an expansion joint
Look at a long bridge and you will find a toothed metal gap crossing the roadway; along a steam pipe you will find a big U-shaped loop. These are expansion joints — deliberate gaps or flexible sections that give a structure room to grow and shrink with temperature instead of tearing itself apart. The design philosophy is simple: if you cannot stop the movement, let it happen freely.
Recall that a fully clamped part builds stress sigma = E x alpha x delta_T regardless of its length, but a free part moves delta_L = alpha x L x delta_T, which does grow with length. A 100 m steel bridge span over a 40 degrees C seasonal swing moves delta_L = 12e-6 x 100 x 40, which is 48 mm — nearly 5 cm. An expansion joint provides at least that much clearance, so the structure expands into the gap at near-zero stress rather than pushing on its supports. Pipelines use expansion loops or bellows for the same reason, and long buildings use sliding bearings and joint gaps.
Expansion joints appear in bridges, railways (the old click-clack gaps, though modern continuous welded rail instead uses heavy anchoring and breather switches), pipelines, long buildings, and even concrete sidewalks. The trade-off is that a joint is a discontinuity that must be sealed, maintained, and can wear, so engineers balance let it move against keep it simple. Omitting them is a classic cause of buckled track and cracked masonry.
The finger-like steel comb joints you drive over at the ends of a highway bridge open and close by tens of millimeters between winter and summer, letting the whole deck breathe without cracking its abutments.
A designed gap lets a structure move at near-zero stress instead of building it up.
The clever twist is that modern continuous welded rail removes the gaps and instead constrains the rail so hard it cannot buckle, accepting the thermal stress on purpose — the opposite strategy, chosen for a smoother, quieter ride.