Commonsense would dictate that structure would have to be adequate to support us and our things against the pull of gravity. There is one notable exception in regions prone to earthquakes where all those physics formulae have to revisited and terms like acceleration and mass, not to mention Alquist Zones, but for today let’s just look at a commonsense way of looking at building structures:
- The predominant direction of force of building weight and its dancing occupants and stuff is downward. Through the finish floor to floor sheathing, to joist, to beam, to post, to foundation wall, to footing, to well compacted subsoil, to mother earth. This being said, the aforementioned earthquakes and stormy winds can produce such a significant uplift as to counteract the weight of materials in place and throw them into the neighbors yard or street. Seen it. Beautiful Deck House, roof system in place, windows in on the lee side. Comes a gale and the roof flips over the shrubbery to the neighbor’s lawn. Fortunately no one was nearby. So we tie our structures down through an uninterrupted series of tension members to something weighty enough to prevent flight; usually the foundation.
- If the path downward lines up the structure can be most efficient. Posts are way strong in holding up weight. Beams, not so much, so if posts land in mid-span of the beam below, prepare for a super-sized header. Post over beam over post is ideal. If the architects can’t make up their collective minds on floor plans, we assume the worst and put the point load at mid span, because if we don’t, that’s where they will want to put the post. Seen it.
- This load transfer paradigm applies to all situations, Had to provide a triple top plate on a building where the constructors couldn’t land the roof trusses directly on top of the 16″ o.c. studs. Usually happens when someone decides to frame the walls at 16″ o.c. and the roof at 24″.