Modelling of 25 storey building with the specified properties using ETABS

MODELLING OF 25 STOREY BUILDING WITH THE SPECIFIED PROPERTIES USING ETABS

Aim:

• To model a 25 storey building on ETABS 2018 with the specified structural properties. 
• To carry out the following exercises:
  • Check if the translational fundamental natural period of vibration in both the x and y direction is approximately the following:

    • Tx₁ = 2.98s

      Ty₁ = 3.14s

  • Check if the following provision in Table 5 of IS 1893 (part 1) – 2016, on torsional irregularity satisfied:
    • The natural period corresponding to the torsional mode of oscillation must be less than the two fundamental translational modes of oscillation along each principal plan directions.
  • Check if the two provisions of clause 7 in Table 6 of IS 1893 (part 1) – 2016, regarding vertical irregularity is satisfied.
  • Check if provision on inter-storey drift limitations in clause 7.11.1.1 satisfied. 

Introduction:

• Etabs is an engineering software product that caters to multistorey building analysis and design.
• Fundamentals to etabs modelling is the generalization 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.
• Ouputs and display formats are also practical and intuitive. Moment, shear and axial force diagrams, presented in 2D and 3D views with corresponding data sets, may be organised into customisable reports.
• Here, in this challenge, a 25 story building is modelled using ETABS and various checks are performed confirming to codal provisions - IS1893-2016.

Procedure:

• Initially open the Etabs file and go to New model option.

• A model initialization window opens where we have to input the units and code details to be used and click ok.

• Then, another window - New mode quick template, opens where we have input the grid dimensions and storey dimensions by choosing the custom grid spacing option.

• Now, input the grid dimesions as given in the question.

• Similarly input storey dimensions.

• Hence, both grid and storey details have been filled.

• Next thing that we need to input is the material property. For this, go to define tab and select material properties and then go to add material option in that window.

• Hence, the two materials are added. Now, click ok.

• Now, we need to add the frame members, i.e beams, columns and slabs. Go to Define tab --> Section properties --> Frame sections (beams and columns) 

• Frame properties window opens and in that go to add new material property.
• The reinforcement details and cover to the reinforcement is also provided.

• Similarly, add column to the list by providing the reinforcement details.

• Hence, the materials added to the list is as given below:

• Next we have to add the slab details. For this go to define tab --> section property --> slab sections --> Slab property data dialogue box opens. In that edit the slab properties.

• So all the properties of frame members have been added correctly, now we have to place the structural members.
• For this choose quick draw column option from the tools availabe at the left side of the screen and place the columns at all the grid intersections.
• Since columns extends throughout the height of the structure, so care should be taken to check whether columns are assigned to all storeys.

• Next, we have to place the beams in position , for this also quick draw beams option can be chosen.

• Now, we have to create the slabs, for this initially change the all storeys option to similar storeys which may exclude the base.
• Then, choose quick draw floor or wall option to easily draw the floor slabs.

• Next, we have to assign loads to the frame. For this, go to define tab --> load patterns --> Define load patterns dialogue box appears. Then, apply the loads.

• Now, provide the mass source value to calculate the seismic weight.

• Next, we need to define the load combinations. For this, go to define --> load combination. Provide the default load combination itself.

• Choose Concrete frame design and click ok.

• Select all the floors. Select --> Select --> object type --> Floors. Hence all the floors are selected.
• Then go to assign --> Shell loads --> uniform --> Live loads --> 3kN/`m^2` -->apply --> ok.

• Select all the floors. Select --> Select --> object type --> Beams. Hence all the beams are selected.
• Then go to assign --> Frame loads --> distributed --> load pattern - Brick wall load -->load -  10kN/m -->apply --> ok.

• We have to keep the support as pinned, so for this select all joints by keeping in view the base storey.
• Then Go to assign --> joint --> restraint --> select pinned joint.

• No, go to analyse tab --> check model --> it is checked for any errors and warnings.

• Now, the model can be analysed. For this, go to analysis --> Run analysis.
• Hence, analysis has been completed.
• Now, we need to check the details given in the question.
• So, for this initially go to display tab --> Show tables --> Analysis result --> Structure output --> Modal information --> Modal periods and frequencies.

• Check if the translational fundamental natural period of vibration in both the x and y direction is as given in the question:

  • 1st mode corresponds to translation in y direction, hence Ty₁ =  3.912 s

  • 2nd mode corresponds to translation in x direction , hence Tx₁ = 3.756 s

• Check if the following provision in Table 5 of IS 1893 (part 1) – 2016, on torsional irregularity satisfied:The natural period corresponding to the torsional mode of oscillation must be less than the two fundamental translational modes of oscillation along each principal plan directions.
  • The natural period corresponding to two translational modes of oscillation is obtained as:
    • Tx₁ = 3.756 s
    • Ty₁ =  3.912 s
  • The natural period corresponding to the torsional mode of oscillation is obtained as:

    •  Txy₁ = 3.009

  • Hence, it is verified that the natural period corresponding to the torsional mode of oscillation is less than the natural period corresponding to two translational modes of oscillation.
• Now, modal participation ratios have to be taken, for this go to display tab --> Show tables --> Analysis result --> Structure output --> Modal information --> Modal participating mass ratios.

• Check if the two provisions of clause 7 in Table 6 (shown below) of IS 1893 (part 1) – 2016, regarding vertical irregularity is satisfied.
  • Modal mass participation ratios of first three modes in X direction is obtained as obtained are: 
  • • 1st = 0.2038
    • 2nd = 0.8004
    • 3rd = 0
  • Summing these three we get, 0.8004+0.2036+0 = 1.004 `~~`100% > 65% . Hence it satisfies the criteria i.e, the first three modes together contributes atleast 65% mass participation factor in each principal plan direction.
  • Now, the difference between X and Y time period is given by, 3.912-3.756 = 0.156 `~~`3.98%. Hence, it does not satisfy the criteria of the code i.e the fundamental natural periods of the two buildings in the two principal plan directions are away from each other by at least 10% of the larger value.
• Check if provision on inter-storey drift limitations in clause 7.11.1.1 satisfied. Please note that this check is for Serviceability Limit State and hence is for situation when structure is subjected to unfactored seismic base shear
  • For obtaining the story drift graph go to display --> Story response plots --> Change the display type to Max. story drift --> load case combo - Eqx.

• Here, the maximum storey drift obtained is 0.0027 which is less than 0.004, hence codal provision is satisfied.

  •  

Result:

The analysis of the 25 storey building is completed and the specified checks were also carried out. The structure satisfies all the codal provision except the second provisions of clause 7 in Table 6 of IS 1893 (part 1) – 2016, regarding vertical irregularity, which is given as the fundamental natural periods of the two buildings in the two principal plan directions are away from each other by at least 10% of the larger value.