Conceptual design of a building with columns and shear walls

CHALLENGE-5

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.

ANSWER:

CRITERIA
Static eccentricity	Observation	Structurally symmetric	Structurally symmetric	Un-symmetric
	Conclusion	Only Option(c) will have static eccentricity and it is significant
Systems redundancy	Observation	Uniform placement of lateral load supports in both the axes.	Uniform placement of lateral load supports in both the axes	Concentrated lateral load supports in both the axes
	Conclusion	Option(a) and (b) are more redundant than Option(c)
Foundation systems	Observation	Due to 'L' shaped Walls, the geometry of the foundation and its design will be critical	Walls are in line. Easy to fix the geometry and design of the foundation	Walls that are in line & concentrated. Easy to fix the geometry of the foundation but critical in design
	Conclusion	Option(b) and Option(c) shall be incorporated with isolated footings/ strip footings. Option(a) is recommended to have a peripheral box foundation
Architectural Constrints	Observation	Walls obstruct only corners.	Walls obstruct the faces of building	Vertical wall will not allow passage
	Conclusion	Architecturally speaking, Option(a) is the preferred one. Option(b) is subjected to discussion. Option(c) will not be accepted both architecturally and commercially

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:

Issues in the given Framing plan:

Indirect lateral load Transfer in X and Y directions :

• The beams are connecting to other beams which are ineffective during lateral load transfer. In such conditions, the primary beam is prone to shear failure.

Issues in the given Framing plan:

Eccentricity:

• The longer the distance between the center of mass and the center of stiffness, the higher the torsional moment of the building.
• As per the given plan eccentricity, the building is subjected to higher torsional moment (Torsional flexibility) in both X and Y directions.

Issues in the given Framing plan:

Strong beam Weak Column possibility:

• The size of columns is majorly mentioned as 25 cm x 25 cm. But all the beams have a much deeper section (50cm). The inertia of the beam is visually higher than the column. If the column reinforcement is adequately greater than that of beams, we are okay and since we do not have the reinforcement detail, this is doubtful.
• It is recommended to have slightly bigger sizes for columns.

ALTERNATIVE SCHEME:

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:

Observations in the Given Floor plan:

• Floor plan:

• The floor plan of the building is not either uniform or symmetric. So naturally, any lateral load induced in the floor will not have 100% influence in the 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 responses. So we have to assume and design those columns for only gravity loads.

• Floor-to-floor variations in Partition walls and Beams:

• Partition walls provide important mass control in a building and beams play a 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 on 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.

proposed schematic plan for the lateral load system:

........................................THE END.....................................................................