stay-rod

from The Century Dictionary.

• noun In steam-engines: One of the rods supporting the boiler-plate which forms the top of the fire-box, to keep the top from being bulged down by the pressure of steam.
• noun Any rod in a boiler which supports plates by connecting parts exposed to rupture in contrary directions.
• noun A tension-rod in a marine steam-engine.
• noun A tie-rod in a building, etc., which prevents the spreading asunder of the parts connected.