Some fascinating stuff coming in from readers. Sample:

“John—A hole can be drilled through the center of a beam (wood or metal)

without substantially reducing its capacity to bear a load. At the risk of

getting too deep into structural design theory, there exists a neutral axis

that runs through the longitudinal center of a beam. In a typical beam

loading condition, the top of the beam will be in maximum compression and

the bottom will be in maximum tension. As this transition occurs in a

linear fashion through the cross-section of the beam, there is zero stress

at the neutral axis, to comply with the ‘laws’ of statics. Therefore, any

removal of material at or near the neutral axis will not meaningfully affect

the load carrying capacity of the beam. I mention this to you because it

may be quite cumbersome to design all your connections to avoid drilling

through the center of a beam. On the other hand, this could become an

interesting design detail that would distinguish your tree house.”

Other readers have warned me that a tree house is every wasp colony’s

favorite place to build a nest, so when you come back from summer vacation,

check carefully before entering.