What is the name of a static anti-tank obstacle defense made of metal angle beams or I-beams?

The Czech hedgehog (Czech: rozsocháč or ježek) is a static anti-tank obstacle defense made of metal angle beams or I-beams (that is, lengths with an L- or I-shaped cross section). The hedgehog is very effective in keeping light to medium tanks and vehicles from penetrating a line of defense; it maintains its function even when tipped over by a nearby explosion. Although Czech hedgehogs may provide some scant cover for infantry, infantry forces are generally much less effective against fortified defensive positions than mechanized units.

The Czech hedgehog's name refers to its origin in Czechoslovakia. The hedgehogs were originally used on the Czech–German border by the Czechoslovak border fortifications – a massive but never-completed fortification system that was turned over to Germany in 1938 after the occupation of the Sudetenland as a consequence of the Munich Agreement.

The first hedgehogs were built of reinforced concrete, with the shape similar to later metal version. However, the concrete hedgehogs proved ineffective during tests as they could be substantially damaged by machine-gun fire. Moreover, even when turned into debris it provided more cover for the enemy infantry than the metal counterpart. Therefore, only the oldest sections of the Czechoslovak defensive line, built in 1935–1936, were equipped with concrete hedgehogs, and usually only in the second line.