Project 1_Comparative study of different storey buildings for Seismic forces

Question

Factors influencing the dynamic characteristics of a building

Factors influencing the Natural Period of a building

Factors influencing the Mode shape of oscillations.

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 base

Five storey benchmark Building: Elevation and plan of benchmark building showing the structural moment frame grid(All dimensions are in mm)

Buildings considered to illustrate concept of natural period: Details of 10 buildings considered.

Effect of stiffness on T : Compare fundamental natural periods of buildings E and F as well as G and H. Why is there a marginal or significant difference in the fundamental natural periods?

Effect of mass on T : Compare fundamental natural periods of buildings H,J and K. Have the buildings become more flexible or stiff due to change in mass?

Effect of Building Height on T : How does the fundamental natural periods of Buildings A,B,F and H change with in building height?

Effect of Column Orientation on T : How does the fundamental natural periods of Buildings B,C and D change with change in column orientation?

Ans:

The following building task is given and analysing about three point.

Project-1 Comparative Study of Different Story 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 characteristics of the earthquake shaking (beyond the control of an engineer) itself.
• The important dynamic characteristics of buildings are modes of oscillation and damping (assumed constant in most practical cases).
• A mode of oscillation of a building is defined by associated Natural Period and Deformed Shape in which it oscillates.
• Every building has a number of 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 largest 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 its horizontal translational oscillation along X and Y directions, respectively, and one fundamental rotational natural period Tθ1 associated with its rotation about an axis parallel to 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 base

Five storey benchmark Building: Elevation and plan of benchmark building showing the structural moment frame grid(All dimensions are in mm)

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 natural period. But, in the mathematical modelling of building, usually the building is discretized into a number of elements. The junctions of these elements are called nodes. Each node is free to translate in all the three Cartesian directions and rotate about the three Cartesian axes. Hence, if the number of nodes of discretization is N, then there would be 6N modes of oscillation, and associated with these are 6N natural periods and mode shapes of oscillation.
• Effect of Flexural Stiffness of Structural Elements on mode shapes: Compare fundamental mode shape of Building B in two situations when flexural stiffness of beams relative to that of adjoining columns is very small versus when it is large.
• Effect of Axial Stiffness of Vertical Members on mode shapes: Compare fundamental mode shape of Building H in two situations when axial cross-sectional area of columns is very small versus when it is large.
• Effect of Degree of Fixity at column bases on mode shape: Compare fundamental mode shape of Building B in two situations when base of columns is pinned versus when it is fixed.

We taken the above data check the Mode shape of oscillations, Natural Period of a building, dynamic characteristics of a building by using ETABS.

Introduction:

• The structural configuration of the building plays a major role in determining the overall response during an earthquake.
• Different building geometries behave differently when induced by the same seismic forces.
• Hence, it is very important to properly understand the response of buildings related to their structural configurations for a safer design approach.
• Therefore in this project, we have modelled different buildings with varying geometries using ETABS.
• Furthermore, their responses are analyzed with the help of the software and the results are then compared to grasp an understanding of the structural behaviour of buildings with different storeys.

Procedure:

• For an easier approach, this project is divided into two phases. In the first phase, we have covered the modelling part of the buildings
• where the structures are drawn from scratch as per the pre-specified geometrical and structural conditions.
• Moreover, running the analysis and generating the structural output with regard to all the buildings is also covered in the first phase itself.
• Moreover, a comparative analysis of the structural results obtained in the first half is executed in the second phase of this project.
• And based on the comparative results, various structural properties of the buildings pertaining to their seismic response are concluded.

Cases:

Modelling the Buildings and Generating their Respective Structural Outputs using ETABS

• There are a total of 10 buildings included in this project to achieve a more comprehensive conclusion.
• The buildings are listed in alphabetical order starting from A and ending at K.

A complete stepwise procedure to model all the buildings and generate their respective structural outputs is discussed in detail as follows:

1) Building - A

In model 1 having 2 storey building. which have 4 byes in x - direction and 3 byes in y- direction having a column 400x400mm and beam 300x400mm.

Step - 1) Open ETABS and Set up the Model

• Open ETABS from the desktop shortcut or by searching for it from the start menu.
• Once the software is up and running, click on the new model option.
• Once you click on the new model, a dialogue box will pop up where you can set up the codes and units to be used in your project.
• Thus, do the needful settings and click ok to start modelling the first building in this project.

