By the 1910s, many states developed standard plan truss bridges, including steel Warren pony truss bridges. As the 1920s and 1930s progressed, some states, such as Pennsylvania, continued to build steel truss bridges, including massive steel through-truss bridges for long spans. Other states, such as Michigan, used standard plan concrete girder and beam bridges, and only a limited number of truss bridges were built. Roadbed types The truss may carry its roadbed on top, in the middle, or at the bottom of the truss. Bridges with the roadbed at the top or the bottom are the most common as this allows both the top and bottom to be stiffened, forming a box truss. When the roadbed is atop the truss it is called a deck truss (an example of this was the I-35W Mississippi River bridge). When the truss members are both above and below the roadbed it is called a through truss (an example of this application is the Pulaski Skyway), and where the sides extend above the roadbed but are not connected, a pony truss or half-through truss. Sometimes both the upper and lower chords support roadbeds, forming a double-decked truss. This can be used to separate rail from road traffic or to separate the two directions of road traffic. Since through truss bridges have supports located over the bridge deck, they are susceptible to being hit by overheight loads when used on highways. The I-5 Skagit River bridge collapsed after such a strike; before the collapse, similar incidents had been common and had necessitated frequent repairs. Gallery Multiple spans Truss bridges consisting of more than one span may be either a continuous truss or a series of simple trusses. In the simple truss design, each span is supported only at the ends and is fully independent of any adjacent spans. Each span must fully support the weight of any vehicles traveling over it (the live load). In contrast, a continuous truss functions as a single rigid structure over multiple supports. This means that the live load on one span is partially supported by the other spans, and consequently it is possible to use less material in the truss. Continuous truss bridges were not very common before the mid-20th century because they are statically indeterminate, which makes them difficult to design without the use of computers. A multi-span truss bridge may also be constructed using cantilever spans, which