The diagram above shows three beams.  The top is a wood beam.  The middle is a reinforced concrete beam as you would see on any number of local freeway overpasses.  The bottom is a steel beam.  For this example they are ordered by increasing ability to resist bending forces.  Why might one be stronger than the others?

The change in the cross-sections from rectangular to I shape is our focus.  Whereas stresses are proportionately distributed across the solid wood beam (with the maximum stresses occurring along the top and bottom faces), the concrete beam tends to concentrate its high stress regions in the enlarged regions top and bottom.  A steel beam takes this concentration to the extreme by concentrating the majority of the stresses in the flanges top and bottom.  The trend for ever stronger steel beams is to push the flanges farther apart and to increase their cross-sectional area.  The penalty is increased weight of material that detracts from the working load the beam can carry.

Hold that thought, because we about to take the idea of forces separated by a distance to the next level.