Hi all,
I have 2X8 joists spanning roughly 15 feet in what will soon be a master bedroom 3rd floor. I would like to stengthen/stiffen the joists and the situation is this:
I have access to the joists from the top (floor is up) but not from underneath because of finished ceillings on the second floor of the house. There are numerous wires running through the existing joists. For the most part, these wires are concentrated in the bottom half of the joist.
From reading previous posts on stiffenning joist, I realize you get the biggest increase in strength by increasing the depth of the joist. Unfortunately, with the finished ceilling below and clearance issues above not to mention an existing open rail stair increasing the depth of the joists would not appear to be an option.
I guess my question is can these joists be strengthened/stiffened in a satisfactory manner without increasing their depth? Would bolting continuous 2x4s the length of the joists along the top edge (both sides???) greatly increase their perfomance or not? Would this approach create sort of a T-beam or am I crazy!
Dan
Replies
Have you thought about putting in bridging? A row or two will do wonders if they are put in tight.
If you don't need the space above the ceiling, i.e. it is attic space, then put some hangers in from the rafters. We just framed my buddy's house over the summer and with what he wanted, we used 2x6 ceiling joists the full depth of the house all the way across the house (box colonial). The we used 1x8 spruce boards and nailed them into the rafter right next to the ridge and nailed them into the ceiling joists. You alternate putting the boards on each side of the ridge. One board on each joist is plenty and by alternating them, you distribute the load evenly. The roof pitch was a 6 and the rafters were 2x6 I believe, maybe 2x8 though.
By doing this, there were no inside bearing walls, the floor plan could be set however we wanted, and the ceiling is extremely strong. The only downside is the attic space was a pain to move around in when we ran the duct work. At only a 6 pitch, it could never be used for anything other than duct work so nothing else mattered.
15' is way to long a span for 2x8s ( I presume on 16" centers but even at 12" the 2x8s wiould be inadequate)
the only good way I see to do this, given your spatail constraints, is to reroute wiring and sister more 2x8s to double them all up. there are ways electricians can help out with fusing wires or re-running them.
I don't imagine you want to support these up to rafters both because that would be inconvenient in a master bedroom, and because if the floor was under designed, the roof may be too, and you definitely don't want to be pulling the roof in on yourself.
;)
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Thanks Piffin for the response. Though it isn't the repsonse I was hoping for it was the one I was preparing myself for. I can reroute the wiring myself (it is my own house) I was just hoping I wouldn't have to .... lots more work. Someone else suggested the possibility of sistering with lvls ... do you think that would be worth the added expense or not.
Oh forgot to mention, the joists are 16 inches on center give or take an inch or so ... 100 year old house.
Dan
I have done LVL floor framing. The cost is roughly three times as much for about double the strength in such a small size. the suggestion to use on every other one helps modify the ciost, but leaving an uneven loading, compromised upwards by adding blocking but still uncertain in terms of performance. I would want an engineer to say whether it would be worth the pro/con /cost. I have to buy min LVL sizeof 11-7/8" and rip down when overspanning in a short space like that. this spells a lot of waste for you.obviously, I would do the same as another previous poster and recommend an engineer or going with standard load tables if this were a total renovation or new construction, but since it is already there, and you or the house are placing limitations on acceptable elevations, the doubled 2x8s will perform much better than what is there now, even if sklightly less than ideal.But using 2x4 sisters or notching tops or bottoms for wiring will make the effort a wasted one. Drill through.
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Piffin,They don't have 1-3/4" x 7-1/4" lvl's by you?I just had some dropped off yesterday and one of them was 36'.Joe Carola
I figured they are made somewheres, but my suppliers don't stock them, so it would be a special order for me.
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2x8 spanning 15' are undersized for floor framing...span tables indicate 2x12 as the acceptable sized members...
Call an engineer...
2x12 would be nice but there is no way I can take down the existing ceillings. It would entail ripping out the ceillings in all three of our existing bedrooms and with three little kids in the house that is just not feasible. Contacting a engineer is probably the way to go but I thought I would run it by fellow BTers first.
Thanks
Dan
You don't need 2x12's for a 15' span 2x10's work just fine but your going to get a million ways to do it but your best bet is to ask an Architect or Engineer and let them see exactly what you have.Joe Carola
Edited 1/23/2005 2:40 pm ET by Framer
I'm a little confused by the "2nd floor bedrooms" part of the equation. Do any of these rooms support the ceiling joists mid span or is the area underneath the proposed master bedroom "clear span 16ft.?
