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
Observations
Option A-Full restrainS at the corners
Option B-Corners are left free but restrained at edges
Option C-Non uniform restrains
Conclusions
Option A will have higher restraint to shrinkage ,thus lead to cracks in the floor
Option B will have average restraint
Option C will not effectively restrained to shrinkage, so possibly less cracks
Lateral Stiffness
Observations
Option A-Comparatively lower lateral stiffness in both X and Y directions.
Option B-Uniform and comparatively higher lateral stiffness in Both X and Y directions
Option C-Non uniform lateral stiffness in both X and Y direction
Conclusions
Option B will provide higher lateral stiffness for a pure seismic acceleration in X and Y directions
Torsion with respect to vertical axis
Observations
Option A-Corner walls provide more resistance to twist
Option B-Edge walls provide good resistance to twist
Option C-Inner walls provide very poor resistance to twist
Conclusions
Option A will be best option with respect to torsional resistance whereas Option C is worst one
Vertical Reinforcement for similar base capacity
Observations
Option A-Walls are subjected to nominal flexural moment and heavy twisting moment
Option B-Walls are subjected to nominal twisting moment
Option C-Walls are subjected to heavy flexural moments and twisting moments.
Conclusions
Option C will require higher vertical reinforcement
Option B will require less vertical reinforcement
Static eccentricity
Observations
Option A-Structurally symmetric
Option B-Structurally symmetric
Option C-Structurally unsymmetric
Conclusions
Only option C will have static eccentricity and it is significant
System’s redundancy
Observations
Option A-Uniform placement of lateral load supports in both the axes
Option B-Uniform placement of lateral load supports in both the axes
Option C-Concentrated lateral load supports in both the axes
Conclusions
Option A and Option B are more redundant than Option C
Foundation System
Observations
Option A-Due to L shaped walls the geometry of foundation and its design will be critical
Option B-Walls are in line.Easy to fix the geometry and design of foundation
Option C-Walls are in line and concentrated .Easy to fix the geometry of foundation but critical in design
Conclusions
Option B and Option C shall be incorporated with isolated footing or strip footing
Option A is recommended to have peripheral box foundation
Architectural Constraints
Observations
Option A-Walls obstruct only at corners
Option B-Walls obstruct the faces of building
Option C-vertical wall will not allow passage
Conclusions
Option A is most preferred one in architectural aspects
Option B is subjected to discussion
Option C will not accepted both in 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.
Ans
Indirect Lateral Load Transfer in X and Y Directions
Beams are connected to another beams and it is ineffective during lateral load transfer.in such conditions
The primary beams is subjected to shear failure
Eccentricity
The longer the distance between the centre of mass and centre of stiffness the higher is the torsional moment in the building
As per the given building plan, the building is subjected to higher torsional moment in both X and Y directions
Strong Beam Weak Column
The size of columns majorly mentioned is 25cmx25cm,but the beams are of depth 50cm
Inertia of beams is visually higher than columns
If the reinforcement of the column is higher than that of the beam, then it is ok.
Since we don't have any reinforcement details it is doubtful. And it is recommended to have slightly bigger columns
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
Observations
Floor plan
The floor plan is not either uniform or symmetrical . So naturally any lateral load acting in the building will not take 100% influence in the same direction. So a special lateral load resisting system for these torsional moments needs to be accommodated.
Random Internal Columns
Since the internal columns are random, they are unable to transfer the lateral load effectively. So we have to assume that the columns are designed only for gravity loads.
Floor to Floor Variations in Partition Walls
Partition walls are an important mass control element in the building and play a vital role in the transfer of shear force from floor to beam.All these elements are not in order,there is heavy distortion in the lateral load distribution.
Conclusions and suggestions
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.So corner shear walls on the left side should be heavier than right
Corner shear walls shall preferably supported over a box foundation,Shear walls should be connected by link 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
