Project 1_Comparative study of different storey buildings for Seismic forces

Etabs project 1

Aim- To model and analyse multiple buildings with different floors and different column dimensions subjected to seismic loads and compare the results of the buildings in seismic behaviour.

Procedure-

Modelling Building A- Number of storey=2 and column dimension= 400x400mm

• Open etabs 2019.
• Set up new model initialization> use built in setting. Specify the model as Indian code.
• 
• Set the grid system and storey system of the model.
• 
• 
• Now, add the material property in the model.
• Define>material properties>add new material>m30 and HYSD 415.
• Define > section properties>frame sections > add new property> select rectangular section.
• Define beams, columns and slab and modify the frame section data. Set the depth, width and reinforcement data.
• 
• Now, add the load patterns in the model.
• Define> load patterns> add new load.
• Dead load is automatically defined, rest are to be added.
• 
• Modify the seismic loads for eccentricity and directions.
• 
• Add mass source.
• Define> mass source> modify mass source.
• Add the load patterns in the mass source. Dead load, live load and wall load.
• 
• Add load combination.
• Define>load combination>add default load combinations.
• 
• Now add the columns, beams and slab in the model.
• Carefully place the columns, beams and slabs.
• 
• Add restraint of pin foundation in the model.
• 
• Add loads on the building.
• Apply 10kn/m2 distributed frame loads on beams (dead)
• Apply 3kn/m2 uniform shell load on the slab (live)
• 
• Add the diaphgram on the building for centre of mass.
• Check the model for any errors.
• Run analysis on the model.
• Similarly draw All the models and check the results.

Model A

Model B

Model C

Model D

Model E

Model F

Model G

Model H

Model J

Model K

Comparisons-

Effect of stiffness on T: Compare fundamental natural periods of buildings E & F as well as G & H. Why is there a marginal or significant difference in the fundamental natural periods?

• Model E has varying size of columns and Model F has uniform sizes of column.
• Model E and F has a time period of 1.731sec and 1.735 sec respectively.
• Model G has varying size of columns and Model H has uniform sizes of column.
• Model G and H has a time period of 4.171sec and 4.416sec respectively.
• Varying column sizes are more stable or equally stable compared to uniform column sizes along the building.

Effect of mass on T: Compare fundamental natural periods of buildings H, J and K. Have the buildings become more flexible or stiff due to change in mass?

• Model H, model J and model K has a time period of 4.416 sec, 4.425 sec, 4.435 sec respectively.
• We can observe as the time period increases with the increase in the imposed load we can assume that the building becomes more flexible.
• Since the mass of the structure is directly proportional to the flexibility of the structure.

Effect of Building Height on T: How does the fundamental natural periods of Buildings A, B, F and H change with change in building height?

• Time period of model A= 0.5 sec 2 story
• Time period of model B= 1.036sec 5 story
• Time period of model F= 1.736 sec 10 story
• Time period of model H= 4.416 sec 25 story
• We can see as the height of the building increases the time period also increases.

Effect of Column Orientation on T: How does the fundamental natural periods of Buildings B, C and D change with change in column orientation?

• Model b uniform column of 400x400= 1.036sec
• Model c uniform column of 550x300 in x direction= 1.145 sec
• Model d uniform column of 300x550 in y direction= 1.12 sec
• We can observe there are some changes in natural periods of the building depending upon the column orientation.

Effect of Flexural Stiffness of Structural Elements on mode shapes: Compare fundamental mode shape of Building B in two situations when flexural stiffness of beams relative to that of adjoining columns is very small versus when it is large.

• Flexural stiffness of beams is 100%= 1.03sec
• Flexural stiffness of beams reduced by 90%= 2.62 sec.
• As observed when flexural stiffness is very small the time period increases and the structure becomes more flexible.

Effect of Axial Stiffness of Vertical Members on mode shapes: Compare fundamental mode shape of Building H in two situations when axial cross-sectional area of columns is very small versus when it is large.

• Full axial cross section for columns = 4.41 sec
• Axial cross section is only 10% for columns = 5.99 sec
• Reducing axial stiffness increases the time period and also affect the gravity design of the building.

Effect of Degree of Fixity at column bases on mode shape: Compare fundamental mode shape of Building B in two situations when base of columns is pinned versus when it is fixed.

• The time period If the Model B is pinned = 1.03sec
• The time period if the Model B is fixed = 0.941 sec
• The model becomes less deflected and more stiff if the restraint are set as fixed.