Conceptual design of a building with columns and shear walls

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.

A.) FIGURE (A):

- This structure will have higher restraint to shrinkage, thus would lead to cracks in the floor.

- It would be the best structure with respect to torsional resistance when compared to the other two structures.

- It is more redundant.

- Structure (A) is recommended to have peripheral box foundation.

- It occurs lack of tensile capacity and force configuration.

FIGURE (B):

- This structure would have an average restraint to shrinkage.

- It would provide higher lateral stiffness for a pure seismic acceleration in X and Y direction.

- It would require lower vertical reinforcement.

- Uniform placement of lateral load supports in both axis. So it is more redundant.

- The walls are in line. So its easy to fix the geometry and design the foundation. It would be incorporated with isolated footings or strip footings.

FIGURE (C):

- This structure will not effectively restrain shrinkage so there would less cracks.

- It would be the worst structure with respect to torsional resistance.

- It also would require higher vertical reinforcement.

- When compared to the three structures, this structure would have static eccentricity and is significant.

- It is also incorporated with isolated or strip footings.

- It will not be accepted both architecturally and commercially.

2. Discuss the suitability for earthquake resistance of the moment resisting framing plan of a three-storey building depicted here (cross-sectional dimensions in cm), the eccentricity of the centre of mass (as centroid of floor plan) to the centre 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.

A.) The issues in the given framing plan are:

- The beams are connecting to another beams which is ineffective during lateral load transfer. So in these conditions, the primary beam is prone to shear failure.

- Eccentricity:

- As per the given plan eccentricity, in both the X and Y axis, there is a greater torsional moment applied to the structure.

- Because the beams in this structure are stronger than the columns, the design is weak column strong beam in nature.

- Therefore this structure need some shear wall to resist the column failure, when the lateral forces acting about the structure.

- In order to achieve lateral stiffness, mass regularity, and moment resisting factor, the connections between the column beams must be in the proper cross section area.

Alternative plan:

3. A multi-storey building with basement, with a quadrilateral (non symmetrical floor plan) plan as, has interior columns in an irregular (not in a grid) pattern in plan that serves architectural and functional considerations. Partition walls and interior beams supporting the slab have different layout 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.

A.) The observations from the given plan are:

- Floor plan:

The given floor plan is not either uniform or symmetric. A special lateral load resisting system for these torsional moments should be accommodated.

- Floor to floor variations in partition walls and beams:

All the elements of the plan are not in order which would cause distortion and distribution of lateral loads will happen.

- This plan is not suitable for heavy seismic zone areas.

- The flat slab is a better choice for this design than a regular beam-column junction since it supports the columns without the need of a beam, lowering the building's weight and indirectly enhancing the structure's flexibility.

- Corner shear walls are suggested.

- The internal walls can be supported by isolated footings.