GOODWILL AND INTANGIBLE ASSETS - Carrying Amount of Goodwill by Segment (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 26, 2020	Jun. 27, 2020	Sep. 26, 2020	Dec. 28, 2019
Goodwill
Gross balance, beginning				$ 478,111,000
Accumulated impairment losses				(49,247,000)
Balance, ending	$ 421,947,000		$ 421,947,000	428,864,000
Carrying amount of goodwill
Balance at the beginning of the period			428,864,000
Acquisitions			6,138,000
Asset impairment	0	$ (12,575,000)	(12,575,000)
Foreign currency translation			(480,000)
Balance at the end of the period	421,947,000		421,947,000
Engineered Support Structures Segment
Goodwill
Gross balance, beginning				228,634,000
Accumulated impairment losses				(18,670,000)
Balance, ending	195,784,000		195,784,000	209,964,000
Carrying amount of goodwill
Balance at the beginning of the period			209,964,000
Acquisitions			0
Asset impairment			(12,575,000)
Foreign currency translation			(1,605,000)
Balance at the end of the period	195,784,000		195,784,000
Utility Support Structures Segment
Goodwill
Gross balance, beginning				130,594,000
Accumulated impairment losses				(14,355,000)
Balance, ending	118,968,000		118,968,000	116,239,000
Carrying amount of goodwill
Balance at the beginning of the period			116,239,000
Acquisitions			1,100,000
Asset impairment			0
Foreign currency translation			1,629,000
Balance at the end of the period	118,968,000		118,968,000
Coatings Segment
Goodwill
Gross balance, beginning				93,747,000
Accumulated impairment losses				(16,222,000)
Balance, ending	77,116,000		77,116,000	77,525,000
Carrying amount of goodwill
Balance at the beginning of the period			77,525,000
Acquisitions			0
Asset impairment			0
Foreign currency translation			(409,000)
Balance at the end of the period	77,116,000		77,116,000
Irrigation Segment
Goodwill
Gross balance, beginning				25,136,000
Accumulated impairment losses				0
Balance, ending	30,079,000		30,079,000	$ 25,136,000
Carrying amount of goodwill
Balance at the beginning of the period			25,136,000
Acquisitions			5,038,000
Asset impairment			0
Foreign currency translation			(95,000)
Balance at the end of the period	$ 30,079,000		$ 30,079,000