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
IST STRCTURE:
For this structure is considered as it is asymmetric in nature.
Here cannot to be considered as a horizontal irregularities and the shear wall shape like L.
There is some uncertainty concerning certain features of the seismic response of non-rectangular walls.
It occurs lack of tensile capacity and force configuration.
These walls have high stiffness and strength in both direction, hence are subjected biaxial bending and bi-directional shears during earthquakes.
2ND STRUCTURE:
For this structure frame can be considered as symmetric in placed the shear walls.
The walls of a wall system of this frame could be arranged in two horizontal directions with as much two-way symmetric as possible.
The shear walls are arranged in similar and symmetric, so it can be considered at every storey to fairly uniform resistant in seismic forces and deformation demands, minimizing the uncertainty about the seismic response.
3RD STRUCTURE :
For this structure can be considered as asymmetric in nature, because of the shear wall are provided are not arranged in a two orthogonal horizontal directons.
The center of mass and center of gravity of such structure will not lie far away from each other enabling transitional modes in the first two principles modes.
The foundation , shear wall and columns are arranged different arrangements, so here take up huge amount moments in case of during earthquakes,
This type of shear wall system will not carry the load equally in either directions. So this structure have lack of redundancy.
On left hand side the wall provide more eccentricity, lateral stiffness is more. Therefore it occurs then the damage or failure will be less.
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 directions - The beams are connecting to another beams which is ineffective during lateral load transfer. In such conditions, th primary beam is prone to shear failure.
Issues in the given Framing plan:
Strong beam Weak Column possibility - 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.
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:
Observations in the given 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 columns: -
Since the internal columns 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 variations in Partition walls and Beams: -
Partition 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 happen.
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 area is greater on the left side than the right side (Irregular quadrilateral). So Corner shear walls on the left side should be heavier than the right.
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 the other.
The internal columns shall be supported by isolated footings. Those isolated footings can be connected with Shear walls through Tie beams.