DYNAMIC ANALYSIS OF DIAGRID STRUCTURAL SYSTEM IN HIGH RISE STEEL BUILDINGS

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 8, August 2018, pp , Article ID: IJCIET_09_08_009 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed DYNAMIC ANALYSIS OF DIAGRID STRUCTURAL SYSTEM IN HIGH RISE STEEL BUILDINGS Akshat Post Graduate Student, Department of Civil Engineering, Chandigarh University, Mohali, India Gurpreet Singh Assistant Professor, Department of Civil Engineering, Chandigarh University, Mohali, India ABSTRACT Now a days, the rate of population is increasing day by day due to which the access for land is decreasing. Due to this, tall structures are preferred. As it is known that the effect of lateral load is more on the tall structures because with the increase in height of structure, the effect of lateral load increases. The lateral load on the structure can be due to the wind and earthquake. In this paper, the study is made on the basis of lateral load due to earthquake. There are various structural systems for resisting the lateral load but the diagrid structural system is in trends nowadays and adopted for research work. In this paper, a 60 storey tall building of height 216 m is analyzed. The plan dimension of the building is 48 m 48 m. The building is analyzed for lateral load due to earthquake in seismic zone IV. Various patterns of the diagrid were used in the dynamic analysis by varying the angles of the diagonal elements. The analysis is performed by using ETABS software. Response Spectrum Method is adopted for the dynamic analysis of the structure. The number of diagonal elements are also varied on the façade of the structure for the assessment of the economy. At last, secondary bracing system was also added to it. The results of analysis are discussed in terms of Maximum storey displacement, maximum storey drift and maximum storey shear. Key words: Diagrid Structural System, Variable Angle Diagrid, Granularity, Structural Design, Secondary Bracing System, Response Spectrum Analysis. Cite this Article: Akshat and Gurpreet Singh, Dynamic Analysis of Diagrid Structural System in High Rise Steel Buildings. International Journal of Civil Engineering and Technology, 9(8), 2018, pp editor@iaeme.com

2 Akshat and Gurpreet Singh 1. INTRODUCTION The advancement of the shape and articulation of the high rise buildings is just slightly more than 100 years old. Older high rise buildings were typically dependent on steel structures based on portal frame system in which connections were made up of reinforced material to resist lateral loads. After few years, gravity and lateral load resisting systems were separated in buildings using additional bracings usually in the form of diagonals, which resists the lateral loads. For making high rise structures, it is necessary to design them for stability. In ancient days, designing of high rise structures was done by only the engineers. It was only the job of engineers. In older days, terra cotta tiles were used with punching system as a glazing in the high rise buildings. When aluminum curtain wall system was invented, then this system was replaced with higher amount of glazing. As the height of the building is increased, higher wind load will act over the structure. For increasing the strength of the structure, different types of bracings were used in the high rise buildings due to which the structure will behave as like a cantilever. There are also some locations and joints where it is difficult to provide moment resisting connections. So, at that places K and X bracings were used to provide a connection. These bracing were placed internally near the core of the structure due to which there is no obstruction to the flow of traffic at the façade of the building. For increasing the stability and strength of these structures, trusses were used in the building. Truss band floors can be used in the exterior of the structure due to which it will also supports the glazing and curtain walls of the structure. This was done only in the John Hancock tower in the year 1960, where braced tubes were introduced in the building located in Chicago having 100 storeys in it. It was a challenge for architects to introduce bracings in the design of façade of the building. Thus, it pushes the choice of engineer into the design of Architecture. The Shukhov tower in Polibino is the world s first hyperboloid diagrid structure built in the period of It was designed by Russian engineer and architecture Vladimir Shukhov in The first building in which diagrids were used at the façade of the structure is in Pittsburgh, constructed in year around This method of construction was not used again after this till the mega projects of diagrid and their patterns were designed. The designing and construction of diagrids was started again in the year The IBM building was constructed with the help of diagrids in which diagrids were coordinated with the curtain wall system due to which the shapes of windows get disturbed and becomes odd. This building has covered maximum areas of design phase of the diagrid buildings except the curtain wall system. After this, diagrid structure came in trends for using in high rise structures. The IBM building is currently known as United Steel Workers building. Curtis and Davis were the designers of IBM building. Lesslie Robertson was the engineer during its construction work. This is the first diagrid building in which the diagrids were placed at the perimeter of the building. The main purpose of using diagrid structural system is to reduce the weight of the structure by reducing steel in it which results in removal of maximum number of vertical columns from the structure. There are some of the vertical columns at the core of the diagrid structure whose purpose is only to transfer gravity loads to the foundation. These columns at the core of the structure are not able to resist the horizontal load due to wind and earthquake editor@iaeme.com

