 Steel girders used in the construction of buildings often are I-beams ', which have a cross-section shape resembling the letter 'I'. Most of the material in the beam is at the top and the bottom, which would seem to make the beam weaker. Why are girders shaped like this? 
When a girder is used in a building, it must support a load; the girder will bend slightly as it takes up this load. The top region of the girder gets compressed, while the bottom region expands due to the tension force. 
Most of the material making up the center of the girder doesn't change length, and is useful only in connecting the top and bottom layers, which are just about the only parts of the beam actually under load.Because the center portion of the beam isn't really needed to support the load, most of it is removed to make the beam lighter. The beam is almost as strong as a solid length of steel would have been. Moreover, the top and bottom surfaces are now easily accessible for riveting beams together. |