبقات کاهش پیدا کرد و عملکرد این سیستم در کنترل تغییر مکان‌های جانبی بسیار ملموس می‌باشد.

Accumulation of growing population especially in developing countries has resulted in an increased height of buildings, this need creating impact on structural development of tall building. As building increases in height there is effect of wind and earthquake forces, to increase stiffness of building against lateral load additional structural system such as belt truss and outriggers is required. This paper presents the review of various techniques and methods used to investigate uses of belt truss and outrigger system in a tall building. The various parameters like lateral displacement, storey drift, core moment and optimum position related to outrigger and belt truss are reviewed. The reviewed approach for the design and development of tall building using outrigger and belt truss is useful to provide a potential solution. The study in turn is useful for various research persons involved in design the tall buildings by using outrigger and belt truss system.

In building design, an outrigger denotes a rigid horizontal structural element used to improve strength and resistance to overturning. Because they don’t require closely spaced perimeter columns and spandrel beams, outrigger-braced systems have replaced the framed tube as the premier lateral system in tall buildings (Choi, 2012). These outriggers, comprised of multiple story deep trusses or girders, connect a building’s lateral-load resisting core to perimeter columns to prevent free cantilever rotation (Taranath, 1998). Outriggers can connect to perimeter columns on mechanical floors, saving the structural system from intruding on rentable floor space. An outrigger-braced building has less story drift and will have 30 to 40 percent less overturning moment in the core when compared to a free cantilever (Lame, 2008). In the case of supertall towers with mega columns designed for drift control, the reduction in core overturning can be up to 60 percent (Choi, 2012). Outriggers are an efficient structural solution that dramatically increases architectural possibilities. However, their use ushers in new space-planning complications and structural challenges associated with differential shortening between core and perimeter column members (Nair, 1998). This paper will highlight the benefits, challenges, and solutions associated with outrigger-braced systems in tall buildings.

The outrigger systems, which is widely used with shear wall-framed systems at the tall buildings, increase the lateral stiffness of the structural bearing system and reduce the lateral drift of the structure under lateral loads. However, the traditional outrigger systems, besides these positive contributions, also create some limitations and problems affecting the modeling of the structure. Some of these; more interior space occupying as an architect, problems arising in the connection of outrigger and center core (especially when a concrete shear-wall core is used). On the other hand, the belt trusses known as "Virtual Outriggers" which have recently been used to build high-rise structures, have removed these problems. Unlike the traditional outrigger systems, belt trusses are formed between the outer columns. In this way belt trusses eliminate the problems arising from the direct connection of the outriggers to the center core and other problems associated with using outriggers. Extensive studies have been carried out on the examination of outrigger and belt truss systems used in high-rise buildings under static and dynamic loads. In this study, the linear earthquake responses of three structural models, which are shear wall-framed system, shear wall-framed system with traditional outriggers and shear wall-framed system with belt trusses, were performed by using modal time history analysis method. Lateral displacements and drifts of the structure, internal forces of the structural elements were obtained. These results of three structural models were compared with each other and the effectiveness of outrigger and belt truss systems were assessed. For earthquake input, three real earthquake records were selected. These records were scaled in accordance with the DD2 level earthquake design spectrum defined in Turkish Building Earthquake Standards (2018) and used in the analyses.

The outrigger and belt truss system is one of the most efficient systems used to effectively control the excessive drift due to lateral load, so that, during small or medium lateral load due to either wind or earthquake load, the risk of structural and non-structural damage can be minimized. For high-rise buildings, particularly in seismic active zone or wind load dominant, this system can be chosen as an appropriate structure. This paper studies the efficient use of outrigger and belt truss system for high-rise concrete building subjected to wind or earthquake load. Seven 44 storey two dimensional models of outrigger and belt truss system are subjected to wind and earthquake load, analyzed and compared to find the lateral displacement reduction related to the types of outrigger and belt system. The analysis has been carried out to study the effect and performance of outrigger system in 44 storey building. The outrigger system is provided at different levels along the height of the building. The depth of the Outrigger and belt trusses is equal to the height of the typical story and maintained same in all the models. The key parameters discussed in this paper include lateral deflection, storey drifts, base shear and fundamental time periods. Loads are considered as per Indian Standards IS: 875(Part1)-1987, IS: 875(Part2)-1987, IS: 875(Part3)-1987 and IS: 1893(Part-1)-2002. The modeling and analysis were performed using finite element software ETABS 15.2.2.

During the past two decades, outrigger-belt truss system has been investigated and used in design of tall buildings. Most of the studies focused on the optimization of the system for minimum displacement and some of them proposed the best locations. In this study, however, multi-objective optimization of tall steel frames with belt trusses is investigated to minimize displacement and weight of the structure. For this purpose, structures with 20, 30, 40, and 50 stories are considered as models, based on the suggestions in the literature. The location and number of trusses and cross section of all structural elements are considered as design variables. After sizing of the structure for a specific topology and shape, weight and displacement of the structure are obtained and plotted in a diagram to illustrate trade-off between two objective functions. The results show the optimal Pareto-front solutions for different stories. Smooth trade-off and optimal number of trusses and their locations obtained.

