 Open Access
 Total Downloads : 540
 Authors : Nimisha P, Namitha Krishnan
 Paper ID : IJERTV5IS040196
 Volume & Issue : Volume 05, Issue 04 (April 2016)
 DOI : http://dx.doi.org/10.17577/IJERTV5IS040196
 Published (First Online): 01042016
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Structural Comparison of Diagrid Building with Tubular Building
Nimisha. P

ech Structural Engineering and Construction Management
Dept. Of Civil Engg.
Vimal Jyothi Engineering College Kannur, Kerala, India
Abstract Todays tall building concept is rooted in architectural features of building geometry along with stiffness and lightness. Hence, architectural concept and structural concept must go hand in hand. Based on this many lateral load resistance systems have been developed. Tubular system is the latest technology in this area. Recently, the high rise building technology relies on this system. Diagrid is another latest invention in this area, which is a mutation of Tubular system. Diagrid is a better choice, where Tubular system failed to fulfil the requirements, especially in the case of complex geometries. In this paper the Diagrid and Tubular buildings are compared in order to study the structural efficiency of both type of buildings. For this purpose, comparison is made among different models of Diagrid building and respective models of Tubular building. 24, 30, 36, 42, 48, 54, 60, 66 storey models of both Diagrid and Tubular buildings are generated in ETABS2015 software and analysed. The loads are taken from IS: 875 1987. The earthquake load is applied as per 18932002. The analysis results are compared in terms of Displacement, Storey drift, Time period and Storey shear. The values of these parameters of Tubular building are found to be larger than that of Diagrid building under same loading condition. The comparison of analysis results revealed that the Diagrid building is structurally more efficient than the Tubular building.
KeywordsDiagrid Building; Tubular Building; Lateral Load Resistance; Tall Building

INTRODUCTION
The complex geometries of buildings along with high cost of land emphasis the need for considering architectural ideas and structural concepts hand in hand. As the building height increases, the lateral load resistance system is more important than the gravity load resistance system. There are a number of lateral load resistance systems, such as Moment resisting frame system, Braced frame system, Shear wall system, Advanced structural formstubular systems. In these the tubular system is found to be more efficient both in terms of reduction in self weight and better lateral load resistance [3]. They are made with strong exterior frame to resist lateral loads, allowing the interior frame to take only the gravity loads. The distance between the interior and the exterior is spanned with beams or trusses and intentionally left column free. This maximizes the effectiveness of the perimeter tube by transferring some of the gravity loads within the structure to it and increases its ability to resist overturning due to lateral loads. Diagrid or Exo diagonal system is a new concept in lateral load resistance of high rise buildings. These are the
Namitha Krishnan Assistant Proffessor Dept. Of Civil Engg,
Vimal Jyothi Engineering College Kannur, Kerala, India
latest mutation of tubular structure, in which tubes are arranged diagonally across the building perimeter [3]. i.e. the columns are placed in an inclined position to make a triangular structural configuration, so that all the loads acting on the diagrid will get distributed as axial forces; rather than bending or shear [4]. Tubular configuration utilize overall building plan dimension to counteract the overturning moment. But this potential bending efficiency is not fully achievable because of shear deformation that arise in the building webs. On the other hand the diagrid systems, which provide shear resistance and rigidity by means of axial action in the diagonal members, rather than bending moment in beams and columns, allows for a nearly full exploitation of the theoretical bending resistance [3].
Todays building technology prefer tubular concept for high rise buildings. But diagrid is also an important lateral load resistance system, which can be used for complex geometries of building. Hence, in this study the diagrid building and tubular building are compared in order to reveal the structural advantages of diagrid building, if any, over the tubular building, so that to add the importance of diagrid as a lateral load resistance system. For the comparison eight models of diagrid building and eight models of tubular building are generated in ETABS2015 software. The number of stories is taken as the criteria for the selection of models. 24, 30, 36, 42, 48, 54, 60 and 66 storey models are used for the study.

RELATED WORKS

Optimum Angle of Diagrid Structural System [1]
This work has been done by Nishith B. Panchal et.al. This paper involves the modeling of diagrid structures of 24, 36, 48 and 60 stories. The diagrid structure of each storey height is designed with diagonals placed at various uniform angles as well as gradually changing angles along the building height in order to determine the optimal uniform angle for each structure with a different height and to investigate the structural potential of diagrids with changing angles. In this paper, the comparison study of 24storey, 36storey, 48storey and 60storey of diagrid structural system with a diagrid angle 50.2Â°, 67.4Â°, 74.5Â° and 82.1Â° is presented. The comparison of analysis of results in terms of top storey displacement, storey drift, time period, angle of diagrid and steel and concrete consumption are included in this paper. From the study it was observed that the Diagrid angle in the region of 65Â° to 75Â°
provides more stiffness to the diagrid structural system which reflects the less top storey displacement, storey drift, time period and storey shear. It also indicates economy in terms of consumption of steel and concrete.

