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-
Restraint of floor shrinkage -
obervation :
- structure (a)= full restraint at all corners
- structure (b)= corners are left free but restraint along the edges
- structure (c)= non uniform restraint
conclusion :
- structure (a) will have higher restraint to shrinkage, thus leads to high cracks. structure (b) is average. stucture (c) will not effectively restrain the shrinkage, so possibly less cracks.
the lateral stiffness-
obervation :
- structure (a)= comparatively lower lateral stiffness in both x and y direction
- structure (b)= unifrom and comparatively higher lateral stiffness in both x and y direction
- structure (c)= non unifrom lateral stiffness in both x and y direction
conclusion :
- structure(b) will provide higer lateral stiffness for a pure seismic acceleration in x and y direction.
torsion with respect to vertical axis-
obervation :
- structure (a)= corner walls provide more resistance to twist
- structure (b)= edge walls provide good resistance to twist
- structure (c)= irregular wall provide very poor resistance to twist
conclusion :
- structure(a) will be best option with respect to torsional resistance where as the structure c will be worst one.
vertical reinforcement for similar base capacity
obervation :
- 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 higher vertical reinforcement and structure (b) will require the least.
static eccentricity
obervation :
- structure (a)= structurally symmetric
- structure (b)= structurally symmetric
- structure (c)= un-symmetric
conclusion :
structure (c) will have static eccentricity and it is significant
systems redundancy
obervation :
- structure (a)= uniform placement of lateral loads supports in both the axes
- structure (b)= uniform placement of lateral loads supports in both the axes
- structure (c)= concentrated lateral load support in both axes
conclusion :
structutre (a) and structure (b) are more redundant than strucuture (c)
foundation systems
obervation :
- 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 structure (c) shall be incorporated with isolated footings /strip footing. structure(a) is recommended to have peripheral box foundation.
architecural constraints
obervation :
- structure (a)= wall obstruct only corners
- structure (b)= wall obstruct the faces of building
- structure (c)= vertical wall will not allow passage.
conclusion :
architecturally speaking structure (a) is 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.
ans-
- indirect lateral load transfer in x and y direction - 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 centre 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 torsional moment ( torsional flexibility) in both x and y direction.
Strong beam and weak column possibility-
the size of column majorly mentioned as 25x25cm. but all the beams have much deeper section (50cm). inertia of beam is visually higher than column. if the column reiforcement is adequately greater than that of beams, we are okay and since we donot have the reinforcement details, this is doubtful. it is recommended to have slight bigger sizes of column.
alternate scheme
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-
observation :
floor plan - the floor plan of the buildong is not either uniform or symmetric. so naturally any lateral load included in the floor will not have 100% influence in same direction. definately a special lateral load resisting system for these torsional moments should be accommodated.
random internal columns - since the interanl columns are random, they are unable to transfer lateral load efficiantly and cannot 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 - partitions walls provide an important mass control in a buildings and beams play key role in shear transfer from floor to beam. all these elements are not in order. so heavy distortions and distribution of lateral loads will happen
suggestion on the lateral load system and its foundations :
- heavy torsional forces shpuld be controlled by respective lateral load system. so corner shear walls are suggested.
- the proportion of floor area is greater on left side than the right side ( irregular quadrilateral). so corner shear walls on left should be heavier than the right
- corner shear walls shall preferably be supported over a box foundation. shear walls should be connected by link beam to transfer shear from on to other.
- the internal column shall be supported by isolated footing. those isolatedd footing can be connected with shear walls through tie beams.
proposed schematic plan of lateral load system
