Concept on positioning of Columns

CONCEPT ON POSITIONING OF COLUMNS

AIM-

TO STUDY THE CONCEPT OF POSITIONING OF COLUMN

OBJECTIVE -

• To understand the impact of vertical and horizontal members on the seismic behavior of a structure
• To understand different irregularities in a building.

Question 1:

Comment on the layout of the framing plan concerning earthquake resistance in the two horizontal directions X or Y (dots are columns, and lines depict beams). Look into aspects of symmetry, redundancy, and bi-directional stiffness and ductility.

SYMMETRY -

• The plan is symmetric in X as well as in Y direction.
• This structure has an exact matching of form and arrangement of parts on opposite sides of the boundary at X and Y.
• The load resistance framing in this symmetrical structure is distributed evenly throughout the structure.

REDUNDANCY

• The column in the x-direction are comparatively more and even more space. So, the structure is more redundant in the x-direction. Except for the top and bottom columns, rows are having quite longer spans.
• The column in Y-direction are comparatively lesser but evenly spaced. So the structure is redundant but lesser than that of in the x-direction.

BI-DIRECTIONAL STIFFNESS

• The column in the horizontal directions are connected with straight-to-straight beams. This means any lateral load (seismic force in X-direction) passing from the left end of the building will have its path straight till it reaches the right end of the building. So no or less possibility of force diversion taking place.
• The column in the vertical direction are not connected with straight-to-straight beams. So a lateral load passing from the south side of the building will get diverted to the east and west direction to reach the next bay. There will be a diversion taking place with seismic force in the y-direction. The framing plan is weak concerning Bi-direction stiffness in Y-direction.

DUCTILITY-

• Prima facie, the aspect ratio of the plan hint at more column in the x-direction compared to the y-direction. It will result in higher stiffness of the building in the x-direction, which would reduce ductility in the x-axis.
• Moreover, the x-force will easily pass from one bay to the next due to beam alignment. Whereas, the force will struggle to pass through. The north-south beams are arranged in a zig-zag which leads to discontinuous bracing.
• We can conclude that the given framing plan is more ductile in the y-direction than that in the 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 the bottom stories. The floor plan area is getting reduced with an increase in height. These building offsets have to be crosschecked concerning IS-1893-2016 Table 6 serial number (iii)
• SOFT STOREY - The building has an uncovered opening space on the 6th floor. As there is no occupancy or a partition wall, this will lead to plastic hinge formation at the column(concerning IS-1893-2016 Figure 4A). This has to be checked and sufficiently designed.
• SLOPY BASE - The building is constructed on sloping ground at one side. This will lead to significant variation in the stiffness (concerning IS-1893-2016 Figure 5E). Few levels may be subjected to a soft story mechanism.
• Failure of a soft story -The ground floor of the building is mostly considered a soft story because it has a short column, no shear wall, and a lot of mass properties. Damage may also occur, such as a collapse, buckling of the column, etc.

Question 3:

A 4-story hotel building has an open ground floor for the restaurant. Story 2 to 4 has one row of rooms along each long side in the plan, separated by a corridor. The two short sides of the perimeter are fully in-filled in all stories, except for certain openings at the ends of the corridor at stories 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 structural design features and 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)?

• OPEN GROUND FLOOR - This denotes a soft story mechanism acting in the building
• ROOMS ALONG 2 BAYS SEPARATED BY AN OPEN CORRIDES - This bay filled with rooms will have higher mass compared to the middle bay left open for the corridor. This includes mass irregularity along height uniformly.
• TWO SHORT SIDES COVERED WITH INFILL WALLS - This means the buildings left and right sides' periphery is 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 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 dimensions in Y-direction. This means they are more stiffer along the y-direction. But the beams are running only along the x-direction which would attract much lesser lateral forces 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 a significant difference in the lateral drift of columns. So the concrete floor might have suffered heavy out-of-plane shear which would lead to collapse.