Installing a laminated beam mid span might be your best option.
By only removing the material that would be between the beam and the joists, creating slightly oversized pockets at each end to allow for overtravel at one end and once the beam was installed trim with a small cove trim to cover all edges and it's finished.
Gabe
Gabe, sorry I wasn't clear in my description. The actual size of the third floor is 30 feet by 32 foot with a beam running down the center dividing the 30 foot dimension in half. Thus the 15 foot clear span. Adding a beam to cut down on the span would not be practical since carry the load path to the basement would involve placing a column in the middle of my dining room.
Dan
Again, not knowing the layout of the rooms on the 2nd floor makes it hard to solve your problem but if you could use any of the wall to your advantage, you can transfer the loading to the outside walls by simply re-enforcing the wall with a layer of plywood at the pockets.
Gabe
2x4 on the bottom edge both sides would help greatly. Not the top. Glue and screw. That isn't code of course. If you got an engineer to look at it, they may suggest steel on the bottom edge. It may be possible to bend it under the wires, rotate it vertical and bolt it. I'd recommend having an engineer look at it. This isn't something where a guess would be good. Any reinforcement on the top edge is much less efffective than reinforcement on the bottom. 2x4 on the top would help a little, but I don't know if it would be enough.
The purpose of the 2x3 or 2x4 in a wood I-joist is to stabilize the plywood web which has strength against deflection- the 2x flange also provides a surface to glue & nail the subfloor. Your 2x8 joist isn't adequate to resist deflection in a 15' span, so I don't see a top flange doing much. You can get 1 3/4" x 7 1/4" LVL and glue and nail that to your joists- maybe every other bay, then do solid cross blocking to tie everything together. PL structural adhesive works well. The solid row or rows of cross blocking will direct the loads to the stronger LVL reinforced joists.
It might work out that glue/screwing the narrow edge of a 2/4 to the bottom edge of the joist would work. This would hopefully come in under most of the wiring, though getting the 2x4s in there would be a challenge.
Otherwise, remove the wiring & double up the joists or install LVLs.
One other possibility would be to install a "decorative" beam in the room below, running along the middle of the joists.
Bridging may be enough but if not is there any chance of simply shortening the span by simply installing a beam below the downstairs joists'? Consider trying to incorporate the beam and its covering into the details downstairs.
I'd consider steel (with fire rated covering) with appropriate jacks at ends. Make sure these jacks are well supported in the cellar. Depending on the girder length you may not need much of a web. It sure seems like a lot less work if feasible. Good luck
Take a stick of #5 re-bar. Cut it about 16ft or so. bend the last four inches or so at right angles and place the rebar alongside the joist so that the ends with the 4inch right angle bends pointing up and resting on the top-plates of the walls.
Go to the middle of the joist span, and glue and screw a small block of 5/8 ply, leaving an inch or so to the top of the joist. tack or staple the re-bar in place so that it rests atop the plywood block, but does not protrude above the top of the joist (I assume you want to deck it afterwards)
Now go back to the ends, that rest atop the wall plates and push them down until they rest on the plate and tack them in place. At this point you should have a two by eight joist with a re bar sistered alongside with about a six inch camber in it.
Now go back and fasten some plywood blocks tight along both sides of the rebar, all way across. Make sure to put plywood blocks tight against the out sides of the right angle bends of the ends of the bar so it will not spread horizontally when loaded.
Hopefully, you will be able to manage this by working around the exisiting wire runs ( be careful not to pinch the wires anywhere)
Now take a fresh crowned two by eight and sister this alongside the other creating a "sandwich" glue and screw everything.
deck the attic. call the fiddler. have a squaredance.
I thought your approach was interesting... is it kind of a home spun twist on a post tensioning system?
I think your idea should work.
I would just nail the 2X4's to both sides, and add jacks to transfer their load to the plates.
100yr. old 2X8's are probably almost equal to today's 2X10's.
This is your own place, I'm assuming.
If it isn't, then time to pass the buck to somebody who is "certified".
If anybody ever said
That I give a damn,
Well they damn sure told you wrong.
I've had ramblin' fever all along.
- Merle Haggard
Dan,
I have a similar situation on the third floor of my old house. I found this article that describes how to imbed a steel rebar into the bottom of a wood joist using epoxy. By strengthing the sistered joists this way you could avoid having to put in a deeper joist.
