Concept on positioning of Columns
Question 1-
Aim- 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.
Answer-
1) Symmetry:- If we split the plan vertically half and horizontally half it look like as a mirror. So we conclude that the given plan is symmetric in x and y direction .
2) Redundency:-
Redundancy in X direction: If we see the plan the column in X direction is more than in Y direction. Also more space in x direction, except top and bottom column.
The redundancy at the top and bottom is less as compair to middle part in the X direction .
Redundancy in Y direction: If we see the plan the column in y direction are comparatively less but having evenly spaced.
The structure in Y direction is redundant but comparitively less to X direction.
3) Bi-Directional stiffness:- The column in horizontial direction are connected to stright beam if any lateral load in X direction which are passing from left to right direction of the building , we see that path is stright there was less or no possibility of the force diversion, but in y direction the column is not connected to stright beam so, the lateral load passing through south side of the building get diverted into the opposite direction that is east and west direction to reach at a next bay. Therefore major force diversion is take place with sesmic force in Y direction. The framing plan is weak with respect to the bi-directional stiffness in Y direction.
4) Ductility:-
We see that more column in X direction than Y direction it will results in higher stiffness in X direction that reduces the ductiliy in X axis. The given floor plan is more ductile in Y direction than X direction
Question 2-
Aim- 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.
Answer-
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-
Aim- 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.
omment 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).
Answer-
Looking toward the plain we see that it consist of one way slabs, beam and columns in the structural plan the reinforced concrete beam is provided in x direction, no beams are provided in y direction thats way structure is failure. We see at bottom floor looking open at may lead soft story failure mechanism acting in the building. as per the given instructions, the middle portion is the corridor which means that portion has no mass as compaired to other sides having room and which results in greter mass, due to this irregularity building may collapse.
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 the 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 bay Staircase filled with the Concrete:- The extreme left hand bay has Staircases (made of concrete) due 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.