Conceptual design of a building with columns and shear walls
Aim:
To investigate the give plan with respect to seismic behavior.
To schematically draw a seismic viable structural plan.
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:
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 transformation in both X and Y direction:
The shown beams are connected to anther beam with out the direct support from respective column. When the lateral load is applied the primary beam will attain shear failure.
Beams are supposed to be connect as shown below.
Eccentricity:
The longer the distance between the center of mass and center of stiffness, the higher torsion moment of the building.
As per the given plan the building tends to attain higher torsional moment in both x and y direction.
Strong beam weak column failure:
As shown above the size of the beams are greater than size of the column which might result in total collapse of the building when the seismic load is applied.
Alternative plan:
- Here it has strong column of 250x500 and weak beam of 250x250 which are aligned according to the plan.
- Beams are connected to the columns.
- Remaining area are covered by cantilever beams.
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:
Floor plan:
- The floor plan of the building is neither uniform nor symmetric. Hence it can not support any lateral load acting on it.
- It needs load resisting system to avoid torsional moment.
Random internal columns:
- Since the internal columns are randomly placed it can only support the gravity load but can not take part in lateral resisting system.
Floor to floor variations in partition walls and beams:
- Partition walls plays important role in mass contribution.
- Shear is transferred from floor to beam.
- All these elements are not in order, hence heavy distortion and distribution of lateral loads will happen.
Suggestions on the lateral load system and its foundation:
- Heavy torsional forces should be controlled by the “corner shear walls”
- The proportion of floor area in the left is greater than the right. So, corner walls on the left side should be heavier than the right.
- The corner shear walls preferably supported by a box foundation.
- Shear walls should be connected by the link beams to transfer shear from one to the another.
- Internal columns shall be supported by isolated footings. Those isolated footings can be connected to the shear walls through tie beams.
- Conclusion:
- Hence the given plans are analyzed and provided with alternative solution as required.