Expansion joints in masonry walls are provided if length exceeds

In building construction, expansion joints are provided to accommodate the expansion and contraction of materials due to temperature variations and other factors. These joints help prevent cracks and damage to the structure. The general guideline for providing expansion joints in masonry walls is when the length of the wall exceeds a certain threshold.

The correct option is 30 m. Expansion joints are typically provided if the length of the masonry wall exceeds this limit. This allows for the necessary movement and helps maintain the structural integrity of the wall.

Therefore, in the context of the question, the expansion joints in masonry walls are provided if the length exceeds 30 m.