Conceptual design of a building with columns and shear walls

OBJECTIVE: 1] To investigate the suitability of the given plan With respect to seismic behavior

2] To schematically draw a viable seismic structure.

Question 1:
The building shown, 20 × 35 m in plan, has columns on a 5 × 5 m grid and shear walls (with dimensions shown in m, 250 mm in thickness) in three alternative arrangements, (a), (b), (c), all with the same total cross-sectional area of the shear walls. Compare the three alternatives, taking into account the restraint of floor shrinkage, the lateral stiffness, and the torsional one with respect to the vertical axis, the vertical reinforcement required for the same total flexural capacity at the base, the static eccentricity, the system’s redundancy, foundation systems, architectural constraints, etc.

ANSWER=

Figure A:

1. The structure is considered asymmetrical according to the regulations and the grid system.

2. Structural components do not have sufficient tensile capacity and force configuration.

3. The design will have some uncertainty regarding certain seismic response properties of non-rectangular walls.

4. This construction cannot be considered as horizontal unevenness and shear wall shape as L.

5. Structural walls have high stiffness and strength in both directions and are therefore subject to biaxial bending and bidirectional shear during earthquakes.

Figure B:

1. The structure's frame can be considered symmetrical in the grid-located shear wall according to the symmetrical ideology of the design.

2. The wall system of this frame can be arranged in two horizontal directions with as much two-way symmetry as possible to distribute the load in a balanced transfer from multiple sides.

3. Shear walls are arranged similarly and symmetrically so that they can be considered at each level fairly, minimizing uncertainty about seismic response and transferring loads uniformly across the frame.

Figure C:

1. In the floor plan of the structure, the placement of the columns is inherently asymmetrical and is not acceptable by ordinance and is a code provision.

2. The foundation or foot may interfere with the placement of the foundation foot, and in practice, this may not be possible

3. The left wall provides more eccentricity, and the lateral stiffness is greater. Therefore, if this happens, then the damage or failure will be less.

4. The center of mass and center of gravity of such a structure will not be reliable according to the structure design, it does not satisfy the equation of equilibrium for the transfer of loads

5. This type of shear wall system will not carry the load equally in both directions. the structure lacks redundancy.

Question 2:
Discuss the suitability for earthquake resistance of the moment-resisting framing plan of a three-story building depicted here (cross-sectional dimensions in cm), the eccentricity of the center of mass (as the centroid of floor plan) to the center of stiffness (from the moments of inertia of the columns) are shown. Suggest an alternative. Also, is there torsional flexibility? Are the two fundamental translational modes of vibration larger than the fundamental torsional mode of vibration? Discuss qualitatively.

ANSWER=

Issues in the given Framing plan:

Indirect lateral load Transfer in X and Y directions - The beams are connecting to another beam which is ineffective during lateral load transfer. In such conditions, the primary beam is prone to shear failure.

Issues in the given Framing plan:

Eccentricity - The longer the distance between the center of mass and the center of stiffness, the higher the torsional moment of the building. As per the given plan eccentricity, the building is subjected to higher torsional moment (Torsional flexibility) in both X and Y directions.

Issues in the given Framing plan:

Strong beam Weak Column possibility - The size of columns is majorly mentioned as 25 cm x 25 cm. But all the beams have a much deeper section (50cm). The inertia of the beam is visually higher than the column. If the column reinforcement is adequately greater than that of beams, we are okay and since we do not have the reinforcement detail, this is doubtful. It is recommended to have slightly bigger sizes for columns.

ALTERNATE SCHEME:

Question 3:

A multi-story building with a basement, with a quadrilateral (nonsymmetrical floor plan), plan as, has interior columns in an irregular (not in a grid) pattern plan that serves architectural and functional considerations. Partition walls and interior beams supporting the slab have a different layouts in different stories. However, there is no constraint to the type, location, and size of the lateral force-resisting components and sub-systems on the perimeter. Proposals are to be made and justified for the choice of the lateral-load-resisting system and its foundation.

ANSWER=

Observations in the given Framing plan:

Floor plan - The floor plan of the building is not either uniform or symmetric. So naturally, any lateral load induced in the floor will not have 100% influence in the same direction. Definitely, a Special lateral load-resisting system for these torsional moments should be accommodated.

Random Internal columns - Since the internal columns are random, they are unable to transfer lateral loads efficiently and can not achieve desirable building responses. So we have to assume and design those columns for only gravity loads.

Floor-to-floor variations in Partition walls and Beams - Partition walls provide important mass control in a building and beams play a key role in shear transfer from floor to beam. All these elements are not in order. So heavy distortion and distribution of lateral loads will happen.

Suggestion on the Lateral load system & Its foundation:

1. Heavy torsional forces should be controlled by the respective lateral load system. So "Corner Shear walls" are suggested.

2. The proportion of floor area is greater on the left side than on the right side (Irregular quadrilateral). So Corner shear walls on the left side should be heavier than the right.

3. Corner Shear walls shall preferably be supported over a box foundation. Shear walls should be connected by Link beams to transfer shear from one to the other.

4. The internal columns shall be supported by isolated footings. Those isolated footings can be connected with Shear walls through Tie beams.