Conceptual design of a building with columns and shear walls

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.

As the structure is more restrained it will be more shrinking, expanding and cracking.

There will be more floor shrinkage and expanding in structure A whereas there will be minimum cracks in structure B and C.

The shear walls in the structure also decide whether the floor vibration is more or less in the structure it will be better placing a column at the corners.So that it will reduce the cracks instead of having a shear walls at the corner in the structure.

The Torsion in the structure C is more when compared to structure A and B because in the structure C there will be an eccentricity.

The redundancy in structure A and B is more when compared to structure C because there will be an eccentricity in structure C in the left side where the length of the wall 10 meters which cannot carry load equally in either direction.

For Lateral stiffness structure C is more laterally stiff when compared to the structure A and structure B because there will be an eccentricity in structure C in the left side where the length of the wall 10 meters which cannot carry load equally in either direction.

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.

As we can see in the above figure the centre of mass and centre of stiffness has some eccentricity in the plan thus it leads to torsional failure of the building

Here in the above picture the internal forces are not transferred to other beam so it will lead to bad structure design in consideration of the earthquake design

the secondary beam which acts on the secondary beam will create the torsional effect in the primary beam of the structure

There is a two way eccentricity in the frame along X direction and Y direction also with stiffness the eccentricity in X direction is larger when compared to Y direction.

There is only two continuous frames in the structure that is B1 and B2 in the X direction and B9, B10 and B11,B12 in the Y direction and all other beams are broken at the midway in the frame.

There will be Torsional flexibility in the structure.To avoid torsional restraints it is better to make the room dimensions even in both X and Y direction and placing the columns at the junction of two wall if it is feasible better to provide shear wall at appropriate location.

The fundamental translation mode of vibration is larger than the fundamental torsional mode of vibration

Question 3:

A multi-storey building with a 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.

Observations in the given Framing plan:

• 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. 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 response. So we have to assume and design those columns for only gravity load floor-to-floor
• Floor to floor variations in Partition walls and Beams - Partition walls provide an important mass control in a building and beams play 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:

• Heavy torsional forces should be controlled by the respective lateral load system. So "Corner Shear walls" are suggested.
• The proportion of floor area is greater on the left side than the right side (Irregular quadrilateral). So Corner shear walls on the left side should be heavier than the right.
• 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.
• The internal columns shall be supported by isolated footings. Those isolated footings can be connected with Shear walls through Tie beams.