Conceptual design of a building with columns and shear walls

CHALLENGE ON:-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
Restraint of floor shrinkage	obseravtion-	Full restraints at all corners	Corners are left free but restraints along the edge	Non-uniform restraints	Conclusion- Option (a) will have higher restraint to shrinkage, this lead to crackin the floor. Option (b) is average. Option (c) will not effectively restrain the shrinkage, so possibly less cracks
The lateral stiffness	obseravtion-	Comparatively lower lateral stiffness in both x and y	Uniform and comparatively higher lateral stiffness in both x and y	Non-uniform lateral stiffness in both x and y	Conclusion- Option (b) will provide higher lateral stiffness for a pure seismic acceleration in X and Y directions
Torsion with respect to vertical axis	obseravtion-	Corner walls provide more resistance to twist	Edge walls provide good resistance to twist	Irregular walls provide very poor resistance to twist	Conclusion- Option (a) will be the best option with respect to the torsional resistance wheras the Option (c) is the worst one
Vertical reinforcement for similar base capacity	obseravtion-	Walls are subjected to nominal flexural moment and heavy twisting moment	Walls are subjected to nominal twisting moment	Walls are subjected to heavy flexural moment and twisting moment	Conclusion- Option (c) will require higher vertical reinforcement and Option (b) will require the least
Static eccentricity	obseravtion-	Structurally symmetric	Structurally symmetric	Un-symmetric	Conclusion- Only option (c) will have static eccentricity and it is significant
System's redundancy	obseravtion-	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	obseravtion-	Due to "L" shaped walls, the geometry of foundation and its design will be critical	Walls are in line. Easy to fix the geometry and design of foundation	Walls are in line &Concentrated. Easy to fix the geometry of foundation but critical in the design	Conclusion- Option (b) and (c) shall be incorporated with isolated footings/strip footings. option (a) is recommendate to have peripheral box foundation.
Architectural constraints	obseravtion-	Walls obstruct only corners	Walls obstruct the faces of building	Vertical wall will not allow passage	Conclusion- Architecturally speaking, Option (a) is the prefered one. Option (b) is subjected to discussion option (c) will not be accepted both architecturaly and commerciallu.

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 direction - The beams are connecting to another beams whichineffective during lateral load transfer. In such conditions, the primary beam is prone to the shear failure.

Eccentricity- The longer the distance between centre of mass and the centre of stiffness, The higher 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.

Strong beam weak column possibility - The size of columns mjorly mentioned as 25cm x 25cm, but all the beams have much deeper section (50cms). Inertia of beam is visually higher than 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 slight bigger sizes for column.

Alternate 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:-

observation 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 lateral 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 variations in the partition walls and beams - Partition walls provide an important mass control in a building and beams play and key role in shear transfer from floor to beam. All these elemets are not in order. So heavy distortion and distribution of the lateral loads will happen.

Suggestion on the lateral load system and 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 the link beams to transfer shear from one to the other.
• The internal columns shall be supported by the isolated foortings. Those isolated footings can be connected with shear walls through tie beams.