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.

Ans: 

Figure A:

The structure is considered asymmetrical according to the regulations and the grid system.

Structural components do not have sufficient tensile capacity and force configuration.

The design will have some uncertainty regarding certain seismic response properties of non-rectangular walls.

This construction cannot be considered as horizontal unevenness and shear wall shape as L.

Structural walls have high stiffness and strength in both directions and are therefore subject to biaxial bending and bidirectional shear during earthquakes.

Figure B:

The frame of the structure can be considered symmetrical in the grid-located shear wall according to the symmetrical ideology of the design.

The wall system of this frame can be arranged in two horizontal directions with as much two-way symmetry as possible to distribute the load in a balanced transfer from multiple sides.

Shear walls are arranged similarly and symmetrically so that they can be considered at each level fairly, minimizing uncertainty about seismic response and transferring loads uniformly across the frame.

Figure C:

In the floor plan of the structure, the placement of the columns is inherently asymmetrical and is not acceptable by ordinance and is a code provision.

The foundation or foot may interfere with the placement of the foundation foot, and in practice, this may not be possible

The left wall provides more eccentricity, and the lateral stiffness is greater. Therefore, if this happens, then the damage or failure will be less.

The center of mass and center of gravity of such a structure will not be reliable according to the structural design.It does not satisfy the equation of equilibrium for the transfer of loads

This type of shear wall system will not carry the load equally in both directions. the structure lacks redundancy.

Criteria
Resistant of Floor Shrinkage	Observations	Full restraints at all corners	Corners are let free but restraint along the edge	Non - Uniform  restraints
	Conclusion	Option (a) Will have higher restaint to shrinkage,thus lead to cracks in the floor. Option (b) Is average. Option (c) Will not effectively restraint the shrinkage.So possibly less cracks.
The Lateral Stiffness	Observations	Comparatively Lower lateral stiffness in both X and Y.	Uniform and Comparatively Higher lateral stiffness in both X and Y.	Non - Uniform lateral stiffness in both X and Y.
	Conclusion	Option (b) Will provide higher vertical reinforcement and option (b) will require the least.
Torison with respect to vertical axis	Observations	Corner wall provide more resistance to twist.	Edge walls provide good resistance to twist.	Irregular walls provide very poor resistance to twist.
	Conclusion	Option (a) Will be the best option with respect to torsional resistance whereas. Option (c) Is the worst one.
Vertical reinforcement for similar base capacity	Observations	Walls are subjected to nominal flexure moment and heavily twisting moment.	Walls are subjected to nominal twisting moment.	Walls are subjected to heavy flexure moment and heavily twisting moment.
	Conclusion	Option (a) Will have higher restaint to shrinkage,thus lead to cracks in the floor. Option (b) Is average. Option (c) Will not effectively restraint the shrinkage.So possibly less cracks.
Static Eccentricity	Observations	Structurally symmetric	Structurally symmetric	Un - symmetric
	Conclusion	Only Option (c) Will have static eccentricity and its significant.
System Redundancy	Observations	Uniform placement of lateral load supports in both the axes.	Uniform placement of lateral load supports in both the axes.	Concentrated lateral load supports in both the axes.
	Conclusion	Option (a) and Option (b) are more redundant tha Option (c).
Foundation systems	Observations	Due to 'L' - shaped walls,the geometry of  foundation and design will be critical	Walls are in line,Esay to fix the geometry and  design of foundation	Walls are in line & concentrated.Esay to fix the geometry of foundation but critical in design.
	Conclusion	Option (b) and (c) shall be incorporated with isolated footings/strip footings. Option (a) Is recommended to have peripheral box foundation.
Architectural constraints	Observations	Walls obstruct only corners	Walls obstruct the faces of building	Vertical walls will not allow passage
	Conclusion	Architecturally speaking,Option (a)  is preferred one Option (b) Is subjected to discussion.. Option (c) Will not be accepted both architecturaly and commercially.

Question -2

Discuss the suitability for earthquake resistance of the moment resisting framing plan of a three-story building depicted here (cross-sectional dimensions in cm), the eccentricity of the center of mass (as the centroid of floor plan) to the center 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.           

Ans:

Indirect lateral load transfer in X and Y direction

From the given plan observation, we can find that some of the beams which not directly connected to beam-column junction, they we connected to other beams so they act as secondary beams. In these cases, there will be a heavy shear force acting on the primary beams due to the secondary beams such that the primary beams are prone to the shear failures. The alternative for this condition is to keep all the beams in a straight line such that all the axes of the beams should pass in a line.

Eccentricity

As provided in the above image, the center of mass of the is not matching with the center of the stiffness of the building, so due to this some eccentricity gets developed leading to the torsional moment in the structure. To reduce the eccentricity of this plan is done by some modifications in Plan. This can be done by make the structure more symmetrical in the geometry of the beams and columns provided.

Strong beam weak column

In the given plan the dimensions of beams and columns were provided. We can clearly observe that the cross section i.e., depth of beam is of bigger size than that of the column, so we can understand that the beams present are stronger than the columns available in the building. The size of columns majorly mentioned as 25 cm x 25 cm. But all the beams have much deeper section (50cm). Inertia of beam is visually higher than 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 slight bigger sizes for column. This could lead to the strong beam-weak column mechanism. To make the columns stronger than beams to adequate reinforcement is provide for columns.



Alternative 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.

Ans : 

Floor plan :The floor plan of the building is not 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 resistance system for these torsional moments should be accommodated.

Random internal columns :Since the internal Columns are random, they are unable to transfer the lateral loads efficiently and cannot achieve desirable building response. So we have to assume and designed those columns for only gravity loads.

Floor to floor variation in partition walls and beams :partition was provide an important mass control in and building and Beams plan key role in shear transfer from floor to Beam. All these elements are not in order. So heavy distorsion distribution of loades will happen.

• This plan is totally unacceptable for heavy seismic zone areas.
• In this plan interior columns are in unsymmetric.
• The partition wall are provided through to the base of beam, should support the lateral deformation due to the structure.
• The Interior beams will support the slab, then the lateral load resisting the failed due to earthquakes.
• For this irregular columns in plan, there we provide the flat slab because it directly stay in column centoidal Axis.
• For here the beams at different locations is irregular in plan are the contineuous and is disallowed to resist the seismic load in the region.
• The solution of the structure, we provide the shear wall from the foundation of the structure through vertical components, it should minimise the lord of second failure occur due to lateral forces acting on structure.