Plural blocks (14) of friction material are mounted inside an elongate hollow element (12), such as a box section steel housing, in a substantially parallel array. Respective spring means (16) hold the blocks (14) under spring bias. Such elements (12) are intended to be mounted to or built into the walls of security enclosures or buildings, or into security doors. If a would-be thief attempts to cut through the element (12) with known cutting or grinding equipment and cuts into any one of the blocks of friction material (14), the equipment will cease to operate effectively and will be immobilised because of the compressive force under which the block is held. The blocks (14a, b, c) are arranged with associated spring means (16a, b, c) in parallel rows with the respective blocks and springs of each row offset so that the spring or springs (16) of any one row overlap the location of the friction material blocks (14) of the adjacent row so that wherever a cutting tool may penetrate it will always encounter a friction block, even in cases when it first encounters a spring. The blocks (14) are advantageously enveloped in non-friction material (18) to ensure that parallel blocks can slide relative to each other.