I-beams size when smaller termination

[TexanJohn]

Novice question: I noticed on all the highway construction around me that the I-beams get 'smaller' where they mount to base/beginning of an overpass or even an intermediate supporting structure. For instance, the vast majority of the I-beam appears to be 8' tall, but at the termination points it might only be 4' tall.

Why not just make the same size all the way?

 

[FredGarvin]

I can't say for sure, but the times I have seen varying cross sectional beams was an effort to establish a constant stress across the span. In the situation of overpasses etc...the loading schemes they have to take into account must be pretty complex.

 

[haynewp]

It has to do with the moments/forces varying along the length of the beam and saving money. (More depth is required where certain demands on the beam are greater.)

Lots of beams are made to be constant along the entire length, they are just standard sizes an engineer can pick to meet the loading demands (none 8ft deep though). The size is chosen by the highest loading demand on the beam, and the entire beam is the same section all the way through and cut to a desired length. But those beams are not an example of maximizing the efficient use of material.

There is also the option to vary a beam's depth (among other things) and have it custom fabricated. This can sometimes be cheaper because there is less material, but this strategy may also be offset by the higher fabrication costs of creating a "customized" beam. For a beam the size you describe, you would have to have it custom built anyway so why not save on material by varying the section, and have a bridge that does not look overwhelming to people driving under?

HTH