*
This is the scenario: 2 – 1/2 story home with 1st and 2nd load bearing walls 30″ to one side of basement beam. Result: area of first floor right above actal beam location is bowed up, some of the joists are cracked. What would you do?
I thought about sistering (sandwiching, actually) existing 2 x 8s on either side with 8′ lengths of 3/4 inch ply, jacking level, glueing and bolting. Do you foresee any problems or better options to this? By the way, does anyone know the approximate load capacity of 3/4 plywood versus similar depth lumber?
Replies
*
Perhaps you should pose this this question in the Woodshed Tavern?
Sandwiching?
*
Ouch! mcgough, your sarcastic humour really burns! But seriously, folks, what would the advantages be of sistering both sides of a joist with ply for reinforcement/repair vs sistering 2x lumber? Is this overkill? What would you use for fasteners, nailing/bolting pattern, adhesives, etc?
*
Can you install new a beam under load bearing walls?
I'm a little confused about your plywood sandwiching idea. How long are these joists? My idea of "sistering" is to add a joist right next to the first one, bearing the way the first one does, adding strength by doubling the framing members. If you only through bolt, screw or nail the second one alongside without bearing, it seems to me you would loose most of the strength you hoped to gain.
So you'll point to TJIs and say they are stronger than lumber because the flanges keep the web from rolling and that is what you propose to do by fastening plywood to the joist. I only think that logic holds true if the plywood gets direct bearing on each end. So...how far do these joists span? - jb
*
I think you're probably right, my span is 12 feet, so the sistered plywood (in this case, 8' length) will bear only on the beam and not on the foundation wall. I will have to use lumber and double or triple up on points from beam to foundation wall to support this cantilever.
I can almost get a new beam in, but, this is complicated by the fact that the joists do not have alot of overlap over the existing beam so that any beam change more than 8" leaves one side floating. What about a second beam 30" over? I've thought about this. Is there anything wrong with having 2 beams so close together (other than the number of posts increasing and making the basement look like a forest (unless I do a steel beam or microlam and size it to free span the 32 feet beam span). Oh, and yes, I will be talking to an engineer eventually.
*
No, in fact, adding a beam was my first thought. Look, those joists failed due to too much weight, right? Well, when you sister in new ones, they will be ok at first, but eventually they will probably fail too. Put in a beam where it should have been in the first place, under the bearing walls. (You still shoud sister in joists next to any cracked or broken ones though) - jb
*
I'd be a little leery of sticking columns 30" away from the foundation beam, which I assume is under the existing beam and columns.You could end up with a pretty good point load sitting on 3"-4" of concrete slab.I'd seriously think about spanning the whole distance,with columns at the perimeter walls, where hopefully they'd be resting on something fairly solid.
I'm interested in how you plan to jack it up level,BTW good luck with the sheetrock repair.
*
semtex - good point about bearing on the slab. May have to pour pier pads there. - jb
*Hi Gio.I am working on a 1920 Colonial Craftsman home.In the back of the house, there is a garrison overhang. This extends the back of the house, on the second story, 36" past the foundation.(Add to that, directly below, on first floor, was mostly windows and a door!)I found that in this cantilever situation, the failure was caused by lack of counterweight. In other words, the new joists are 6' longer than the old warped ones I took out.I was fortunate to have opportunity to replace the roof, so I framed to carry load to the foundation. I also used lots of PL400 and blocking.I used shear walls to stay the walls that extend in overhang.Hope this helps.
*alan - you can rely on PL400 for sheer resistance? - jb
*Hi jim "crazy legs" blodgett.I have been.Why? Do you know something I dont?Some time ago, someone told me about "creep", regarding the PL400.So, I called the chemists at ChemRex, the manufacturer of the product, whose number is on the side of the tube.The chemists told me that the glue has to be used in conjunction with mechanical fasteners.Accordingly, that is how I have been using it.Since I started using it a lot, for this project, ChemRex has come out with some sort of polyurethane version of the PL400 which is supposed to be stronger with less glue.I tried it on a pressure treated wall recently. It dries very shiney. Seems strong. I would use it again, so that I could run some tests. But, if anyone has any doubts, call the manufacturer. They love glue talk!Hope this helps.
