Learn English – Abbreviation “mm o.c.” in construction (spacing of ducts etc)

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What does o.c. stand for and what does this mean?

These are some examples of its use:

Space end bells approximately 250 mm o.c. for 125-mm ducts.

Space fasteners at 10" (250 mm) o.c. on intermediate supports for single layer floors and 12" (300 mm) o.c. for subfloors.

Cold joints in flat insulating concrete form foundation walls shall be reinforced with no less than one 15M bar spaced at not more than 600 mm o.c. and embedded 300 mm on both sides of the joint.

It seems to be impossible to find a definition for this term, but it's widely used in construction-related papers.

Best Answer

O.C. stands for on center.

From About's woodworking glossary:

Definition: abbreviation for "on-center", which means that an object is placed at the center of the mark. For instance, in building a wall, a building plan may call for the wall studs to be places every 16-inches OC. This means that the center of each stud is placed every sixteen inches.

On center measurement results in different spacing than using the same space between the edges of the pieces:

Space end bells approximately 250 mm o.c. for 125-mm ducts.

If you left 250 mm and weren't spacing on center, then four ducts would require 1.25m (250mm for each of 3 spaces and 125mm for each of 4 ducts.)

Spacing on center, 4 ducts would fit into 875mm: 250mm for the 3 spaces (which the ducts fit into) and 125mm for the edges of the ducts on the ends.

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