Conceptual design of a building with columns and shear walls

 AIM :- Conceptual design of a building with columns and shear walls

INTRODUCTION:-

• Use of shear walls in RC buildings is one of the most commonly used strategies for earthquake mitigation. o avoid torsion in buildings, shear walls must be placed symmetrically in plan.
• we also make a deep look at  the columns sizes and positioning in this projects 

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.

                             a                                                             b                                                        c

AIM:

To 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.

INTRODUCTION:-

1) Restraint of floor shrinkage :-

  For Case A :-

• In this case, since the shear walls are placed at the corners, it has good restraint against floor shrinkage.
• These walls at the corners will tightly hold the floor in position and possess good control of shrinkage.

  For Case B :-

• Here, the shear walls are placed along the edges of the Floor.
• This wall will provide better stability to the floor near it but the corner portions of the floor are prone to shrinkage.

  For Case C :-

• Here, the floor is not enclosed with shear walls in both principal directions.
• This floor will have a high shrinkage effect.

  2) Lateral Stiffness :-

  For Case A :-

• As the shear walls are placed at the corners of the floor, the edges of the floor do not have any support.
• During the earthquake excitation, the forces tend to act along the edges of the floor, this placement of shear walls will have the least stability and hence the lateral stiffness will also be less.

  For Case B :-

• The shear walls here are placed along the edges which is the same direction of earthquake excitation, the structure will have more stability and thus possess high stiffness in both the principal directions.

  For Case C :-

• In this system, the stiffness is effective along the X-direction as there are two shear walls at both edges of the floor.
• Whereas in the Y-direction, there is only one shear wall which leads to the concentration of seismic force in that wall and reduces the lateral stiffness in that direction.

  3) Torsion with respect to vertical axis :-

  For Case A :-

• Here the placement of shear walls is symmetric, hence there is no eccentricity, so the centre of mass and centre of stiffness coincide, so it is less prone to torsion.

  For Case B :-

• The symmetricity is maintained in this arrangement also, hence there is no eccentricity, so the centre of mass and centre of stiffness coincide, so it is less prone to torsion.

  For Case C :-

• In this arrangement, the shear wall placement is unsymmetrical along the Y-direction. So, so the centre of mass and centre of stiffness will not coincide. Thus this arrangement will experience torsion during the earthquake.

  4) Vertical Rebars required at the base :- 

  For Case A :-

• As the shear reinforcements are placed at the corners, not in the principal plane, it does not take part much in the resistance of the lateral forces as compared to the other parts of the walls.
• Thus, the amount of reinforcement required for these walls will be less.

  For Case B :-

• Here, a huge amount of the lateral forces are taken by the shear walls as they are placed along with the principal directions.
• Thus, these walls required more vertical reinforcement than system A.

  For Case C :-

• In this arrangement, as the shear walls are placed asymmetrically, so these walls will experience extreme lateral forces.
• These walls need the highest amount of vertical rebars.

  5) Static Eccentricity :- 

  For Case A :-

• The shear walls are symmetrically placed at the corners and the centre of mass and centre of stiffness coincide and there is no eccentricity.

  For Case B :-

• The shear walls are symmetrically placed along the edges and the centre of mass and centre of stiffness coincide and there is no eccentricity.

  For Case C :-

• Here, the shear walls are not symmetrically placed in the Y-direction and the centre of mass and centre of stiffness didn't coincide and thus eccentricity occur in this arrangement.

  6) System's Redundancy :-

  For Case A :-

• In this arrangement, shear walls are placed at the corners of the floor and thus, these walls do not actively take part in resisting the lateral force.
• Hence, a huge amount of the lateral force has to be taken care of by the other part of the structure and thus, the structure is less redundant.

  For Case B :-

• In this arrangement, the shear walls are along the mid half of each edge and the lateral forces will be equally shared among them.
• Hence, this arrangement has really good redundancy.

  For Case C :-

• This arrangement of shear walls is asymmetric and also the concentration of load is at the left edge.
• Thus, this arrangement has poor redundancy.

  7) Foundation System :-

  For Case A :-

• As the shear walls are placed at the corners of the structure, an isolated footing with a tie beam arrangement is suitable for this system.

  For Case B :-

• Since the shear walls are spread across the 4 edges of the structure, a raft foundation or box type foundation is suitable for this system.

  For Case C :-

• In this arrangement, two shear walls are provided along the X-direction so for this a combined footing can be provided along Y-direction connecting these two shear walls.
• The other shear walls at the left t edge can be provided with isolated footing.

  8) Architectural Constraints :-

  For Case A :-

• The wall at the corners of the floor, thus, is Architecturally very effective as it will not obstruct any other elements.

  For Case B :-

• From the architectural point of view, this arrangement may not be recommended as there is a chance that these shear walls may obstruct the other elements.

  For Case C :-

• This arrangement has two walls on the right side along X direction is architecturally fine. But the shear wall at the left edge along Y-direction occupies almost half of the wall, which can cause hindrance to the open space.

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.

AIM:

To 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) Suggest an alternative. Also, is there torsional flexibility? Are the two fundamental translational modes of vibration larger than the fundamental torsional mode of vibration

ANSWER:-

1. Indirect lateral load transfer in X and Y direction:-

• 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.

 2. Strong Beam - Weak Column Possibility:-   

• The size of columns majorly mentioned is 25 cmX25 cm.
• But all the beams have much deeper section.
• 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 detail, this is doubtful.
• It is recommended to have slight bigger sizes of columns as centricity.

  3. Eccentricity:-

• The longer the distance between centre of mass and centre of stiffness, the higher torsional moment of a building.
• As per given plan is eccentricity, the building is subjected to high torsional moment in both X and Y direction.

For the Alternate option of this plan, we can add some changes as per following;

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 :-

Observation in the given farming plan:-

Floor plan:-

•  The floor plan building is not either uniform or symmetric. So naturally any lateral load include in the floor plan is not have 100% influence in the same direction. Definitely a special lateral load resisting system for these torsional moments should be accommodated

Random internal column:

•  Since the internal column are random they are unable to transfer lateral load efficiently and can not be achieve desirable building response. So we have to assume and design those column for only gravity load.

Floor to floor variation is partition wall and beams:-

• Partition wall provide an important mass control in the building and play the key role in the shear transfer to floor to beam. All this elements are not in order so heavy distortion and distribution of lateral load will happen.

Suggestion on the lateral load system & its foundation:

• Heavy torsional force should be control by respective lateral load system. So “corner shear wall “ are suggested.
• The proportion of floor area is greater then the left side of the then the right side. So the conner shear wall should be heaver then the right
• Corner shear wall shall be preferably supported over box foundation shear wall should be connect by link beams to transfer the shear from one to other.
• The internal column shall be supported by isolated footing. Those isolated footing can be connect with the shear wall through the tie beam.

PROPOSED SCHEMATIC  PLAN OF LATERAL LOAD SYSTEM ARE AS FOLLOWS:

RESULT:- 

We understand Conceptual design of a building with columns and shear walls