Analysis and Design of Diagrid Structural System for High Rise Steel Buildings [2]
This is the work done by Khushbu Jani and Paresh V. Patel. The analysis and design of 36 storey diagrid steel building is presented in this paper. A regular floor plan of 36 m Ã— 36 m size is considered for the study. The modeling and analysis of structural members have been done in ETABS software. All structural members are designed as per IS 800:2007 considering all load combinations. Dynamic along wind and across wind are considered for analysis and design of the structure. Load distribution in diagrid system is also studied for 36 storey building. Similarly, analysis and design of 50, 60, 70 and 80 storey diagrid structures are also carried out. The analysis results are compared in terms of time period, top storey displacement and interstorey drift. From the study it is observed that most of the lateral load is resisted by diagrid columns on the periphery, while gravity load is resisted by both the internal columns and peripheral diagonal columns. So, internal columns need to be designed for vertical load only. Lateral and gravity load are resisted by axial force in diagonal members on periphery of structure, which make system more effective.


MODELLING AND ANALYSIS
Building models of plan 32 m x 32 m are generated in ETABS2015. The storey height is 3.8 m. Four interior columns are used for carrying a part of gravity load. In the present study models of both the diagrid buildings and tubular buildings are modelled. The models of various number of storey ie, 24, 30, 36, 42, 48, 54, 60, 66 are generated in ETABS 2015, which are represented by D1, D2, D3, D4, D5, D6, D7, D8, T1, T2, T3, T4, T5, T6, T7 and T8. Here D
represents, diagrid building model and T represents tuular building model. The digits 1 to 8 represents the number of storey from 24 to 66 respectively.
In the diagrid models the peripheral diagonals are placed at 8 m spacing. The diagonal angle is kept uniform for each model. The end conditions for diagonals are assumed as pinned and the support conditions as fixed. 6storey diagrid module is used for modelling. Based on the 6storey module the diagonal angle provided is 70.6Â°.
In tubular building models columns are placed at closer spacing of 4 m on outer periphery of the building. In order to model these buildings, the member sections are kept same as that of diagrid building for the purpose of comparison. Then the analysis results of two types of buildings are compared in terms of displacement, storey drift, time period and storey shear.
The characteristic strength of concrete and steel are taken as 40N/mm2 and 415N/mm2 respectively. The design of building models have been done based on Indian Standards. The design dead load and live load are taken from IS: 875 (Part 1) 1987
[7] and IS: 875 (Part 2) 1987 [8] respectively. The live load assigned on roof level is 1.5KN/m2 and that on typical floor level is 2KN/m2. The earthquake loads are taken from IS:18932002 [5]. The earthquake design is based on zone III. Zone factor of 0.16, Importance factor of 1, Soil type II and response reduction factor 5 are adopted as per IS 1893 (Part 1): 2002 [5]. The design wind load is computed based on location Calicut, Wind speed 39 m/s, Terrain category 2, Structure class B, Risk Coefficient 1 and Topography factor 1 as per IS: 875 (Part 3) 1987 [9]. The models are generated in ETABS 2015 software and linear static analysis have been done.
The structural plan used for the present study is given in the Fig.1. The member sections used for diagrid building models and tubular building models are given in Table I and Table II. Initially, preliminary member sections are assumed and modelling and analysis have been done by using ETABS 2015 software. The member sections are revised for failed members. The Table 1 and Table 2 represents the revised member sections for diagrid and tubular models presented in this study. The member sections of tubular building are kept same as that of diagrid building for the purpose of comparison so that the variation in results can be studied under same building configuration and loading condition. The models D1 and T1 used for the study are shown in Fig.2. Similarly, all other models are also generated in ETABS2015 software.
Fig. 1. Structural plan.
TABLE I. MEMBER SECTIONS FOR DIAGRID MODELS
Model
Member Section
Beam
Column
Diagrid
B 1
B 2
B 3
D 1
300X500
300X1000
300X500
1200X1200
500X500
D 2
300X500
300X1000
300X500
1250X1250
540X540
D 3
300X500
300X1000
300X500
1380X1380
600X600
D 4
300X500
300X1000
300X500
1510X1510
660X660
D 5
300X500
300X1000
300X500
1650X1650
690X690
D 6
300X500
300X1000
300X500
1790X1790
720X720
D 7
300X500
300X1000
300X500
1900X1900
760X760
D 8
300X500
300X1000
300X500
2200X2200
810X810
Model
Member Section
Beam
Column
B 1
B 2
B 3
Core
Peripheral
T 1
300X500
300X1000
300X500
1200X1200
500X500
T 2
300X500
300X1000
300X500
1250X1250
540X540
T 3
300X500
300X1000
300X500
1380X1380
600X600
T 4
300X500
300X1000
300X500
1510X1510
660X660
T 5
300X500
300X1000
300X500
1650X1650
690X690
T 6
300X500
300X1000
300X500
1790X1790
720X720
T 7
300X500
300X1000
300X500
1900X1900
760X760
T 8
300X500
300X1000
300X500
2200X2200
810X810
Model
Member Section
Beam
Column
B 1
B 2
B 3
Core
Peripheral
T 1
300X500
300X1000
300X500
1200X1200
500X500
T 2
300X500
300X1000
300X500
1250X1250
540X540
T 3
300X500
300X1000
300X500
1380X1380
600X600
T 4
300X500
300X1000
300X500
1510X1510
660X660
T 5
300X500
300X1000
300X500
1650X1650
690X690
T 6
300X500
300X1000
300X500
1790X1790
720X720
T 7
300X500
300X1000
300X500
1900X1900
760X760
T 8
300X500
300X1000
300X500
2200X2200
810X810
TABLE II. MEMBER SECTIONS FOR TUBULAR MODELS
Fig. 2. Model D1 and T1.
The two types of models are compared under WINDX load case. The displacement of tubular models are found to be more than that of diagrid models. For example, the maximum displacement of model D1 is 28mm. The maximum displacement obtained for model T1 is 113.2mm.i.e. the maximum displacement of model T1 is 304.3% more than that of model D1. While, the maximum displacement of model T8 is only 175.9% more than that of model D8. i.e the variation in displacement values is found to decrease from 24 storey model to 66 storey model. Though the variation in displacement decreases from lower to higher models, the displacement of tubular building is found to be of very large value while comparing with the diagrid model results.
As per code IS: 4562000 [6], clause: 20.5, page no. 33, the maximum top storey displacement due to wind load should not exceed H/500, where H is the total height of the building. Both the building models satisfy this criteria. But the lateral displacement of tubular bilding model is very larger than that of diagrid building model. i.e. the tubular building model displaces more than that of diagrid model under same load.
B. Storey Drift
The storey drift values of Diagrid building models and Tubular building models are compared under EQ X load case in the Fig.4.

