Diagrid structural system

Diagrid

1. Diagrid Structural System

2. Contents Introduction Diagrid Components Triangular Diagrid Module Structural action of Diagrid Module Node Design Node construction Varied Angle Diagrid Merits and demerits of Diagrid Structures Examples Conclusion References

3. Introduction Tall commercial buildings are primarily a response to the intense pressure on the available land. Advances in materials , construction technology, analytical methods and structural systems for analysis and design accelerated the development of tall structures. The lateral loading due to wind and earthquake is the major factor that causes the design of high-rise buildings. These lateral loads are resisted by exterior structural system or interior structural system. The lateral load resisting systems that are widely used are mainly rigid frame, shear wall , wall-frame, braced tube system, outrigger system, diagrid system and tubular system . Recent trend shows that the Diagrid structural system is becoming popular in the design of tall buildings due to its inherent structural and architectural advantages. Diagrid is an exterior structural system in which all perimeter vertical columns are eliminated and consists of only inclined columns on the façade of the building. Shear and over-turning moment developed are resisted by axial action of these diagonals compared to bending of vertical columns in framed tube structure.

4. Vertical columns in the core are designed for carrying gravity loads only and the diagrid is useful for both gravity and lateral loading . Diagonalized applications of structural steel members for providing efficient solutions both in terms of strength and stiffness are not new, however nowadays a renewed interest in it and a wide spread application of diagrid is registered with reference to large span and high rise buildings, particularly when they are characterized by complex geometries and curved shapes.

5. The diagrid systems are the evolution of braced tube structures. The major difference between a braced tube building and a diagrid building is that, there are no vertical columns present in the perimeter of diagrid building. The diagonal members in diagrid structures act both as inclined columns and as bracing elements and due to their triangulated configuration, mainly internal axial forces arise in the members . Diagrid structures do not need high shear rigidity cores because shear can be carried by the diagrids located on the perimeter. Braced Tube Structure( John Hancock Center, Chicago) Diagrid Structure ( Hearst Tower, New York)

6. Diagrid has good appearance and it is easily recognized. The configuration and efficiency of a diagrid system reduce the number of structural element required on the façade of the buildings , therefore less obstruction to the outside view. The structural efficiency of diagrid system also helps in avoiding interior and corner columns, therefore allowing significant flexibility with the floor plan. Perimeter “diagrid” system saves approximately 20 percent structural steel weight when compared to a conventional moment-frame structure . An early example of the diagrid structure is the IBM Building in Pittsburgh built in the early 1960s, with its 13-storey building height. IBM Building , Pittsburgh, Pennsylvania

7. The Swiss Re tower in London, Hearst tower in New York, CCTV headquarters building in Beijing, Mode Gakuen Spiral Tower in Aichi, West tower in Guangzhou, Lotte super tower in Seoul, Capital Gate in Abu Dhabi etc. are some other popular diagrid buildings. Mode Gakuen Spiral Tower Capital Gate Super Lotte Tower CCTV Headquarters

8. History The Shukhov tower in Polibino  is the world's first  diagrid hyperboloid structure designed in 1896 by Russian engineer and architect Vladimir Shukhov. B uilt in the period 1920–1922. 160 m high. Its steel shell experiences minimum wind load.

9. Diagrid Components Nodes Diagonal Members Ring Beams Tie Beams Core Floor Slab

10. Nodes Joints that connects all the members. Typically formed by bolting or welding the ends of the members to a gusset plate . When diagrid is to be exposed internally or externally then we have to weld to achieve seamless connections otherwise bolting can be done easily. Nodes can be designed as hinged or fixed depending upon condition. In case of concentric load and high diagrid angles hinged support is enough but when loads is eccentric and diagrid angle is small then we have to provide fixed nodes as it becomes difficult to erect diagonal beams at nodes.

11. Diagonal Members Members that transfer both lateral and gravity loads through axial action . Can be made of steel , concrete , timber and composite materials. Usually steel diagonal members are used.

12. Perimeter Girders or Ring Beams They comprise of ring structure at the periphery of the building connected at the nodes which are further connected with the diagrid columns . Extremely important in maintaining the stability of the system . C onstrain the shape and act in a very similar manner to the hoops in the Shukhov towers . In smaller modules, those with heights tip to tip of the diamond in the range of 2 to 4 floors, the beams are designed in conjunction with the edges of the floors. For taller modules there may need to be some interaction between the floor edge beams and the long diagonals, in addition to the dedicated function of the horizontal members that frame directly into the node. The longer the diagonals the more likely they are to require additional lateral bracing if the desire is for slenderness . Alternately, if unbraced, the member size must be substantially increased to provide self-support. This is common if the diagrid is used in conjunction with a large atrium space where there are no floors to assist with this function.

