It takes more than slapping a few logs across a ditch to carry the loads of highway traffic. Even in pioneer days, it was not uncommon to transport great loads of merchandise and raw materials in huge freight wagons hitched in tandem. The weight these ancient vehicles carried could make any bridge groan.
Oregon bridges chiefly employ three truss methods to bear the loads, the Kingpost, the Queenpost, and the Howe truss. The Kingpost only exists on one Oregon bridge now, the Neal Lane Bridge. Five bridges use the Queenpost, Antelope Creek, Fourtner, Lost Creek, North Fork of the Yachats, and Wimer bridges. Most of the remaining use the Howe truss.
The simplest and oldest bridge truss is the Kingpost. It is very efficient at transferring the load from the center of the bottom chord to the bridge abutments. Its failure is as the span to be traversed increases, so does the height. Since there is a limit to the height which trees grow, another method needed to be found to bridge greater distances.
The answer came in the form of the Queenpost. It uses an upper chord to foreshorten the height which would be necessary if using a Kingpost. This upper member allows the use of two uprights in the truss and thereby increases the distance which may be spanned, but this truss also fails for similar reasons. The Queenpost only allows two uprights, so like the Kingpost, the Queenpost's height must increase as its length increases
Around 1840 William Howe developed a further variation on the Kingpost truss. His method of transferring load from the lower chord through upright threaded steel rods into a series of diagonal allowed for much greater distances to be bridged. At last, with this truss, the distance it spans can be increased by simply adding more uprights and diagonal. The height can remain consant. This is the most common truss used in Oregon's covered bridges.
In addition to the wooden trusses used in "true" covered bridges, several additional methods have been employed. A steel girder is used to support the Milo Academy Bridge. Wooden deck girders are used to support the Rock O' the Range Bridge. Portland's Cedar Crossing is a concrete span covered with a wooden structure. Finally, the Lake Creek Bridge has a pre-cast concrete deck with a concrete center pier to help support it as it often accommodates heavy log trucks.