Every material can span onto a certain point, they all have limits. Wood for example can only have a span of about 40 feet (depends on the load); that is the max in order to carry the wood onto a truck. Rules of thumb means the general concepts on making something, this can vary depending on the material (wood, steel, concrete) you use. In other words, you have to use your own understanding on how to make such material, rules of thumb is not something that is strictly intended to be accurate or reliable. it’s more like a guide on how to start things. Example (Page 4.04 on Building Construction Illustrated) “Rule of thumb for estimating depth of a concrete beam: span/16′”- Francis Ching. What Ching means is that 16′ feet is the “standard” principle on making the depth of the concrete beam.