13. Tie Beams Transfer load from RC core to Diagrid structures. Unbalanced forces can be balanced by ring beams and tie beams.

14. Core T o carry gravity loads To break up the span of the floor beams. Can be made of RCC ( Canton Tower in Guangzhou, China) or Steel Columns ( Swiss Re Tower, London) Capital Gate in Abu Dhabi, UAE designed by RMJM Architects has also used a reinforced concrete core, in this instance prestressed to counterbalance the 18 degree lean of the tower. Fig: . BIM Model of Al Dar Headquarters and Capital Gate Building

15. Triangular Diagrid Module A diagrid structure is modelled as a vertical cantilever beam on the ground, and subdivided longitudinally into modules according to the repetitive diagrid pattern . Each Diagrid module is defined by a single level of diagonals that extend over ‘n’ stories . The geometry of the single module plays a major role in the internal axial force distribution, as well as in conferring shear and bending rigidity to the building structure. While a module angle equal to 35° ensures the maximum shear rigidity to the diagrid system, the maximum engagement of diagonal members for bending stiffness corresponds to an angle value of 90°, i.e. vertical columns . Vertical columns are almost eliminated and both shear and bending stiffness must be provided by diagonals, a balance between this two conflicting requirements should be searched for defining the optimal angle of the diagrid module. 

16. Optimal Angle : Usually adopted range is 60 to 70 degrees The diagonal member of the diagrid carries both shear and moment. So the optimal angle of placing of the diagonals is dependent of building height. The optimal angle of the columns for maximum bending rigidity in the normal building is 90 degree and for the diagonals for shear rigidity is 35 degree. It is assumed that the optimal angle of the diagrid falls in between the both. A sample study done of a 60 storey tall building, measuring 36m x 36m with an 18m x 18m gravity core at the center with floor to floor heights of 3.9m confirmed 69 degree as the most effective angle for a uniform diagrid as it results in the least amount of steel by weight. (Moon, 2007, 2008, 2009, 2010,2011 ) This angle changes as a function of the building height as well as its width to height ratio .  

17. Module Dimensions : Height of the module :    It depends on the number of stories stacked per module. Usually 2 – 6 stories are stacked per diagrid with average floor height varying from 3.5 - 4.15 m on an average. The relationship between the overall height and number of floors of a tower is more critical in the case of a diagrid tower as the façade expression needs to terminate at a full module. Therefore the number of storeys will directly influence the decision on the primary module height. Larger modules allow more flexibility in the choice of curtain wall. They are easier to “fit” on the façade and can more easily accommodate standard rectilinear based curtain wall systems as infill. Smaller modules are more restrictive and invariably lead to more complex glazing systems that tend to be triangulated. This impacts labour costs as these tend to be more time consuming to install and maintain as they cannot be fitted with standard window washing equipment.   Base of the module :    It depends on the height and optimal angle (apex angle) of the diagrid.

18. Structural action of a Diagrid module Effects of gravity loading, lateral loading and shear loading Effect of non-apex loading : Bending moment and shear force are expected due to this load condition. However the introduction of a horizontal member at each floor girder to diagonal intersection allows for the absorption of the force component orthogonal to the diagonal direction, thus preserving the prevailing axial force condition. Courtesy: Kyoung Sun Moon

19. Node Design The load path can be divided into two main scenarios, vertical load and horizontal shear . The vertical load will be transferred in the form of an axial load from the diagrid members above the node to the gusset plate, then to the diagrid members below the nodes. The horizontal shear will be in the form of axial loads in the diagrid members above the node with one in compression and one in tension to the gusset plate and. The force will then be transferred as shear force in the gusset plate and then to the other pair of tensile and compressive forces on the diagrid members below the nodes.

20. From this load path, the shear force at the location of bolt connections is high under lateral loads. Because this may create weak points at the node particularly during earthquakes, the strength of the bolts should be designed carefully.

