Now a day’s earthquakes are very frequently occurred and maximum loss of life and loss of property occurred due sudden failure of the structure, therefore special attentions are required to evaluate and to improve the seismic performance of multistoried buildings. Hence in this paper the seismic analysis of G+4 story office building is carried out using composite structure in which composite beams (RCC slab rest over steel beams) and composite columns (encased composite columns) are used. The 3-D model static analysis is carried with the help of advanced analysis software (SAP software) according to codal provision by considering different load combination. The results obtained from this type of structure are compared with results of same R.C.C. structure to describe earthquake resistant behavior and performance of the structure.
Such type of constructions has many advantages like high strength, high ductility and stiffness, ease in erection of high rise buildings, fire resistance, and corrosion resistance and helps to achieve modern trend in architectural requirement.
KEY WORDS
Composite structure, problem, composite beams, encased composite column, earthquake analysis, codal provision, different load combination, comparison with RCC building.
INTRODUCTION
In India, earthquakes occurrence have been increased during last few years and it has been studied that maximum loss of life and property occurred due to sudden failure of structure. In composite construction economy of the construction and proper utilization of material is achieved. The numbers of structures are constructed using composite structure in most of the advanced countries like Britain, Japan and America but this technology is largely ignored in India despite its obvious benefits (1).
In composite structure the advantage of bonding property of steel and concrete is taken in to consideration so that they will act as a single unit under loading. In this structure steel is provided at the point where tension is predominant and concrete is provided at the point where compression is predominant. In conventional composite construction, concrete rests over steel beam (2), under load these two component acts independently and a relative slip occurs at the interface of concrete slab and steel beam, which can be eliminated by providing deliberate and appropriate connection between them. So that steel beam and slab act as composite beam and gives behavior same as that of Tee beam. In steel concrete composite columns both steel and concrete resists external loads and helps to limit sway of the building frame and such column occupies less floor area as compared to reinforced concrete columns. The number of studies related to economy of the composite construction shows that the composite construction are economical, light weighted, fire and corrosion resistant and due to fast track construction building can be utilize or occupied earlier as compared to reinforced concrete structure(3).
In this paper an office building considered and seismic analysis is carried using composite beam (RCC slab rest over steel beam), encased composite column (concrete around Hot Rolled steel I section) and the results obtained from this type of structure are compared with the results of same RCC structure.
EXAMPLE OF BUILDING
The building considered is the office building having G+4 stories. Height of each storey is 3.5m. The building has plan dimensions 24 m x 24 m, which is on land area of about 1200 sqm and is symmetric in both orthogonal directions as shown in the figure 1. Separate provisions are made for car parking, security room, pump house and other utilities. However they are excluded from scope of work. The building provision is made for 180 employees and considered to be located in seismic zone III built on hard soil. In composite structure the size of encased composite column is 450mm x 450mm (Indian standard column section SC 250+ 100mm concrete cover), size of primary composite beam is ISMB 450 @72.4 Kg/m and size of secondary composite beam is ISMB 400 @61.6 Kg/m. Here channel shear connector ISMC 75 @ 7.14 Kg/m are used. Concrete slab rest over steel beam having thickness of about 125mm. The unit weights of concrete and masonry are taken as 25 kN/m3 and 20 kN/m3 respectively. Live load intensity is taken as 5 kN/m 2 at each floor level and 2 kN/m2 on roof. Weight of floor finish is considered as 1.875 kN/m2 (4). In RCC structure the size of column is decided by taking equivalent area of encased composite column that is 400mmx 700mm; size of primary beams is 300mm x 600mm and secondary beams is 300mm x 450mm with slab thickness is about 125mm. The unit weights of concrete and masonry are taken as 25 kN/m3 and 20 kN/m3 respectively. Live load intensity is taken as 5 kN/m 2 at each floor level and 2 kN/m2 on roof. Weight of floor finish is considered as 1.875 kN/m2. In the analysis special RC moment-resisting frame (SMRF) is considered.
MODELLING OF BUILDING
The building is modeled using the software SAP 2000. Beams and columns are modeled as two noded beam element with six DOF at each node. Slab is modeled as four noded shell element with six DOF at each node. Walls are modeled by equivalent strut approach (5). The diagonal length of the strut is same as the brick wall diagonal length with the same thickness of strut as brick wall, only width of strut is derived. The strut is assumed to be pinned at both the ends to the confining frame. In the modeling material is considered as an isotropic material.
