Solution:
Columns BC and DE are perpendicular to beam AC,
∴BC∥DE,
∫D is the midpoint of AB,
∴AD=BD,
∴AE:CE=AD:BD,
∴AE=CE,
∴DE is the center line of △ABC,
∴DE= 1/2 years BC,
In Rt△ABC, BC= 1 /2 AB=4,
∴DE=2.
Comments: This question examines the proportion theorem of parallel lines and the midline theorem of triangles. The opposite side of a right triangle with an angle of 30 is equal to half of the hypotenuse. The key to solve the problem is to prove that DE is the center line of △ABC.
I think it is to find the length.