21. Node construction for diagrid structure Both the engineering and fabrication of the joints are more complex than for an orthogonal structure, and this incurs additional cost . Due to the triangular configuration of the diagrid structural system, rigid connections are not necessary at the nodes, and pin connections using bolts can be made more conveniently at the jobsite.  In order to reduce jobsite work, prefabrication of nodal elements is essential . The prefabricated nodes are connected to the large built-up diagonal members by bolts at the jobsite . The precision of the geometry of the connection nodes is critical, so it is advantageous to maximize shop fabrication to reduce difficulties associated with erection and site work. Some nodes are many tonnes and it is desirable to be able to lift and turn the elements with a crane to provide access for fabrication, welding and finishing.

22. As this type of structure is more expensive to fabricate, cost savings are to be realized if there is a high degree of repetition in the design and fabrication of the nodes . An architecturally exposed diagrid is more likely to require fully welded connections – both within the node and for the connections between the nodes and the diagonals. This puts added pressure on the fabrication tolerances in order to ensure that the butt welds are closely aligned to ensure a seamless transition. Site welding may also require pre-heating of the material, so scaffolding and weather protection might also be required.

23. Storey Drift for different grid systems under earthquake load case The  drift ratio  is defined as the  ratio  of maximum lateral  drift  to total height of the specimen.     Normal conventional building without any load resisting system [R1]  Diagrid building with diagonal angle of 45  [M1]  Diagrid building with diagonal angle of 63  [M2]  Diagrid building with diagonal angle of 73  [M3]  Diagrid building with diagonal angle of 75  [M4]  Diagrid building with diagonal angle of 78  [M5]  Diagrid building with diagonal angle of 81  [M6]  Hexagrid building [M7] Courtesy : Deepika R1, Shivanand C.G2, Dr.Amarnath K3 P G Student1, Assistant Professor2, Professor and HOD3 Department of Civil Engineering The Oxford College of Engineering, Bangalore, Karnataka India

24. Storey Drift for different grid systems under wind load case PARAMETERS   Seismic parameters Wind parameters Location: Amritsar Wind speed, V b : 47m/s Zone: IV Terrain category: 3 Importance factor: 1.0 Class C Response reduction factor: 5  < 3  Soil type: medium soil k1,k3 = 1 Wind and seismic parameters Table II: wind and seismic parameters Courtesy : Deepika R1, Shivanand C.G2, Dr.Amarnath K3 P G Student1, Assistant Professor2, Professor and HOD3 Department of Civil Engineering The Oxford College of Engineering, Bangalore, Karnataka

25. Varied Angle Diagrid As a tall building functions as a vertical cantilever, the taller and more slender the building, the greater will be the differentiation between the function of the diagrid elements at the base to those towards the top. This has suggested that a variation in the inclination angle of the diagonals can be used to reduce the amount of steel and provide needed resistance in the structure. The members and connections at the base of the building must be designed to resist moment while those at the top to resist shear . The diagonal members towards the top of the tower should be more inclined and the module smaller as these members have less gravity loading associated with their function and must resist more wind loading .

26. Comparison between uniform and varied angle diagrids Fig. Different configurations of diagrids for an 80 storey building A comparative study was conducted on 20 models ( 4 different configurations of 36, 50, 60, 70 and 80 storey buildings) in terms of top storey displacement and maximum forces developed on the diagrid member.

27. Maximum storey drift for different models Courtesy : Nijil George Philip, Dr.Shashidharan

28. Maximum force on diagrid for different models Courtesy : Nijil George Philip, Dr.Shashidharan

29. Results B uilding of 36 and 50 storeys yield lowest displacement when configured with uniform diagrid angle. In 60, 70 and 80 storey buildings, non-uniform diagrid configuration provided lower displacement and diagrid forces when the diagrid angle changed from 75° to 50°. The forces on diagrids reduced around 2.5%, 3% and 5.5 % in 60, 70 and 80 storey buildings respectively. So designing the diagrid building with changing diagrid angle could reduce the section size required and thereby reduce the material required for the building.

30. Twisted Diagrids vs Braced Tube Courtesy: Kyoung Sun Moon

31. Examples Swiss Re Tower , London First modern application of diagrid structure. Designed by Sir Norman Foster, with 40 stories and an inter-story height of 4.15 m, the tower is 180 meters tall. C onsists of a central core and a perimeter “diagrid ’. Core is required to act only as a load-bearing element and is free from the diagonal bracing, producing more flexible floor plates . Tower’s circular plan widens as it rises from the ground and then tapers toward its apex . The aerodynamic form of the tower encourages wind to flow around its face, minimizing wind loads on the structure and cladding, and enables the use of a more efficient structure.

