فِي عَمَدٍ مُمَدَّدَةٍ
Amid extended columns or braces. (الهمزة: 9)
The final phrase follows the declaration that the fire is closed over its occupants, intensifying the image of confinement. The expression names extended pillars or supports; detailed accounts differ on how this is pictured or syntactically attached.
No single elaborated physical reconstruction should be presented as a necessary grammatical fact. For this lesson the defensible rhetorical observation is that the phrase completes the preceding scene of enclosure and severity.