Project 1_Comparative study of different storey buildings for Seismic forces
Factors influencing the dynamic characteristics of a building:-
* Buildings oscillate during earthquake shaking. The oscillation causes inertia force to be induced in the building.
* The intensity and duration of oscillation and the amount of inertia force induced in a building depend on features of buildings, called their dynamic characteristics, in addition to the parts of the earthquake shaking (beyond the control of an engineer).
* The essential dynamic characteristics of buildings are modes of oscillation and damping (assumed constant in most practical cases). A mode of oscillation of a structure is defined by the associated Natural Period and Deformed Shape in which it oscillates.
* Every building has several natural frequencies (how many?), at which it offers minimum resistance to shaking induced by external effects (like earthquakes and wind) and internal effects (like motors fixed on it).
* Each of these natural frequencies and the associated deformation shape of a building constitute a Natural Mode of Oscillation.
* The mode of oscillation with the smallest natural frequency (and most significant natural period) is called the Fundamental Mode; the associated natural period T1 is called the Fundamental Natural Period.
* Regular buildings held at their base from translation in the three directions have two fundamental, translational natural periods, Tx1 and Ty1, associated with their horizontal translational oscillation along X and Y directions, respectively, and one entire rotational natural period, Tθ1 associated with its rotation about an axis parallel to the Z axis.
Factors influencing the Natural Period of a building:-
Structural Element Sizes
- Beams: 300x400mm
- Columns: 400x400mm
- Slab: 150mm thick
Material Properties
- Grade of concrete: M30
- Grade of steel Reinforcement Bars: Fe415
Loading
- Dead Load on beams from infill wall: 10kN/m
- Live load on Floor: 3kN/m2
(what is the seismic mass?)
Buildings are assumed to be pinned at the base.
Five-storey benchmark Building: Elevation and plan of benchmark building showing the structural moment frame grid(All dimensions are in mm)
Note
1. Bay length in each plan direction is 4m(center to center).
2. All columns at each story are of the same size.
3. All beams in all buildings are of the same size(300mmx400mm).
Buildings considered to illustrate the concept of natural period: Details of 10 buildings considered.
* Effect of stiffness on T: Compare fundamental natural periods of buildings E and F and G and H. Why is there a marginal or significant difference in the entire natural period?
* Effect of mass on T: Compare fundamental natural periods of buildings H, J, and K. Have the facilities become more flexible or stiff due to change in Mass?
* Effect of Building Height on T: How do the fundamental natural periods of Buildings A, B, F, and H change within building height?
* Effect of Column Orientation on T: How do the fundamental natural periods of Buildings B, C, and D change with a change in column orientation?
Factors influencing the Mode shape of oscillations:-
* Mode shape of oscillation associated with a natural period of a building is the deformed shape of the building when shaken at the natural period. Hence, a building has as many mode shapes as the number of natural periods.
* For a building, there are infinite numbers of the natural period. But, in the mathematical modeling of structure, the structure is usually discretized into several elements. The junctions of these elements are called nodes. Each node can translate in all three Cartesian directions and rotate about the three Cartesian axes. Hence, if the number of discretization nodes is N, then there would be 6N modes of oscillation, and associated with these are 6N natural periods and mode shapes of change.
* Effect of Flexural Stiffness of Structural Elements on mode shapes: Compare the fundamental mode shape of Building B in two situations when the flexural stiffness of beams relative to that of adjoining columns is minimal versus when it is significant.
* Effect of Axial Stiffness of Vertical Members on mode shapes: Compare the fundamental mode shape of Building H in two situations when the axial cross-sectional area of columns is very small versus when it is significant.
* Effect of Degree of Fixity at column based on mode shape: Compare the fundamental mode shape of Building B in two situations when the bottom of columns is pinned versus when it is fixed.
Aim:-
Structural Element Sizes
- Beams: 300x400mm
- Columns: 400x400mm
- Slab: 150mm thick
Material Properties
- Grade of concrete: M30
- Grade of steel Reinforcement Bars: Fe415
Loading
- Dead Load on beams from infill wall: 10kN/m
- Live load on Floor: 3kN/m2
(what is the seismic mass?)
Buildings are assumed to be pinned at the base.
Five-storey benchmark Building: Elevation and plan of benchmark building showing the structural moment frame grid(All dimensions are in mm)
Note
1. Bay length in each plan direction is 4m(center to center).
2. All columns at each story are of the same size.
3. All beams in all buildings are of the same size(300mmx400mm).
Buildings considered to illustrate the concept of natural period: Details of 10 buildings considered.
We took the above data to check the Mode shape of oscillations, the Natural Period of a building, and the dynamic characteristics of a building by using etabs.
Procedure:-
* Open the etabs software.
* Click on the new model.
* Model installation dialog box shows.
* In this dialog box, select the use built-in settings option to change the display units as metric SI, steel section database as Indian, Steel design code as IS800:2007, and Concrete design code as IS456-2000, then click on ok.
