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 | Observation | 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, thus lead to cracks in the floor. Option(b) is average. Option(c) will not effectively restrain the shrinkage, so possibly less cracks. | |||
| The Lateral stiffness | Observation | 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 | Observation | 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 best option with respect to Torsional resistance whereas Option(c) is the worst one. | |||
| Vertical reinforcement for similar base capacity | Observation | 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 | Observation | Structurally symmetric | Structurally symmetric | Un- symmetric |
|---|---|---|---|---|
| Conclusion | Only Option(c) will have static eccentricity and it is significant | |||
| System's redundancy | Observation | 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 | Observation | 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 design |
| Conclusion | Option(b) and Option(c) shall be incorporated with isolated footings/strip footings. Option(a) is recommended to have peripheral box foundation. | |||
| Architectural constraints | Observation | Walls obstruct only corners. | Walls obstruct the faces of building. | Vertical wall will not allow passage |
| Conclusion | Architecturally speaking, Option(a) is the preferred one. Option(b) is subjected to discussion. Option (c) will not be Accepted Both 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.
Answer:-
ISSUES IN THE GIVEN FRAMING PLAN:
Indirect lateral load transfer in X and Y direction – the beams are connecting to another beams mixes ineffective during natural loads transfer in such condition, the primary beam is prone to shear failure
Eccentricity:-
The longer the distance the distance between center of mass and centre the centre of stiffness, the high the higher torsional moment off the building come on as per the given plan eccentricity, the building is subjected to higher torsional and moment (torsional flexibility ) invert x and y directions.
Strong beam weak column possibility: –
The size of column majorly mentioned as 25 cm x 25 cm but all the beams have much deeper section 50 cm. Inertia of beam is visually higher than column. If the column reinforcement is adequately greater than that of beams, we are OK And since we do not have the reinforcement details, this is doubtful It is recommended to have slight bigger size of column.
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:-
Floor plan :-
The floor plan of the building is not 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 resistance system for these torsional moments should be accommodated.
Random internal columns :-
Since the internal Columns are random, they are unable to transfer the lateral loads efficiently and cannot achieve desirable building response. So we have to assume and designed those columns for only gravity loads.
Floor to floor variation in partition walls and beams :-
partition was provide an important mass control in and building and Beams plan key role in shear transfer from floor to Beam. All these elements are not in order. So heavy distorsion distribution of loades will happen.
- This plan is totally unacceptable for heavy seismic zone areas.
- In this plan interior columns are in unsymmetric.
- The partition wall are provided through to the base of beam, should support the lateral deformation due to the structure.
- The Interior beams will support the slab, then the lateral load resisting the failed due to earthquakes.
- For this irregular columns in plan, there we provide the flat slab because it directly stay in column centoidal Axis.
- For here the beams at different locations is irregular in plan are the contineuous and is disallowed to resist the seismic load in the region.
- The solution of the structure, we provide the shear wall from the foundation of the structure through vertical components, it should minimise the lord of second failure occur due to lateral forces acting on structure.