Conceptual design of a building with columns and shear walls

CHALLENGE WEEK 4

ETABS

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:

• RESTTRAINT OF FLOOR SHIRINKAGE

• OBSERVATION

1. I) Full restraint at all corners

ii)corners are left free but restraints along the edge

iii)non uniform restraints

• CONCLUSION

1. will have higher restraint to shrinkage thus lead to cracks in the floor
2. Is average
3. Will not effectively restrain the shrinkage so possibly less cracks

• THE LATERAL STIFFNESS

• OBSERVATION

I)comparatively lower lateral stiffness in both X and Y

ii)uniform and comparatively higher lateral stiffness in both X and Y

iii) Non Uniform later stiffness in both X and Y

• Conclusion

1. Will provide higher lateral stiffness for a pure seismic acceleration in X and Y direction

• Torsion with respect to vertical axis

• Observation

1. Corner walls provide more resistane to twist
2. Edge walls provide good resistance to exit
3. Irregular walls provide very poor resistance to twist

• Conclusion

Option (a) will be best option with respect to Torsional resistance wheareas option (c) is the worst one

• Vertical reinforcement for similar base capacity

• Observation

Walls are subjected to nominal to flexural moment and heavy twisting moment

Walls are subjected to nominal twisting moment

Walls are subjected to heavy flexural moment and twisting moment

• Static eccentricity

• Observation

Structurally symmetric

Structurally symmetric

Un-symmetric

• Conclusion

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

• System's redundancy

• Observation

Uniform placement of lateral Uniform placement of Concentrated lateral load load supports in both the lateral load supports in supports in both the axes axes

both the axes Option(a) and (b) are more redundant than Option ()

• Foundation systems

• Observation

Due to 'L shaped Walls, the Walls are in line. Easy to Walls are in line & geometry of foundation and fix the geometry and

concentrated. Easy to fix its design will be critical design of foundation

the geometry of foundation

• Conclusion

but critical in design Option(b) and Option(c) shall be incorporated with isolated footings / strip footings Option(a) is recommended to have peripheral box foundation

• Architectural constraints

• Observation

Walls obstruct only corners

Walls obstruct the faces of building

Vertical wall will not allow passage

• Conclusion

Architecturally speaking Option(a) is the preferred one Option(b) is subjected to discussion Option(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-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.
• ESSENTRICITY- the longer the distance between center of mass and center of stiffness. The higher torsional moment of the building as per given plan eccentricity the building is subject to higher torsional moment in both X and Y directions.
• STRONG BEAM WEAK COLUMN POSSIBILIITY-the size of column majorly mentioned as 25x25 cm. but all the beams have much deeper section(50cm). inertia of beam visually higher than columns reinforcement is adequately grater than that of beams we are okay and since we do not have slight bigger sizes for column.

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 from givn 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 column:

Since the internal column 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 variation in partitiobn walls and beams:

Parttion 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 ahppen.

• 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 are is greater on the left side than the right side(irregular quadrilateral). So corner shear walls on the left side than the right side.
• 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 teh other.
• The internal columns shall be supported by isolated footings. Those isolated footings can be connected with shear walls through tie beams.