We are remodeling the first floor in our 2 flat. We use it as a single family. Tearing out a wall between kitchen and "second" bedroom from when it was originally built in 1926. The current floor joists are 2×8 and span the 24 feet, but there is a wall mid way. If we were to double up the joists to add additional support would that work? These are 16" on center. We have steel columns in the basement supporting the house. Or do we need to do a header to support the second level? Any help would be greatly appreciated.
Ceiling – span 23 feet with doubled 2×8 floor joists
ceilingjoists
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Blocking typically refers to the structure that prevents the joists from twisting, which would be a concern over a load bearing beam. A solid piece of wood, often the same material as the joists themselves, is installed perpendicular to the joists without any gaps (e.g. I joists would use a piece of I joist for blocking). Two 2x4's installed in an X could also be done, and I've seen that with engineered joists, but check with your local building codes to be sure it complies.
I could also see blocking refer to additional vertical members (2x4's) to carry the load from the beam up to another load bearing object above the floor (e.g. a column). The last type of blocking I can think of would be fire blocking to prevent fire from traveling through the ceiling between parts of the home, though it's unlikely you'd see this requirement in a single family structure outside of a garage.
Like Tester101 says, call the inspector and ask for clarification, they should be happy to provide it.
400 lbs is not so large (consider that 2 or 3 people standing close to each other could easily exceed that), but you must be careful not to mount the hammock in a way that compromises the joists. Since the hammock supports pull at a diagonal and not straight down, the force exerted at each contact point is actually much more than the weight of the occupants.
It would be best to mount the hammock in the direction of the joists, so that one end is near the outer wall and one end is near the main center beam. The bending load on the joist will be far less than if you mount at the midpoint. Plus, the load from the hammock will be mostly in the axial direction, which is not a big deal.
If you must mount the hammock perpendicular to the joists, suspended from their midpoint, it would be best if the hammock attach point were at the location of cross bracing between the joists, and/or blocking if you have that instead. The bracing prevents the joists from twisting or shifting laterally, which otherwise would be a weakness. If you have no bracing between joists or they're not in a convenient spot, I would add some at the suspension points——say, 2 joists in each direction. If you mount the hammock this way, you must ensure that the joist do not twist or get deflected laterally, as that will severely compromise their stability.
As for attaching to a joist, I would not drill a large hole through the joists if you can avoid it, although it would probably be OK. Try to find a metal bracket at a construction supply store that will fit on the 2x8s and provide a support for your hammock chain. E.g. Home Depot carries several Simpson Strong-Tie products that look promising.
(If this is sounding like too much work or too risky, you could always pick up a free-standing hammock stand.)
Best Answer
No, you cannot span 23 feet with conventional lumber, even doubled. It'll be as bouncy as a trampoline and won't meet code, and that's if you can find such lengths.
You'll need at least a 12" (nominal) I-joist or an engineered floor truss to span that far, or you'll need an LVL or steel beam midway.
None of us can offer more specific advice without a site visit, and most of us aren't engineers anyway. There's really no way around having a qualified local expert stop by.