Shear walls are vertically oriented elements that apart from their ability to bear vertical loads, they also limit the horizontal deformations of the structural frame. Since as a rule, they carry gravity loads, their large size is not entirely required. On the other hand, that large size is abso-lutely necessary in order for them to resist the horizontal seismic forces. Ground earthquake motions cause severe flexural and shear stresses to the shear walls. These stresses can only be carried by strong reinforcement, properly placed inside their entire mass.
Shear wall refers to any vertical element with a length to thickness ratio of 4 or more. The clas-sification of an element as shear wall determines the way that the reinforcement will be placed inside its concrete mass. In order for a shear wall to behave in the required way, it must have two columns embedded inside its ends or otherwise called two boundary elements.
In case the shear wall does not have clearly defined boundary elements, two hidden columns are formed at the edges of the wall’s mass. Their width is equal to the shear wall’s thickness and their length must be at least equal to one and a half times the wall’s thickness (≥ 1.5b).
The boundary columns apart from assisting in the assembling of the shear wall also ensure a minimum strength capacity. During a severe seismic event, it is possible for wall consistency degradation to happen. In such a case, at the lower critical level of the building, the two embed-ded columns, with the high ductility that they have, will continue to bear the largest amount of the applied vertical gravity loads and seismic forces.
a) Forming a 450 mm column (regular or hidden), has the advantage that it is the maximum length to place a three-legged stirrup and the distance between the stirrup legs to be around 200 mm as required by the regulation.
b) Forming a 400 mm column usually does not provide adequate anchorage length. On the other hand, the use of a 500 mm column requires a four-legged stirrup.