3 Dynamic Analysis of Diagrid Structural System in High Rise Steel Buildings (a) Shukhov Tower in Moscow (Source: Figure 1 Diagrid buildings (b) IBM Building in Pittsburgh (Source: When diagrid structures were compared with the normal conventional moment resisting framed structure, it was observed that the diagrid structure resists more lateral loads than the conventional moment resisting framed structure. By the axial action of diagonals, diagrid structure resists more of the shear force by which shear deformation of the structure get reduced. On the other side, conventional structure resists shear force by the bending action which occurs in vertical columns of the structure. Diagrid structural system is defined as a framework made by the intersections of diagonal members made up of different materials like concrete, steel, metals or wooden beams which are used in the construction work. Diagrid is used for the construction of skyscrapers. Steel is mainly used as a construction material for these structures. In these structures, a triangular truss type structural system is formed in which there is also a supporting beam for the diagonals at their base. Diagrid structures of steel members are generally used for increasing the strength and stiffness of the structure. But nowadays diagrids are used in the case of high rise buildings whose span and heights are larger. More specially, this system is useful in the case of more complicated patterns and profile of the structure. Diagrid structural system of tall structures have inclined columns which are known as diagonal members of the structure. Due to inclination of these columns, axial force is produced along the direction of the column under the action of lateral load by means of which diagrid structural system resists the horizontal wind load and seismic load. Diagrids are set with an ideal angle at the façade of the building. With the help of these diagonal members, the building is divided into number of parts along the height of the building which are known as modules. The connection between the diagonal members is considered as a pin connection. 2. DESCRIPTION OF BUILDING MODEL In this paper, a 60 storey building model of interstorey height 3.6 m is analyzed and studied. A structure model having a square plan dimension of 48 m 48 m is assumed for the study of structural performance of diagrid structural system. It consists of a core at the centre of model having square plan dimension of 24 m 24 m. The total height of the building is 216 m. The dead and live loads acting on the floor of the structure are assumed as 4 kn/m 2 and 3 kn/m 2 respectively. The lateral load due to earthquake acting on a structure is calculated for seismic zone IV. The structure is modeled and analyzed by using ETABS software. The static and 73 editor@iaeme.com

4 Akshat and Gurpreet Singh dynamic, both the analyses are performed on the building models. Response Spectrum Method is used for dynamic analysis of the structure. (a) Figure 2 (a) Square Plan of the Structure (b) Internal Column of the Structure I-sections are used as beams in the construction of building model. ISWB 400 (B1), ISWB 450 (B2), ISWB 500 (B3), ISWB 550 (B4) and ISWB 600 consists of cover plates at the top and bottom (B5) are considered for the modeling of structure. 3. ANALYSIS OF BUILDING FOR EARTHQUAKE LOAD The equivalent static method is used for finding the response of the structures against earthquake loads. This method is also known as seismic coefficient method (SCM). In this study, first the design base shear is determined for the structures and then this base shear will be distributed in various floors of the structure and finally this design base shear will be distributed to each lateral load resisting element at each floor level through the structural analysis [9]. (b) For static and dynamic analysis, the design seismic force is calculated by adding the dead load fully and some percentage of imposed load [9]. The weight of each floor used in seismic analysis is equal to its dead load (which is taken as 100%) plus some percentage of load which is imposed on the floors slab of the building. In this study, the percentage of imposed load is taken as 25%. When computing the seismic weight of floor, it should be kept in mind that the weight of the columns must be distributed in equal parts to the floors above and below the columns [9]. In general, the sum of seismic weight of all the floors is considered in analysis as a seismic weight of the building. Damping Ratio: The damping ratio is considered as 5% for finding the value of A h for steel structures in the base shear of a building for both static and dynamic analyses. In this study, the plan of the building model is considered as symmetrical and assumed to be non-sensitive against torsion. The steel building is modelled as a special moment resisting frame without any masonry walls as infills and the fundamental natural time period of oscillations T a (in seconds) for the assumed building is as [9]: 74 editor@iaeme.com