2.1 Shell Element
Slab modeled as shell element of 125mm thickness having mesh of 1mx1m of this shell element. Material used for shell element is M25 grade cement concrete in both composite and RCC structure
2.2 Beams
In composite structure beams are steel I section from IS code and steel table. The length of each beam is divided into small parts of 1m intervals and connected with concrete slab so as to get composite action. In RCC the length of each concrete beam is divided into small parts of 1m intervals and connected with concrete slab so as to get behavior same as that of Tee beam action.
2.3 Columns
In composite structure column is modeled by giving section properties of both steel and concrete to the software. Also in RCC structure column is modeled by giving sectional properties to the software
ANALYSIS OF BUILDING
Equivalent static analysis is performed on the above 3D model. The lateral loads are calculated and is distributed along the height of the building as per the empirical equations given in the code (IS 1893:2002). The building modeling is done then analyzed by the software SAP 2000. The bending moment and shear force of each beam and column are calculated at each floor and tabulated below.
RESULTS AND DISCUSSION
4.1 Results of Composite Structure:
Floor Level
Max. Shear Force
(kN)
Max. Bending Moment (kN-m)
+ve B M
-ve B M
Plinth Level
73.32
19.64
168.6908
1
177.925
134.31
306.1174
2
175.075
132.34
299.477
3
165.571
132.34
274.038
4
153.64
132.39
236.546
Roof Level
65.59
82.15
125.52
Table 1: Bending Moment and Shear Force of Beam
4.2 Results of RCC Structure:
Floor Level
Max. Shear Force
(kN)
Max. Bending Moment (kN-m)
+ve B M
-ve B M
Plinth Level
115.00
62.45
230.42
1
244.772
177.96
449.82
2
236.744
183.89
418.69
3
223.675
175.28
380.04
4
207.023
174.63
324.58
Roof Level
119.83
115.1004
181.00
Table 2: Bending Moment and Shear Force of Beam
4.3 Results of Composite Structure:
Column No.
Max. Axial Force (kN)
Max. Shear Force (kN)
Max. Bending Moment
(kN-m)
Column-1
1462.307
83.868
251.1801
Column-2
2865.903
101.64
271.4602
Column-3
2828.667
100.091
269.33
Column-4
2865.903
101.64
271.46
Column-5
1462.307
83.87
251.18
Table 3: Axial Force, Shear Force and Bending Moment of Column
4.4 Results of RCC Structure:
Column No.
Max. Axial Force (kN)
Max. Shear Force (kN)
Max. Bending Moment
(kN-m)
Column-1
2453.516
148.942
495.89
Column-2
3526.32
161.64
510.50
Column-3
3538.64
160.995
509.61
Column-4
3519.463
161.83
511.142
Column-5
2455.27
149.047
496.432
Table 4: Axial Force, Shear Force and Bending Moment of Column
From above results of bending moment and shear force of composite structure and RCC structure it is found that bending moment and shear force for composite structure
are less than RCC structure. Hence the cross section area of section and amount of steel for structural element reduced in composite structure than RCC structure so that large space meets for utilization.
CONCLUSIONS
In this paper a three dimensional model is analyzed using SAP 2000 software in terms of the structural characteristics of encased composite column and composite beam. It is concluded that:
The dead weight of composite structure is found to be 15% to 20% less than RCC structure and hence the seismic forces are reduced by 15% to 20%. As the weight of the structure reduces it attracts comparatively less earthquake forces than the RCC structure.
The axial force in composite columns is found to be 20% to 30% less than RCC columns in linear static analysis.
The shear force in composite column is reduced by 28% to 44% and 24% to 40% in transverse and longitudinal directions respectively than the RCC structure in linear static analysis.
The bending moment in composite column in linear static analysis reduces by 22% to 45%.
In composite beams the shear force is reduced by 8% to 28% in linear static analysis.
It also provides fire, corrosion resistance, sufficient strength, ductility and stiffness.
Hence Composite structure is one of the best options for construction of multistory building as well as for earthquake resistant structure.
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