Conceptual design of a building with columns and shear walls
Conceptual design of a building with columns and shear walls
Jagadeep.M.M
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.
For Arrangement(a)
Restraint of floor shrinkage:
- Observation: Full restraints at all corners.
- Conclusion: Arrangement (a) will have higher restraint to shrinkage, thus lead to cracks in the floor.
The Lateral stiffness:
- Observation: Comparatively lower lateral stiffness in both X & Y
Torsion with respect to vertical axis:
- Observation: Corner walls provide more resistance to twist.
- Conclusion: Arrangement(a) will be best option with respect to Torsional resistance.
Vertical reinforcement for similar base capacity:
- Observation: Walls are subjected to nominal flexural moment & heavy twisting moment.
Static eccentricity:
- Observation: Structurally symmetrical.
System's redundancy:
- Observation: Uniform placement of lateral load supports in both the axies.
- Conclusion: Arrangement (a) has more redundant.
Foundation systems:
- Observation: Due to "L" shap walls, the geometry of foundation & it's design will be critical.
- Conclusion: Arrangement (a) is recommended to have peripheral box foundation.
Architectural constraints:
- Observation: Walls obstruct only comers.
- Architecturally speaking.Arrangement(a) is the preferred one.
For Arrangement(b)
Restraint of floor shrinkage:
- Observation: Corners are left free but restraints along the edge.
- Conclusion: Arrangement (b) will have average restraint to shrinkage, thus average cracks.
The lateral stiffness:
- Observation: Uniform & comparatively higher lateral stiffness in both X & Y.
- Conclusion: Arrangement (b) will provide higher lateral stiffness for a pure seismic acceleration in X & Y.
Torsion with respect to vertical axis:
- Observation:Edge walls provide good resistance to twist.
Vertical reinforcement for similar base capacity:
- Observation: Walls are subjected to nominal twisting moment.
- Conclusion: Arrangement (b) will require the least.
Static eccentricity:
- Observation: Structurally symmetric.
System's redundancy:
- Observation: Uniform placement of lateral load supports in both the axes.
- Conclusion: Arrangement (b) has more redundant.
Foundation systems:
- Observation: Walls are in line, Easy to fix the geometry & design of foundation.
- Conclusion: Arrangement (b) shall be incorporated with isolated footing/strip footings.
Architectural constraints:
- Observation: Walls obstruct the faces of building.
- Conclusion: Architecturally speakign. Arrangement (b) is subjected to discussion.
For Arrangement (c)
Restraint of floor shrinkage:
- Observation: Non - uniform restraints.
- Conclusion: Arrangement (c) will not effectively restrain the shrinkage, so possibly less cracks.
The lateral stiffness:
- Observation: Non uniform Lateral stiffness in both X & Y.
Torsion with respect to vertical axis:
- Observation:Irregular walls provide very poor resistance to twist.
- Conclusion: Arrangement (c) is the wost one.
Vertical reinforcement for similar base capacity:
- Observation: Walls are subjected to heavy flexural moment & twisting moment.
- Conclusion: Arrangement (c) will require higher vertical reinforcement.
Static eccentricity:
- Observation: Un-symmetric.
- Conclusion: Only arrangement (c) has static eccentricity & it is significant.
System's redundancy:
- Observation: Uniform placement of lateral load supports in both the axes.
- Conclusion: Arrangement (b) has more redundant.
Foundation systems:
- Observation: Walls are in line, Easy to fix the geometry & design of foundation.
- Conclusion: Arrangement (b) shall be incorporated with isolated footing/strip footings.
Architectural constraints:
- Observation: Walls obstruct the faces of building.
- Conclusion: Architecturally speakign. Arrangement (b) is subjected to discussion.
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.
Issues in the given Framing plans:
- Indirect lateral load transfer in X & Y directions - The beams are connecting to another beams which is ineffective during lateral load transfer.
- In such conditions, the primary beam is prone the shear failure.
Issuces in the given Framing plan:
- Eccentricity : The longer the distance between center of mass & center of stiffness,the higher torsional moment at the building.
- As per the given plan eccentricity, the building is subjected to higher torsional moment( Torsional flexibility)in both X & Y directions.
Issues in the given framing plan:
Strong beam weak column possibility
- The sizes of column majorly mentioned as 25cm X 25cm.
- but all the beams have much deeper section (50cm).
- Inertia of beam is visually higher than column.
- If the column reinforcement is adequately greater than that of beams,we are okey & 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.
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 internal columns - since the intrnal columns are random, they are unable to transfer lateral loads efficiently & cannot achieve desirable building response.
- So, we have assume & design those columns for only gravity loads.
- Floor to floor variations in partition walls & beams - partition walls provide an important mass control in a building & beams play key role in shear transfer from floor to beam.
- All these elements are not in order. So heavy distortion & distribution of lateral loads will happen.
Suggesion 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 than the right side( irregular quadrilateral). So corner shear walls on the left side should be heavier than the right.
- Corner shear walls shal 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 of lateral load system:
Superstructure
Sub-structure
