How do Columns in RC Buildings Resist Earthquakes?

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 Reinforcement

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).

Columns Reinforcement2

 

Design Strategy

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.

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Here below we will mention some requirements of the American Standard ACI 318 – Chapter 21: Earthquake Resistant Structures:

21.6.1.1 — the shortest cross-sectional dimension, measured on a straight line passing through the geometric centroid, shall not be less than 300 mm.

21.6.1.2 — the ratio of the shortest cross-sectional dimension to the perpendicular dimension shall not be less than 0.4.

21.6.3.1 — Area of longitudinal reinforcement, Ast , shall not be less than 0.01Ag or more than 0.06Ag.

21.6.4.1 — Transverse reinforcement required in 21.6.4.2 through 21.6.4.4 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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How to Make Buildings Ductile for Good Seismic Performance?

 

Post Author: Zahi Baroudi

11 thoughts on “How do Columns in RC Buildings Resist Earthquakes?

  • […] Posts you may like: How do Beam-Column Joints in RC Buildings Resist Earthquake? Vertical Distribution of Seismic Forces: Linear or Parabolic? Combining Horizontal and Vertical Earthquake Effects How do Columns in RC Buildings Resist Earthquakes? […]

  • […] Differs in ASD Load Combinations ? How do Earthquakes Affect Reinforced Concrete Buildings ? How do Columns in RC Buildings Resist Earthquakes? How do Beams in RC Buildings Resist Earthquake […]

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  • amar nath jha

    (July 30, 2016 - 12:38 PM)

    Very much useful.learnt a lot.thank you.

  • […] Poor behavior of short columns is due to the fact that in an earthquake, a tall column and a short column of same cross-section move horizontally by same amount Δ (Figure 2). However, the short column is stiffer as compared to the tall column, and it attracts larger earthquake force. Stiffness of a column means resistance to deformation – the larger is the stiffness, larger is the force required to deform it. If a short column is not adequately designed for such a large force, it can suffer significant damage during an earthquake. This behavior is called Short Column Effect. The damage in these short columns is often in the form of X-shaped cracking – this type of damage of columns is due to shear failure (see How do Columns in RC Buildings Resist Earthquakes?). […]

  • […] How do Columns in RC Buildings Resist Earthquakes? How do Beams in RC Buildings Resist Earthquake ? How do Earthquakes Affect Reinforced Concrete Buildings ? How Buildings Twist During Earthquakes? How to Make Buildings Ductile for Good Seismic Performance? […]

  • R. P. Sharma

    (May 21, 2016 - 8:05 PM)

    Good article

  • Rodney T

    (May 18, 2016 - 7:27 PM)

    Very good article. Thanks for posting

  • tesfaye

    (May 17, 2016 - 7:47 AM)

    wow a good

  • Tara Puri

    (May 15, 2016 - 7:49 AM)

    Yeah, the correct bending of loop ends and their extension into the concrete is vital in resisting seismic forces. During the devastating earthquake that hit Nepal a year ago, failure of R.C.C. buildings reveal that columns failed due to diagonal shear and buckling of longitudinal bars showing that the hooks were opened up easily. This is mainly due to lack of technical construction supervision prevailing in Nepal in private house construction.

  • Tara Puri

    (May 15, 2016 - 7:39 AM)

    Yeah, the correct bending and extension into the concrete of loops is vital in resisting seismic forces. During the devastating earthquake that hit Nepal a year ago, most R.CC. columns showed failure and buckling of vertical bars as well as severe diagonal shear cracks due to defective loops.

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