*Well, I built a box beam several years ago and sheated it and stood it up and sheated the back and took out the temporary legs and it sagged. So I jacked it back up, troweled on PL400 with a notched trowel (this thing is 4' X 32' and resheated it and nailed the begeebers out of it and took the temporary legs out and it has held just fine for 15 years. I always figured it was the glue.But then I read in "Offcuts" or "Tools + Materials", or one of those, that the strength to construction adhesives is that they never completely harden and get brittle, but that also means they cannot be counted on for shear. So it interests me that you mentioned it, that's all. What did those folks say about this "creep". That sounds like what I remember reading. - jb
*I don't know if this expains anything about creep, I think alan's post makes it pretty clear but I'll throw this in.Creep is when a material changes it's shape over time. All materials flow like a liquid, some faster than others. There was a post here a couple of weeks ago about a granite bench that after 100 years was curved. That's creep.The thing with creep is that it's permanent. If you take the load off of a sagging joist, it will spring back up. If you take the load of a joist that has crept, it won't.When materials that are crystaline creep, the molecules keep shifting and realigning with the row of molecules next to it. Diamonds become perfect over millions of years because broken chains of carbon "flow" to the outside or line up the way we like um.I had a college professor who used to call creep "historisis". Throw that one out the next time you've talking to a client.
*That's not the "creep" I was thinking of. Maybe the term "elasticity" would be a better one. Something about the way glue retains elasticity, allowing framing members in a vertical plane to sag independenly of each other. I don't remember for sure, I just remember bein' convinced that what the writer said made sense about not relying on that type of adhesive. Anyway, they have probably changed the glue by now. - jb
*Your concerns are valid. At this stage, sistering may or, more likely, may not, cure the problem. To go through all that work and have it not work would be painful.I'd opt for a new beam directly under the load bearing walls. Figure out where you want your support columns. I'd then take a concrete blade and cut a 2' square out of the 4" basement slab in the proposed location of each column. Dig down 12-18" and pour a new footing, keeping the top of the footing even with the bottom of the slab. Jack up the sagging joists slowly, over a period of several days or even weeks, depending on how much you need to bring them up to get back to level. By going slowly, you can sometimes minimize then number or severity of plaster/drywall cracks. Place your new beam and lally columns, with the columns sitting on the footing. Plan on using something (LVL, steel, etc) other then 2-by stock for your beam. If you do use 2-by stock, plan on it shrinking slightly as it dries out, once again causing your joists to settle. Once done, pour the final 4" concrete layer over the footing and around the base of the lally column to bring it level with the top of your current basement slab.I don't like support columns sitting on top of a slab, even if the footing is underneath. If the column/footing settles at all, you're looking at a cracked slab. By placing the column on the footing, if it settles, typically the column will "slide" through the slab, not cracking it. If it does happen to crack, it won't be as severe. If the column did indeed settle due to uncompacted soil under your newly poured footing, you could jack the beam off the column and properly place and secure a steel plate (shim) between the top of the column and the bottom of the beam.If you proceed and try this yourself, study up on jacking. You could easily maim or kill yourself if you don't do it properly. You don't ever want to find yourself in the path of an airborne jack.
*
This is the scenario: 2 - 1/2 story home with 1st and 2nd load bearing walls 30" to one side of basement beam. Result: area of first floor right above actal beam location is bowed up, some of the joists are cracked. What would you do?
I thought about sistering (sandwiching, actually) existing 2 x 8s on either side with 8' lengths of 3/4 inch ply, jacking level, glueing and bolting. Do you foresee any problems or better options to this? By the way, does anyone know the approximate load capacity of 3/4 plywood versus similar depth lumber?