GOODWILL AND INTANGIBLE ASSETS (Details 3) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 29, 2018	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
Goodwill [Line Items]
Goodwill, Gross			$ 434,454	$ 372,612
Accumulated impairment losses			(49,247)	(34,892)
Carrying amount of goodwill
Balance at the beginning of the period		$ 385,207	337,720
Acquisitions		42,133	87,119
Impairment	$ (14,355)		(14,355)
Divestiture of grinding media			(15,814)
Foreign currency translation		1,524	(9,463)
Balance at the end of the period		428,864	385,207
Engineered Support Structures Segment
Goodwill [Line Items]
Goodwill, Gross			204,735	170,076
Accumulated impairment losses			(18,670)	(18,670)
Carrying amount of goodwill
Balance at the beginning of the period		186,065	151,406
Acquisitions		21,870	42,216
Impairment			0
Divestiture of grinding media			0
Foreign currency translation		2,029	(7,557)
Balance at the end of the period		209,964	186,065
Utility Support Structures Segment
Goodwill [Line Items]
Goodwill, Gross			123,618	90,248
Accumulated impairment losses			(14,355)	0
Carrying amount of goodwill
Balance at the beginning of the period		109,263	90,248
Acquisitions		7,889	34,280
Impairment			(14,355)
Divestiture of grinding media			0
Foreign currency translation		(913)	(910)
Balance at the end of the period		116,239	109,263
Coatings Segment
Goodwill [Line Items]
Goodwill, Gross			80,937	76,696
Accumulated impairment losses			(16,222)	(16,222)
Carrying amount of goodwill
Balance at the beginning of the period		64,715	60,474
Acquisitions		12,374	5,120
Impairment			0
Divestiture of grinding media			0
Foreign currency translation		436	(879)
Balance at the end of the period		77,525	64,715
Irrigation Segment
Goodwill [Line Items]
Goodwill, Gross			25,164	19,778
Accumulated impairment losses			0	0
Carrying amount of goodwill
Balance at the beginning of the period		25,164	19,778
Acquisitions		0	5,503
Impairment			0
Divestiture of grinding media			0
Foreign currency translation		(28)	(117)
Balance at the end of the period		25,136	25,164
Other
Goodwill [Line Items]
Goodwill, Gross				15,814
Accumulated impairment losses				$ 0
Carrying amount of goodwill
Balance at the beginning of the period		$ 0	15,814
Acquisitions			0
Impairment			0
Divestiture of grinding media			(15,814)
Foreign currency translation			0
Balance at the end of the period			$ 0