CONCEPT ON POSITIONING OF COLUMNS
Concept on positioning of Columns
Q.1 Possible sources of irregularity in the provided building.
Ans:
Aim: The goal is to assess the framing plan's layout for earthquake resistance in both horizontal directions, X and Y. This evaluation will focus on factors like symmetry, redundancy, and the ability to withstand forces in both directions.
Introduction: We'll be explaining the layout of the framing plan, as depicted in the figure above, in terms of its ability to resist earthquakes in the horizontal X and Y directions. This will be done through a step-by-step procedure.
1) Symmetry
i. Investigation of the framing plan with respect to symmetry in X-direction:-
* If we look closely at the plan above and divide it into two equal parts horizontally, we can see that it looks like a mirror image.
* This shows that the plan is symmetric in the X-direction.
ii. Investigation of the framing plan with respect to symmetry in Y-direction:-
* If we examine the plan above closely and then cut it in half horizontally, creating Section 1 and Section 2, we'll notice that the plan appears mirrored.
* This means that when we divide it into two equal parts in the Y-direction, the plan displays symmetry.
* So here we can conclude that the given above plan in the Y-direction is also symmetric.
2) Redendency
i. Investigation of the framing plan with respect to redendency in X-direction:-
* The plan shows dots and lines. The dots are columns, and the lines are beams.
* If we look at the plan, we can see that there are many evenly spaced columns in the middle in the horizontal direction (X-direction).
* So, we can say the structure has extra support in the middle.
* But at the top and bottom, there are fewer columns, although they are evenly spaced, so the structure also has extra support in the horizontal direction
ii. Investigation of the framing plan with respect to redendency in Y-direction:-
* The plan above has dots and lines. The dots stand for columns, and the lines represent beams.
* When we look at the plan, we notice that the columns in the vertical direction (Y) are evenly spaced, but there are fewer of them compared to the horizontal direction (X).
* This means that there is less redundancy in the vertical direction.
3) Bi-directional stiffness
i. Investigation with respect to Bi-directional stiffness:-
* In simpler terms, if you look at a structure from the side (horizontal view), it tends to be wider from left to right (X-direction) than from top to bottom (Y-direction).
* Because of this, there are more support elements like columns and beams arranged horizontally.
* This arrangement makes the structure better at resisting movement or deformation in the left-right direction.
* So, we can say that the structure is stiffer in the left-right direction (X) compared to the up-down direction (Y).
4) Ductility
* Ductility and stiffness have an inverse relationship.
* When a structure is very stiff, it tends to be less ductile, and the opposite is true as well.
* So, if we assess the ductility in both the horizontal (X) and vertical (Y) directions, we can conclude that the frame is more ductile in the X-direction.
* On the other hand, in the Y-direction, it is not ductile because there are fewer beams providing support.
Q.2 Explaining the possible sources of irregularity in the building shown.
Ans:
Aim: The objective of this study is to examine the potential sources of irregularity in the depicted building and to outline how, as an engineer, each of these irregularities should be addressed.
Introduction: Irregularities in building layout or elevation are known to be significant contributors to earthquake-related failures. Buildings located in seismic zones are especially worrisome in this regard. Structures often have multiple irregularities, and addressing just one may not accurately predict seismic performance. The choice of irregularity type, extent, and placement in structural design is vital for both function and aesthetics. This study examines how reinforced concrete structures with different irregularity combinations react to seismic forces.
1] Vertical Geometrical Irregularity:-
* If we take a close look at the provided plan, we'll notice that the building's footprint is larger at the bottom and gradually decreases as it goes upwards.
* In other words, as the height increases, the floor area decreases, which is evident in the image.
* The green rectangle encloses a larger floor area compared to the area within the yellow rectangle.
* It's important to verify the building offset using IS 1893-2016 table no.6 as a reference.
2] soft storey:-
* A soft storey refers to a floor level within a building that is significantly more flexible or less rigid compared to the floors above or below it.
* In the provided image, it's evident that the 6th floor of this building qualifies as a soft storey due to the presence of an uncovered opening.
* This floor lacks partition walls, occupancy, shear walls, or any other type of structural support elements. Consequently, there's insufficient support for the upper sections of the building during an earthquake, potentially leading to the formation of plastic hinges at the columns.
* According to IS 1983-2016 Figure 4A, this condition needs to be thoroughly evaluated, and the building should be designed accordingly to ensure that no mistakes occur in the design process. This is crucial for the overall structural integrity of the building.
3] Sloppy Base:-
* A "sloppy base" refers to the condition where a building is constructed on uneven or sloping ground.
* In the case of the building on the right side, there is an embankment, indicating that the building is erected on such terrain.
* This situation can result in a considerable difference in stiffness across the building, which should be assessed with reference to IS-1893-2016 Figure 5E.
* Due to this variation in stiffness, some levels of the building may be vulnerable to what is known as a "soft storey" mechanism.
* In simpler terms, certain floors might experience more significant structural flexibility or weakness compared to others, which can pose a risk during seismic events.
* Proper evaluation and design considerations are necessary to address this potential issue and ensure the building's safety.
Q.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 resulted from ground excitation in both orthogonal horizontal directions and torsional effects. Despite being designed to resist these actions, several contributing factors led to the collapse:
a] Open Ground Floor
b] In-filled Short Perimeter Sides
c] Concrete-filled Staircase on Extreme Left Side
d] Misalignment of Columns:
a] Open Ground Floor:
* The ground floor of the building is open, potentially classifying it as a soft storey.
* This means that the long columns on the ground floor may have been unable to effectively withstand the earthquake forces, leading to collapse.
* This is known as the soft storey mechanism. To prevent this, it is important to implement a strong column-weak beam mechanism.
b] In-filled Short Perimeter Sides:
* The presence of infill material on two short sides of the perimeter resulted in an uneven distribution of mass throughout the building.
* This is referred to as mass irregularity.
* To address this, shear walls should be incorporated along these sides to provide stiffness to the building frame, helping it withstand lateral loads from the infill material.
* This also helps correct the mass irregularity in the plan.
c] Concrete-filled Staircase on Extreme Left Side:
* The extreme left bay of the building contains a concrete-filled staircase, which concentrated mass in that area.
* This led to the formation of torsional effects, as the base of mass concentration was located within the staircase.
* To avoid this, staircases should ideally be positioned at the center of the building, minimizing torsional effects and rectifying irregularities in the plan.
d] Misalignment of Columns:
* There is a noticeable deviation in the alignment of columns in the extreme left-middle line due to the presence of the staircase.
* This deviation contributes to torsional flexibility.
* To mitigate this issue, it is crucial to maintain proper eccentricity within the limits of beam and column sizes.
* Additionally, the sizes of beams and columns in the plan should be re-evaluated and adjusted accordingly.