5 Dynamic Analysis of Diagrid Structural System in High Rise Steel Buildings T a seconds The data considered for the seismic analysis of the building model as per IS:1893 (Part 1) are: Earthquake Zone IV; Factor for Zone, Z = 0.24; Factor of importance for the building, I = 1.5; Factor of Response Reduction for the building, R = 5.0; Soil type = Medium Stiff Soil Site (i.e. Soil type II); ( S a g ) = 0.34 for T a > 4.00 seconds. The design base shear computed above will be distributed by using the formula as per IS:1893 (Part 1)-2002 along the height of the building. In this study, the assumed building is in Zone IV and has height of 216 m which is more than 40 m. So it is required to perform dynamic analysis on the building model [9]. There are two different methods for performing the dynamic analysis i.e. Response Spectrum Method and Time History Method. However, any of the method can be used. The computed design base shear should not be less than the design base shear which is calculated by using fundamental natural time period T a. If is smaller than, then the force parameters of response such as base shear, base reactions, and member stress resultants should be multiplied by. Response Spectrum Method is adopted for the dynamic analysis of the structures. The Response Spectrum Analysis (RSA) is also known as Modal Analysis Procedure. This analysis is performed as per the codal provisions. This method of analysis is based on superposition of modes. For doing the analysis by eigen value method, it is necessary to calculate modes of free vibration The higher value of response of a modal λ k is obtained for each mode (say k th mode). The number of modes to be considered is based on the term total mass participation factor for each mode. The number of modes must be selected in such a way that at least 90% of the seismic weight of the building will get added in analysis as a total participating mass of the building [9]. In this study, 40 modes are considered for analysis, and 99% of the seismic weight of the building is utilized as a total participating mass of the building. The modal responses can be combined together from all the considered modes by using two different methods i.e. SRSS (Square root of the Sum of the Squares) and CQC (Complete Quadratic Combination). The quantities of force response such as displacement, reactions, member forces are combined together by SRSS method or CQC method. CQC is also considered as the extension of SRSS. In this study, the natural periods of the modes of the building are found to be very closely spaced. So, CQC method is used for calculating the earthquake loads acting over the structure for very closely spaced modes. Scale factor is a term which is required in seismic analysis by Response Spectrum Method. If the dynamic base shear of the structure is more than 80 % of static base shear then there is no need of calculating the scale factor. If the dynamic base shear is less than 80 % of the static base shear of the structure, then the following formula is used for the calculation of scale factor: Scale Factor g Static ase Shear esponse Spectrum ase Shear 75 editor@iaeme.com

6 Maximum Storey Displacement, Δ h (mm) Akshat and Gurpreet Singh ISWB 600 is used as a member for secondary bracing system. The X-type bracing system is used at the central core of the structure to increase its stability. With a very small amount of structural steel, the performance of the structure is modified greatly. (a) (b) Figure 3 (a) w/o SBS, and (b) w SBS 4. LATERAL DISPLACEMENT AND STOREY DRIFT DUE TO EARTHQUAKE As per codal provisions, the lateral displacement due to seismic load at the top of the building should not be more than H/250, where H is the total height of the structure. The interstorey drift of a building should not exceed h i [9], where h i is the storey height of that building as per clause of IS:1893 (Part 1) So, the maximum permissible lateral displacement for a building of height 216 m is 864 mm and, the maximum allowable interstorey drift for a storey height of 3.6 m is 14.4 mm. 5. COMPARISON OF ANALYSIS RESULTS FOR SEISMIC LOADS Δ h,lim = [Y VALUE] mm Figure 4 Maximum Storey Displacement for Granularity Variation w/o SBS 76 editor@iaeme.com

