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-->
1.RESTRAINT OF FLOOR SHRINKAGE
OBSERVATION;
(a) Structure (A): Full restraint at all corners
(b) Structure (B): Corners are left free but restraint along edges
(c) Structure (C): Non-uniform restraint
CONCLUSION;
(a) Structure (A): will have higher restraint to shrinkage,thus lead to crack in the floor.
(b) Structure (B): is average
(c) Structure (C): will not effectively restraint the shrinkage,so possibly less cracks.
2.THE LATERAL STIFFNESS
OBSERVATION;
(a) Structure (A): Comparitively lower lateral stiffness in both x and y
(b) Structure (B): Uniform and comparitively higher lateral stiffness in both x and y
(c) Structure (C): Non-uniform lateral stiffness in both x and y
CONCLUSION;
Structure (b) will provide higher lateral stiffness for a pure seismic accelaration in both x and y direction
3. TORSION WITH RESPECT TO VERTICAL AXSIS
OBSERVATION;
(a) Structure (A): corner walls provide more resistance to twist
(b) Structure (B): edge walls provide good resistance to twist
(c) Structure (C):Irregular walls provide very poor resistance to twist
CONCLUSION;
Structure(a) will be best option with respect to torsion resistance whereas structure (c) is the worst one
4.VERTICAL REINFORCEMENT FOR SIMILAR BASE CAPACITY
OBSERVATION;
(a) Structure (A): Walls are subjected to nominal flexural moment and heavy twisting moment
(b) Structure (B): Walls are subjected to nominal twisting moment
(c) Structure (C): Walls are subjected to heavy flexural moment and twisting moment
CONCLUSION;
Structure(c) will require heavy vertical reinforcement and structure (b) will require least.
5.STATIC ECCENTRICITY
OBSERVATION;
(a) Structure (A): Structurally symmetric
(b) Structure (B): Structurally symmetric
(c) Structure (C): Asymmetric
CONCLUSION;
Only Structure(c) will have static eccentricity and it is significant
6.SYSTEM'S REDUNDANCY
OBSERVATION;
(a) Structure (A): Uniform placement of lateral load supports in both the axes
(b) Structure (B): Uniform placement of lateral load supports in both the axes
(c) Structure (C): Concentrated of lateral load supports in both the axes
CONCLUSION;
Structure(a) and (b) are more redundant than structure (c)
7. FOUNDATION SYSTEM
OBSERVATION;
(a) Structure (A): due to L shaped walls , the geometry of foundation and its design will be critical
(b) Structure (B): walls are in line ,easy to fix the geometry and design of foundation
(c) Structure (C): walls are in line and concentrated,easy to fix the geometry of foundation but critical in design
CONCLUSION;
- Structure(b) and (c) shall be incorporated with isolated footings/strip footings
- structure (a) is recommended to have peripheral box foundation
8. ARCHITECTURAL CONSTRAINTS
OBSERVATION;
(a) Structure (A): walls obstruct only corner
(b) Structure (B): walls obstruct the faces of building
(c) Structure (C): vertical wall will not allow passage
CONCLUSION;
- Architectural speaking Structure(a) is the preffered one
- Structure (b) is subjected to discussion
- structure (c) will not be accepted both architecturally and commercially
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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-->
Indirect Lateral Load Transfer in X and Y Directions -
The beams are connecting to other beam which is ineefective during lateral load transfer . In such conditions ,the primary beam is prone to shear failure
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 eccentricity , the building is subjected to higher torsional moment (torsional flexibilty ) in both x and y direction
Strong beam weak column possibilty-
- The size of columns majorly mentioned as 25cm*25cm,but all the beam have much deeper section (50cm)
- 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 details, this is doubtful
- it is recommended to have slightly bigger sizes of columns
Alternative scheme
To provide the columns in proper alignment of structure in x and y direction of building
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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 framing 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. A special lateral load resisting system for these torsional moments should be accomodated.
RANDOM INTERNAL COLUMNS
Since the internal columns are random , they are unable to transfer lateral loads efficiently and cannot achieve desirable building responses. 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 important mass control ina 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 and its foundation:
- Heavy torsional forces should be controlled by the respective lateral load systems. 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 the beams
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