Concept on positioning of Columns

CHALLENGE 4:-

AIM:- To understand the Concept on positioning of Columns.

INTRODUCTION:-

• It is very important to consider the effect of earthquake in proportioning of columns and their reinforcement.
• Columns should be preferred to be positioned at the corners of the building and at the intersection of beams.
• They should be placed in such a way that the bending moments will be reduced in the beams.
• Avoid larger spans of beams.
• Avoid larger centre to centre distance between columns.
• Columns on property line.
• The main aim of the strong column weak beam strength concept in codes is to reduce the formation of mechanisms.
• The formation of plastic hinges in beams helps to build the most desired and suitable energy dissipating mechanism for structure.
• Strong column weak beam means that the flexural strength in column end should be greater than that of beam end.
• If a beam fails it will only affect that particular story. But if a column fails then the complete structure may fail.
• It is always preferable that hinges form in beams before they are formed in columns.
• At the plastic hinge locations, it is very important to provide sufficient confining reinforcement such that the required ductility is achieved.

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.

In the above given plan there are multiple dots and the lines. So these dots represents the columns and the line represents the beams.

Investigation of framing plan with respect to Symmetry in X-direction:-

• If we split the plan vertically half, it will look mirrored.
• So we conclude the given plan is symmentric in X-direction.

Investigation of the framing plan with respect to Symmetry in Y-direction:-

• If we split the plan horizontally half, it will look mirrored.
• So we conclude the given plan is symmetric in Y-direction.

Investigation of the framing plan with respect to redundancy in X-direction:-

• The columns in x-direction are comparatively more and evenly spaced.
• So the structure is more redundant in x-direction.
• Except the top and bottom column rows are having quite longer spans.
• The redundancy at the top portion is less when compared to middle portion for the x-direction.

Investigation of the framing plan with respect to redundancy in Y-direction:-

• The columns in y-direction are comparatively lesser bot evenly spaced.
• So the structure is redundant but lesser than that of in x-direction.

Investigation with respect to the Bi-directional stiffness:-

• The column in horizontal direction are connected with striaght to striaght beams.
• This means, any lateral load (Seismic force in x-direction) passing from left end of the building will have its path striaght till it reaches right end of the building.
• So no or less possibility of force diversion takes place.
• The column in vertical direction are not connected with striaght to striaght beams.
• So a lateral load passing from south side of building will get diverted to east and 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.

Investigation of the framing plan with respect to ductility:-

• Prima facie, the aspect ratio of the plan clearly hints more column in x-direction compared to y-direction. It will result in higher stiffness of the building in x-direction, which would reduce ductility in x-axis.
• Moreover, the x-force will easily pass from one bay to its next due to beam alignment. Whereas, the y-force will struggle to pass through.
• The north-south beams are arranged in zig-zag which leads to discontinuous bracing.
• We conclude that the given 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.

Vertical irregularity:-

• The building plan area is large in bottom stories.
• The floor plan areas are getting reduced with increase in height.
• With respect to vertical irregularities there are limitations on the change in the horizontal dimension of the lateral force resisting system of any storey as compared to storey below.

• The seismic response of buildings with such irregularity must be done using dynamic analysis that is these building offsets has to be crosschecked with reference to IS-1893-2016 Table-6 serial number(iii).

Soft storey:-

• The building has uncovered opening space in 6th floor.
• As there is no occupancy or a partition wall, this lead to plastic hinge formation at the columns (with reference to IS-1893-2016 Figure 4A).
• This has to be checked and sufficiently designed.

• A soft storey is a storey whose lateral stiffness is less than that of storey above.
• This has to be checked and we have to design the building accordingly. So that there will be no mistake in designing the buiding.

Slopy base:-

• The building is constructed on sloping ground at on e 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.
• In the right side of the building, there is an embankment.
• It means the building is constructed over a sloping ground and this will lead to a significant variation in the stiffness.
• So, 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.

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).

Structure failure reasons for the given figure are:

• Improper arrangement and placement of structural elements
• Structure is not symmetric Which must required for standard structure
• Larger strength is provided hence low ductility.
• No proper placement of masonry infill walls.
• Redundancy in the vertical direction of this building is not provided properly.

Open ground floor:-

• This denotes soft storey mechanism acting in the building.
• 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.

Rooms along 2 bays seperated by a open corridor:-

• As we can see at the above image that the upper portion and the bottom portion is for the rooms and because of that rooms these portions have higher mass because of the various furniture load,live loads etc.
• And the middle portion they left for the corridor as we can see in the above given image.
• The bays filled with room will have higher mass compared to middle bay left open for cooridor.
• This induces mass irregularity along height uniformly.

Two short sides covered with infill walls:-

• As we can see that the two shorter sides of the perimeter are not in filled in all the storeys, except for the certain openings at the ends of the corridor at the 2nd and the 4th floor and along the right-hand side of the ground floor.
• This means that the building at the left and the right of the given building is more stiffer during the earthquake.
• And they have built non structural wall.
• This means the building's left and right sides perphery is more stiffer seismic forces (especially along North-South).

Staicase portion at left top corner:-

• As we can see that the staircase and the lift portion of the building is not affected with the seismic force.
• These are the only elements that are not affected during the earthquake force.
• This is because the usually the staircase and the lift portion are more stiff than the other members of the building.
• It means it does not bend easily with the external forces and that is the reason that the staircase and the lift is intact in the position with much harm.
• Usually staircase and lift partions are stiffer than other parts because of concentrated concrete elements.
• So the staicase portion contains more stiffness in the provided building plan.

No framing connection along Y-direction:-

• 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.
• 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 dimension 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 ateral forces and hardly transfers it.
• As we can see in the above image that the columns have the longer direction in the Y-direction.
• But there is no direct connection or frame in the Y-direction which may connects the beam in the Y-direction.
• There are columns that are connected in the X-direction but the large side of the column is in the Y-direction which means that they are more stiffer in that direction whic can attract more lesser lateral force and this is one of the reason for the collpase of this building.

Variation in column sizes:-

• As we can see in the above given image that the sizes of the columns are different.
• We can say that the size of the interior column is big in size and the size of the exterior column is smaller as we can see in the above image.
• And because of the difference in the size of the columns the structure collapse because it may suffered heavy out of plan shear.
• 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 columns.
• So the concrete floor might have suffered heavy out-plane shear which would led to collapse.

RESULT:-

The column positioning concept is understood well.