Structural Modelling using Etabs 2018

Solution:

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.

Procedure:

• Initially open the given AutoCAD file and locate the positions of columns beams and possible loaction of shear wall.

• Now, open the Etabs file and provide the grids and storey information as required and given.

• Next we need to define the material property for concrete and reinforcement.

• Then, define the section property for column, beam and wall.
  • For this, go to define tab -> section property --> frame section --> Beam 300X450mm and column 400X600mm.

• Now, define the slab properties. Define --> section properties --> slab sections --> Slab150 and slab 125.

• Next thing that we need to define is the wall properties.
  • Go to Define --> section properties --> wall sections --> wall250.

• Hence we have now defined the material as well as section properties, so now we need to place the components in the suitable positions.
• So, for this go to quick draw columns and place the columns at suitable positions(C400x600).
• Similarly place the beams(B300x450) and shear walls at suitable locations without obstructing the parking area.

• After this we need to place the slabs , for staircase slab we use slab150 and for remaining area we use slab125.
• Now make the staircase slab as one way slab.

• Next, we need to provide loads before carrying out the analysis.
• Define --> load patterns.(Include wall and load floor finishes as super dead loads)
• After defining assign the load cases.
  • Brickwall load is asssigned as 3.5kN/m.
  • Floor finishes - 1.5 `(kN)/m^2`
  • Live load - 2 `(kN)/m^2` and 5 `(kN)/m^2`for terrace as it contains mumty and lift unit.
  • Also provide the liveload reduction factor as per IS875 part II.Design --> Live load reduction factor.

• Next we need to define the mass source.

• After defining mass source, next we have to define the modal case. Keep the max no. of modes as 20.
• Next we need to define the response spectrum function. Define--> Response spectrum --> Func. name: RS function -->ok
  • Then go to load case --> add new case --> RSx (U1)--> load case type - Response spectrum-->Then fill load applied details.similarly add another load case as RSy (U2).

• Now, Assign --> shell --> diaphragms --> D1 --> Apply.
• Then assign pier labels.
• Now, go to define --> load combinations --> add default load combinations --> choose concrete frame design and concrete shear wall design --> convert to user combination.

• All the inputs have been provided, now we can check the model. Analyze --> check modal.

• Now, we can run the analysis.
• Once, the analysis is over, go to display --> show tables -->analysis result --> structure output --> modal information --> modal mass participating ratio.
  • Mode 1 - 69.14% ; Mode 2 - 63.96% ; Mode 3 - 63.03%
  • Hence, the mass participation of translational mode (first two fundamental modes) is greater than torsional mode(third mode).

• Now, in display choose base reactions. Go to display --> show tables -->analysis result --> structure output --> base reactions.
  • Eqx obtained is 1159.66kN and RSx = 1163.24kN. Hence, the scale factor = 1159.66/1163.24=0.9969.
  • Similarly, Eqy = 1265.27kN anf RSy = 1128.56kN. Scale factor = 1265.271128.56=1.12.
  • So, now, keep these values as scale factor.

• Now, both Eq and RS values are made same using the scale factor hence, the Eqx and Eqy can be removed from the load case and combinations.

• Now, we need to check the interstory drift. For this display --> story reponse plot -->maximum stry drift
  • Here, for both RSx and RSy the max story drift is within 0.004 as given in IS1893-2016

• Now, the axial force diagram is given by display --> force/stress diagram --> frame/pier/spandrel/link forces.

• Hence the completed modal of the G+7 building is as given below:

Result:

• The structural modal of the G+7 building has been developed that complies with IS1896 and IS13920.
• After framing the layout, analysis has been carried out.
• Necessary checks have been carried out and also verified the values with the code.