Concept on positioning of Columns

CHALLENGE WEEK 4

ETABS

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.

Answer:

• INVESTIGATION OF THE 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 symmetric in X-direction

• INVESTICATION 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

• INVESTIGATION OF THE FRAMING PLAN WITH RESPECT TO REDUNDANCY IN Y-DIRECTION

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

• INVESTIGATION WITH RESPECT TO BI-DIRECTIONAL STIFFNESS

• the columns in horizontal direction are connected with straight to straight beams. This means, any lateral load (seismic force in x-direction) passing from left end of the building will have its path straight till it reaches right end of the building.so no or less possibility of forces diversion takes places
• the columns in vertical direction are not connected with straight to straight beams.so a lateral load passing from south side of building will get diverted to east &west direction to reach next bay. There will be major forces diversion take place with seismic forces 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 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-forces will struggle to pass through. The north south beams are arranged in zigzag which leads to a discontinuous bracing.
• We can conclude that the 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.

Answer:

• VERVTICALLY IRREGULARITY-

• the building plan area in bottom is large in bottom stories. The floor plan areas are getting reduced with increase in height. These building offsets has to be crosschecked with reference to IS-1893-2016 table-6serial number(iii)
• Vertical geometric irregularity shall be considered to exist, when the horizontal dimension of the lateral force resisting system in any storey is more than 125 percent of the storey below.
• In buildings with vertical geometric irregularity and located in Seismic Zones III, IV and V, the earthquake effects shall be estimated by Dynamic Analysis Method

• SOFT STOREY-the building has uncovered opening space in 6^th floor.as there is no occupancy or a partition wall ,this will lead to plastics hinge formation at the columns (with reference to IS-1893-2016 figure4A).this has to be checked and sufficiently designed

SLOPY BASE-the building is constructed on a slopping ground at one side this well lead to significant variations in the stiffness (with reference to IS1893-2016 figure 5E). 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.

Answer:

• OPEN GROUND FLOOR-this denotes soft storey mechanism acting in the building

• ROOMS ALONG 2 BAYS SEPARATED BY AN OPEN CORRIDOR – the bays filled with room 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 buildings 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 portion are stiffer than other parts because of concentrated 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 AXIS -the columns are having longer dimension in y-direction. this means are more stiffer along y-direction. but the beams are running only along x-direction which would attract much lesser lateral forces and hardly transfers it.
• VARIATION IN COLUMN SIZES-this internal columns are having higher sizes compared to exterior columns (this is common because of higher load demands ). But this will leads to significant differences in lateral drift of columns,. So the concrete floor might have suffered heavy out-of-plan shear which would led to collapse