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;

1. RESTRAINT OF FLOOR SHRINKAGE

Observation;

• Structure(A): Full restraint at all corners
• Structure(B): Corners are left free but restraint along edges
• Structure(C): Non-uniform restraint

 

Conclusion;

• Structure(a)will have higher restraint to shrinkage, thus lead to crack in the floor.
• Structure (b) is average.
• Structure (c) will not effectively restraint the shrinkage, so possibly less cracks.

 

 

2. THE LATERAL STIFFNESS

Observation;

• Structure(A): comparatively lower lateral stiffness in both x and y
• Structure(B): uniform and comparatively higher lateral stiffness in both x and y
• Structure(C): Non-uniform lateral stiffness in both x and y

 

Conclusion;

• Structure(b)will provide higher lateral stiffness for a pure seismic acceleration in bath x and y direction

 

3. TORSION WITH RESPECT TO VERTICAL AXIS

Observation;

• Structure(A): corner walls provide more resistance to twist
• Structure(B): edge walls provide good resistance to twist
• Structure(C): irregular walls provide very poor resistance to twist

 

Conclusion;

• Structure(a)will be best option with respect to torsion resistance whereas structure(c)is the worst one.

 

4. VERTICAL REINFORCEMENT FOR SIMILAR BASE CAPACITY

Observation;

• Structure(A): walls are subjected to nominal flexural moment and heavy twisting moment
• Structure(B): walls are subjected to nominal twisting moment
• Structure(C): walls are subjected to heavy flexural moment and twisting moment

 

Conclusion;

• Structure(c)will require heavy vertical reinforcement and structure(b) will require least.

 

5. STATIC ECCENTRICITY

Observation;

• Structure(A): structurally symmetric
• Structure(B): structurally symmetric
• Structure(C): un-symmetric

 

Conclusion;

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

 

6. SYSTEM’S REDUNDANCY

Observation;

• Structure(A): uniform placement of lateral load supports in both the axes.
• Structure(B): uniform placement of lateral load supports in both the axes.
• Structure(C): concentrated lateral load supports in both the axes.

 

Conclusion;

• Structure(a) and (b) are more redundant than structure(c).

 

 

7. FOUNDATION SYSTEM

Observation;

• Structure(A): due to ‘L’ shaped walls, the geometry of foundation and its design will be critical.
• Structure(B): walls are in line, easy to fix the geometry and design of foundation.
• Structure(C): walls are in line and concentrated , easy to fix the geometry of foundation but critical in design.

 

Conclusion;

• Structure(b) and (c) shall be incorporated with isolated footings/strip footings.
• Structure (a) is recommended to have peripheral box foundation.

 

8. ARCHITECTURAL CONSTRAINTS

Observation;

• Structure(A): walls obstruct only corner
• Structure(B): walls obstruct the faces of building.
• Structure(C): vertical wall will not allow passage.

 

Conclusion;

• Architectural speaking structure(a) is the preferred one.
• Structure (b) is subjected to discussion.
• Structure (c) will not be accepted both architecturally and commercially.

 

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 transfer in x and y axis direction;

• The beam are connecting to another beams which is ineffective during lateral transfer load. In such conditions, the primary beam is prone to shear failure.

 

Eccentricity:

• The longer the distance between centre of mass and centre 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 direction.

 

Strong beam week column possibility:

• The size of columns majorly mentioned as 25cm X25cm, but all the beam have much deeper section (50cm).
• Inertia of beam is visually higher than column.
• If the column reinforcement is adequately greater than that of beam, we are okay and since we do not have the reinforcement details, this is doubtful.
• It is recommended to have slightly bigger sizes of columns.

 

ALTERNATIVE SCHEME:

• To provide the columns in proper alignment of structure in both x and y direction of building.

 

 

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: 

• The above plan that is given is totally unsuitable for normal areas and also in Earthquake prone area.
• In seismic region the lateral and vertical loads play important criteria because it helps in resisting the failure against the earthquake as the loads.
• Resultant will not co inside with the centre of the mass of the structure it becomes eccentric and not suitable for any type of areas.
• In this plan a flat slab is more appropriate than a regular beam column junction as it provides support to the columns without any provision of the beam
• which reduces the weight of the building and also helps in the ductility
• which indirectly helps to maintain the structure in the earthquake prone areas.
• Apart from flab slab a wooden structure is also appropriate which reduces the weight of the building .
• Also helps to maintain the structure in the earthquake prone areas.