Report - PEER - University of California, Berkeley

Report - PEER - University of California, Berkeley Report - PEER - University of California, Berkeley

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4.2 Incremental AnalysisBase shear versus roof displacement for the longitudinal direction of the building,calculated from applied uniform and inverted triangular distributed loads are shown infigure 4. Additional marks are included in the figure to show the maximumdisplacement demands for the records, calculated with a non-linear dynamic analysis,and the results of an elastic linear analysis with the elastic spectrum of the ChileanCode.Base Shear [ T ]1500120090060030000,00 0,55 1,10 1,65 2,20Shear failure incolumnsLateral Displacement/building height %Elastic response NCh433.Of96Ultimate capacitycolumn 7- 2th storyExpected displacement Viña del Mar (0,55g)Expected displacement Llolleo N10E0 10 20 30 40Lateral Displacement [cm]120967248240Base Shear [% weight]Figure 4. Basal shear vs. displacement of the roof.5. SHEAR STRENGTH IN COLUMNSOnly the concrete contribution, Vc, to shear resistance of columns was considered inthe analysis. Since shear resistance depends on axial forces, their magnitude wasobtained from the Llolleo record response, considering 2% of damping ratio. Withthese values available, shear resistance varies between 43 [tons] and 22 [tons]. Theavailable shear resistance of the existing columns, calculated with ACI 318-99, doesnot allow the columns to reach their flexural capacities.Since ACI 318 is conservative, to estimate the shear in columns that do not suffertension axial forces, the internal column shear resistance was also calculated withexpressions obtained experimentally by Ang, Priestley and Paulay [2],[3]. In this casethe contribution of the concrete to the strength of the shear can be estimated as:438

Vc⎛ P ⎞⎜3= 0.37⋅α⋅ 1+⎟ ⋅ f 'fcA⎝ ' ⋅g ⎠⋅ Ag[ MPA]Eq.(1)where:α =⎜⎛ M⎝2( V ⋅ D)≥ 1⎟⎞⎠Eq.(2)P is the axial compression force, Ag the gross area, Ag the effective area (0,8⋅Ag), Mthe moment, V the shear and D the diameter of the confined concrete area.[story]Shear in column D 2Desing Shear ACI 318Shear Capacity ACI 318Shear Capacity according to Ang.....543210-150 -100 -50 0 50 100 150[ton]Shear in column D 2Desing Shear ACI 318Shear Capacity ACI 318Shear Capacity according to Ang...543210-150 -100 -50 0 50 100 150[ton][story]Llolleo, β = 2% Viña del Mar, β = 2%Figure 5. Shear capacity and shear demands in columns.Column shear capacity of existing columns is compared in figure 5 to the sheardemands of the considered records.The exterior columns, which could be in tension during an earthquake, wouldhave less shear resistance than the interior ones, which remain under compressionduring the whole response. The record of Viña del Mar S20W scaled to 0.55g has adisplacement demand on the structure of 14.1cm (0.8% of height). If columns at theperimeter are not reinforced with jackets, they could resist the earthquake without ashear failure only for displacements below 2.5 [cm]. Therefore, all exterior columnsof the building and in all floors must be reinforced.An option to meet the required shear strength, as indicated in ACI 318 -Chapter21, would be to add jackets to the columns with reinforced concrete, steel plates orcarbon fibers.439

4.2 Incremental AnalysisBase shear versus ro<strong>of</strong> displacement for the longitudinal direction <strong>of</strong> the building,calculated from applied uniform and inverted triangular distributed loads are shown infigure 4. Additional marks are included in the figure to show the maximumdisplacement demands for the records, calculated with a non-linear dynamic analysis,and the results <strong>of</strong> an elastic linear analysis with the elastic spectrum <strong>of</strong> the ChileanCode.Base Shear [ T ]1500120090060030000,00 0,55 1,10 1,65 2,20Shear failure incolumnsLateral Displacement/building height %Elastic response NCh433.Of96Ultimate capacitycolumn 7- 2th storyExpected displacement Viña del Mar (0,55g)Expected displacement Llolleo N10E0 10 20 30 40Lateral Displacement [cm]120967248240Base Shear [% weight]Figure 4. Basal shear vs. displacement <strong>of</strong> the ro<strong>of</strong>.5. SHEAR STRENGTH IN COLUMNSOnly the concrete contribution, Vc, to shear resistance <strong>of</strong> columns was considered inthe analysis. Since shear resistance depends on axial forces, their magnitude wasobtained from the Llolleo record response, considering 2% <strong>of</strong> damping ratio. Withthese values available, shear resistance varies between 43 [tons] and 22 [tons]. Theavailable shear resistance <strong>of</strong> the existing columns, calculated with ACI 318-99, doesnot allow the columns to reach their flexural capacities.Since ACI 318 is conservative, to estimate the shear in columns that do not suffertension axial forces, the internal column shear resistance was also calculated withexpressions obtained experimentally by Ang, Priestley and Paulay [2],[3]. In this casethe contribution <strong>of</strong> the concrete to the strength <strong>of</strong> the shear can be estimated as:438

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