***Of course make sure you have an engineer approve this approach***
http://www.westsystem.com/ewmag/17/beam.html
Mike K
Amateur Home Remodeler in Aurora, Illinois
Mike,
this sounds like a facinating idea but I wonder how much it would end up costing. The 3rd floor I am talking about is 32x30 so it would be an awfull lot of epoxy and work. If I just had one beam that needed strengthened using epoxy and rebar could be just the ticket.
Dan
I'm with Mike on this one. Fasten the 2x4s on top -- it should work exactly the same as 2x4s on the bottom (note that steel I-beams have the same cross-section for the top flange and the bottom flange because they have similar contributions to preventing deflection of the beam).
I agree with adding blocking -- be sure to glue it on each end.
One of the greatest contributions you can make to stiffening your floor is to use a high quality 3/4 inch or thicker plywood as a subfloor and glue and screw it to the joists. Use PL polyurethane glue. Don't use CDX plywood. Glue and screw another layer of plywood with staggered joints if you want more stiffness.
Billy
Edited 1/23/2005 11:36 pm ET by Billy
Edited 1/23/2005 11:37 pm ET by Billy
Edited 1/23/2005 11:44 pm ET by Billy
I don't know where you guys got the idea that the flanges on an I-beam are only there to provide stiffness while the web does all the work. It's actually the other way around. The web (osb in an I-joist) is only there to hold the flanges apart without slipping. Almost all the stress is in the flanges (the LVL part).
Once the subfloor is glued down, it acts as a lateral stiffener. The 2x4's on edge aren't going to do anything.
I agree with you though on the high-quality plywood glued down with PL Premium. Love that stuff.
Mike
53045.25 in reply to 53045.20
I don't know where you guys got the idea that the flanges on an I-beam are only there to provide stiffness while the web does all the work. It's actually the other way around. The web (osb in an I-joist) is only there to hold the flanges apart without slipping. Almost all the stress is in the flanges (the LVL part).
===================
Mike,
If you read my post carefully you will see that we are in agreement... I think? That's why I said that adding 2x4s at the top will help -- if you think about it they are equivalent to the flanges in an I-beam.
If you look at the cross-section of a beam the greatest contribution to stiffness comes from the area of cross-section at the top and the bottom of the beam cross-section -- the flanges. The stiffness is realed to the distance from the center of the beam *squared* which is why beams become so much stiffer from increases in depth.
I agree -- PL Premium is great stuff... unless you get it on your hands...
Billy
Or in your hair.If anybody ever said
That I give a damn,
Well they damn sure told you wrong.
I've had ramblin' fever all along.
- Merle Haggard
Laughed out loud when I read "in your hair." Been there, done that. I wear goggles when I use it now, especially if overhead. I'd hate to think what would happen with poly in the eye. The moisture cure would kick in... I hate to think about it.
Billy
OK, sorry. I guess I misunderstood.
I'm not out to get you, I swear, but I do have to get you on the stiffness factor (modulus of elasticity). It's actually the CUBE of the distance from the centroid, so height is that much more important than depth!
Mike
Mike,Thanks for keeping me honest and setting the record straight!
Something in the back of my head was asking me "are you sure it's the square?" -- it's been a long time.
By the way, does wood act about the same in tension and in compression along the grain when applied as a "flange" on the top or bottom of a beam?
Billy
Edited 1/24/2005 5:04 pm ET by Billy
Billy, wood does act about the same in tension and compression, in the sense that in most species of wood the "extreme fiber in tension" and the "extreme fiber in compression" are similar values. In steel the "extreme fiber in tension" is a higher than in compression, in concrete compression is the big one.
If you added a single flange like you suggested, if you could guarantee a perfect joint without slippage, you would increase the moment of inertia (stiffness) but it be eccentric--in other words, when loaded it would want to roll. In reality the floor sheathing would keep it from rolling, and the joint would slip a bit so the flange becomes less effective than just sistering a whole new member.
Just re-read your question. The short answer is that adding a single flange, to either the top or bottom of a wood joist, would add about the same amount of strength. Getting a perfect joint and ensuring lateral stability would be the hard parts.
Mike
Lots of good ideas showing up. I'm curious have you checked the level and sag of the floor? Use some string and check diagonals and side to side etc..??? Then when you start you may want to temp jack before locking in defect.
Lvls sound good to me, or some site built in place plylams. Would take a little engineering for the exact specs but may let you use shorter sections to place in floor. Sounds like you would have enough room to work. Say something like 2' 10' 2' and then 18" gusset blocks of same plywood glued and screwed.
Sorry, I didn't realize about the beam carrying two sets of joists. I wouldn't go adding anything else to the existing system without professional help at this point. too much dead load on the beam could spell disaster.
