Conceptual design of a building with columns and shear walls
1)
• Design codes define a wall as a vertical element with an elongated cross-section. A lower limit of 4 for aspect ratio (long to short of a rectangular cross-section is used for a vertical element to be considered as a well.
• If the cross-section consists of rectangular parts, one of which has an aspect ratio greater than 4, the element is also classified as a wall.
• With this definition on the basis of the cross-sectional shape alone, a wall differs from a column in that it resists lateral forces mainly in one direction (parallel to the long side of the section) and can be designed for such a unidirectional resistance by assigning the flexural resistance to the two edges of the section ('flanges, or 'tension and compression chords) and the shear resistance to the ' beam. web' between them, as in a Beam.
• In the first figure the L shape walls are given, these walls have high stiffness and strength in both horizontal directions, hence are subjected biaxial bending and bi-directional shears during the earthquake.
• They are most cost-effective than the combination of their constituent parts as individual rectangular walls. However, present-day knowledge to of their behaviour under cyclic biaxial bending and shear is very limited, and the rules used for their dimensioning and detailing still lack a sound basis.
• For this reason, it recommended to make limited use of such walls in practical design. If non-rectangular walls are chosen, they should have a fairy simple section.
• The large openings should be avoided in ductile walls, especially near the base, where the plastic hinge forms.
If they are necessary for functional reasons (doors and windows), they should not be staggered vertically, but should be arranged at every floor in a regular pattern, creating a coupled wall, with the lintels between the openings serving as coupling beams and designed as such.
• The high lateral stiffness of walls reduces inter-storey drifts and structural or non-structural damage.
• It also overshadow the contribution of masonry infills to the lateral stiffness of the building and reduces its adverse effects due to their
potential irregularity in plan (essentric placement) or elevation (open storey).
• Soft storey mechanisms are precluded by the absence of wall counter-flexure within a storey. This way it helps reduce irregularity in elevation.
• It is difficult to place several walls around the periphery without compromising the architectural function of the building.
2)
• In a figure, If a beam does not continue straight from span to span, but its axis is offset at the column between them, there is no smooth flow of beam internal forces througn a proper beam-column joint, neither continuity of the beam longitudinal bars across the column from one span to the next: these bars have to terminate there and be separately anchored at the joint.
• Even when the beam axis is not offset from span to span, the smooth flow of internal forces from the beams to the column is impaired by a large eccentricity between the axis of the beam and the supporting column.
• In approximation, an indirect support may be considered as a simple support; the indirectly supported beam is less effective in frame action than one connected to columns at both ends.
• Beams with long span may have their top reinforcement at the supports governed by factored gravity loads, rather than by the 'seismic design situation.
• This will result in beam overstrength, relative to the moment demand in the 'seismic design situation.
• The overstrength is carried over to the capacity design shears of beams and columns, penalizing them and creating some uncertainity whether plastic hinges will form in the beams or the columns. There is possibility of plastic hinge occuring away from face of column.
• Unless diagonally reinforced, short beams have low deformation capacity and poor ductility.
3)
• The general layout and certain details of the geometry (shape, size and typology) of an individual plane frame have a major impact on its seismic behaviour. Very important also is the overall layout of the frames in a structural system.
• Frames place few constraints on a building's architectural design, including the facade and parking space in ground floor.
• Frames may be cost-effective for earthquake resistance, because beams and columns are placed anyway for gravity loads; so, they may also provide earthquake resistance in both directions, if their columns are large.
• Frames with concentric connections and regular geometry have well-known and understood seismic performance.
• It is easier to design an earthquake-resistant foundation element for a smaller vertical member than for a larger one.
• Frames are inherently flexible; the cross-section of their members may be governed by the inter-storey drift limitations under the moderate earthquake for which limitation of damage to structural and non-structural elements is desired.
• Column counter-flexiure in the same storey allows soft storey mechanisms, which lead to collapse.
• Earthquake-resistance requirements on frames lead to large columns, interfering with architectural functions.
• The reinforcement detailing of frames for ductility requires workmanship of high level for its execution and good supervision on site.
The walls and frame systems each have their advantages and disadvantages as lateral load resisting systems.
• Walls seem to have a better balance of advantages against drawbacks; nevertheless, a concrete building always has beams and columns to support the gravity loads; it is a waste not to use them for earthquake resistance.
• Therefore frames and walls may well be cost-effectively combined in a single lateral-load-resisting
system.
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.