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:-

Restrain of floor shrinkage	Observation	Full restrain at all corners	Corners are left free but restrain along the edge	Non-uniform restrains
	conclusion	Option(a) will have higher restrain to shrinkage, thus lead to crack in the floor, Option(b) is average. Option(c) will not effectively restrain the shrinkage, so possibly less cracks
The lateral stiffness	Observation	Competitively lower lateral stiffness in both x and y	Uniform and competitively higher lateral stiffness in x & y	Non uniform lateral stiffness in x and y
	Conclusion	Option(b) will be provide higher lateral stiffness for a pure seismic acceleration in x and y direction
Torsion with respect to vertical axis	Observation	Corner wall provides more resistance to twist	Edge walls provides good resistance to twist	Irregular wall provide very poor resistance to twist
	Conclusion	Option(a) will be best option with respect to torsional resistance Option(c) is the worst one
Vertical R/F for similar base capacity	Observation	Wall are subjected to normal flexural moment and heavy twisting moment	Wall are subjected to nominal twisting moment	Walls are subjected to heavy flexural moment and twisting moment
	Conclusion	Option(c) will require higher vertical reinforcement Option(b) will require the least.
Static eccentricity	Observation	Structurally symmetric	Structurally symmetric	Un-symmetric
	Conclusion	Option(c) will have static eccentricity and it is significant
System’s redundancy	Observation	Uniform placement to lateral load support in both axes	Uniform placement to lateral load support in both axes	concentrated lateral load support in both axes
	Conclusion	Option (a) and (b) are more redundant then option (c)
Foundation systems	Observation	Due to ‘L’ shape wall the geometry of foundation and its design will be critical	Wall are in line easy to fix the geometry and design of foundation	Wall are in line & concentrated easy to fix the geometry of foundation but critical in design
	Conclusion	Option(b) and (c) shall be incorporated with isolates footing/strip footings Option(a) is recommended to have peripheral box foundation
Architectural constraints	Observation	Wall are only corners	Wall obstruct faces of building	Vertical wall will not allow passage
	Conclusion	Option(a) is the preferred one Option(b) is subjected to discussion Option(c) will not be accepted both architectural 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.

Ans:-

Issues in the given framing plan:

• Indirect lateral load transfer in X and 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.

• Eccentricity-

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

• Strong beam weak column possibility-

The size of column majorly mentioned as 25cm x 25cm. but all the beams have much deeper section inertia of beam is visually higher than column. If the column reinforcement is adequately greater than that of beam 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.

Ans:-

Observation

• 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 achive desirable building response. So we have to assume and design those column for gravity loads.

• Floor to floor variation in partition wall and beams-

Partition walls provides an important mass control in 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.

• Suggestions on the lateral load system & its foundation
  • Heavy torsional forces should be controlled by respective lateral load system.so “corner shear walls” are suggested.
  • The proportion of floor area is greater on the left side then the right side. So corner shear walls on the on the left side should be heavier then the right.
  • Corner shear walls shall preferable 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 wall through tie beams.
• Proposed systematic plan of lateral load system-

Superstructure

Sub-structure