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.
Ans:
Symmertry:
Investigation of the framing plan with respect to symmetry in X- direction:
If we split the plan vertically half, it will look mirrored. So we conclide the given plan is symmetric in X-direction.
Investigation of the framing plan with respect to symmetry in Y-direction:
This tends to reduce the horizontal displacements in both of the two horizontal displacements in both of the two Orthogonal Horizontal Directions.
It also has a very good uniformity also.
Redundancy:
There are missing structural elements in the Periphery of the plan.This causes the uneven distribution of earthquake action effects in non linear range across the plan.
The energy dissipation across the structure is very low which results in non -reliable structure.
3.Bi-directional Stifness:
The columns in horizontal direction are connected with straight to straight beams . This means, any lateral load ( seismic force in X-direction) passing from left end of the building will have its path stright till it reaches right end of the building. So no or less possibility of force diversion takes place. ]
The columns in vertical direction are not connected with stright to straight beams. So a lateral load passing from south side of building will get diverted to east & west direction to reach next bay. There will be major force diversion take place with seismic force in Y-direction. The framing plan is weak with respect to bi-directional stiffness in Y-direction.
4.Ductility:
The building may not be able resist the Elastic earthquake forces in Shorter direction (Y-axis) so it may result in Inelastic Response.The reduction in Design force in the shorter direction (Y-axis) is to be compensated by Ductility.
We can conclude that the given framing plan is more ductile in Y-direction than that of in X-direction.
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.
ANS.........
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
soft storey- The building has uncovered opening space in 6th floor. As there is no occupancy or a partition wall, this will lead to plastic hinge formation at the columns (with reference to IS-1893-2016 fogure A4) . This has to be checked and sufficiently designed.
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
Vertical irregularity- The building plan area is large in bottom stories. The floor plan areas are getting reduce with increase in height. These building offsets has to be crosschecked with reference to IS- 1893-2016 table-6 serial number (3)
3.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.
Slopy base - The building is constructed on sloping ground at one side. This will lead to significant variations in the stiffness (with reference to IS-1893-2016 figure 5E. Few levels may be subjected to soft storey mechanism.
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.
ANS.............
The Building collapse occured due to the Ground excitation in Orthogonal horizontal directions and also due to the .Inspite of being having the Earthquake resistance for the two mentioned actions
following factors:
1.Open Ground Floor
2.Two short sides of the perimeter in-filled
3.Extreme left hand bay Staircase filled with the Concrete
4.Columns not in the same line
1.Open Ground Floor
The ground floor of the building is open,which may be a softstorey (having long columns) due to this the Ground floor excitation the columns could have been unable to resist the Earthquake and form collapse.
This is called as soft storey mechanism.
This can be avoided by having Strong column-weak beam mechanism.
2.Two short sides of perimeter in-filled
The mass has been undistributed in all the sides of the Building in all storeys because it is given that the two sides of the perimeter of the building is in-filled ,may be wih the soil, which causes the Irregular distribution of mass called as Mass Irregularity.
This can be avoided by providing the shear walls in the directions of Two short sides of the perimeter which can resist the Lateral loads ,soil due to Infill, by providing stiffness in the Building frame and helps to resist the Earthquake actions.
Also it adjusts the Mass Irregularity for the plan.
3.Extreme left hand Staircase filled with Concrete
The extreme left hand bay has Staircases made of concretedue to this the mass got concentrated in the staircase area due to which torsional effects are formed having the base of the concentration of mass in the Staircase.
To avoid this type of failure the staircases should be kept in the centre of the Building which makes the Torsional effect as Zero by Uncoupling the moment.It also makes the Implications of the Irregularity in plan of the Building.
4.Columns not in the same line
It is observed that the columns in the Extreme left hand middle line columns has the deviation in the line of the Construction at the staircase which again a reason for the Irregularity in plan having the Torsional flexibilty.
To avoid this the Eccentricity should be maintained for the deviation to be with in the limits of Beam size and the Columns sizes.
Also the sizes of the Beams and Columns in the Plan should be redesigned in such a way that the line of the columns doesn't deviate even in the staircase area.
No framing connection along Y-direction- We could see no structural beams provided along the vertical direction between columns. This makes the building more vulnerable during the north-south seismic force.
Beams connected along column minor axes- The columns are having longer dimention in Y-direction. This means they are more stiffer along Y-direction.But the beams are running only along X-direction which would attract much lesser lateral force and hardly transfers it.
Variation in column sizes- The internal columns are having higher sizes compared to exterior columns ( This is common because of higher load demands). But this will lead to significant difference in lateral drift of columns. So the concrete floor might have suffered heavy out-of-plane shere which would led to collapse.