Conceptual design of a building with columns and shear walls

AIM :

To investigate the suitability of given plan with respect to seismic behavior

To schematically draw a seismic viable structural plan

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 -
Plan 1 (20 × 35 m) for shear wall

Restraint of floor shrinkage :

• Fully restrained at all corners.
• Have higher restrained to shrinkage, leads to partially non-structural cracks.

The lateral stiffness :

• Medium to lower lateral stiffness in both x & y.

Torsion with respect to vertical axis :

• Corners walls provides high resistance to twist.
• This Plan is best option with respect to Torsional resistance.

Vertical reinforcement for similar base capacity :

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

Static eccentricity :

• Structurally symmetric

System's redundancy :

• Uniform placement of lateral load support in both the axis.
• This Plan more redundant than Plan 3.

Foundation system :

• Due to 'L' shaped wall, the foundation design will be complex.
• This Plan shall be incorporated with peripheral box foundation.

Architectural constraints :

• Wall obstructs only corners. Architecturally this Plan is preferred.

Plan 2 (20 × 35 m) for shear wall

Restraint of floor shrinkage :

• Partially restrained, free along the edges but restrained along the edges.
• Average restrained to shrinkage.

The lateral stiffness :

• Uniform and comparatively higher stiffness in both x & y.
• This Plan provides higher lateral stiffness for a pure seismic acceleration in x & y directions.

Torsion with respect to vertical axis :

• Edges walls in x & y direction provides good resistance to twist.

Vertical reinforcement for similar base capacity :

• Walls are subjected to nominal twisting moment.
• This Plan will require lest vertical reinforcement.

Static eccentricity :

• Structurally symmetric

System's redundancy :

• Uniform placement of lateral load support in both the axis.
• This Plan more redundant than Plan 3.

Foundation system :

• Walls in line, easy to fix the geometry and design of foundation.
• This Plan shall be incorporated with isolated footings/ strip footings.

Architectural constraints :

• Wall obstructs the faces of building. Architecturally this Plan is can be preferred upon proper discussion.

Plan 3 (20 × 35 m) for shear wall

Restraint of floor shrinkage :

• Non-uniform restraints.
• Poorly restrained to shrinkage, so possibly less non-structural cracks.

The lateral stiffness :

• Non-uniform Lateral stiffness in both x & y.

Torsion with respect to vertical axis :

• Irregular Walls provides very poor resistance to twist.
• This Plan is worst option with respect to Torsional resistance.

Vertical reinforcement for similar base capacity :

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

Static eccentricity :

• Structurally symmetric
• This Plan will have static eccentricity and it is significant.

System's redundancy :

• Concentrated lateral load support in both the axis.
• This Plan less redundant than other Plan.

Foundation system :

• Walls in line & concentrated, easy to fix the geometry of foundation but critical design of foundation.
• This Plan shall be incorporated with isolated footings/ strip footings.

Architectural constraints :

• Vertical wall will not allow passage .Architecturally this Plan is not preferred.

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 :

Issues in the given framing Plan

Indirect lateral load transfer in X & Y direction: The beams are connecting to another beams which is ineffective during lateral load transfer. In such conditions, the primary beam is prone to shear failure. As boxed out in the figure.

Eccentricity : The longer the distance between center of mass and center of stiffness, the higher torsional moment of the building.

Strong beam and weak column possibility : All the columns in the plan is 25/25 C1-C9 and all the beams have the size of 25/50 B1-B12. So inertia of beam is higher than column. So as a solution we can increase the size of column to make column strong.

Also we can avoid this issues by designing the Plan like this -

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 :

Floor Plan - The Floor plan of building is not either uniform or symmetrical. So naturally any laterally any lateral load induced in the floor will not have 100% influence in same direction.

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 loads.

Floor to Floor variation 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 Plan -

Corner Shear walls are suggested to control heavy torsional forces.

The left side of floor area is greater than the right side of floor area. So corners shear walls on the left side should be heavier than the right.

Corner Shera 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 footing can be connected with Shear walls through Tie beams.