Step - 2) Setting up the Grid Data

• To set up the grid data, you need to click on the custom grid option.
• Once you do that, a menu with all the grid settings will open up.
• In the same menu, you can fix the length, numbers of grids in both the orthogonal directions, spacing between all the vertical and horizontal grids, etc.
• So, by following the same approach, we have set up a 4 x 3 bay grid with 4m centre to centre spacing in-between.

Step - 3) Fixing the Story Data

• Similar to what we did while setting up the grids, click on the custom story data option.
• Then, specify the base level, plinth level, height of each floor, and the number of floors to be included in the model.
• Thus, we have arranged for a building with a 1.5m plinth height from the base.
• Also, you need to fix a master story and all the similar storeys in the same menu.
• And for our project, story one is the master and all other remaining floors are similar storeys.
• Once, you do this, your model is set to be executed in ETABS with the grid and storeys as specified.

Step - 4) Defining the Material Properties

• It is important to set details of the materials you are going to use for the project in ETABS.
• And for our model, we have to use the M30 grade concrete and Fe415 HYSD steel as per the specifications.
• So, in order to define the same, go to define → material → properties → add new → concrete/steel → grade → ok.
• However, since we have already specified the Indian Standard Codes of practices, the M30 concrete and 450 steel is added by default into the project.

Step - 5) Defining Section Properties 

• We are using the rectangular concrete sections for all the structural elements like beams, columns, and slabs.
• So, we need to define that in ETABS software so that it allows us to use these sections repeatedly throughout the project.
• And to do so, go to define → section properties → frame sections → concrete rectangular.
• Then, a dialogue box will appear on your ETABS screen where you can specify the column/beam dimensions and the type of reinforcement (HYSD415) to be used in the model.
• So, you can define both columns and beams from the same menu.
• Now, to define the slab section, again go to define → section → slab.
• Again a menu will open up where you need to set the slab specifications required for the project.
• Here, you need to specify the slab thickness as 150mm and the section as a membrane.

Beam 300X400mm

Column 400X400mm

Slab 150mm

Step - 6) Execute the Modelling

• Now that we have defined everything from materials to beams, columns, and slabs to be needed for our structure,
• we can go ahead and draw each structural element as per the specifications.
• And to do that, we can use the commands in ETABS named quick draw tool.
• As the name suggests, you can draw all the beams, floors, and columns with the help of this tool.
• All you need to do is select the relative tool that is quick draw columns for drawing columns and vice versa. Then choose the section from the dropdown list.
• Then, simply go to the grid system in plan view and start clicking on the grid locations where you want to build these structural elements.
• So, the same approach was followed and the following beams, floors, and slabs were drawn in our ETABS model.

Step -7) Define the Load Patterns

• After completing the model, now we need to list down the types of loads that are expected on the structure during its service period.
• And to do so, go to define → load patterns  → add loads.
• Here, you can add all the loads and for our project, we are having the dead load, super dead load (Brick wall load), live load, and the earthquake load in x and y directions.
• Also, you need to define the mass source by adding 100% of dead and super dead load as well as 25% of the imposed or live load.

Step - 8) Generate Load Combinations

• Go to define → load combinations → default combinations  → check the convertible option.
• A list of load combinations will appear which you can look at and modify anytime during the project if needed.

Step -9) Assign Loads to the Structural Members

• You can select a particular structural member from the model and assign it the load that is imposed on it.
• For example, select the beam and assign a udl of super dead load due to the brick walls resting upon it.
• Thus, a similar technique is used and the loads on beams and slabs are assigned as follows:
• Dead Load on beams from infill wall : 10kN/m
• Live load on Floor : 3kN/m2

Step - 10) Assign the Floor Diaphragm

• Select the S150 slab section from the select option.
• Go to assign  → shell → diaphragm.
• Select the default D1 diaphragm already incorporated in ETABS and press ok.
• It is now done, and you can see the centre of mass for our model has been automatically calculated as we assigned the floor diaphragm.

Step - 11) Run Analysis

• Now that everything with regards to our first model is complete, we can proceed to run the analysis for this building.
• But we'll have to first save our project before running the analysis so do it at the preferred location within your system.
• Then, you can simply press F5 on your keyboard or click on the arrowhead available on the ETABS screen to run the analysis for the current model.

Plan View

3D View

Modal Response for Building A

Time Period After Modifiying the Building - A

• Time Period - 0.5 second 
• It means structure is not having flexibility. Because time period is 0.5 sec.

Note:Building(B - H) Building data are in given below pdf file.