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: From the above fig (a),

1. Restraints of floor shrinkage: This rc wall system poses good control of shrinkage because all the corners of the floor is restricted to experience shortening.
2. Lateral stiffness: This system will posses less stiffness because the rc wall is not in the principal plane but in the corners. Only the principal planes are subjected to lateral movements.
3. Vertical rebar at base : Due to lesser lateral forces the vertical rebar will be lesser in comparision to other systems.
4. Static eccentricty:T he system is eccentric with respect to mass stiffness comparision.
5. Foundation system: Isolated footing with tie beam is suitable for this system.
6. Architectural constraints: From architectural point of view this arrangement is fine because only corners are obstructed.

•  From fig (b),

1. Restaint of floor shrinkage: Edges of floor is resistricted to the floor shortening so therefore the corners are more prone to shoretening with frame system.
2. Lateral stiffness: The systems will have better stiffness compared to other systems because the rc wall are in the direction of lateral force direction.
3. Torsion with vertical axis: Center of mass and center of stiffness will coincide so less prone to torsion.
4. vertical rebar at base: Verticlal rebar are more than previous system as rc wall will take all the lateral loads in principal direction.
5. Static eccentricity: The system is centric with respect o mass -stiffness comparision.
6. Foundation system: box type foundation is suitable for this foundation.
7. Architectural constraints: This layout is not recommended because it hinders with entrance.

•  From fig (c),

1. Restrain of floor shrinkage: This system will have more shrinkage effect because the floor is not enclosed with rc walls at two sides.
2. Lateral stiffness: RC wall is stiffer in x-direction for one side only as per arrangement.
3. Torsion with vertical axis: center of mass and center of stiffness will not coincide in both the principal directions. so this has more torsion.
4. Static eccentricity: The system is more eccentic eith respect o mass stiffnee comparision.
5. Foundation system: Provide strip footing for Y-direction RC wall and isolated footing in x-direction.
6. Architectural constraints: This layout is fine with the archtectural point of view as only left side is obstructed. 

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 observe the given structural framing in terms of seismic influence and to suggest better alternative for the same and in addition telll whether the given system need to obbssed for torsional flexibility.

1. Suitability for earthquake:

     From the seismic point of view as it a 3 store building so earthquake of moderate nature will effect this structure due to its complexiity in load transferring mechanism as we can see in fig. 

    The coloumns and beams are not in rigid pattren they have offsetted for their path due to whic load does not transfer effectively.

    Alternative is that we can rearrange the columns so the load will transfer effectively

    Main focus should be on making the grid of column and beam for effective resistance aganist lateral forces providing walls at possible locations for mor resistance aganist lateral force.

    2. Torsional flexibility:

    Since the center of mass and the centre of stiffness of the building does not coincide so definitely the buildind experience torsion eith respect to vertical axis. The plan aspect ratio is less than 3 so we should ssume that the given system is well proportioned.

    The given vertical floor system if not effective in transferring in lateral loadsto vertical loads due to the offset and beam to beam connection. The response of the building in fundamental mode of translation is worst.

    Yes, The two fundamental modes of translation may be still higher than the fumndamental mode of torsion.

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:

1. As we can see from the plan that he floor area is not uniformly distribited so because the centre of mass will be some where on the left side pf the plan. 
2.  So position of thr lateral loadin system be in such a way that their stiffness center is near to the mass centre in orde to avoid torsional irrerugularity.

• Lateral loading system:

1. As we can see in the fig the floor plan if an irregular quadilateral so during the occurence of seismic forces both the direction of earthquake will have influence on each other because of geomentry.
2. So the best position is to provide L-shaped corner RC wall arrangement which will have bidirectional strebgth to resists seismic force.
3. The foundation that can be taken for this arrangment are either isolated footing with tie beam connection or a box type foundation.