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.

Ans:

Restraint of floor shrinkage - In (a),the restraint of floor shrinkage will be more as it will restrain in all the corners leading to more cracks on the floor.

Lateral Stiffness - (b) will provide higher lateral stiffness as it is uniformly distributed.In option (a),shear walls are at the corners which will not resist much of the lateral loads.

Torsion - Option (b) will be the best option as the shear walls are present at the corners which will provide more resistance to twisting moment than the other two options.

Vertical reinforcement required - Option (c) will require the highest reinforcement as due to its irregular placement,both torsion as well as flexural moment will be heavy.Option (b) will require the least reinforcement.

Static eccentricity - Option (c) will have static eccentricity as it is unsymmetric.

System redundancy - Option (c) is least redundant than (a) and (b).

Foundation System - Option (b) and (c) will be incorporated with isolated footings and option (a) is recommended to have peripherial box foundation.

Architectural Constraints - Option (a) is the preferred one.

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: The issues in the given framing plan are:-

Indirect lateral load transfer in X and Y directions

Here,beams are connected to the other beams instead of columns.Direct beam to column connection is absent.In such conditions,primary beam is prone to shear failure.

Eccentricity

The longer the distance between centre of mass and centre of stiffness,the higher is the torsional moment of the building.In the plan,the building is subjected to higher torsional moment in both X and Y direction.

Strong beam weak column possibility

The sizes of the beam are higher than that of the columns.

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: Observations in the given floor plan are:-

Floor plan

The floor plan of the building is neither uniform nor symmetric.Any lateral load induced in a particular direction will not have 100pc influence in the other direction.

Random internal columns

The internal columns are not uniform and does not have uniform transfer of lateral load from one part to other.These columns are constructed for the sole purpose of withstanding gravity loads.

Floor to floor variations in partition walls and beams

Partition walls play an important role in the transfer of shear from floors to beams.If all these elements are not in proper order than heavy distortion of distribution of lateral loads will happen.