* Next, a new model quick templates dialog box will appear in these dialog box grid dimensions below. Select custom grid spacing, then click on edit.
* Another dialog box will appear, i.e., Grid System Data. Change the x and y grid data values in this dialog box, then click ok.
* In the new model quick templates dialog box, click on custom store data in the story dimension below, then click on edit story data.
* Another dialog box will appear, i.e., Story data change the story names and height master story similar to options, then click ok.
* Right-click on any story, then click on add story again and click on keep existing story heights.
* Add story dialog box will appear to change the values and enter the story number(i.e., floors), then click ok.
* In the story data dialog box, story 25 details are shown. Change the similar options, then click on ok.
* Model will be shown.
* Goto define tab; click on a material property to determine the material dialog box will appear in the define tab material property determine material dialog box will appear. In this dialog box, click on add new material, select material, then click ok.
* Go to section properties in the defined tab and click on frame sections.
* Frame properties dialog box will appear in the dialog box. Click on add a new property. Another dialog box shows select and given sizes and material of beams and columns, then click on ok.
* In the defined tab, go to section properties, and click on slab sections.
* The slab property data dialog box will appear in the slab sections, then change the properties.
* In the defined tab, go to section properties and click on load patterns.
* Provide the loads and properties in the define load patterns dialog box.
* In the defined tab, go to section properties and click on load combination.
* Select concrete frame design in the load combination dialog box, then click ok.
* Select quick draw columns, then change the property and assign it to the model.
* Select quick draw beams, then change the property and assign it to the model.
* Select the first story in the plan view, then click ok.
* Select quick draw slab, then change the property and assign it to the model.
* Goto select the tab, click on the select option, and select properties dialog box will appear. Select the beam and column assigned to the model.
* Goto select the tab, click on the select option, select frame load assignment(distributed), select the load pattern, and given the load value, click on ok.
* Goto select the tab, click on the select option, select frame load assignment select the slab section, and given the load value, click ok.
* Goto select the tab, click on select option shell load assignment(uniform), select the live load section, and given the load value, click ok.
* In the assigned tab, select diaphragm select D1, then ok the load shown.
* Save the model.
* In the analyze tab, click on run analysis.
* The given question verify in the result.
1. BUILDING "A" (G+2)
2. BUILDING "B" (G+5)
3. BUILDING "C" (G+5 with column orientation in X-direction)
- Create a Copy Model by Save As
- As we did for building B, now do it for building C through building B.
- Choose the appropriate location and save the file as building c.
- Now, the model is set to be edited as needed.
4. BUILDING "D" (G+5 WITH COLUMN ORIENTATION IN THE Y-DIRECTION)
In this model, we don't need to alter anything other than the column orientation. The column sizes will remain the same. However, we have to shift the column orientation from X to Y direction. So follow the same procedure as we did for the previous model and save it as the C file into the D file. Then, select the column section and assign the local axes and bring the origin back to 0. This way, all the columns shall be oriented in the Y-direction. Then like usual, check for the loads and the diaphragm before running the analysis. Then, could you note the modal period for building D?
5. BUILDING "E" (G+10)
Column sizes for top 5 stories: 400x400
Column sizes for bottom five stories: 600x600
6. BUILDING "F" (G+10)
Consistent Column size throughout the structure: C600X600
PROCEDURE:
This building has the same columns for the entire structure. Therefore, we don't need to do much editing regarding the sections. We need to adjust the view settings>hide beams and floors>select all the columns> and assign them the frame section properties of C600x600.
7. BUILDING G (G+25)
Columns for top 5 stories: C400X400
Columns for middle ten stories: C600x600
Columns for bottom ten stories: C800x800
PROCEDURE:
We must add 15 new stories for this model and assign different column sizes on other floors. We can follow the exact procedure that has already been explained for the previous project to create this model by editing building F. The same is discussed below:
8. BUILDING "H" (G+25)
Consistent column size throughout the building: C800X800
PROCEDURE:
This is the same model as the previous one. All we need to do is assign the column section as C800x800 for the entire structure wh; ich can be done by simply selecting all the columns>go going assign> selecting C800x800>cli clicking and done. Thus the same procedure is listed down below for beta ter understanding.
9. BUILDING "J" (G+25)
10% increment in the imposed loads
PROCEDURE:
Building J is the exact copy of model H; we don't need to alter anything related to the modeling. However, we need to increase the imposed load on the building by 10%, which is assigned as 3KN/m^2 on all building slabs. So, the procedure to do the same is listed below:
10. BUILDING "K" (G+25)
20% increment in the imposed loads
PROCEDURE:
For this model, we can follow the exact procedure as we did for building j. However, we need to increase the live load by 20% instead of 10% this time. So, the stepwise procedure has been listed down below:
Models Periods and Frequencies
