# Closely spaced perimeter columns primary beams rc

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Closely-spacedPerimeter ColumnsPrimary BeamsRCCoreGravityColumnsPrimary BeamsSecondary BeamsTertiary BeamsLarge lever arm betweenperimeter column pairsPerimeterBeams
105(a)(b)Figure 3.55:Comparison of Structural Systems: (a)TraditionalFrame System with a central core, and(b)TubeSystem
106Figure 3.56:Concrete Core in Tube Systems: Bernoulli action absent near the base of the CoreConsider a square plan 160m tall building with 40m each plan side, having 40 storeys eachof 4m. The columns are hinged at the base of the building. For the sake of simplicity, all columns,beams and slabs in the building are of the same size in all storeys in this example. Two structuralsystems are considered, namely thetraditional systemsand thetube systemsimilar to those shown inFigure 3.55. Sizes are listed in Table 3.10 of columns, beams, slabs and RC cores in these twostructural systems. Both buildings are subjected to a lateral load of 1% of the weight of the building;the distribution of this lateral force along the height is parabolic (as per IS:1893 (Part 1) – 2007).Table 3.10: Sizes of structural members in example 40-storey tall buildings consideredItemTraditional Frame SystemTube SystemPlan GeometryNumber of Column in plan3292Size of all Columns (all storeys)1500 mm × 1500 mm1200 mm × 1200 mmSize of all Beams (all storeys)1000 mm × 1400 mm300 mm × 800 mmThickness of RC Core Walls1500 mm1200 mmCorners draw more forcethan centers(near base of building)Plan of CoreElevation of Core5 @ 8m5 @ 8m
107TheStructural Plan Density(considering both columns and core walls) of the TraditionalFrame Building is 6% and that of the Tube Building 9.48%. The share in the SPD is 75% of framecolumns and 25% of core walls in the Traditional Frame Building, and 87% of frame columns and13% of core walls in the Tube Building. Thus, the share of column members is more in the TubeBuilding. Linear static analyses of the two buildings show that(1)in the Traditional Frame Building, columns take only 7% of the lateral shear as against corewalls that take the remaining 93%; and(2)in the Tube Building, columns takeincreased44% of the lateral shear as against thereducedremaining 56%.Thus, increasing the structural plan density allowed more columns to be provided in the TubeBuilding. Even though the relative area share of the columns increased from 75% to 87% (i.e.,byabout 12%) in the Tube Building, the share of the lateral shear increased significantly from 7% to44% (i.e.,by about 37%). This shift of large shear from the inner core to the perimeter tube isattributed primarily to the different and more efficient structural system in the Tube Building.Another important behavioural aspect of these two structural systems is theshear lag effectincolumn axial forces. Under the earthquake induced lateral inertia forces, the axial forces areexpected to be uniform inallcolumns on the leeward face of the building. But, it is not so in normalbuildings (Figure 3.57). There is difference in axial forces amongst columns on the leeward and onthe windward faces; the corner columns have larger axial force than the interior columns. Inaddition, farther the spacing of columns, the larger is the difference in column axial forces. Thus,buildings withtraditional frame structural systemhave larger shear lag effect than buildings withtubestructural system. The earthquake induced lateral inertia force is carried most by the stiffer frames. In

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Earthquake engineering