Structural Modelling using Etabs 2018
Structural Modelling using Etabs 2018
AIM:
- To design a structural model on the basis of provisions from IS 1893 and IS 13920 in seismic zone 5.
- To Run a modal response spectrum analysis and check analysis results of the Etabs model for inter-storey drift requirements, vertical & planar irregularities, minimum shear wall requirements, maximum axial load ratio for column and shear walls, effective moment of inertia for beams and columns etc.
INTRODUCTION:
ETABS is an engineering software product that caters to multi storey building analysis. Etabs consists of Modelling tools and templates Code based load prescriptions, analysis methods etc.
PROCEDURE:
Initially open the ETABS application and click on the new model option from the file menu.
Now in the model initiation window desired units, Design codes which are to be used was selected and then click OK.
In New Model Quick templates under grid dimensions select the custom grid spacing and then click edit grid data.
The grid dimensions were inserted according to CAD file drawing as follows and click OK.
In New Model Quick templates under Story dimensions select the custom story data and then click edit story data.
Now input the storey details as given in question and story 1 was selected as the master story and was applied to rest story from 2nd story to roof and then click OK.
Now to define the properties of elements, go to define menu and then select material properties. The grade of concrete was selected and click ok.
Steel(HYSD 500 and HYSD415) and Concrete (M25) properties was added to the list now as shown and then click OK.
To add frame members like beams, columns and slab go to Define Properties- Section Properties- frame sections options. From the frame properties dialogue box click on add new property option.
From the frame property shape type select the rectangular concrete option and click OK.
Beam details (250X450) was provided as shown in figure and then click on Modify/ Show reinforcement under reinforcement section.
Beam reinforcement data’s:
Select M3 Design Only (Beam) and chose the rebar materials as shown and click OK.
The torsional constant was reduced to 0.001, The Moment of Inertia about 2 and 3rd axis was reduced to 0.35 respectively
The beam of size 300x500 was also designed as per the same steps above
In the similar manner apply the column data’s (400X400) as shown below
The Moment of Inertia about 2 and 3rd axis was reduced to 0.75 respectively
In the similar way 400X 600 column was also detailed.
From the list we can see the columns of size 400x400 and 400x600 and Beams of size 250x450 and 300x500.
To design the slab initially For Select Slab section from the section properties under Define menu.
Select add new property option.
Slab properties like thickness and other details are added as shown below and then click OK.
In the similar manner a slab of 200mm thickness was made for the staircase.
Walls:
For that click on Define followed by section properties- Wall sections-Wall 3000mm
So all the properties of frame members have been added correctly, now we have to place the structural members. For this select quick draw column option from the tools available at the left side of the screen and place the columns at all the grid intersections. Also select all storeys at the right bottom of screen to apply it to all storeys.
After placing columns, Beams (the quick draw beams option beams) was placed followed by Shear walls and slabs (Quick draw floor option slabs).
For placing secondary beams functions like replicate, move, mirror etc was used.
Now to Change the base connections initially go to the base view in the plan and select the base view. Now in assign tab- select joints- restraints. Now from the dialogue box that appears select fixed connections and then click OK.
To assign loads to the structure, select define followed by load patterns to open the Define load pattern dialogue box.
The various loads which are applied are as shown below
The direction and eccentricity were selected as the X direction
The Response Reduction Factor R was taken as 5
Seismic zone is 5 so Z=0.36
Soil type was taken as III
Importance factor for residential building with less than 200 is taken 1 and time period is taken as 0.5
In order to assign the loads, select the frame in which brick wall falls by using the CAD drawings and provide 3.5kn/m^2
Live loads were applied to slabs next.
As per IS 875 in residential buildings its 2kn/m^2 for rooms and bathrooms whereas its 3Kn/m^2 for staircase, lift and balcony
In the same way select slabs to assign floor finish which is 1.25Kn/m^2 for rooms and bathrooms whereas its 3Kn/m^2 for staircase balcony
Live load reduction factor as per IS 875 code
To assign mass source to model follow these steps.
In define menu select mass source options to open the mass source dialogue box.
Now select Modify mass source data option and click OK
Now mass source data was provided as shown and load pattern was also added to it.
To assign diaphragm to the model follow these steps:
In assign menu select Shell followed by Diaphragm to open the shell Assignment diaphragm dialogue box
Select D1 from the diaphragm assignments and click apply followed by OK.
The Diaphragm D1 is applied and is as shown below.
Load combinations was defined for both frame and shell.
To create the modal analysis, go to define tab- modal cases- and select modify and enter the data’s as shown in image. Model number was taken as 20.
Here both spectrum analysis and equivalent lateral force are done at the same time
New function was defined as per the IS 1893 for response spectrum analysis.
Two load cases was defined for response spectrum analysis and set default scale factor initially
To check the model for errors, go to Analysis tab and select check model.
In order to obtain the new scale factor, go to display- show table- structural output- base shear.
As per IS 1893 design base shear should not be less than design base shear, If so then scale factor needs to be multiplied with calculated base shear by estimated base shear.
Modal number check for that go to Display-Show Tables-Analysis Results-Structure Output-Modal Information-Modal mass participation ratio.
According to IS 1873 clause 7.1, the natural period corresponding to fundamental torsional mode of oscillation should not be greater than sum of two fundamental translational mode of oscillation (Tx+Ty>Txy)-from table 1.25+1.115=2.365>0. 832. hence no torsional irregularity.
- IS 1873-clause 7.1, table 6 the first three model contribution should be more than 65% of mass participation factor in each principal plan direction. Here it is 0.738>0.65.
Here Tx=1.254 and Ty=1.115, differ by 12%, hence conclude that building is regular in mode of oscillation in two principal plan direction.
- From modal number, maximum required is 13 number as it captures more than 90% of mass in all 6 degrees of freedom.
Storey response plot:
For this go to Display-Story Response Plot- Maximum Storey drift
Here storey response for maximum storey drift is less than 0.004 for both X and Y
Base Shear Checking:
click on Load Display >Load Type as Eqx > Show Fx load.
From code- shear load should take at least 80% of the load.
In order to find the percentage,
3174/3800-83.5%>80%, provided shear wall is sufficient in x-direction.
3181/3800=83% which is greater than 80% so the shear wall provided in Y direction is sufficient.
Check for Columns under tension:
Under loading conditions 4b and 4d are found to be under tension.
check for aspect ratio for both column and shear wall.
Column:
The aspect ratio should be less than 0.4.(aspect ratio=force/cross section area*fck).
considering first load combination(1.5DL),force 1580kN.
aspect ratio=1580*1000/400*600*30-0.22< 0.4, hence ok.
Shear wall:
The aspect ratio should be less than 0.28.
Force is 7.22kN.
Aspect ratio=7.22*1000/1200*300*25=0.08-0.28, hence ok.
RESULT:
The structural model of the G+7 model is designed for given architectural planning using Etabs.
Frame model which satisfies both code provision are analysed.
Columns, shear walls and beam are provided in possible symmetrical way without much offsets.
Analyses is done for modal number, base shear storey drift aspect ratio, irregularity etc.,
As per the analyses result, building is bit flexible in y-axis, as shear wall are sufficient add additional column as provided in y-direction to meet the maximum storey drift limit.