How do Columns in RC Buildings Resist Earthquakes?
Columns, the vertical members in RC buildings, contain two types of steel reinforcement, namely:
(a) Long straight bars (called longitudinal bars) placed vertically along the length, and
(b) Closed loops of smaller diameter steel bars (called transverse ties) placed horizontally at regular intervals along its full length (Figure 1).
Columns can sustain two types of damage, namely:
(a) Axial-Flexural (or combined compression bending) failure, and
(b) Shear failure. Shear damage is brittle and must be avoided in columns by providing transverse ties at close spacing (Figure 2b).
Designing a column involves selection of materials to be used (i.e, grades of concrete and steel bars), choosing shape and size of the cross-section, and calculating amount and distribution of steel reinforcement. The first two aspects are part of the overall design strategy of the whole building.
For example: The Indian Ductile Detailing Code IS: 13920-1993 requires columns to be at least 300mm wide. A column width of up to 200mm is allowed if unsupported length is less than 4m and beam length is less than 5m. Columns that are required to resist earthquake forces must be designed to prevent shear failure by a skillful selection of reinforcement.
Vertical Bars tied together with Closed Ties
Closely spaced horizontal closed ties help in three ways, namely
(i) they carry the horizontal shear forces induced by earthquakes, and thereby resist diagonal shear cracks,
(ii) they hold together the vertical bars and prevent them from excessively bending outwards (in technical terms, this bending phenomenon is called buckling), and
(iii) they contain the concrete in the column within the closed loops. The ends of the ties must be bent as 135° hooks (Figure 2). Such hook ends prevent opening of loops and consequently buckling of concrete and buckling of vertical bars.
Here below we will mention some requirements of the American Standard ACI 318 – Chapter 21: Earthquake Resistant Structures:
188.8.131.52 — the shortest cross-sectional dimension, measured on a straight line passing through the geometric centroid, shall not be less than 300 mm.
184.108.40.206 — the ratio of the shortest cross-sectional dimension to the perpendicular dimension shall not be less than 0.4.
220.127.116.11 — Area of longitudinal reinforcement, Ast , shall not be less than 0.01Ag or more than 0.06Ag.
18.104.22.168 — Transverse reinforcement required in 22.214.171.124 through 126.96.36.199 shall be provided over a length lo from each joint face and on both sides of any section where flexural yielding is likely to occur as a result of inelastic lateral displacements of the frame.
Length lo shall not be less than the largest of (a), (b), and (c):
(a) The depth of the member at the joint face or at the section where flexural yielding is likely to occur;
(b) One-sixth of the clear span of the member; and
(c) 450 mm.
Ast = total area of nonprestressed longitudinal reinforcement (bars or steel shapes), mm2
Ag = gross area of concrete section, mm2 For a hollow section, Ag is the area of the concrete only and does not include the area of the void(s)
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