The minimum thickness of web in a plate girder, when the plate is accessible and also exposed to weather, is 8 mm. This is according to the American Institute of Steel Construction (AISC) Specification for Structural Steel Buildings, which states that the minimum thickness of webs in plate girders shall be 8 mm when the web is accessible and also exposed to weather. This requirement is intended to ensure that the web is thick enough to resist the effects of corrosion and fatigue.
Option A, 5 mm, is not a valid answer because it is less than the minimum thickness required by the AISC Specification. Option B, 6 mm, is also not a valid answer because it is less than the minimum thickness required by the AISC Specification. Option C, 8 mm, is the correct answer because it is the minimum thickness required by the AISC Specification. Option D, 10 mm, is not a valid answer because it is greater than the minimum thickness required by the AISC Specification.