I have a 70+ year old story and a half with some seriously sagging floors. When I bought it 15 years ago, I’m sure the home inspector mentioned something about the overspanned joists, but it didn’t sink in until recently when I was considering a bathroom renovation.
The joists are 16″ OC 2×8’s and have up to a 16′ span, resting on a 5 1/2″ x 7 1/2″ beam supported by columns on about 8′ centers. Regardless of the bathroom renovation, the sag has to be fixed, as I am contemplating selling the house in 2-3 years. I don’t want to be hammered then by somebody demanding a $25,000 allowance to remedy the floor as I watch my closing date sail off into the sunset. The sag is apprciable; long pieces of furniture have a noticeble “list”.
Using the Southern Pine Span Tables, even the highest grade of MSR lumber only gets me 13’10” for 40psf live load and 10psf dead load with a 360 deflection (worst case: 10’3″ for No.3 Visually Graded). My question is this: If I sister all the 2×8’s with 2×8’s, will I be able to cover the span adequately? I plan to nail the heck out of everything and thru bolt as well. I’ll also be replacing the center beam with a builtup of 3 or 4 2×12’s. Intuitively, it would seem that, all other things being equal, a 4×8 should span 27’8″, best case (20’6″, worst case). In any event, by sistering the beams I’ll be reducing the effective spacing to 12.5″ OC. Your comments are appreciated.
Replies
> Intuitively, it would seem that, all other things being equal, a 4x8 should span 27'8", best case (20'6", worst case).
It looks like your reasoning is that doubling the width should allow you to double the span. Sorry, but the math involved isn't that simple.
If there's a building department for your location, check with them. They'll probably require you to have an engineer design and wet stamp anything that isn't in the span tables.
Even if there's no government agency requiring you to hire an engineer, you're looking at a long, steep learning curve to get qualified to do it yourself, potentially some real danger if you make an algebra mistake, and all for something you're probably only going to need to do once in a lifetime. While I'd never discourage anyone from trying to learn something, this time it looks like it would be a bunch easier to make the money to pay someone who already knows. Go to the library and look at some engineering books. The one I have is "Simplified Engineering for Architects and Builders" by Parker and Ambrose. My guess is that a few hours spent reading will convince you to hire an engineer, and make you a better informed customer for that engineer.
It sounds like you want to retain the 8" joist width rather than raising the floor or lowering the ceiling. In that case, your engineer may suggest flitch plates, basically sistering your joists with steel plates.
The actual job is going to mean either tearing up the floor upstairs or the ceiling downstairs to get access to the joists, so you're looking at some major mess. If you do it from above, be aware that any little slip, dropping a hammer or something, can result in falling plaster downstairs.
-- J.S.
You're right, intuition can be a dangerous thing. I should know, because I *am* an engineer, but my specialty is chemical, not mechanical. I dusted off my old statics text book, and the increase in allowable span would be about the fourth root of the ratio of new width to old width, based on the same allowable deflection. Or, for No. 3 visually graded, from 10'3" to 12'2". This isn't quite right because it makes the simplifying assumption that the two deflections are equal, rather than proportional to the span. I've ordered some span tables and a design guide and I'll just have to run the math from square one when I get everything.
I wasn't clear in my original post but I'm trying to level the first floor from the basement. It's a full basement, and everything is accessable. I really don't want to crank in more columns 'cause it'll look like a forest down there.
By the way, how long should this take? I know that lifting everything in one shot is probably going to crack more plaster etc. than necessary, but how much should I lift at a time and how long should I wait in between?
Since you're an engineer, I'll run the formulas by you that I've been taught to use for simple beams.
To figure the moment on a beam, the formula I was taught was Span (in feet) squared X loading (In PLF)divided by 8. In this case, 16 squared X 66.67 / 8 gives you a moment of 2,133 foot pounds. (The 66.67 is 50 PSF at 16" O.C.)
To figure the moment capacity of a board, you first need the section modulus. That's the width times the depth squared divided by 6. Works out to 13.14 for a 2X8.
Then the moment capacity can be figured with this formula: Section modulus X bending value for the lumber divided by 12. If we assume the bending value for your 2X8s is about 1,200 (SYP #2) we would have a moment capacity of 1,314 foot pounds.
So your 2X8s aren't even close at 16" O.C. But doubling them up would cut the bending moment in half, so they would work in bending.
As for deflection - The max live load deflection allowed by code is L/360 for the live load, or .53" for a 16' span. (L/480 would be better, at .4")
Couldn't find my notes about deflection, so I cheated and used a beam program. It shows a live load deflection of 1.18" for your joists, which is way over the allowable. Should be half of that with a double joists, or about .59". So you're still over the allowable. You could probably make it if you assumed SYP #1, but.......
But my point in both of these discussions has been that the length over depth ratio is bad. A 2X8 spanning 16 feet has a ratio of about 26 to one. No way is that acceptable - I don't like to see more than 18 to one. Doesn't matter how many more joists you throw in there, you can't change that ratio.
We can span chart and calc the thing to death, but common sense and experience has to take over. My experience tells me that what you're trying to do isn't a good idea.
Why is the person who invests your money called a broker?
Thanks for the quick formula tutorial. It sure looks like doubled 2x8's aren't going to pass muster. Someone suggested sistering with 2x10's, and I *so* don't want to deal with compound beams. It seems like I'll have to call "Beams 'R Us" or some such to get it all straightened out (no pun intended). Thanks for all your help.
Pardon my jumping in here but it's time for a little attitude adjustment on you part, I'm afraid.
You don't want 'compound beams'
You don't want a forest of beams and posts in the cellar.
You don't want a sag in your floor.
You don't want to drive off a potential buyer
It's starting to look like what you do want is for the laws of physics to conform to your desires...and I'm just guessing that you want to do it cheaply too.
It's not the width you need to deal with for this span, it's the depth. Either decrease the span with a beam and support posts, increase the depth with 2x12s, or gert used to sags and bounces.
Since you need to place a temp beam to jack up and straighten the floor anyway, that is the easiest, cheapest way to deal with this.
Please don't shoot the messanger....
Excellence is its own reward!
Piffin:
DITTO!
Charlie
Exactly how are you guys planning on dealing with the permanent set of those joists?
Jacking the joists up? What are ya gonna do when the ENDS come up right along with the mid section of joist?
Just curious...
James DuHamel
J & M Home Maintenance Service
Jim, He doesn't exactly say how much the bow is other than to describe leaning furniture, not exactly an engineering term so I was left to presume that it was less than 3/4" which will likely come out ok. anything more than that would require use of the more precise engineering description, "furniture falling over"
Excellence is its own reward!
"furniture falling over"
Heh heh... Boy is that an engineering term or what!
I've actually seen a floor that was that bad. That sucker was 6" off over a span of 10'. It was in a bathroom, and the old lady finally had enough of the leaning toilet. The water level was almost at the rim on one side, and barely in the bottom on the other. I got it all on video, and after I repaired/replaced everything, she was amazed at how straight the floor was.
I was just curious...
James DuHamel
J & M Home Maintenance Service
> By the way, how long should this take? I know that lifting everything in one shot is probably going to crack more plaster etc. than necessary, but how much should I lift at a time and how long should I wait in between?
I've heard that 1/8" to 1/4" per day is supposed to be OK, but I don't see any mechanism why that should be true. IMHO, it's more important to work when it's quiet, pull down slowly on the jack handle, and listen carefully.
I took my kitchen wall up about 3/4" in a few minutes, and got a new crack on one side, but not the other. (I didn't care about saving the plaster on that side, it's already slated for replacement.) The difference was that on the side that didn't crack, I had removed the baseboard, and raked out all the plaster from the bottom to about 1/2" above the sole plate. So, I'd recommend doing that, and also if there are existing cracks that will need to be repaired, rake them out before the lift, and repair them afterward.
Even in an old bearing wall, the studs aren't all exactly the same length and they aren't all completely and equally snug up against the plates. Depending on where the joists below are relative to the studs, you might also get a little bending of the sole plate and sub floor. So, raking out that plaster at the bottom gives things the tiny bit of wiggle room they need and keeps the forces involved from getting transferred to the plaster you want to protect. Existing cracks are evidence of things having moved, so likewise, it makes sense to give them room to move some more.
-- J.S.
JOHN SPRUNG is correct - The math doesn't work that way.
Check out this recent thread:
http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=20397.1
That guy had 2X10s spanning 17' and wanted to do roughly the same thing you are proposing.
Adding a beam under the joists is by far the easiest. That way you can use adjustable posts and bring the span back up slowly over a long period of time.
You read about all these terrorists--most of them came here legally, but they hung around on expired visas, some for as long as 15 years. Now, compare that to Blockbuster; you are two days late with a video and those people are all over you. Let's put Blockbuster in charge of immigration.
From the NAHB wood beam series book, pg 36 Table 25....
Allowable live loads (lb/ft) No 1 Southern Pine
1-2x8 @ 16' span......................24 lb/ft
2-2x8 @ 16' span......................48 lb/ft
From Southern Pine Assoc....Max Spans Pg 7 , Table 2 Floor Joist @ 40psf l/360
1-2x8 12"oc No1 Dense KD span rated to 15'/0, No.2 SYP rated to 14'/2
" 16"oc " " " ' " 13'/7 " " " " " 12'/10
I assume that your stated span is actually the length of the joist, if so then your span is actually less than 16 ft. What code/inspector specs do you need to conform your repairs? If Sistering values bring you close to the require span values then consider adding a supporting Closet or storage shelf system at the wall and or center beam to reduce the span.
More data to digest.........................Iron Helix
I'm currently working on a very similar project so thought I would chime in on what I plan to do to rectify my bouncing and slightly out of level floor.
The home is west coast 1920s so no perimeter foundation. I started by making the house level (used screw jacks w/4x6 cribbing). I've completed the foundation and I'm ready to install new beams to support the second story. I decided to install DF #1 or #2s 4x10 beams at 7' on center. The joists for the second floor are 2.5"x6" so I need to keep the new beams pretty close.
I decided on the beam size after consulting a couple of different beam span charts and a LVL beam calculator. I'm not an engineer just a guy who enjoys learning new things and needs a project to work on. If one of you real engineers think I'm off base on my beam selection or layout I would appreciate your input.
John
Edited 6/19/2002 1:46:35 PM ET by john m
John M, that's nowhere near enough information on which to tell if your beams are sized adequately.
-- J.S.
Does this mean that your beams4x10 will be perpendicular to and placed under the existing 2.5x6 joists?
What is the span of each?
LVL charts won't help a bit for finding load bearing on doug fir. That's like asking a blind man to describe the sunset.
Excellence is its own reward!
Yes the 4x10 beams will be perpendicular to the 2.5x6 joists and placed 7' oc. I am still deciding whether to install the new beams by running them under the joists or cut the joists and use Simpson hangers. I see an advantage of installing the beams under the joists for extra space to install plumbing, wiring and radiated heat on 2nd floor. The longest run I have for any section of beam is 12', next longest beam run is 8'. If the 12' span is to long I can either go to 4x12s or build in an additional structure (closet/entertainment center) with post support to shorten the span to 10'.
John
> I see an advantage of installing the beams under the joists for extra space to install plumbing, ....
If you put your girders under the joists instead of cutting them, the joists would be continuous beams rather than simple beams. All else being equal, the continuous beam version will have a smaller maximum bending moment and smaller deflection. Bottom line, that's a much better structure. That's the main reason to not cut your joists.
-- J.S.
"the continuous beam version will have a smaller maximum bending moment and smaller deflection. Bottom line, that's a much better structure."
Technically, that's correct. But the reality is that a continuous floor joist isn't always better.
For instance - Put a kitchen on one side of the beam, with the fridge right in the middle of the span. The resulting deflection will sometimes cause the other side of the joist to bow upwards. Then when you walk on it, the floor feels springy.
I've had this happen more than once.
Edited 6/21/2002 2:34:31 PM ET by Boss Hog
Boss Hog "The resulting deflection will sometimes cause the other side of the joist to bow upwards."
This is exactly the problem I am currently dealing with. The joists lap over the beam and some overlap by a foot or more. At the overlapping 1'+ I have a bump (for lack of a better term) on the 2nd story floor. Of course my current spacing between beams is 14' so I'm hoping that going to 7' oc will remove most of the highs and lows in the floor joists. If after installing the new beams I still have the bump problem I was considering cutting the extra 1'+ overlap,which is causing the bump, from the floor joists. Any structural reason I shouldn't cut the extra lengths of joist?
"Any structural reason I shouldn't cut the extra lengths of joist?"
Not that I can think of. But I really prefer to look at stuff before giving an opinion fer shure.........
> The joists lap over the beam and some overlap by a foot or more. At the overlapping 1'+ I have a bump (for lack of a better term) on the 2nd story floor.
I have those bumps, too. They're common in pre-WWII buildings, from before the Skil saw became commonplace. In those days, any cut made on the job site was sawn by hand, so they left wild ends like that. Today framers cut them off. You can cut yours off, or you can just relieve the tops to below the level of the adjacent joist, whichever is easier to do.
-- J.S.
You accounted for all of the seismic upgrades rewuired by code during such a renovation? Hmm...maybe professionals should do professional work....that's not a mistake, it's rustic
I'm following my county code book and building to if not beyond the codes. The foundation was installed with an 8" stemwall w/all the rebar, and Simpson seismic bolts and straps required by code. Each floor will be tied together with seismic tie downs following a complete load path back to the foundation. Shearwalls too.
Wiring, plumbing will follow codes as well.
Thanks for checking.
Your comment "maybe professionals should do professional work" is a bit off base since you have no idea what I do for a living. I have had so called "professionals" bid some of my jobs and I have to tell you I haven't been impressed. I like to do the job or see the job done right the first time and many of the professionals seem to want to take shortcuts. The biggest problem with many of these local "professionals" is just getting them to show up. Just to be clear, I'm not saying all contractors do shotty work or don't show up but in my recent experience with local contractors this has been the norm. Maybe its just the area I'm in.
John