Column Design Details

Column Design Details Limits for main reinforcement (BS 8110 : Clauses 3.12.5 and 3.12.6) (a) The total area Asc of the longitudinal bars should not be less than 0.4% of the crosssectional area of the column. (b) The longitudinal bar area Asc should not exceed 6% of the cross-sectional area of a vertically cast column, nor 8% of that of a horizontally cast column, except that at laps of reinforcement bars (in both types of columns) the limit may be increased to 10%. (The I.Struct.E. Manual recommends an upper limit of only 4%). (c) Designers normally consider that a minimum of four longitudinal bars should be used in a rectangular column and six bars in a circular column. The I.Struct.E. Manual further recommends that longitudinal bars should not be smaller than size 12 and their spacing should not exceed 250 mm. Lateral ties or links (BS 8110 : Clause 3.12.7) (a) All longitudinal bars should be enclosed by links (sometimes called ties or stirrups), which should be so arranged that every corner and alternate bar shall have lateral support provided by the corner of a link having an included angle of not more than 135˚. No bar shall be further than 150 mm from a bar restrained by a link. (b) For circular columns, where the longitudinal reinforcement is located round the periphery of a circle, adequate lateral support is provided by a circular link passing round the bars. (c) Links should have a minimum diameter of at least one-quarter of that of the largest longitudinal bar, and the maximum link spacing should not exceed 12 times the diameter of the smallest longitudinal bar. (The I.Struct.E. Manual further recommends that the link spacing should not exceed the smallest cross-sectional dimension of the column). Concrete cover for durability (BS 8110 : Clause 3.3) Table 2.5-7 in Section 2.5(e) gives the concrete covers to meet the durability requirements for the columns and other structural members. Note that in Table 2.5-7, the

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meaning of nominal cover is as defined in BS 8110 : Clause 3.3.1.1, namely: the nominal cover is the design depth of concrete to all steel reinforcement, including links. It is the dimension used in design and indicated on the drawings. Table 2.5-7 Durability requirements (BS 8110 : Clause 3.3.3)

Exposure condition Mild Moderate Severe Very severe

25 – – –

Maximum free w/c ratio Minimum cement content (kg/m3) Concrete fcu (N/mm2)

Nominal cover (mm) 20 20 35 30 – 40 50 –

20 20 25 30

0.65

0.60

0.55

0.45

275 30

300 35

325 40

400 50

Fire resistance (BS 8110 : Clause 3.3.6) The fire resistance of a column depends on its minimum dimension and the concrete cover, as shown in Table 3.5-1. Note that the fire resistance requirements may in practice dictate the size of the beam and the concrete cover. Table 3.5-1 Fire resistance requirements for columns (BS 8110 : Part 2 : Clause 4.3.1)

Fire

Minimum dimension

rating (hours) 1 2 3 4

(mm) Fully exposed 50% exposed 200 160 300 200 400 300 450 350

Concrete cover to MAIN reinforcement (mm) 25 35 35 35

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