I need to stiffen a portion of the 2nd floor of my 100 year old 2 1/2 story victorian house, framed with 3X8 rough-sawn white pine joists @ 16″ o.c., cross-bridged @ mid-span. Joists run long dimension of the room I’m working on, from an inside wall to a 3-sided bay, spanning 16′ at the center, 13′ at the “short” corners. Preformance is marginal, due to over-span, low quality material (many knots and other defects) and inherent weakness of this species. A century of dancing is enough! Joists look impressive (when they have square edges), and work well in rooms with shorter spans. No help here!
Proposed fix: Apply 2 layers of 3/4″ plywood to one side of each joist, using construction adhesive and staggered joints; jack the system at mid-span to reduce sag and engage the sister “joists”.
Question: Would this approach work better if the plywood were deepened to, say, 12″ between the end walls of the room and notched over the supports (ledger @ outside wall, interior partion @ opposite end); or simply matched to the existing joist depth (averaging 7 3/4″ in these old members) and stopped inside the end walls? The “deep” option would probably require addition of a solid wood nailer to carry the new drywall ceiling (old lath and plaster removed for this work). Some favor the first choice, saying it would boost the stiffness of the plywood “helpers” because of their greater depth; I find it counter-intuitive, because the helpers would be bearing on thin air for the entire clear span, adding, it would seem, nothing but weight and cost to the project.
Comments welcome, including other approaches (no matches or kerosene, please).
Replies
Unless you can get 16' long plywood, you are better off sistering solid lumber
Construction adhesive won't hold the plywood to the beams -- you need fasteners.
The "taller" plywood would be considerably stiffer -- stiffness is proportional to the square of the beam height (but only linearly proportional to beam width).
Sistering will be much better than plywood.
Deeper joists sistered on will be the only thing that helps much, IMHO.
I'd bolt steel straps to the bottom of the joists, most fasteners at the ends, calculate for lateral shear.
Beefing up floor joist
What Morson said. LVL's much straighter and stronger than lumber. LVL's +/- 3000 f and 2,000,000 Mod. of Elas. You will pay more but you will not loose as much ceiling height.
A 1&3/4x9&1/4 LVL 16" o.c. will span 16'-0" with a +/-50# L&D Load with a 1/480" deflection. But don't take my word for it. I don't have all the info. needed to properly advise you. Go to you local lumber dealer and have them size the joist. They should have an Engineering source for their products. Depending on your requirements, you might get by with even a 7&1/4 deep LVL.
Of course, fitting an LVL into there, and especially getting it to bear on the ends, may be a challenge. (Though this is likely an easier scenario than many where the work is done in a crawlspace.) The plywood approach has the advantage that it can be notched around wires, etc, without losing much stiffness.
beefing up floor joist
"notched around wires, etc, without losing much stiffness". Dan, if you don't mind, I would like to question that statement.
Depending on where the notch occurrs within the span of the joist, the depth of the notch will reduce the strength/stiffness of the joist as if the overall size of the joist had been reduced by the debth of the notch.
Also, since the vertical shear requirements for "normal" joist installations is not a limiting factor, it is not necessary for the new sistered joist to bear on the supporting walls. If the sistered joist are properly fastened to the original joist, the new joist can just be fitted between the bearing supports.
To me, properly fastened means screwed and glued. And glued with real glue, not construction adhesives as mentioned in the original post. A glued joint needs to act the same as a weld does with steel. That is to say that the joint is to be at least as strong as the materials being joined.
I agree with most of what you say. But with two layers of plywood you can make an L cut in one piece, going one direction (the L passing above the wire), then another L cut in the other piece going the other direction (the L passing below the wire). WIth appropriate glue and fasteners there's very little loss of strength.
Do you have room to add a beam or a wall under (perpendicular to the floor joists)the floor joists? Cutting the floor joist span down would be a significant help.
Perpendicular Beam Idea
That would work if you can carry the loads on both ends of the beam down to the foundation. But that would require investigation and opening up walls and stuff. It's the kind of thing that really has to be looked at in person to say....
-- J.S.
sistering floor joists
I think I know what you are talking about, but what is the definition of SISTERING///