Structural Modelling using Etabs 2018
The architectural drawings for a G+7 residential building in zone 5 has been provided. The floor plan is the same for all 7 floor levels. Possible column positions have also been provided. Conceptualize a framing plan layout and then model the building in Etabs. The following architectural drawings have been provided:
- Typical floor plan – The location and thickness of the brick walls and the use of the
space (for estimating live load) can be obtained from this plan - A building section through the staircase and lift core is provided
- Parking layout in the ground floor – the column/shear wall arrangement must not
obstruct entry/exit of vehicles.
The following loadings are to be taken into account: - Brickwall made of AAC blocks
- Finishes – floor tiles of 75 mm thickness
- Live loads as per IS 875 part 2
- Water tank of 20 KL above the staircase at Mumty level
- Lift Machine room at mumty level above lift core
-
The structural model must comply with provisions of IS 1893 and IS 13920.
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.
Please note that the column positions shown in the architectural typical floor plan are only to provide an initial starting point for modelling. Participants will have to change its size, introduce shear walls, and add new vertical elements after running the analysis. You will have to run the analysis multiple times until a structural model is obtained that meets structural code requirements without compromising architectural functions. Also, there is no one ‘correct’ structural system for this challenge. There can be multiple possible solutions for this problem.
Aim :- i) To design a structural model on the basis of provisions from IS 1893 and IS 13920 in seismic zone 5.
ii) To Run a modal response spectrum analysis and check analysis results of the Etabs .
Introduction:-
1) 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.
2) For a sophisticated assessment of seismic performance, modal and direct-integration time-history analyses may couple with P-Delta and Large Displacement effects.
3) Nonlinear links and concentrated PMM or fiber hinges may capture material nonlinearity under monotonic or hysteretic behavior.
4) Dynamic considerations may include modal, response-spectrum, or time-history analysis. P-delta effect account for geometric nonlinearity.
5) Analysis capabilities then offer advanced nonlinear methods for characterization of static-pushover and dynamic response.
Procedure: -
Step 1): Open Etabs:
- Click on ETABS SOFTWARE then ul of software is open shown below
STEP 2: Model initialization:
- 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.
STEP 3: To draw grids and storeys:
- 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 .
- in that click on CUSTOM GRID SPACING and click on EDIT GRID DATABASE
- In this box we set the 5 no of grids in x direction and 5 no of grids in y direction having distance is 4m. - for adding new grid click on add
- as shown below - click ok
STEP 4:
- Now we have to enter the Story data as per cad files. ( It's G+7 building)
- Floor to floor height is given as 3m
- For that click on 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 add mummty level then click OK.
- After that plan and eleveation of each story are shown
- shown below
STEP 5: Add material properties
- Now we have to define material properties
- For that Define the properties of elements - Define menu
- Select material properties.
- Once you select material properties a following dialog box will appear
- Define the materials - Add New material -
- Select M25 grade of concrete -
- Similarly for Steel select Rebar Steel(HYSD 500 and HYSD415) properties was added to the list now as shown and then click OK.
- Material Properties is added
STEP 6: Add section properties of column and beam:
- Now we have to add section properties -
- For that Click on Define Properties- Section Properties- frame sections options.
- From the frame properties dialogue box click on add new property option.
- Click on Add new Property
- Select frame property shape type select the rectangular concrete option and click OK.
- Beam details (300X500) was provided as shown in figure and then click on Modify/ Show reinforcement under reinforcement section.
- Click on Modify / Show Rebar -
- Select M3 Design Only (Beam) and chose the rebar materials as shown and click OK.
- Click on Modify / Modifier
- The torsional constant was reduced to 0.001, The Moment of Inertia about 2 and 3rd axis was reduced to 0.35 respectively
- Similarly the beam of size 250x450 is also added by following same steps
- In the similar manner Column properties is also added by following same steps
- Column 400 x 600 is added as shown below
- In case of Column the Moment of Inertia about 2 and 3rd axis was reduced to 0.75 respectively .
- In the similar way 400X 400 and 900x 500 column was also detailed.
- From property list we can see the columns of size 400x400 and 400x600 and 500x 900 and Beams of size 250x450 and 300x500. are added
STEP 7: Add slab section
- 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.
- Give the thickness =125mm and 150 mm
- Similarly a slab of 200mm thickness is also added for the staircase.
STEP 8: Add shear wall section:
- Now we have to add the Shear walls
- For that click on Define - Section properties- Wall sections-
- Assign the Wall thickness as 3000mm
STEP 9:Draw quick columns:
- First we have to draw the columns by using Quick draw column option - Select properties according to required column
- Change orientation of some column - select columns then follow following process
- Repeat process for beam Next we need to add the beams using quick draw beams option as shown below
- Next we need to add the secondary beams by using quick draw secondary beams option as shown below
- Next we have to add the slab by using Quick draw floor plan / walls
- Next we have to add the shear walls in required locations as shown below
- And add the column shear wall balcony wherever required
STEP 10: Apply support
- 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 Base connection as fixed
- 3D view after assigning all properties -
STEP 11: Define load pattern
- Now we have to assign loads patterns
- Select Define - 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
- Rpeat process for other eartquake forces
STEP 12: Assign brickwalll load
- Now we have to assign the loads to the strucure -
- Brick wall load - 5 kn/m2
- Live loads were applied to slabs next.
- As per IS 875 in residential buildings its 2.5kn/m^2 for rooms
- Live loads are applied repectively .
3) Floor Finish
- In the same way select slabs to assign floor finish which is 1.5Kn/m^2 for rooms and
STEP 13: Live load reduction factor:
- Live load reduction factor as per IS 875 code
- For Building Above 10 Storey we have to reduced live load by 50%
STEP 14: Define mass source
- 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 15: Assign diaphragm
- 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 O
- The Diaphragm D1 is applied and is as shown below.
STEP 16:Create load combination
- Now we have to add the load combinations
- Click on Add default Design Combo
STEP 17:
- Befor that visible pier labels create all pier lables
- 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 shear walls as P1,P2.......P19 etc .
STEP 18: Create model analysis
- 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 19: Create response spectrum:
- 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 factor initially
STEP 20: Check model:
- Now we have to check the model for errors, go to Analysis tab and select check model.
STEP 21:
- Now Run the model
- Click on Run Analysis
STEP 22:
- 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 23: Check Modal Participation ratio -
i) Modal number check for that go to Display-Show Tables-Analysis Results-Structure Output-Modal Information-Modal mass
participation ratio.
ii) 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.
iii) 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 0.832>0.65.
iv) 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 24: Check all reults
1) 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
2) Base Shear Checking:
- click on Load Display >Load Type as Eqx > Show Fx load.
- 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.
3) Check for Columns under tension:
- i) Under loading conditions 4b and 4d are found to be under tension.
4) Check for aspect ratio for both column and shear wall.
A) Column -
- i) 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=1634.9*1000/400*600*30-0.22< 0>
B) Shear wall:
- i) The aspect ratio should be less than 0.28.
- ii) Force is 7.22kN.
- iii) Aspect ratio=7.22*1000/1200*300*25=0.08-0.28, hence ok.
5) SFD And Bmd of building
RESULT -
i) The structural model of the G+7 model is designed for given architectural planning using Etabs.
ii) Frame model which satisfies both code provision are analysed.
iii) Columns, shear walls and beam are provided in possible symmetrical way without much offsets.
iv) Analyses is done for modal number, base shear storey drift aspect ratio, irregularity etc.,
v) 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