Precast double-tee units are generally used as structural floors when:. a. floor spans are short and loads are high.
b. floor spans exceed that which is economical with site-cast or hollow-core slabs.
c. floor spans exceed 60 feet.
d. floor spans exceed that which is economical with site-cast or hollow-core slabs and exceed 60 feet.

d. floor spans exceed that which is economical with site-cast or hollow-core slabs and exceed 60 feet.

Explanation:

Double tee slabs are prestressed concrete castings usually about 100 m to 150 m long. They are usually used in the construction industry in structures requiring a long floor span (or roof span) exceeding 60 feet in length, that is required to bear additional load (additional load carrying capability). Double tee slabs are usually employed in construction because they shorten construction time. reduce congestion at the construction site, and also reduce construction cost.

They are the required choice in construction when the cost of site casting or hollow-core slabs is no longer economical due to the length of the structure exceeding 60 feet.

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explanation: