We have a kitchen floor with 2 x 8 floor joists, 16″ on center, spanning 11’6″. Consequently the floor is bouncy. The homeowners wants to replace the kitchen linoleum floor with ceramic tile. The basement area is heavily used and they don’t want us to support the floor with a new beam and lally columns. I am wondering if we could sister both sides of each joist with 1/2″ plywood to help stiffen them up? We can’t use steel flitch plates because there are too many obstacles in the way. Does anyone have a suggestion?
Thanks,
P.S Both homeowners are on the hefty side and they entertain alot in the kitchen so the additional weight can be considerable.
Replies
They don't want this or that and everything is maxed out, so they are forcing the nmost expensive solution.
Sister LVLs to the existing joists.
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Bolt a 10 ft long 1-1/2 wide steel strap to the bottom edge of each beam. LOTs of bolts, this will make each joist into what is termed a "built up beam". The steel does not have to be the full length of the joist.
A first 'off the top of head, non calculated' fastener schedule is 2" long 1/4" diameter lag bolts thru tight fitting holes in the steel into the bottom edge of the joist (NOT thru the joist). Spacing of about 6 inches apart for the center, gradually getting closer till the bolts have only 2" spacing at the ends of the steel.
Edit PS: is there proper bridging in place? If so, every other joist (or even every 3rd joist) is likely good to try first.
Edited 3/29/2007 9:40 am ET by junkhound
That's an interesting approach. Have you tried it? I remember several years back someone on this forum recommending attaching 2x4s (flat-wise) along the bottom of the floor joists but the steel approach would preserve more headroom. Initially, I was thinking that steel stock on the flat wouldn't help much- just bend to conform to the deformed shape of the joist when it flexes but I guess if you attach it really well (very tight fitting holes in the steel as you mentioned), when the joist wants to deflect, the steel is put in tension and helps that way. Still though it seems that any slop in the bolt holes or wiggle in the screw would undermine the approach. Just wondering if you've done this with success in the past...
Yep have done it many times.
The method can also take an existing sag out of a joist or a beam. Did just that on brothers house (he bought a fixer) that has a 2 inch sag in a doubled joist under a lath and plaster wall (plaster had cracked badly so no structure contribution from the wall). Put the steel in place, jacked up the center of the doubled joist with a 10 ton hydraulic jack till it was actually bowed upwards about 1/4 inch, then bolted with jack in place. Setteled right down to perfectly flat when jack was removed.
The attachment is a 1/8 by 5" plate I bolted to the bottom of a 6x12 at son's house. We added a floor and storage above the garage, and the calculations showed the beam would have been too weak (and likely would have sagged) and needed to be about 2x as strong, which the steel plate did.
Notice on the pix that there are many bolts (these were 6 inch 3/8 lag bolts) near the ends of the steel plate, many fewer near the center. The bolts take the horizontal lateral shear forces at the ends of the beam.
The "bub2" pix is at one end, notice the many bolts. The "built up beam' pix is from the center of the beam ooking outwards, fewer bolts at the center.
View ImageView Image
Edited 3/29/2007 1:05 pm ET by junkhound
Thanks very much for the suggestion and the very helpful photos. My question is how long ago was the fix done and is it still holding up? Another thought I had - could a u shaped channel of steel bolted underneath and on the sides of the joists work as well? My concern is that lags will loosen up over time and cause the floor to flex again.
Wouldn't 1 1/2" x 1 1/2" steel angle or channel work even better?
Edited 3/29/2007 2:30 pm ET by PGproject
how long ago was the fix done and is it still holding up
long-long ago in one case (57 years), 19 years ago in another, the garage 6 years ago.
I learned the technique in 1950, when one of the bridge engineers my dad worked with on the railroad told him how to do make a "built up beam".
Pop did put a U channel on the bottom of a 6 by 8 that went across the whole house (he took out a brick wall beneath) and even chiseled some of the beam off the side so the U channel would fit - easier to use a flat plate. Mom still lives in the house, 57 years later there is zero sag in the beam. Pop used 1/2" lag bolts vertically into the bottom of the beam, non have loosened. On green wood there probably would be some loosening that would require re-tightening when the wood was dry. Edit - Pop used the channel 'cause he got it free off the railroad scrap pile from an old water tower (diesel coming in, the water towers coming down)
The example fix on my brother's house was done in 1988, again it is still in place with no problems and no re-sag. Recall we also added on bolte on strap at a center of the room joist to get rid of some bounce, just the one joist did wonders with good bridging.
The garage pix fix was done about 6 years ago on sons house, again, no problems, no sags.
When my brother did an addition in 1992, he needed a beam to span 26 feet. He could have bought a big glue lam, but we made a built up beam with flat 2x6s and plywood webs, glued and screwed - that also has not moved. A glue lam would have pretty much precluded any outlets in that wall under windows.
The angle iron idea is fine if you dont have to remove any bridging (or molding as in the garage case) to put it on , a few extra bolts at the end sideway thru the joist and angle leg wouldn't hurt.
Edited 3/29/2007 2:52 pm ET by junkhound
Again, thanks a million for the input! You plan sounds like a very viable solution. I'm meeting with the clients early next week and will try to sell them on this. As usual, what people want and what they really need can be vastly different. I cannot in good conscience proceed with the job without taking care of the sag. I'll let you know how it progresses.
What they want, and what they need, are at odds. They need to be made aware of this. They are going to have to comprimise.
Plywood isn't going to solve the problem. Steel flitches si going overboard....larger 2Xs, or LVLs sistered will help....Ideally a girder or supporting wall below.
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