Three Pinned Arches

The railway system in Germany would be lost without the structural capacity of the three pinned arch.  Most people are familiar with the arches in stone which were the structural system of choice for bridges and buildings prior to the 19^th century.  They were sufficiently massive to ensure that the forces in the arch were dominated by the downward force of self-weight and as a result resisted all loads applied while still generating a compressive force in the stone.

Bottom Pin

When arches were built from cast iron, they became lighter and stronger, but since the major load was no longer self-weight, the arches began experiencing tensile forces.  Cast iron can resist some tension, but not much, so the span and height of the arch was limited.  Enter the three pinned arch.  It can be identified by the support at the bottom of the arch, which looks like a pin around which the arch can move. 

There is a similar pin at the base on the other side, and a third one somewhere near the top.  These pins combine to allow the arch to move and therefore eliminate bending forces, and thus allowing all forces to be resisted by compression.  The result is the huge spanned light structures which can be reproduced side by side to cover several platforms, and duplicated along the length of the platform infinitely. 

Top Pine visible along the centre line

The have the advantage of off-site manufacturing and easy assembly – no welding or riveting up there at the top during assembly.  That is why you find them everywhere in the Deutsche Bahn railway stations such as Frankfurt and Leipzig both shown here.

The final result of the arch structures