You get the biggest gain in floor strength by going deeper but it is not the only solution!
thickness of a timber affects strength*.. as does closeness of them I don't have the chart here but try doing it every 6 inches or so and see what numbers you get..
Variety of wood also affects strength, I used ash to gain the highest numbers I could..
Green ash isn't too hard to nail into but don't put any real load on it untill the wood has been thru a heat cycle (the first winter here in Minnesota) green wood will develop a sag so I plan on using temprary shoring and after the first winter I'll knock it down and go ahead and do the finish work..
* I needed to span 20 feet with a timber no thicker than 8 inches I wound up going to a 8x 14 inch thick timber 20 feet long. to get the required strength..
Another option is to bite the bullet, rip out the ceiling below, and glue/screw plywood to the bottoms of the joists, then re-rock. (Be sure to get all the wiring you may want in the future in place before closing up the top. You could even install some empty boxes (with conduit) so they're flush to the back of the rock, and could be cut open in the future if needed (eg, for a ceiling fan).
I'll see your 15' span and raise ya.
How about a 90 year old house with a 17' span in living room...bedroom above is sitting on 2 x 6's at 16" on center?
:)
Yeah, the living room ceiling is going to get lower. Definitely.
I was faced with a simular problem with my garage ceiling. The attic over the garage was never intended for this type of loading. The ceiling was 2x6's@ 24" oc with a 18' span. Before I was able to replair the ceiling, a couple of the joist had actually sagged to the point that they broke. The repair was complicated by the wiring that was passing through the joists, vermiculite in the cavity and nailed down attic subfloor with asbestos panels for the garage ceiling. After talking with an engineer, I used some 1/4"x3x3" Angles to strengthen the joists. I used some selftapping and drilling screws to attach them. The screws were 1/4" with a hex head. They were installed every 6" oc, in a staggered pattern. I used a drill to run them in. The screws had a special wing on them so they would strip out the wood and allow the screw to drill into the metal without spreading the two apart. The installation went pretty easily. The small size of the angle allowed us to thread it through the maze of wiring. Because the shear strength of the existing 2x6's was adequate, we were able to cut the angles a couple of feet short to just control the deflection. This allow us to swing them in the attic and then insert them below the rafters. The end result is that the floor is significantly stronger than what it was originally. Because of the broken joists, the deflection is still out of spec-you can get the floor to bounce pretty good. The engineer when he first designed the reinforcement hadn't allowed for the joists actually breaking. We had looked at the use of some heavy gauge studs. The problem was that I only need a dozen of them. They would have been a custom order and the cost was nearly $100 a pop. Instead of using the angle like I did, I would recommend using a 3" channel. Attach it the same way, using the screws.
Certified boat fetish.
Edited 1/25/2005 1:57 pm ET by Timbo
I'll add a couple comments to the good advice you've already received.
Sistering a single 2x8 to each existing joist will roughly double the strength and stiffness of the existing floor. Double 2x8's will be approximately as stiff as a single 2x10.
You worried me a little with the statement "For the most part, these wires are concentrated in the bottom half of the joist." Holes near the bottom of a joist are a serious no-no. If you do sister in some 2x8's and route wiring back through it, be sure to drill your holes near the center of the joist.
Anything you add to the existing joists to stiffen them should be continuous. Sistering 2 8' 2x8's end to end, even if they are gussetted with plywood, are not as good as a 16' 2x8. Same goes for applying plywood to the bottom of the joists. You'd need two overlapping layers, or it wouldn't do you much good.
Another idea, not necessarily better than the sistered 2x8's, is to use 9 1/2" TJI. I know you'd lose 1 1/2" of headroom, but they'd be straight, easy to handle (I think) and rated to handle this span by themselves. You could leave the old 2x8's in place, just to support the ceiling below. Might be tricky getting space for them to bear on the top plate though.
Finally, if this area is an attic being converted to living space, you probably want to check the bearing walls to be sure they're up to the task.
I guess most of this stuff is obvious, but didn't want it to be overlooked.
If it was my house, and I had a source for some straight 16' 2x8's, that's the route I'd go.
Jon
I want to thank Jon and all who have responded. A lot of great suggestions. Will probably end up rerouting wiring and sistering with 2x8's.
Dan
You're welcome. Hope all goes smoothly!"Listen, strange women lying in ponds distributing swords is no basis for a system of government."Jon
Try 3/4" plywood rips on both sides, glue & screw
pretty hard to buy plywood that long
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