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:

A).

A.Restraint floor shrinkage:

• full restraints at all corners remaining space is free.

Conclusion :

It will have higher restraint to shrinkage thus leads to cracks in the floor

B.The lateral stiffness :

• in the first case the center of mass and centre of stiffness at the centre it can prevents the torsional when it comes to the lateral stiffness is low in both direction.

C.Torsion with resect to vertical axis

• i.corner wall provide more resistant to twist.

Conclusion :

It is good resistance to twist.

D.vertical reinforcement for same total flexural capacity at the base :

• walls are subjected to nominal flexural moments and twisting moments.

Conclusion :

heavy reinforcement require to option(iii) because it requires more ductility to resist the lateral loads.

E.static eccentricity

• structurally symmentric

F.system's redunducy :

• symmentric placement of lateral support in both x and y direction.

G.foundation system :

• due to L shaped walls the geomentry of foundation and its design will be ctrictical.

Conclusion :

due to L shape shear wall go for the box type having tie beams connects on both direction.

H.Architectural constraints :

• walls obstruct only corners.

Conclusion :

i.except chances are there.

ii.dicussion will there according to facade aspect.

iii.wont accept because large portion is occupied.

B).

A.Restraint floor shrinkage:

• restraints along the edges but corners are left to free.

Conclusion :

It is average

It will not effectively restraint the shrinkage so possibly less cracks.

B.The lateral stiffness :

• symmentrical in both direction so the higher lateral stiffness in both x and y ditrection.

Conclusion :

provide higher stiffness for pure seismic accerlation.

C.Torsion with resect to vertical axis

• edge walls provide good resistance to twist.

Conclusion :

It is good resistance to twist.

D.vertical reinforcement for same total flexural capacity at the base :

• walls are subjected to nominal twisting moments.

Conclusion :

heavy reinforcement require to option(iii) because it requires more ductility to resist the lateral loads.

E.static eccentricity

• structurally symmentric

Conclusion :

option(iii) is not symmentric.

F.system's redunducy :

• symmentric placement of lateral support in both x and y direction.

G.foundation system :

• walls are in line easy to fix the geomentry and design foundation.

Conclusion :

walls in the same line so go the isolated foundation.

H.Architectural constraints :

• walls obstruct the face of the building.

Conclusion :

The dicussion will there according to facade aspect.

C).

A.Restraint floor shrinkage:

• shear wall are placed at non uniform at the periphary of slab.

Conclusion :

It will not effectively restraint the shrinkage so possibly less cracks.

B.The lateral stiffness :

• non uniform lateral stiffness in both x and y .

Conclusion :

option(ii) provide higher stiffness for pure seismic accerlation.

C.Torsion with resect to vertical axis

• irregular walls provides very poor resistance to twist.

Conclusion :

have to change the configuration.

D.vertical reinforcement for same total flexural capacity at the base :

• walls are subjected to heavy flexural moments and twisting moments.

Conclusion :

heavy reinforcement require to option(iii) because it requires more ductility to resist the lateral loads.

E.static eccentricity

• unsymmetric

Conclusion :

It is not symmentric.

F.system's redunducy :

• concentrated lateral load suort in both the axis.

G.foundation system :

• walls are in line easy to fix the geomentry but design will be difficult.

Conclusion :

• iii.strip footing is prerferable for the third option.

H.Architectural constraints :

• left side more portion is occuied.

Conclusion :

wont accept because large portion is occupied.

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:

Drawbacks:

• the beams are connected to another beam which is ineffective during lateral loads and the primary beams are fail in shear.
• flow of loads in X direction are not in line.It is discontinued.
• There are eccentricityin both X and Y direction.Due to this, when lateral loads are applied there are torsional moment in both X and Y direction.
• As we see, the depth of beam is greater than column.So there is possibility of weak cloumn and strong beam meachnisms.

Issues in the given Framing plan:

Indirect lateral load Transfer in X and Y directions - The beams are connecting to another beam 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 the center of mass and the center of stiffness, the higher the 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.

Issues in the given Framing plan:

Strong beam Weak Column possibility - The size of columns is majorly mentioned as 25 cm x 25 cm. But all the beams have a much deeper section (50cm). The inertia of the beam is visually higher than the column. If the column reinforcement is adequately greater than that of beams, we are okay and since we do not have the reinforcement detail, this is doubtful. It is recommended to have slightly bigger sizes for columns.

To prepare for the alternate plan

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 :-

• The floor plan is neither uniform nor symmetry.so lateral load induced in the floor is not uniform.
• The internal columns are random.They are unable to transfer lateral load efficently.They are designed for only gravity load.
• since the floor plan is not uniform and symmetry.so partition walls provide an important mass control.

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 same direction. Definitely 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 can not achieve ddesirable building responce. So we have to assume and design those columns for only gravity loads.

Floor to floor variation in partion walls and beams :-

Partition walls provide an important mass control in the building and beams play key role in shear transfer from floor to beam. All these elements are not in order. So heavy distortion of lateral loads will happen.

Suggestion on the lateral load system and its foundation :-

• Heavy torsional force should be controlled by lateral load system The corner shear walls are suggested.
• The floor area is greater than on left side than right side.so shear walls on left side is more heavier than right side shear walls.
• corner shear walls shall be supported over a box foundation and they are connected by links beams to transfer shear .
• Internal columns are supported by isolated footing and these isolated footings are connected with shear walls through tie beams.

Proposed schematic plan of lateral load system