Project 1_Comparative study of different storey buildings for Seismic forces
Here we study the factors influencing the dynamic characteristics, natural period , mode shape of oscillations of different buildings and compare their analysis results with respect to seismic forces. We are given 10 buildings with different structural configurations. They are:
MODEL A - 2 Storey building with column dimensions 400x400
MODEL B - 5 Storey building with column dimensions 400x400
MODEL C- 5 Storey building with column dimensions 550x300 oriented along x- direction.
MODEL D- 5 Storey building with column dimensions 300X550 oriented along Y- direction.
MODEL E - 10 Storey building with column dimensions 400X400 upto bottom 5 storeys and 600x600 column in top 5 storeys.
MODEL F - 10 Storey building with column dimensions 600x600
MODEL G - 25 Storey building with column dimensions 400X400 in upper 5 storeys , 600x600 column in middle 10 storeys, and 800x800 in bottom 10 storeys.
MODEL H - 25 Storey building with column dimensions 800x800
MODEL J - 25 Storey building with column dimensions 800x800 with imposed mass larger than model H by 10%
MODEL K - 25 Storey building with column dimensions 800x800 with imposed mass larger than model H by 20%.
Effect of stiffness on T: Comparing the fundamental natural periods of buildings
MODEL E [1.73 seconds] MODEL F [1.73second] vs MODEL G [4.17 seconds] MODEL H [4.41second] - Comparing these we find that varying column sizes are more stable than uniform sized columns. As mass is directly propotional to the lateral load acting on the building that induces more lateral displacement leading to greater time period. Model E and F does not have that much difference but more no. of storeys can be a game changer.
b) Effect of mass on T: Comparing fundamental natural periods of buildings
MODEL H [4.41 seconds] MODEL J [4.42second] MODEL K [4.43 seconds] - There is not significant change in the time periods as because the mposed loads were uniform in all storeys. Change in mass will change the time period but a significant change can be seen if the imposed loads were only on the top storeys.
c) Effect of Building Height on T: Comparing the fundamental natural periods of Buildings
MODEL A [0.5 seconds] MODEL B [1.03 second] MODEL F [1.73 seconds] MODEL H [4.41second] - As height will increase time period will also increase because height of building is directly propotional to time period as per IS code.
d) Effect of Column Orientation on T: Comparing the fundamental natural periods of Buildings with change in column orientation
MODEL B [1.03 seconds] MODEL C [1.145second] MODEL D [1.12 seconds] - In model B we have same column size throughout, analysing its mode shape changes, the first mode deflection is in x direction and 2nd mode is in y direction. Because of same size it will have approximate same response in both directions. Comparing model C and D , the column orientation in changed , here, the lateral deflection in D is higher than C in X direction.
e) Effect of Flexural Stiffness of Structural Elements on mode shapes: Compare fundamental mode shape of Building B in two situations
MODEL B [1.03 seconds] with full flexural stiffness , MODEL B [2.627 second] when flexural stiffness of beams close to nil - Beam is very important structural element in resisting lateral forces. Comparing these two situations gives almost double time period. If stiffness property of beam is not taken into account we get double of the actual time period.
f) Effect of Axial Stiffness of Vertical Members on mode shapes: Compare fundamental mode shape of Building H in two situations
MODEL H [4.41 seconds] with full axial cross section of columns , MODEL H [5.99 second] with axial cross section close to nil - After changing axial cross section to nil , the deformation was very large that its topmost storey came down to 24th . The axial stiffness is very vital for the strength and upliftment of the structure.
g) Effect of Degree of Fixity at column bases on mode shape: Compare fundamental mode shape of Building B in two situations
MODEL B [1.03 seconds] with pinned support , MODEL B [0.941 second] when support changed to fixed - Comparing the situatons, we get lesser time period for fixed support , that means , more fixed support will reduce time period , hence less deflection encountered.
Below are the etabs file attached of all the models with screenshots of the analysis reports.