What makes a truss bridge strong

Addition and decomposition of forces

As early as 2000 BC. the Chinese are said to have dealt with the art of bridge building. In the early days next to the small suspension bridges there were the so-called Girder bridges most common due to their simple construction. In these girder bridges, however, relatively high tensile and compressive forces occur, so that with simple building materials with larger spans one quickly reached limits. By inserting intermediate piers, however, you can achieve considerable spans even with this type of bridge, as demonstrated by the Europa Bridge near Innsbruck. The girder bridge is still the most common type of bridge today.

That meant a considerable step forward Arch bridgewhich found widespread use in the Roman Empire and later among the Venetians. Since compressive forces occur primarily in the arched bridges, materials such as sandstone and granite could be used, which are relatively small in tension but strong in compression. Since the materials used, unlike wood, have a long lifespan, we can still admire bridges from Roman times today.
The arch bridges require abutments, which "absorb" the large horizontally directed forces. These horizontal forces are greater, the flatter (and therefore usually more elegant) the arch. Think about why this is so!
The statics of the arch bridge "works" only when all the stones are fitted between the abutments. During the construction phase, considerable support structures are required, which drive the construction price up significantly.

The most imposing bridge shape is probably that Suspension bridge. In the Himalayas, this type of bridge bridges large gorges (see picture on the right).
With the suspension bridge, very large spans are possible, so that it can also be used, for example, to lead roads across straits. The road used to be hung on chains and later on steel ropes. Only tensile forces occur in the ropes, whereas in the pillars (pylons) on which the supporting structure is suspended, compressive forces are primarily found.
In contrast to the girder and arch bridges, the suspension bridges are not rigid. This can lead to unfavorable wind conditions causing the bridge to vibrate, which can lead to the destruction of the bridge (Tacoma Bridge 1940).




A relatively new type of bridge is the Cable-stayed bridge. As a rule, it is more economical to build than the suspension bridge. In addition, massive anchorages are not necessary at the bridge edges.

Using a section of a cable-stayed bridge, it will now be shown how the weight of the road is converted into a pressure force on the pylon.