The difference between the two is the size of the component. Those who are in the construction agency are usually familiar with these things because they need to know what item they are looking for. The girder is used for items that are much bigger than usual.
The girder is also responsible for making sure that it will provide the needed horizontal support for a given structure. The beans are responsible for providing the support that the structure needs vertically. It is best if people know what they are referring to so that they will know what option is best for them. There are still different types of items that are being used for construction.