Conceptual design of a building with columns and shear walls

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.

a b c

OBJECTIVE:

To investigate the suitability of given plan with respect to seismic behavior

To schematically draw a seismic viable structural plan

Restraint of Floor shrinkage

Observation:

a) Full restraints at all corners there are restraints at the corners of the floor it is highly resistant to shrink in the variation of temperature.

b) Corners are left free but restraints along the edge but this can allow some shrinkage on the floor

c)Non-uniform restraints the vertical direction it is subjected to shrinkage and the left hand side is only arrested efficiently over then right side also in the x- direction in right top corner only got shrinkage

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

a) Comparatively Lower Lateral stiffness in both X and Y because, if wse provide a load in x and y the shear wall of corners cannot effectively arrested

b) Uniform and comparatively higher lateral stiffness in both X and Y

c) Non uniform Lateral stiffness in both X and Y
concrete

Conclusion

Option(b) will provide higher lateral stiffness for a pure seismic acceleration in X and Y directions.
The Lateral stiffness


Torsion with respect to vertical axis

a) Corner Walls provide more resistance to twist

b) Edge Walls provide good resistance to twist good resistance not much comparble to option (a)

c) Irregular Walls provide very poor resistance to twist because they are eccentric in both the x and y directions they cant effectively arrest the twisting force

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

a) Walls are subjected to nominal flexural moment and heavy twisting moment so, the requirments will be considerable

b) Walls are subjected to nominal twisting moment and regular flexural moment.

c) Walls are subjected to heavy flexural moment and twisting moment because torsion also heavy defintley the twsiting moment also heavy because of the eccentric condition and also the shear walls are one sided in both x and y direction so they attract more flexural moment

Conclusion

Option(c) will require higher vertical reinforcement and Option(b) will require the least.

Static eccentricity

a) Structurally symmetric

b) Structurally symmetric

c) un-symmetric

Conclusion

Only Option(c) will have static eccentricity and it is significant

System's redundancy

a) Uniform placement of lateral load supports in both the

b) Uniform placement of lateral load supports in both the axes

c) Concentrated lateral load supports in both the axes


Conclusion

Option(a) and (b) are more redundant than Option(c)

Foundation systems

a) Due to 'L shaped Walls, the geometry of foundation and its design will be critical

b) Walls are in line. Easy to fix the geometry and design foundation

c) 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 have peripheral box foundation Option(a) is recommended to


Architectural constraints

a) Walls obstruct only corners.

b) Walls obstruct the faces of building

c) Vertical wall will not allow passage

Conclusion

Architecturally speaking Option(a) is the preferred one Option(b) is subjected to discussion. Option(e) will not be accepted both architecturally and commercially.

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 plan:
Indirect lateral load Transfer in X and Y directions - The beams are connecting to another beams which is ineffective during lateral load transfer. In such conditions, the primary beam is prone to shear failure.

Issues in the given Framing plan:
Eccentricity-The longer the distance between center of mass and center of stiffness, the higher torsional moment of the building. As per the given plan eccentricity, the building is subjected to higher torsional moment (Torsional flexibility) in both X and Y directions.

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.

Observations 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 internal columns are random, they are unable to transfer

lateral loads efficiently and can not achieve desirable building response. So we have to assume and

design those columns for only gravity loads

Floor to floor variations in Partition walls and Beams-Partition walls provide an important mass

control in a building and beams play 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 & 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 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 Tie beams.