Tuesday, November 19, 2013

Structural Primer

Today you will get a structural lesson. Does that sound boring and too math heavy? Well, it's not!

We could set this post to the tune of that old song: "The Knee Bone's Connected to the...Thigh Bone!"  Why? Because that's what it's like. Trusses or rafters bear on walls or beams; walls bear onto floor joists or joist beams; floor joists rest on girders; beams span openings in walls and they are supported by posts at each end, posts are point loads that need land onto beams or girders; and eventually it all comes down to the concrete footers that are underground.

First some definitions:
  • A Girder is a built-up beam that supports the main floor joists. It is in the crawlspace.You can probably see one in your basement, if it's unfinished.
  • Joists are the repetitive members that make up a floor or ceiling system. They can be supported by beams or walls or a combination of both.
  • Rafters are the repetitive members that make up a roof system (along with a ridge board or beam and possibly collar ties)
  • Beams span an opening (like over a door or window) and joists rest on top of them.
  • Joist Beams are in the same plane as the joists, but are built-up of more than one unit to accept a load that will be there.
  • Flush Beams are not dropped under what they support (like the girder) but rather are flush - or even- with the joists it supports. Hardware like joist hangers are needed for this to work.
  • Posts can be a couple of studs put together to support each end of a beam or such
  • Columns are round (as opposed to square edged posts) but are also vertical point-load carrying members.
  • Bearing Walls are when all the wall studs as a unit provide continuous support
  • Trusses are factory made (usually), can span long distances for floors or roofs, and bear outside wall to outside wall (so interior partitions are just dividers, not structural)
  • Engineered Lumber is made from cut wood, sawdust and glue and is much stronger (really!) and can span farther than 
  • Dimensional Lumber, which is 2x4s, 2x6s, 2x8s, 2x10s & 2x12s comes in a few different species, each with it's own strength and span characteristics.
Well - this is getting long, but stay with me. Here are some photos to put it all together:
Main Girder supports mid-span of floor joists. In this case it is a built-up engineered beam (Three LVLs put together) The framers are cutting a column to the correct height
Girder rests in beam pockets in the concrete wall and onto steel columns that bear onto interior concrete footings. Floor joists will run perpendicular the the girder and across the top of it out toward each side wall

Floor joists span from the foundation wall (sill plate, actually) to the main girder. Here is also a cantilever, which means the joists extend beyond the support  of the foundation wall over open air

Header with posts each end (over opening on the right) Flush beam (on top of wall by John's hand) Second floor joists will attach into that flush beam and run perpendicular to it
Long LVL beam will carry roof, floor and wall loads from above so the space can be open (ie. not have a wall there) This beam has posts at each end and mid-span, buried within the stud walls
Second floor joists - some running east-west landing on a bearing wall, some running north-south where Scott is installing more; also see headers over windows, because regardless of floor joist direction, that exterior wall will be carrying the main roof's load

It's just like Lincoln Logs! Well, maybe not... Did this help any of you understand a bit of how a house is framed? Now you should be able to look around your own house and figure out which walls (non-load bearing) can be changed easily to have a wider opening or a larger window- and which walls or beams are keeping the floor from collapsing when you dance and jump around.


  1. Just curious what you used under the girder to get it even with the sill plate? Do you know if a pressure treated 2x4 can be used or does it have to be a steel plate? Thanks, Scott

    1. Yes, a custom sized piece of pressure treated wood can be used. Whatever you plan to use, the form for the beam pocket in the concrete wall just needs to be properly sized in advance.