32. Guangzhou West Tower The Guangzhou West Tower, designed by Wilkinson Eyre architects, London with 103 stories Height of 440m, is the tallest building in China and one of the tallest in the world . C urvilinear shape along elevation and the floor plate is an equilateral triangle with round-corners. Central concrete core and perimeter diagrid structure. D iagrid module expanding on six stories, 12.4 m wide and 24.8 m high. The diagonals are steel tubular members filled by concrete (CFST), with size ranging between 1080 x 55 mm at the first floor and 700 x 20mm at the top. The concrete core has a triangle shape and fully participates to the lateral resistance up to the seventh floor, where it is eliminated, and leaving place to a central giant atrium for the hotel which occupies the upper floors.

33. Merits The Diagrid structures have mostly column free exterior and interior , hence free and clear, unique floor plans are Possible . The Glass facades and dearth of interior columns allow generous amounts of day lighting into the structure .  The use of Diagrids results in roughly 1/5th reduction in steel as compared to Braced frame structures . Sustainability: - energy efficiency - low environmental cost - efficient use of available resources The Diagrids makes maximum exploitation of the structural Material .  The diagrid Structures are aesthetically dominant and expressive. Redundancy in the Diagrid design is obvious. It is this redundancy then that can transfer load from a failed portion of the structure to another. Skyscraper structural failure, as it is such an important/ prominent topic, can be minimized in a Diagrid design. A Diagrid has better ability to redistribute load than a Moment Frame skyscraper. Thus creating a deserved appeal for the Diagrid in today’s landscape of building.

34. Demerits T he number of storeys directly depend upon the primary module height . It is hard to design windows that create a regular language from floor to floor . Both the engineering and fabrication of the joints are more complex than for an orthogonal structure, and this incurs additional cost. Erection of nodes is a difficult process. As of yet, the Diagrid Construction techniques are not  thoroughly explored . Lack of availability of skilled workers having experience in Diagrid Construction .

35. Conclusion One of the best structural system for high rise buildings especially irregular shaped (tilted, twisted, freeform etc.) The majority of the published research has been conducted within the University setting and has focused primarily on idealized optimization , leaving the applied realities of the practising professionals, hidden or internalized. Using Diagrid we can built skyscrapers even without inner core providing vast floor area. The Leaden hall Building in London is the first skyscraper without a bearing inner core thanks to diagrid structural system. Diagrids help in sustainable development as amount of construction material required is less and energy is saved due to less obstruction to incoming light at the periphery of building. The diagrid Structures are aesthetically dominant.

36. References Analysis of Circular Steel Diagrid Buildings with non-Uniform Angle Configurations Nijil George Philip1, Dr.Shashidharan.2, October-2016 Performance Study of High Rise Buildings with Diagrid and Hexagrid Systems under Dynamic Loading Deepika R1, Shivanand C.G2, Dr.Amarnath K3 P G Student1, Assistant Professor2, Professor and HOD3 Department of Civil Engineering The Oxford College of Engineering, Bangalore, Karnataka, India DOI 10.4010/2016.1084 ISSN 2321 3361 © 2016 IJESC Optimized Topology Extraction of Steel-Framed Dia Grid Structure for Tall Buildings Dong- Kyu Lee1, Uwe Starossek2, and Soo- Mi Shin International Journal of Steel Structures June 2010, Vol 10, No 2, 157-164 Advantage of Steel Diagrid Building Over Conventional Building Saket Yadav1, Dr. Vivek Garg2 1 Post Graduate Student, 2 Assistant Professor, Department of Civil Engineering, M.A.N.I.T., Madhya Pradesh, India Diagrid Structures: Innovation and Detailing, T . M. Boake ,School of Architecture, University of Waterloo, Canada Design and Construction of Steel Diagrid Structures by K. Moon , School of Architecture, Yale University, New Haven, USA Terri Meyer Boake “ DIAGRID STRUCTURES SYSTEM,CONNECTION,DETAILS” Mir M. Ali and Kyoung Sun Moon “Structural Developments in Tall Buildings: Current Trends and Future Prospects” Architectural Science Review Volume 50.3, 205-223 Terri Meyer Boake “Elegant Structures: Diagrids take to the Sky ”

37. Thank You