Conceptual design of a building with columns and shear walls

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.

Ans-

1. Restraint of floor shrinkage Area –

Observation –

(a)Full restraints at all corner. (b) Corners are free but restraints along the edge. (c)Non Uniforms restraints

Conclusion- Option (a) will have higher restraints to shrinkage thus load to cracks in the floor. Option (b)is average , and option (c) will not effectively restraints to shrinkage.

2. Lateral Stiffness –

Observation –

• Comparatively lower lateral stiffness in both X & Y direction.
• Uniform and comparatively higher lateral stiffness in both X & Y direction.
• Non-Uniform lateral stiffness in X & Y direction

Conclusion- Option (b) will provide higher lateral stiffness for a pure seismic acceleration in X & Y direction.

3. Torsion with respect to vertical axis-

Observation –

• Corner wall provide more resistance to twist
• Edge wall provide good resistance to twist
• Irregular wall provide very poor resistance to twist.

Conclusion- Option (a) will be best with respect of twist for torsional resistance.

4. Vertical reinforcement for similar base capacity-

Observation-

• Walls are subjected to nominal flexural moment and heavy twisting moment.
• Walls are subjected to nominal twisting moment.
• Walls are subjected to heavy flexural moment and twisting moment.

Conclusion – Option (c) will require higher vertical R/F and option (b) is least.

5. Static Eccentricity-

Observation-

• Symmetric structure
• Symmetric structure
• Un-symmetric structure

Conclusion- Only option c will have static eccentricity and it’s significant.

6. System Redundancy-

Observation-

• Uniform placement of lateral load supports in both x, y direction
• Uniform placement of lateral load supports in both x, y direction
• Concentrated lateral load supports in both direction

Conclusion- Option (a) and (b) are more redundant than option (c)

7. Foundation System-

Observation-

• Due to L-shaped wall the geometry of foundation and its design will be critical.
• Walls are in line easy to fix geometry and design of foundation.
• Walls are in line easy to fix geometry of foundation but design will be critical.

Conclusion- Option a is recommended to have peripheral box foundation and Option b & c shall be incorporated with isolated footing or strip footing.

8. Architectural constraints-

Observation-

• Walls obstruct only corner
• Walls obstruct faces of building
• Vertical wall will not allow passage

Conclusion- Option (a) is preferred and option b is subjected to discussion & option c is not accepted.

Question 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.

Ans-

1. Indirect lateral load transfer in x,y direction- The beams are connecting to another beams which is ineffective during lateral load transfer in such condition the primary beam is prone to shear failure.
2. Eccentricity- The longer the distance between center of stiffness, the higher torsion moment of the building. As per given plan eccentricity the building is subjected to higher torsion moment in both x, y direction.
3. Strong beam weak column possibility- The size of columns majorly mentioned 25X25 cm, but the entire beam have much deeper section 25X50 cm. Second moment of inertia of beam is visually higher than column so if the column r/f is subsequently greater than beams then ok and since we do not have r/f detail, this is doubtful, so it is recommended to have slight bigger size for column.

Alternative Plan-

Question 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.

Ans-3

Observation-

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

Random Internal Column- Internal column are random, so they are unable to transfer lateral loads efficiently can’t achieve desirable response, so we have to assume and design those column for only gravity.

Floor to floor Variations in partitions wall & Beams- Partitions wall provide an important mass control in a building and beams play key role in shear transfer from floor to bream. These entire elements are not in order, so heavy distribution and distortion of lateral loads will happen.

Suggestion on the lateral load system & its foundation- Heavy torsional forces should be controlled by respective lateral load , so corner shear walls are suggested .The proportion of floor area is greater on the left side than right side, so corner shear wall on the left side should be heavier than right. Corner shear wall shall preferably by support over a box foundation. Shear walls should be connected by link beam to transfer shear from on the other. The internal columns shall be supported by isolated footing .Those isolated footing can be connected with shear walls through is beams.