ANALYSIS RESULTS
A. Displacement
The Diagrid building models and Tubular building models are compared in terms of displacement in the Fig.3. The results are considered under WINDX load case.
Fig. 3. Displacement results of Model D and T.
Fig. 4. Storey drift results of Model D and T.
While comparing the storey drift under EQ X load case, the values are found to increase from lower to higher models for both type of buildings. The storey drift value of tubular building is found to be more than that of diagrid building. For model D1 the maximum storey drift value is 0.000263 and that of model T1 is 0.000514. Similarly, for all other models the storey drift value of tubular building is very larger than that of diagrid building. The maximum storey drift of model T1 is 95.4% more than that of model D1. The variation in storey drift is found to increase upto 48 storey model and reaches 192.7%, then the variation decreases for the remaining three models. For 66 storey building, the variation is 144.7%.
For earthquake load, as per code IS: 18932002 [5], clause: 7.11.1, page no: 27, the storey drift in any storey due to minimum specified lateral force with partial load factor of 1.0 should not exceed 0.004 times storey height that is H/250, where H is thestorey height in meter. The storey drift values are found to be within the permissible limit.

Time Period
The modal analysis results of Diagrid building and Tubular building are compared in Fig.5. The time period for twelve mode shapes are considered for each model.
The storey shear values increases while moving from lower models to higher models for both type of buildings. i.e. the maximum storey shear increases when the height of the building increases. While comparing both the types of building models, the storey shear are found to be almost same for respective models.
Fig. 5. Time period results of Model D and T.
The time period increases from lower to higher models for both type of building models. While comparing the two models, it is clear that the time period of tubular building models are larger than that of diagrid building model. For model D1 the maximum time period is 1.74sec and that of model T1 is 3.579sec. Similarly, the time period of tubular building is more than that of diagrid building for all other models also. The maximum time period of model T1 is 105.7% more than that of model D1. This variation in time period decreases from lower to higher models and reaches to 74.1%. The building natural time period is obtained from the equation;
T = 2 *(m/k) (1)
Where, T is the time period, m is the mass, k is the stiffness. i.e. the time period can be related to the stiffness of the building. Since the time period of tubular building is more than that of daigrid building, its stiffness will be less than that of diagrid building.

Storey Shear
The storey shear of models of both the diagrid building and that of tubular building are compared under WINDX load case in the Fig.6.
Fig. 6. Storey shear results of Model D and T.


CONCLUSION

The models of Diagrid building and Tubular building have been generated in ETABS2015 software. The analysis and design have been carried out based on Indian Standards. For the sake of comparison the member sections and loading conditions are kept the same for both the type of models.
The comparison of analysis results reveales that the displacement, storey drift and time period of tubular building is greater than that of Diagrid building under same member sections and loading condition. The storey shear is found to be almost the same for both type of buildings. Many of the members of Tubular building have been failed while analyzing under the same loading conditions as that of Diagrid models. On the other hand the Diagrid models are found to resist these loads effectively by their axial action. It implies that the member sections of tubular building has to be increased or it has to be strengthened by some other means to carry the same amount of load.
The variation in analysis results of Diagrid and Tubular building decreases while moving to higher models. It indicates that these lateral load resistance systems are best suited for high rise buildings. Hence, the study on the selected models indicates that, the Diagrid building is structurally more efficient than Tubular building.
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