Today, the need of high-rise structures is rising throughout the world day by day. The package programs used in the modelling and analyzing of high structures are being renewed and improved every day. However, analytical solutions are still to be very helpful in the preliminary design of the building model. In this study, the analytical solution approach has assessed for the horizontal displacement determinations of the shear wall-framed systems and shear wall-framed systems with outrigger systems under the static horizontal loads. For this purpose, two building models with 45 floors have been considered. The shear wall-framed system with outriggers has created by adding outrigger system to the shear wall-framed system in two levels. As the horizontal loads to the building models, the triangular distributed load representing the earthquake load and the uniformly distributed load representing the wind load have been applied. Displacement solutions of both models under horizontal loads have been performed using the analytical approach and ETABS program. The results of both models have been compared with each other and the validity of analytical solutions have been evaluated. Internal forces occurred in the both models under horizontal loads have been also assessed. The internal force solutions were obtained by using ETABS program. In addition, the effectiveness of the outrigger system has been examined.

Accumulation of growing population especially in developing countries has resulted in an increased height of buildings, this need creating impact on structural development of tall building. As building increases in height there is effect of wind and earthquake forces, to increase stiffness of building against lateral load additional structural system such as belt truss and outriggers is required. This paper presents the review of various techniques and methods used to investigate uses of belt truss and outrigger system in a tall building. The various parameters like lateral displacement, storey drift, core moment and optimum position related to outrigger and belt truss are reviewed. The reviewed approach for the design and development of tall building using outrigger and belt truss is useful to provide a potential solution. The study in turn is useful for various research persons involved in design the tall buildings by using outrigger and belt truss system. KeywordsTall Building, O...

The sole purpose of the stability improvement techniques system is to the enhance the increasing demand of the
multistoried buildings with building type of semi commercial apartment having different configurations such as horizontal
irregularities, vertical irregularities, different building structural theme and so on. Since the major point in this is living area of
horizontal land is converted now into vertical apartments that lead to new challenges and requirement of new safety measures.
To counteract the seismic forces to the multistoried structures, stiffness of the structure has to increase with increase in elevation
that’s why we need to implement certain structural system add on such as shear wall, bracings, wall belt system, outrigger system
etc. It has been found that the best ideal structure for multistoried buildings is outrigger and wall belt structural stability system
is taken for analysis in the current study approach. External columns are tied to the central inner shear core by the help of wall
outrigger in this system that struggle against rotation and sway action due to lateral effects. Since for the analysis of seismic
effects, total 8 cases have been selected which are abbreviated as Case OTB1 to Case OTB8, that are supposed to be situated at
seismic zone III and rested over medium soil. Various loads along with load combinations applied on all the cases and reflective
result parameters have been analysed with each other to determine the efficient case.

Today, the need of high-rise structures is rising throughout the world day by day. The package programs used in the modelling and analyzing of high structures are being renewed and improved every day. However, analytical solutions are still to be very helpful in the preliminary design of the building model. In this study, the analytical solution approach has assessed for the horizontal displacement determinations of the shear wall-framed systems and shear wall-framed systems with outrigger systems under the static horizontal loads. For this purpose, two building models with 45 floors have been considered. The shear wall-framed system with outriggers has created by adding outrigger system to the shear wall-framed system in two levels. As the horizontal loads to the building models, the triangular distributed load representing the earthquake load and the uniformly distributed load representing the wind load have been applied. Displacement solutions of both models under horizontal loads have been performed using the analytical approach and ETABS program. The results of both models have been compared with each other and the validity of analytical solutions have been evaluated. Internal forces occurred in the both models under horizontal loads have been also assessed. The internal force solutions were obtained by using ETABS program. In addition, the effectiveness of the outrigger system has been examined.

Outriggers along with tube-in-tube (TT) systems became a promising alternative to resist wind and earthquakes loadings in addition to controlling excessive drifts in reinforced concrete (RC) high-rise buildings (HRB). Although yielding inaccurate displacements and straining actions, design engineers used to utilize one-step analysis (OSA) instead of staged-construction analysis (SCA) by assuming both all loads are applied at one stage to a complete building. Including time dependent effects in SCA such as shrinkage, creep, and strength gain (SCAT) in the analysis of RC buildings leads to accurate simulation of those structures. In this research, analysis of 140 m and 175 m HRB with TT and outrigger systems is performed using a robust finite element model (FEM) which utilizes SCAT. Optimum locations of one/two outrigger systems are determined by minimizing difference in displacements and straining actions obtained from the OSA and SCAT analyses. Efficiency of the outrigger systems in reducing differences between differential shortenings obtained from OSA and SCAT for the analyzed buildings are assessed. Three locations of one outrigger (1/2 H, 2/3 H, and 1.0 H) and two locations of two outriggers (1/2 H & 1.0 H and 2/3 H & 1.0 H) along the building height (H) are studied as a mitigation technique. Comparing differences between differential displacements and straining actions obtained from SCAT before and after mitigation to their counterparts obtained from SCAT is conducted. The numerical results showed that using two outriggers at 2/3 H & 1.0 H leads to reducing the maximum reduction in differential displacements by 35% and 42% for the two studied buildings. The study revealed that an optimum solution can be achieved by using two outriggers at 1/2 H & 1.0 H. In this case, the differences between bending moments in horizontal elements before and after mitigation reached 94% for the studied buildings.