Concept on positioning of Columns

Question 1.

Comment on the layout of the framing plan concerning earthquake resistance in the two horizontal directions X or Y (dots are columns, lines depict beams). Look into aspects of symmetry, redundancy and bi-directional stiffness and ductility.

To describe the symmetry, redundancy, bi-directional stiffness, and ductility aspects of a given framing plan layout.

SYMMETRY

• Symmetry

The above two images clearly depicts that the layout is symmetrical about X axis as well as Y axis. Hence the layout has good symmetrical property.

• The framing layout is symmetric in nature
• The load resistance framing in this symmetrical structure is distributed evenly throughout the structure.
• If the reflection of the structure about the axis is identical in geometry, supports, and material properties to the structure itself, the structure is symmetric with respect to an axis of symmetry in its plane.
• This structure has an exact matching of form and arrangement of parts on opposite sides of the boundary at X and Y.

REDUNDANCY

• Because this structure has perfect redundant, the structure becomes more reliable in resisting any lateral forces acting on the structure.
• Furthermore, it tends to prevent any extent of degradation that the structure may suffer without losing some specified elements of its functionality.

BI-DIRECTIONAL STIFFNESS

• Horizontal seismic motion will cause the horizontal axis of X and Y to change direction throughout the layout's framing.
• Because this structure can distribute seismic forces due to its bidirectional nature, it can withstand and resist any horizontal motion of forces.
• The given plan has an orthogonal layout in both the X and Y directions, so it can transfer forces evenly, and its structure will be able to withstand direct action from seismic forces.

Question 2:

What are the possible sources of irregularity in the building shown here? Explain how, as an engineer, each of these irregularities should be dealt with.

Soft Storeys

Flexible structures in Torsion

1.Stiffness Irregularity

The lateral stiffness of the story at the mid height which is open is very less when compared to the above levels bexause the main reason is the structure is discontinued at the midgheight.

It is obseverved that the open storey causes reduction in the stiffness in the Building and attracts lateral seismic Exicitation more in that floor which causes severe damage to the upper floors

The Irregularity can be solved by providing the Shear walls to this high seismic action prone floor

2.Mass Irregularity

The open story also causes mass irregularity in the Building because the high mass concentration is present above the open floor.That is the upper floors carry more mass than the open floor.

Also the no of floors are higher above this open floor.

3.Vertical Geometrical irregularity

There is a change with the horizontal dimension of the lateral force resisting due to the open storey in mid height and also the vertical geometry of the Building which means the length of the Top storeys are varying along the Building unit.

This causes Torsional effects in the Building which need to be Decoupled by providing shear walls or Decoulped moments.

The Flexibility provided due the variation need to be compensated by providing Ductility

Question 3:

A 4-storey hotel building has an open ground floor for the restaurant. Story 2 to 4 have one row of rooms along each long side in plan, separated by a corridor. The two short sides of the perimeter are fully in-filled in all storeys, except for certain openings at the ends of the corridor at storeys 2 to 4 and along the right-hand side of the ground floor. There is a staircase near the upper left-hand corner, with straight flights between landings at floor levels and in-between floors. Cross-section dimensions are written next to the member no. in meters.

Comment on the features of the structural design and of the layout of infills which are important for earthquake resistance and seismic performance. How do they relate to the almost full collapse of this building (the extreme left-hand bay with the staircase survived, as well as one long-side façade in the rear end, and the frame along the right-hand side in plan).

• The structure has rooms on two sides along the long edge which is separated by a corridor at the middle. So, on the two sides, i.e the top and bottom portion of the plan will have heavier mass and hence more stiffness compared to middle portion which is left unoccupied. This leads to irregularity in mass. During earthquake excitation, the middle portion will get collapsed due to lower stiffness whereas the other two sides may sustain.

• The two short sides of the perimeter are fully in-filled in all storeys. Since, this lateral force resisting systems, i.e the walls are continous from the foundation this impart greater strength and stiffness to the structure in these locations. That is the reason why the left and right edges remain intact after the earthquake excitation.

• There is a provision of staircase at the upper left hand corner, this provision increases the weight in that portion which inturn contribute to the stiffness in that portion. There is also redundancy in the staircase portion due to increased number of columns. This is the reason why the left-hand bay survived after the excitation.

• Due to this non-uniform distribution of stiffness in the structure, it is also subjected to torsional irregularity as the centre of stiffness of the structure deviated from ots centre of mass.
• From the plan it is clear that there is no beam along the Y direction i.e, there is no structural member to tarnsfer the load from the floor to columns. This may cause the floor to become flexible and due to this the column will be subjected to irregular loading and hence, increases the chances of collapse.