Project 1_Comparative study of different storey buildings for Seismic forces

1. Factors influencing the Natural Period of a building:

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?

Increase in the column size increases both stiffness and mass of buildings. when the percentage increase in stiffness as a result of increase in column size is larger than the percentage increase in mass, the natural period reduces, buildings are said to have shorter natural periods with increase in column size.

Buildings E and F are two 10-storey buildings with different column sizes along the elevation building F has column size of (600×600) throughout the height, while building E has smaller column size (400×400) in the upper 5 storeys . Thus building F (600×600) is relatively stiffer than Building E and the fundamental period of the stiffer building F (1.99 s) is only marginally smaller than that of the building E (2.17 s).

The deformed shape of the building indicates that most of the deformation is occurring only in the lower storeys because of shear-type of lateral deformation in the building, where the columns size is same, the effect on the overall natural period is not changed.

Between buildings G and H, the occuring is much stiffer. But, while increasing stiffness, the mass is also increased. In arriving at the natural period, the mass and stiffness compete to determine whether the natural period will increase or decrease, when both are changed.

Factors influencing the Natural Period of a building :-

Structural Element Sizes

- Beams : 300x400mm

- Columns : 400x400mm

- Slab : 150mm thick

Material Properties

- Grade of concrete : M30

- Grade of steel Reinforcement Bars : Fe415

Loading

- Dead Load on beams from infill wall : 10kN/m

- Live load on Floor : 3kN/m2

(what is the seismic mass?)

Aim:

Structural Element Sizes

- Beams : 300x400mm

- Columns : 400x400mm

- Slab : 150mm thick

Material Properties

- Grade of concrete : M30

- Grade of steel Reinforcement Bars : Fe415

Loading

- Dead Load on beams from infill wall : 10kN/m

- Live load on Floor : 3kN/m2

Procedure :-

* Open the etabs software.

* Click on new model.

* Model installation dialog box shows.

* In these dialog box select use built in settings option change the display units as metric SI, steel section database as indian,Steel design code as IS800:2007,Concrete design code as IS456-2000 then click on ok.

* Next newmodel quick templates dialog box will be appear in these dialog box grid dimensions below select custom grid spacing then click on edit.

* Another dialog box will be appear i.e., Grid System Data in these dialog box change the x grid data values and y grid data values then click on ok.

* In the newmodel quick templates dialog box In the story dimension below click on custom story data then click on edit story data.

* Another dialog box will be appear i.e., Story data change the story names and also change the height master story similar to options then click on ok.

* In the difine tab, goto section properties click on slab sections.

* In the slab sections, slab property data dialog box will be appear then change the properties.

* In the difine tab, goto section properties click on load patterns.

* In the define load patterns dialog box then provide the loads and properties.

* In the difine tab, goto section properties click on load combination.

* In the load combination dialog box select concrete frame design then click on ok.

* Select quick draw columns then change the property and assign to the model.

* Select quick draw beams then change the property and assign to the model.

* In the plan view select first story then click on ok.

* Select quick draw slab then change the property and assign to the model.

* Goto select tab,click on select option select properties dialog box will be appear select beam and column assign to the model.

* Goto select tab,click on select option select frame load assignment(distributed) select the load pattern and given the load value click on ok.

* Goto select tab,click on select option select frame load assignment select the slab section and given the load value click on ok.

* Goto select tab,click on select option shell load assignment(uniform) select the live load section and given the load value click on.

• Factors influencing the Mode shape of oscillations:

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.

The lateral translational mode shapes depend on flexural stiffness of beams relative to the adjoining columns. The fundamental mode shape of buildings changes from flexural-type to shear-type as beam flexural stiffness increases relative to that of column.

when flexural stiffness of beams is small compared to that of the adjoining columns deformation is predominantly in single curvature bending leading to overall flexure-type deformation behavior of building.

when flexural stiffness of beams is large compared to that of the adjoining columns, deformation is predominantly in double curvature bending within in each storey leading to overall shear-type deformation behavior of building.

Factors Influencing Mode Shapes of Building

1).Effect of Flexural Stiffness of Structural Elements:

The overall lateral translational mode shapes depend on the flexural stiffness of beams relative to that of adjoining columns.The fundamental mode shape of buildings changes from flexural-type to shear-type as beam flexural stiffness increases relative to that of column.

(a) Pure Flexural Response: In Pure Flexural Response (when flexural stiffness of beams is small compared to that of the adjoining columns), column deformation is predominantly in single curvature bending leading to overall flexure-type deformation behaviour of (the cantilever) building.

(b) Pure Shear Response: In Pure Shear-Type Deformation Behaviour (when flexural stiffness of beams is large compared to that of the adjoining columns), column deformation is predominantly in double curvature bending within in each storey leading to overall shear - type deformation behaviour of building But, increasing the flexural stiffness of a beam also increases its strength; this is not desirable when strengths of beams exceed that of columns into which they frame in, especially when beam strengths exceed those of the columns adjoining.

2).Effect of Axial Stiffness of Vertical Members:

• Mode shapes depend on axial stiffness of vertical members in a building (i.e., of columns or structural walls).
• Small Axial Stiffness causes significant axial compressive and tensile deformation in columns in addition to single or double curvature flexural deformations.
• Additional Axial Deformation changes the fundamental mode shape from shear-type to flexural type, particularly in tall buildings.
• Pure Flexural Response is not desirable because of large lateral sway, particularly at higher floors. Hence, designers ensure that the axial areas are large of building columns and structural walls.

1. Highly Flexible soils make column bases as good as hinged, and rocky layers below as good as fixed.
2. Lack of Rotational Fixity at the Column Base (hinged condition) increases the lateral sway in the lower storey than in the higher storey, and the overall response of the building is more of shear-type.
3. On the other hand, full rotational fixity at the column base restricts the lateral sway at the first storey and thus, induces initial flexural behaviour near the base.
4. The overall response of the building is still of shear-type due to flexural stiffness of beams.

• In well-designed Low Height moment frame buildings, the fundamental translational mode of oscillation is of shear-type.
• Buildings become laterally flexible as their height increases. As a result, the natural period of buildings increases with an increase in height. However, the fundamental mode shape does not change significantly (from shear type to flexure type).
• Flexural type behaviour is exhibited only near the lower storeys where the axial deformation in the columns could be significant, particularly in tall buildings. However, at higher floor levels, the response changes to shear type as the axial load level lowers.

• URM Infill Walls are not considered in Analysis and Design of RC Frame Buildings in current design practice in many countries.
• They are assumed to not carry any vertical or lateral forces, and hence, declared as non-structural elements insofar as the transfer of forces is concerned between structural elements (e.g., beams and columns) that are generated in the building during earthquake shaking.

Not consideration of URM Infill Walls in Analysis and Design; This assumption causes a large gap between the building that is considered in analysis and design, and that finally constructed.

Result :-

We have successfully complete the different types of buildings with different conditions Mode shape of oscillations,Natural Period of a building,dynamic characteristics of a building by using etabs.