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.

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1.Symmetry:

Symmetry is the process of comparing two identical objects .Two objects are said to be symmetrical if they are mirror images if each other

a).Symmetry in X-Direction:

• When we split the above given plan along the X- Direction,it seems to be identical
• Hence,it is symmetrical in X-Direction

b).Symmetry in Y-Direction:

• When we split the above given plan along the Y- Direction,it seems to be identical
• Hence,it is symmetrical in y-Direction

2.Redundancy:

Redundancy is a behaviour of the structural arrangment in a building, has sufficient number of supports which are arranged in orders

a).Redundancy in X-Direction:

• From the given plan we can see multiple dots and lines
• Dots represent the columns and line represents the beams
• We can see that the columns in the X direction are more and evenly distributed in the middle portion
• So, we can say that the structure is redundant in the X-direction
• Column arrangment in the top and bottom rows are very less while comparing with the middle portion
• But these columns are evenly spreaded, so we can say that the structure in the X-direction is redundant in nature.

b).Redundancy in Y-Direction:

• When we are observing the Y- Direction columns are lesser comparing with the X-Direction
• Eventhough they are in lesser number columns in Y-Directions are evenly spread
• We can say that the structure is redundant in Y-Direction.But comaring with X-direction lesser redundancy

3.Bi-Directional stiffness:

a).Bi-Directional stiffness along X-Direction:

• From the given plan we can observe that the columns in the X-directions are connected in a straight line which is beams in a continous manner
• So that the loads can be transffered easily in the X-Direction
• We can conclude that the frame is stronger in the Bi-lateral stiffness in the X-Direction

b).Bi-Directional stiffness along Y-Direction:

• From the given plan we can observe that the columns in the X-directions are connected in a straight line which is beams not in a continous manner
• We can see empty spaces after the beam
• So that the loads cannot be transffered easily in the Y-Direction
• We can conclude that the frame is weak in the Bi-lateral stiffness in the Y-Direction

4.Ductility:

Ductility describes the extend to which a material can undergo large deformation without failing.The term used in earthquake engineering to designate how well a building will endure large lateral displacements imposed by ground shakings

a).Ductility along X-Direction:

• As we can see in the above given plan the beams are connecting all the columns in the X-Direction and the stiffness is more in the X-Direction because number of columns are more in the X-Direction
• So the ductility in the X-Direction will be reduced

a).Ductility along Y-Direction:

• As we see in the given plan that the beams are not connected to each and every column of this frame in the Y-Direction
• Stiffness of the frame in the Y-Direction is less because lesser number of beams in the X-Direction
• So we can conclude the frame is more ductile in the Y-Direction than in X-Direction

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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.

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Irregular configuration either inplan or in elevation is recognized as one of the major causes of failure during earthquakes.Thus irregular structures,especially the ones that located in seismic zones are the matter of concern.Structures generally possess combination of irregularities and consideration of a single irregularity may not result in accurate prediction of seismic response. The choice of type ,degree and location of irregularities in the design of structures is important as it helps in improving the utility as well as aesthetics of structures. The possible sources of irregularities in the building shown is

a). Vertical irregularity

• From the above given image of the building we can clearly see that plan area reduces with increase in height
• The building offset has to be checked out with refernce of IS 1893-2016 table no.6

b).Soft storey

• From the given image we can see that the building at the 6th floor has a soft storey because it has uncovered opening
• As we can see there is no partition wall or no occupancy or shaer wall or any other support elements to support the upper blocks of the building during the earthquake and this will lead to the plastic hinge formation at the columns
• According to the reference IS 1983-2016Figure this has to be checked and designed the building accordingly

c).Sloppy base

• From the given image we can see that the building is constructed on a sloping ground at one side
• This will lead to a significant variation in the stiffness(with refernce to Is 1983-2016Figure 5E).So, few levels may be subjected to soft storey mechanism

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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).

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• Open ground floor-This denotes soft storey mechanism acting in the building
• Rooms along two bays separated by an open corridor-The bays filled with rooms will have higher mass compared to the middle bay left open for corridor. This induces mass irregularity along height uniformly
• Two short sides covered with infill walls- This means the building's left and right sides periphery is more stiffer during seismic forces( especially along north-south)
• Staircase portion at left top corner-Usually staircase and lift portions are stiffer than other parts because of concentrated concrete elements.So the staircase portion contains more stiffness in the provided building plan.
• No beam connections in the Y direction .This will make building more vulnerable during north south seismic waves
• Beams connected along column minor axis .This columns are having longer dimension in the Y directions so it is more stiffer in Y direction, but beams are running only along x direction which would attract lesser lateral force and hardly transfer it
• Variation in column size -The internal columns have heavior size than external columns but this will lead to significant difference in lateral drift of columns.So the concrete floor might have suffered heavy out -of- plane shear which would led to collapse.

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