7 Maximum Storey Displacement, Δ h (mm) Dynamic Analysis of Diagrid Structural System in High Rise Steel Buildings Δ h,lim = [Y VALUE] mm Figure 5 Maximum Storey Displacement for Granularity Variation w SBS Maximum Storey Drift, d h d h,lim = [Y VALUE] Figure 6 Maximum Storey Drift for Granularity Variation w/o SBS Maximum Storey Drift, d h d h,lim = [Y VALUE] Figure 7 Maximum Storey Drift for Granularity Variation w SBS 77 editor@iaeme.com

8 Maximum Base Shear, V h (kn) (kn) Akshat and Gurpreet Singh Maximum Base Shear, V h Figure 8 Maximum Storey Shear for Granularity Variation w/o SBS Figure 9 Maximum Storey Shear for Granularity Variation w SBS 6. CONCLUSIONS This study is performed on a 60 storey diagrid high rise steel structure. The modeling and analysis is performed by using ETABS software. The structure is analysed statically and dynamically both for seismic loads. Response Spectrum Analysis is performed for dynamic analysis of the structure. The results obtained by performing the analysis of the structure in terms of maximum storey displacement, maximum storey drift and storey shear are compared and the conclusions are drawn as follows: By reducing the angles of diagrids it is observed that the lateral displacement and storey drift also reduces with a significant amount of reduction. So it is decided to keep the structure under permissible deformation and structural weight of steel is reduced. Hence, the structure becomes economic. There is about 20% less reduction of steel in diagrid structure as compared to conventional structure. With the increase in number of diagonal elements on the web and flange façade of the structure, the top storey displacement and maximum storey drift gets reduced while base shear increases with the increase in number of diagonal elements on the web. Out of 20 different diagrid models and 9 different angles of diagrids, it is found that the efficient results for geometrical diagrid pattern are in between the angles 67.38º and 71.56º editor@iaeme.com

9 Dynamic Analysis of Diagrid Structural System in High Rise Steel Buildings It is found the diagrid structure having 16 number of diagonal elemnts on the façade of the structure gives more efficient results than n x = 32 and n x = 8. REFERENCES [1] D Prajapati, D Panchal, Study of Seismic and Wind Effect on Multi Storey C C, Steel and Composite uilding, nternational Journal of Advances in Engineering & Technology, Sept. 2013, Vol. 6, Issue 4, pp [2] Dr M T huiyan, Dr Leon, Preliminary Design of Diagrid Tall uilding, AJC- ISAM International Conference, Proceedings of The [3] ETA S Nonlinear Ver, Extended Three Dimensional Analysis of uilding Systems, Computers and Structures, nc erkeley, CA USA, [4] G M Montuori, E Mele, G randonisio, A D Luca, Geometrical patterns for diagrid buildings: Exploring alternative design strategies from the structural point of view, Engineering Structures, 71 (2014) [5] G M Montuori, E Mele, G randonisio, A D Luca, Secondary bracing systems for diagrid structures in tall buildings, Engineering Structures, 75 (2014) [6] G Milana, P Olmati, K Gkoumas, F ontempi, Ultimate Capacity of Diagrid Systems for Tall uildings in Nominal Configuration and Damaged State, Periodica Polytechnica Civil Engineering, [7] IS:1893 (Part 1)-, Criteria for Earthquake esistant Design of Structures Part General Provisions and uildings (Fifth evision), ureau of ndian Standard, New Delhi. [8] IS:800-, General Construction in Steel Code of Practice (Third Revision), ureau of Indian Standard, New Delhi. [9] IS:875 (Part 1)-, Code of Practice for Design Loads (Other than Earthquake) for Buildings and Structures Part 1 Dead Loads Unit weights of building materials and stored materials (Second evision), ureau of ndian Standard, New Delhi. [10] IS:875 (Part 2)-, Code of Practice for Design Loads (Other than Earthquake) for uildings and Structures Part mposed Loads (Second evision), ureau of ndian Standard, New Delhi. [11] K Jani, P V Patel, Analysis and Design of Diagrid Structural System for High Rise Steel uildings, Procedia Engineering, ( 3) 100. [12] K Kamath, N Ahamed, Effect of Aspect atio on Performance of Diagrid Structure ecircular in Plan, nternational Journal of Earth Sciences and Engineering, Volume 08, No. 02, April 2015, P.P [13] K S Moon, Diagrid Structures for Complex-Shaped Tall uildings, Procedia Engineering, 14 (2011) editor@iaeme.com