Conceptual design of a building with columns and shear walls

1. The building shown, 20 x 30m in plan, has column on a 5 x 5m grid and shear walls( with dimensions shown in m, 250mm in thickness) in three alternative arrangements. (a, b, c), all with the same cross sectional area of the shear wall. 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 systems redundancy, foundation systems, architectural constraints etc.,

Answer :

a)

• this structure can be considered asymmetrical in nature
• shear wall of the shape “L” cannot be considered
• the non-rectangular walls shows there might be some uncertainty
• lack of tensile capacity can be observed here
• walls have higher strength and stiffness

b)

• shear walls are placed in symmetry
• 2-way symmetry can be done for the shear walls
• shear walls can be considered at every floor due to symmetry and provide uniform restraint in EQ forces and deformation, reducing the uncertainty for EQ

c)

• shear walls provided are not in 2 orthogonal direction and hence the structure is observed to be asymmetric in nature
• during seismic events the structure will have larger moments as the foundations, columns and shear walls are placed differently
• the load carried will not be equal in both directions, hence it will lack redundancy
• center of gravity and mass will not be far from each other
• the lateral stiffness will be more on the left hand side of the structure, leading to less damage

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.

Answer :

• in this frame the eccentricity of x-direction is observed to be more than in y-direction wrt center of mass and stiffness
• weak column - strong beam is observed as the beams provided are weaker than the columns
• hence the structure required shear wall, a combination of columns and shear walls to resist seismic events
• beams provided are observed to have continuity in x-direction, such as B1, B2 & B11, B12, B9 and B1 in Y-direction respectively
• the remaining beams are observed to have discontinuity causing the load transfer difficult evenly to resist lateral forces
• generally, the beams of longer spans govern in gravity loads
• lateral stiffness, mass irregularity and moment resisting factor can be obtained with proper beam-column connections

• after changing the column orientations of C8 and C5 with combined footing to avoid loss of center of stiffness during lateral forces
• additional column C10 will help in center of mass and stiffness

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.

Answer :

• the given columns for the given plan is unsuitable for seismic prone areas
• the plan and columns are unsymmetric
• the interior beams will support the slab and resist the lateral load
• for such structures flat slabs are provided
• beams in this structure that are cantilever will have higher sections and reinforcements
• the most appropriate solution would be to provide shear walls from footings and should help in resisting lateral forces