Project 1_Comparative study of different storey buildings for Seismic forces
Aim:
To perform a comparative study of different story buildings for seismic forces on factors such as dynamic characteristics, natural period and mode shape of oscillations.
Procedure:
i.Initial step is to create the grid using custom grid spacing option according the structural elements. Using custom story height, change the first story into plinth of height 1.5m and all the remaining story height as 3m. Add the story using keep the existing story height apply the height and No. of story required and rename them.
ii.Keep the story 1 as master story and make the remaining stories into similar to story 1. Create the Material properties from Define menu.Create the concrete as M30 and Steel as HYSD 415. Create the Beam size as 300mm x 400mm where 400mm as depth and 300mm as width. Beam size is identical for every building. Create the column dimension as 800 mm x 800 mm, 600 mm x 600 mm and 400 mm x 400 mm. Create the slab properties and keep the modelling type as membrane because to eliminate the flange behaviour and the thickness of slab is 150mm for every building.
iii.Create the load pattern (Wall as super dead load and EQX, EQY as seismic loads). In mass source, Mass multiplier for load patterns, assign Dead load as 100%, Live load as 25% and Wall load as 100%. From load combinations, select the concrete frame design.
iv.Save the model as Modal A and quick draw option draw the columns, beam and slab secions. Before applying slab sections, keep it in story 1 and change to similar stories.
iv.From select option, select the beams and assign the wall load as 10KN/m and in slab sections assign the live load as 3 KN/m. Add diapharms to the slab sections.
v.After assigning the loads, save the modal and analysis the modal. Once the analysis is completed, save as model into Modal B for which the story height and other factors are adjusted according to the requirement.
vi.Same procedure has to carried forward for all the models and create Modal A to Modal H and analysis results are compared as follows:
|
Criteria |
Model |
Analysis Result |
Conclusion |
|
Effect of Stiffness on Time Period |
Building E (10 story with varying column size) |
1.73 Seconds |
For 10 story building having varying and uniform column sizes does not much difference in time period because the height of building is not much. Whereas in 25 story building there is a significant change in time period. This is because in uniform column size the excess masses are lumped at the top story which induces greater lateral displacement and have lesser stiffness than the varying column size building. |
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Building F (10 story with uniform column size) |
1.73 Seconds |
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Effect of Stiffness on Time Period |
Building G (25 story with varying column size) |
4.17 Seconds |
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Building H (25 story with uniform column size) |
4.41 Seconds |
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Effect of Mass on Time Period |
Building H (25 story with 3 KN/m2 imposed load) |
4.41 Seconds |
As we know mass is directly proportional to lateral displacement. When the mass at the top story increases the lateral displacement also increases which results in longer time period. In this case the load on all the story are same and the difference between the loads on building (H, J, K) is uniformly varying i.e., 100%, 110% and 120% therefore the effect on time period is also not so significant. But if the stories have different imposed load i.e., bottom stories have lesser load compared to top stories then the lateral displacement will be more which leads to greater time period. |
|
Building J (25 story with 3.3 KN/m2 imposed load) |
4.42 Seconds |
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Building K (25 story with 3.6 KN/m2 imposed load) |
4.43 Seconds |
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Effect of Building Height on Time Period |
Building A (2 Story) |
0.50 Seconds |
Height of the building is directly proportional to time period. The height of building increases the time period also increases. From the analysis results it is clearly stated that the building with lesser story have lesser time period and time period is higher on building having more stories. |
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Building B (5 Story) |
1.03 Seconds |
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Building F (10 Story) |
1.73 Seconds |
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Building H (25 Story) |
4.41 Seconds |
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|
Effect of Column Orientation on Time Period |
Building B (All floor column size 400 x 400) |
1.03 Seconds |
In Building B, column size on both faces i.e., X and Y directions are uniform hence the response also have same range. Whereas in Building C column size in X-direction got increased and in Building D the column size got reduced in X-direction. This results in greater lateral displacement of floors in building D as compared to Building C in X-direction. Hence the column orientation significantly affects the lateral displacement of building which reflects the time period. |
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Building C (All floor column size 550 x 300) oriented in X-direction |
1.14 Seconds |
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Building D (All floor column size 300 x 550) oriented in Y-direction |
1.12 Seconds |
|
Criteria |
Model |
Case |
Analysis Result |
Conclusion |
|
Effect of Flexural Stiffness of Structural Elements on Mode Shapes |
Building B |
Flexural stiffness of beams close to Nil (Zero Inertia) |
2.62 Seconds |
In regular case applying 100% of flexural stiffness of beam which results in lesser time period whereas in other case considering only 10% of Moment of inertia on both the axis which produces greater lateral displacement results in greater time period. Since the beams provide the flexural stiffness to the building from lateral displacement, minimizing them will results in greater lateral displacement. |
|
Full flexural stiffness of beams (Regular) |
1.03 Seconds |
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Effect of Axial Stiffness of Vertical Members on Mode Shapes |
Building H |
Axial cross section for columns close to Nil (Zero axial area) |
5.99 Seconds |
In regular case applying 100% of axial stiffness of column which results in lesser time period whereas in other case considering only 10% of cross sectional area of columns which produces greater lateral displacement results in greater time period. Since the columns provide the axial stiffness to the building from lateral displacement and uplift, minimizing them will results in greater lateral displacement. |
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Full axial cross section for columns (Regular) |
4.41 Seconds |
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Effect of Degree of Fixity at Column Base on Mode Shapes |
Building B |
Column support pinned (Regular) |
1.03 Seconds |
As we know fixed support provides greater fixity then pinned support. Hence greater the fixity lesser will be the lateral displacement. From the analysis results it is clearly states that the pinned support building having lesser fixity than the fixed support building results in greater time period. |
|
Column support fixed |
0.94 Seconds |
Result:
As the comparative study of different story buildings for seismic forces on factors such as dynamic characteristics, natural period and mode shape of oscillations is performed.