Modelling of 25 storey building with the specified properties using ETABS

Challenge 6 – ETABS modelling of a 25 storey building

Model a 25 storey building on ETABS with structural properties as specified below. Ground floor or plinth floor level can be taken at a height of 1.5 metres from the base. Each of the successive 25 storeys has a storey height of 3 metres.

Structural Element Sizes

• Beams : 300 × 400 mm
• Columns : 800 × 800 mm
• Slab : 150 mm thick

Material Properties

• Grade of Concrete : M30
• Grade of Steel Reinforcement Bars : Fe 415

Loading

• Dead Load on beams from infill wall : 10 kN/m (Let us assume the same for roof floor in view of parapet)
• Live load on floor : 3 kN/m2 (Let us assume the same for roof floor)

a) Buildings are assumed to be pinned at the base

b) All columns in all storey are of the same size

c) All beams in all storeys are of the same size

d) Building is located on Soil type-II and in Zone-III. Assume other factors if required.

Carry out the following exercises:

• Check if the translational fundamental natural period of vibration in both the x and y direction is approximately: Tx1">Tx1Tx1= 4.4 second Ty1">Ty1Ty1= 4.2 second

This check will help confirm if modelling is done correctly. Some deviations from the above values is possible (± 2%). But any major deviation will suggest an error in modelling.

• 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 (shown below) 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. Please note that this check is for Serviceability Limit State and hence is for situation when stucture is subjected to unfactored seismic base shear

Aim:-

To model and analyse a 25 storey building with the given specifications and to cross check the parameters as per IS1893 provisions.

Introduction:- I have used Etabs 2020 version for this 25 storey building modeling and analysis. In this project, I have implemented several clauses from IS 1893 for justifying the various design parameters.

Procedure:-

Step-1:-

Edit > Edit stories and Grid system

Using the user defined grid input, The horizontal and vertical grid legends have been assigned with 4 meter bay on either sides.

Open Etab 2020 and add new Model

Edit the model Initialization window (Select the unit as Metric S.I and Indian codes) and click ok.

Then Edit Grid system as per question data.

Step-2:-

Edit > Edit stories and Grid system

Using the user defined storey input, 26 storeys have been incorporated (plinth at 1.5 meter height + 25 storey with 3 meter height each). Storey 1 has been labelled as master storey and all above levels are labelled similar to that.

Add Storeys from edit tab

Step-3:-

Define > Material properties

Using the material assignments, Concrete grade M30 is included in the material list. The default properties have been checked.

Define material properties (Go to define tab and add concrete(M30) and rebar(Fe 415) as per given data in the question.

Using the material assignments, Rebar grade Fe415 is included in the material list. The default properties have been checked.

Step-4:-

Define > Section properties > Frame sections

Using the rectangular concrete elements, Beam of size 300x400mm size has been added to the list. The design scope and assumed cover, rebar details have been mentioned.

Go to define tab and Go to section property and add frame property and add beam property.

Go to Modify/Show Rebar

Using the rectangular concrete elements, Column of size 800x800mm size has been added to the list. The design scope and assumed cover, rebar details have been mentioned.

Go to Modify/Show Rebar

Define > Section properties > Slab sections

Using the membrane property, Slab size 150mm has been added to the list. "Membrane" property will help to consider only slab self weights and floor weights in analysis of structure while no stiffness component or flange advantage are considered during calculation.

Go to the define tab and select slab section

Step-5:-

Define > Load patterns

With default dead load, additional wall load, live load, seismic load X direction (+/-), Seismic load Y direction (+/-) have been assigned. As the seismic data are not given in the question, it is assumed that zone factor 0.16 (seismic zone-III), soil site type-II, response reduction factor 5, program calculated time period.

Step-6:-

Define > Mass Source

As per the IS 1893 guideline, the reduced live load and all dead loads are considered for mass source. This proportion of building weight is mandatory to calculate seismic forces.

Step-7:-

Define > Load combinations

As per the Indian standards IS 456 and IS 1893, the default combinations are generated using the ETABS predefined set.

Step-8:-

Using the quick draw column option, all the columns have been modelled keeping "all stories" ON.

Select All storey option from bottom right before placing columns

Using the quick draw bemas option, all the beams have been modelled keeping "all stories" ON.

Using the quick draw floors option, all the slabs have been modelled keeping "Similar stories" ON.

Select pre-defined property as "S150" before assigning it.

Step-9:-

Select > Select > Object type > Floors

After selecting all floors, using "Assign > Shell loads > Uniform loads", Live load of 3 kN/sq.m is provided and verified at all levels.

Select > Select > Object type > Beams

After selecting all beams, using "Assign > Frame loads > Distributed loads", Wall load of 10 kN/m is provided and verified at all levels.

Select > Select > Object type > Floors

After selecting all floors, using "Assign > Shell loads > Diaphragm loads

Step-10:-

Using "Move down in List", base level is brought visible. All base supports are selected. Using "Assign > Joints > Restraints" option, pinned suport is assigned to all column bases as per the question description.

Step-11:-

Using "Analyze > Check model" option, the modelled structure is verified without errors and warnings.

Step-12:-

Using "Analyze > Run analysis" option, the model is analysed and deflection patterns, BMD/SFD patterns are cross chdcked logically. It is understood that the worked model provides reliable results.

FINAL RESULTS:-

Step-13:-

Display > Show tables > Analysis results > Structure output > Modal information > Modal periods and frequencies

The time periods for Fundamental Mode-1 Y direction and Fundamental Mode-1 X direction are noted from the table & found to be as follow.

Tx1 = 4.416 seconds

Ty1 = 4.222 seconds

Hence my aforementioned results are satisfying the question required data-

Check if the translational fundamental natural period of vibration in both the x and y direction is approximately:

Tx1= 4.4 second Ty1= 4.2 second

Step-14:-

Display > Show tables > Analysis results > Structure output > Modal information > Modal participation mass ratios

From the table, the natural time period for Mode-1 translation and torsion DOF are noted as follow.

Tx1(Mode 1) = 4.416 seconds

Ty1 (Mode 2) = 4.222 seconds

Txy1(Mode 3) = 3.311 seconds

Thus we can understand, the torsional time period (Mode 3) is less than the two translation time period (Mode 1 & Mode 2) as required by IS 1893 provision Table-5.

Also, the mass participation ratios for first three modes are noted as follow.

Tx1 = 0.8042

Tx2 = 0.8084

Tx3 = 0.0

By summing the three mode mass ratios, we are getting Tx1 + Tx2 + Tx3 = 1.6126, which means first three modes of seismic oscillation contribute more than 65% of building weight as per ETABS analysis. So the code requirement is satisfied.

As we already know the X and Y direction time periods, they differ only by 4.3%. So the code requirement of minimum 10% is not satisfied.

Step-15:

For storey drift in X direction

For storey drift in Y direction

As suggested by the question description, the storey drift for serviceability Base shear case (EQX or EQY) is tabulated from following tool path

Display > Storey response plot

Keeping load case "EQY" and switching to "Max Story drift", the maximum drift value is noted as 0.000987.

As per clause 7.11.1.1 of IS-1893, the maximum inter storey drift is limited to 0.004 for each storey height. But the ETABS value is just 0.000987 which is far lesser than the code limits. Hence OK.

Conclusion:-

Thus, the ETABS model is completed and anlaysed. The requested data are verified as per Indian seismic standards. But the time period separation of at least 10% is not achieved in the given practice.