Semi-trailers can be the same types as trucks. Because they don't cost as much as a truck they can also be special types. Shipping containers are large and heavy. In some countries special laws cover them. Skeleton frame trailers are built to carry them. Long semi-trailers are used for logs. In the UK and EU some logs are smaller. The trailers are 2 or 3 logs long. They have a crane to load them. In the US "lowboys" (the load is just above the road) carry large construction machines. Special trailers can be built for very big loads. Australia "road-trains" can pull 4 trailers.