As per British Standards, how much minimum overlap has to be given for a 32mm rebar and 20mm rebar? Both are present in columns.

It depends on steel bars either in tension or compression.

To avoid this complication in various codes, normal pratice is take 50 times diameter of steel bars eighter in tension or compression. Overlap for 32mm rebar will be =32*50=1600mm or 1.6m

Overlap for 20mm rebar will be =20*50=1000mm or 1.0m. Better to reduce spacing of rings(strips), if ovelap is near the support in case of column or beam.Please refer to details mentioned below by Amercan Concrete Institute(ACI) and British Code (BS)

ACI 318:08 provisions for lap length

Lap length in tension

Section 12.15.1 of the ACI 318:08 specifies the tension

lap lengths for class A and class B splice as 1.0ld and

1.3ld respectively but not less than 300 mm. Section

12.15.2 of CAI318:08 specifies tension lap splice

condition. Since in practice class B splice condition is

more common it was considered in this research.

Lap length in compression

Section 12.16.1 of the ACI 318:08 specifies the compression

lap length as 0.071fydb for fy ≤ 420MPa and 0.13fy – 24

for fy > 420MPa but not less than 300 mm in both cases.

These values are applicable for 21 < fc’≤ 70 MPa (normal

strength concrete). For fc’≤ 21 MPa, lap length shall be

increased by one-third. 48 times diameter of steel bar.

BS 8110:97 provisions of lap lengths

Lap length in tension

As per section 3.12.8.13 of BS 8110:97; in general, the lap

length in tension = 1.0 times tension anchorage length.

Lap length in compression

Section 3.12.8.15 of BS 8110:97 specifies the lap length

in compression = 1.25 times compression anchorage

length.