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.
SOLUTION:
Restraint of floor shrinkage:
Observaation:a) full restraints at all corners. b)corners are left free but restraints along the edges. c)non-uniform restraints.
Conclusion:a)will have higher restraint to shrinkage, thus lead to cracks in the floor. b)Is average c)will not effectivily restrain the shrinkage, so possibly less cracks.
The lateral stiffness:
observation:a)comparatively lower lateral stiffness in both x and y. b)uniform and comparatively higher lateral stiffness in both x and y. c)Non uniform lateral stiffness in both x and y
Conclusion:b) will provide higher lateral stiffness for a pure seismic acceleration in x and y directions.
Torsion with respect to vertical axis:
Observation:a)corner walls provide more resistance to twist. b)edge walls provide good resistance to twist c)irregular walls provide very poor resistance to twist.
Conclusion:a)will be best option with respect to torsional resistance whereas option c) is worest one.
Vertical reinforcement for similar base capacity:
Observation:a)walls are subjected to nominal flexural moment and heavy twisting moment. b)walls are subjected to nominal twisting moment. c) walls are subjected to heavy flexural moment and twisting moment.
Conclusion:c) will require higher vertical reinforcement and option(b) will require the least.
Static eccentricity:
Observation:a)structurally symmetric. b)structurally symmetric. c)un- symmetric
Conclusion:option (c) will have static eccentricity and it is significant.
System's redundancy:
Observation:a)uniform placement of lateral load supports in both the axes. b)uniform placement of lateral load supports in both the axes. c)concentrateed lateral load supports in both the axes.
Conclusion:option(a)and (b) are more redundant than option (c)
Foundation systems:
observation:Due to'L' shaped walls, the geometry of foundation and its design will be critical. b)walls are in line. Easy to fix the geometry and design of foundation. c)Walls are in line &concentrated east to fix the geometry of founndation but critical in design.
Conclusion:option (b and option(c) shall be incorporated with isolated footing/strip footings.Option(a) is recommended to have peripheral box foundation.
Architectural constraints:
Observation:a)walls obstruct only corners. b)walls obstructed the faces of building. c)vertical wall will not allow passage.
Conclusion:Architectural 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.
SOLUTION:
Issue 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, the primary beam is prone to shear failure.
Eccentricity:The longer the distance between center of mass and center of stiffness, the higher torsional moment of the building. As per the given plan eccentricity, the building is subjected to higher torsionnal moment(torsional flexability) in both x and y directions.
Strong beam weak column possibility:The size of column majorly mentioned as 25cm*25cm. 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 size 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.
SOLUTION:
Observation in the given framing plan:
Floor plan: The floor plan of building is not either uniform or symmetric. So naturally 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 beam: Partition walls provide an important mass control in a buildinng and beams play key role in shear transfer from floor to beam. All these elements are not in order. So heavy 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 shell preferable 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 footing. those isolated footings can be connected with shear walls through tie beams.
