This is certainly not a substitute for an engineer doing actual engineering but you can learn a lot about loads, spans and deflection from this online software:
forteweb.com
When companies like StruCalc started marketing engineering software to builders and architects, it was immediately recognized as an affordable, useful tool. Unfortunately, as the software grew in capability, it also became more complex. For those of us who had some training in structural design, using the software was not a problem. But a lot of builders, designers, and even architects had very little training in structures. That became evident when after the 10/93 wind code was introduced in Florida, architects did such a bad job of designing for high wind loads, that the state required a seal from a licensed engineer on every set of plans; and the architect's seal would not pass the structural part of the permitting process.
In states where a builder or designer could design the structural components of homes, even large, complex homes, structural software flourished. Even with the software, the person using it had to be aware of all sorts of factors that would affect the structure. From soil bearing capacity, to wind loads to calculating the live and dead loads on floors, ceilings, roof structures, etc, it was still up to the builder or the designer to do a lot of math. There were also a few cases where a software developer made a mistake and the software was not providing the correct specification for a component. The problems were not common, and usually corrected quickly.
Still, some builders got themselves into trouble when they assumed that the software would do all the thinking for them. In one case that I witnessed, a builder had installed a very large LVL girder in a home that was deflecting over three inches in twenty feet. He had not counted for two large point loads on the girder that were carried down to the girder from the roof framing. No one caught the mistake at the design phase, and the building inspector approved the girder as specified in the plans. It was an expensive repair for the builder.
For most simple span beams, you can pick out your engineered framing member from the manufacturer's design tables.
