Structural Modelling using Etabs 2018

Aim

• To conceptualize a framing plan layout and then model the given G+7 building situated in zone 5 in Etabs
• To develop the structural model that comply with provisions of IS 1893 and IS 13920.
• To run the modal response spectrum analysis and to 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 design and analysis.
• Modelling tools and templates, code base load prescriptions, analysis methods and solution techniques, all coordinated with the grid like geometry unique to this class of structure.
• Basic or advanced systems under static or dynamic conditions may be evaluated using ETABS.
• Fundamentals to ETABS modelling is the generalisation that multi-storey buildings typically consist of identical or similar floor plans that repeat in the vertical direction.
• Once modelling is complete ETABS automatically generates and assigns code-based loading conditions for gravity, seismic, wind and thermal forces. Users may specify an unlimited number of load cases and combinations.
• Outputs and display formats are also practical and intuitive. Moment, shear and axial force diagrams, presented in 2D and 3Dviews with corresponding data sets, may be organised into customisable reports.
• Here, in this challenge we are going to model an entire G+7 building with the floor plan using ETABS software.

Procedure:

Step 1)

1. We have to open the ETABS software and click on the new model option from the file menu.
2. Now in the model initiation window desired units, Design codes which are to be used was selected.

Step 2)

1. In New Model Quick templates under grid dimensions select the custom grid spacing and then click edit grid data.
2. The grid dimensions were inserted according to CAD file drawing as follows.

 

Step 3)

1. Now we have to enter the Story data as per cad files. (It's G+7 building)
2. Floor to floor height is given as 3m
3. For that click on New Model Quick templates under Story dimensions select the custom story data and
4. then click edit story data.
5. Now input the storey details as given in question and story 1 was selected the master story as and was applied to rest story from 2nd story to roof and then click OK.

Step 4)

1. Now we have to define material properties
2. For that Define the properties of elements - Define menu
3. Select material properties.
4. Once you select material properties a following dialog box will appear
5. Define the materialsAdd New material 
6. Select M25 grade of concrete 

Step 5)

1. Now we have to add section properties -
2. For that Click on Define Properties-Section Properties-frame sections options.
3. From the frame properties dialogue box click on add new property option.

1. Click on Add new Property
2. Select frame property shape type select the rectangular concrete option and click OK.

1. Beam details (250x450) was provided as shown in figure and then click on Modify/ Show reinforcement under reinforcement section.

1. Click on Modify/Show Rebar
2. Select M3 Design Only (Beam) and chose the rebar materials as shown and click Ok

1. Click on Modify/Modifier
2. The torsional constant was reduced to 0.001, The Moment of Inertia about 2 and 3rd axis was reduced to 0.35 respectively

1. Similarly the beam of size 300x500 is also added by following same steps

1. From property list we can see the columns of size 400x400 and 400x600 and Beams of size 250x450 and 300x500.are added
2. 

Step 6)

1. To design the slab initially for select slab section from the section properties under define menu

1. select add new property option
2. Slab properties like thickness and other details are added as shown below and then click ok
3. Give the thickness = 125 mm

Step 7)

1. Now we have to add the shear walls
2. For that click on Define- section properties-Wall sections
3. Assign the wall thickness as 300 mm

Step 8)

1. First we have to draw the columns by using quick draw column option

1. Next we need to add the beams using quick draw beams option as shown below

1. Next we need to add the secondary beams by using quick draw secondary beams option as shown below

1. Next we have to add the slab by using quick draw floor plan/walls
2. Next we have to add the shear walls in required locations as shown below
3. And add the balcony wherever required

Step 9)

• Now we go to the base plan and select all base joints
• Next we select the restraints from the joint option as shown below

 

• Select the connection as fixed

 

• 3D view after assigning all properties

 

Step 10)

1. Now we have to assign loads patterns
2. Select Define - by load patterns to open the Define load pattern dialogue box.
3. The various loads which are applied are as shown below

 

1. The direction and eccentricity were selected as the X direction
2. The Response Reduction Factor R was taken as 5
3. Seismic zone is 5 so Z=0.36
4. Soil type was taken as III
5. Importance factor for residential building with less than 200 is taken 1 and time period is taken as 0.5

Step 11)

1. Now we have to assign the loads to the strucure
2. Brick wall load -3.5 kn/m2

 

 

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

 

• Live loads are applied repectively.

 

Floor Finish

• 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.

Step 12)

• Live load reduction factor as per IS 875 code ii) For Building Above 10 Storey we have to reduced live load by 50%

 

Step 13)

• Now we have to assign Mass source
• For that Define menu - 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.
• For live load upto 3Kn/m2- we have to assumed some fraction of live load i.e 25%

 

Step 14)

• Now we haev to assign diaphragm
• 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.

 

Step 15)

• Now we have to add the load combinations

 

• Click on add default design combo

 

Step 16)

• Next we have to select the pier label from the shell option as shown below

 

• Before going to the shear wall design we have to assign the names for the as P1, P2 etc.

 

Step 17)

• Now we have to create a Modal analysis
• For that define tab-modal cases- and select modify
• Enter the data's as shown in image. Maximum number as 20.

 

• Here both spectrum analysis and equivalent lateral force are done at the same time

 

Step 18)

• New function was defined as per the IS 1893 for response spectrum analysis.

 

• After selecting the response spectrum the new tab is visible,
• In that we choose IS 1893:2016 from the "Choose function type" and click on the Modify/Show option as shown below

 

• Two load cases was defined for response spectrum analysis and set default scale

 

Step 19)

• Now we have to check the model for errors, go to Analysis tab and select check model.

 

Step 20)

• Now Run the model
• Click on Run Analysis

Step 21)

• 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.

 

Step 21)

• Modal Participation ratio -

1. Modal number check for that go to Display-Show Tables-Analysis Results-Structure Output-Modal Information-Modal mass participation ratio.
2. According to Is 1873 clause 7.1, the natural period corresponding to fundamental torsional mode of oscillation
3. 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.
4. IS 1873-clause 7.1, table 6 the first three modes contribution should be more than 65% of mass participation factor in each principal plan direction. Here it is 0.738>0.65.
5. 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.

 

Step 22)

Storey response plot:

1. For this go to Display-Story Response Plot-Maximum Storey drift
2. Here storey response for maximum storey drift is less than 0.004 for both X and Y

 

Base Shear Checking:

1. click on Load Display >Load Type as Eqx > Show Fx load.
2. From code- shear wall shoud 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 column under tention:

1. 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.