Residential beam sizing for deflection 3

[Rigger12]

I'm currently sizing a beam to take out a load bearing wall in my own home (to open up between the kitchen and dining room) and have a couple questions about how you size for deflection limits when removing an existing wall, on new construction we size for live load deflection as the dead load deflection wont effect the finishes, but since this is a finished structure should I size the beam to deflection from the combined load, otherwise when we put up the beam and remove the supports the beam will deflect and the upper floor will settle from the dead load deflection and could mess up some finishes or the levelness of the floor above. The span is 16'-6", and it will support a bathroom on one side with a 14' joist span and a bedroom on the other with a 12' joist span, so the tributary width is 7' from the bathroom and 6' from the bedroom, also there is a partition wall that is not load bearing above the existing load bearing wall.

I am getting a live load of 460plf and a total load (live + dead of) 745plf with everything considered (40psf LL bathroom, 30psf LL bedroom, 15psf deadload both in case we ever switch to porcelain wood look tile in the bedroom, plus a few other loads we have), I'm winding up with a 14" or 16" in parallam beam, which I likely won't do because it's 2x8 framing on the floors and we don't want the beam sticking down that far. The limiting factor is deflection limits, not moment or shear. So the question become because it's a long span with tile above in the bathroom do I use L/480 or L/600, and do I use live load deflection or total deflection, L/480=.415" L/600=.33 inches, I'm worried if I size for live load when we remove the supports any settling will disturb the floor above.

Am I being grossly over conservative by sizing for L/600 for total load deflection. Since it's my own home I don't want any chance of the floor bouncing or settling, the house was already built cheap by the builder in the 80's and I'm been improving everything in it as I've been living there.

It seems no matter what I'll have a little of a bump down in the ceiling where the beam is as it's only 8" framing and I'll have to go with a W8 of some weight. I don't mind sizing it a size up as I will be supplying and furnishing the steel myself, just curious for any input as I don't really do any residential work.

 

[BSVBD]

I've commonly heard it said, that the structure neither knows nor cares if it's residential, commerical, industrial or other!

Yes! It's YOUR home! Be satisfied in the end!

I would design for LL Deflection of L/600!

If you need to bump down, you could add pseudo wood or drywall/plaster beams at equal spacing per your preference!

Are you intending on the joists to be top bearing?

Would your floor plan allow for (2) shallower beams with cantilevered joists?

At 16', deflection will TYPICALLY control! But, with half the loads, perhaps you can brainstorm with options!

It looks like a simple matter of aesthetics with limited depth!

 

[jerseyshore]

I have a slightly longer, similar span with 2x8's in my kitchen and went with a W12 + 2x nailer. Creates a nice little visual separation between the spaces.

Going with a W8 in 2x8 framing is going to show a weird 1-3" bump in the ceiling. That will look like crap. Anything less than 4-6" is going to look awkward.

My advice is to size a W10 or W12 with a 2x top flange nailer. You'll save money on the steel (plus it's lighter to pick up), you'll hit L/480 deflection easily (L/600 probably going up one or two sizes), and you'll have a normal little bump of sheetrock in the ceiling.

 

[Rigger12]

We're looking to cut the existing joists, pack the beam with hardwood and put the beam up in the floor and then attach the joists with hangers, that way it will be as little hanging down as possible, we only have 8' ceiling and were trying to not have any bump down, but 2x8's are only 7.25" deep, the W8 will be between 8-8.25" deep. I figure I'll just upsize it one size from the size that satisfies L/600 for LL and that'll give me about L/600 for combined and I won't have any worries, for the little price difference and the beam only being 1/8" deeper.

Not really seeing any way to make it so the ceiling could be completely flat, it'll have to be about an inch deeper where the beam is, maybe we'll just put up one of those fake wood beams that are hollow over it to dress it up a little.

We're going to supply and install the steel, and I'm working with a carpenter/home builder/ general contractor we've done a lot of work with who will be doing the rest of the buildout like the connections of the joists and the finishes, so I've been getting a bit of an education in wood framing as I've never done anything related to wood before other than my studying for the PE exam.

 

[jerseyshore]

The only way to make a flat ceiling with 2x8's is a heavy flitch beam with 7.25" LVL's.

Like I said I have the same exact setup as you 8' ceiling and all. Losing 6" to the ceiling is negligible. A W8 top flush sticking down 3/4" will look awful. If you are going to do a fake wood beam overtop then you are better off utilizing that height with a W10 or W12.

Also I always recommend adding a top flange nailer. Plywood right on a steel beam is a squeaky setup. You'll be cursing yourself forever if the floor squeaks upstairs everytime you walk over the beam.

 

[Rigger12]

Thanks for the info, I'll talk to my GC to see how he would want to do that. I also just looked into doing (2) MC7x22.7's back to back, I'm getting a LL deflection of L/709, and total deflection of L/438. So that may also be a winner since it's only 7 inches deep. My main concern is the total deflection as now there is probably negligible deflection of the floor above since it bears on a wall, with the MC section I will probably see about 3/16" deflection due to dead load, then the LL deflection could be up to almost 1/4" for a total deflection of .452".

I also see a local steel supplier sells what they call a "Super 7" steel beam that's 7 1/8" deep to fit in a 2x8 framed floor, they say it's "structurally equivalent" (in what way I don't know) to a W8x31, and their span & load charts specifies it for my load and span case, but I probably have to call them for more information as they give no information for deflection or structural properties so you could run calculation. It looks like a W6 with plate welded top and bottom.

 

[XR250]

You could do a big W8 and cut out the floor sheathing to make it totally flush. No way a flitch is working if dbl. MC7x22.7 is the ballpark.

 

[phamENG]

L/600 for beams and L/480 for joists (both total load) is my standard for custom homes. The alternative is doing an actual vibration analysis, which can be a bit of a pain (though I did set up a spreadsheet to do it for simply supported beams).

I just renovated my already enclosed garage to split it into a couple of rooms. We had a similar situation...triple 2x12 girder with 2x10 joists, so 2" hanging down. Looks terrible. We wanted a flat ceiling in the new room, so I furred the ceiling down 2.5". Not really noticeable (a 2 car garage with 8' ceilings has terrible proportions anyway), and the room still feels normal even though I'm taller than average. So that could be an option - get as shallow as you reasonably can, and then lose a couple of inches throughout.

 

[DoubleStud]

Also I always recommend adding a top flange nailer. Plywood right on a steel beam is a squeaky setup. You'll be cursing yourself forever if the floor squeaks upstairs everytime you walk over the beam.

So you are suggesting to add a nailer, but nothing will be nailed to it? Interesting. How about adding glue to top of flange? Tape?

I think with 2x8 floor framing you are out of luck. IMO using W10 and let it sticks out below the ceiling is your best bet (wrapped to make it look like heavy timber). I am not sure flitch beam with LVL will do much with only 7.25 deep. Plus more than likely this beam will probably run parallel to a wall that is only 2X6? 3-lvl with two 1/2" plate is already at 6.25" wide. We probably dont want to be that much wider than the 2x6 wall?

 

[phamENG]

Regarding the nailer - as long as you keep the beam down 1/8" to 1/4" from the floor sheathing, you'll be fine without it if you're using packing and face hangers. That way you don't have contact between the plywood and the steel, if there's any shrinkage/settling of the framing into the hangers you won't get a high spot. Shrinkage is unlikely as they are existing and EMC is stable, but settling will happen if they aren't seated just so.

 

[XR250]

A W8x24 is similar I to dbl MC7x22.7.

 

[Rigger12]

Pharmeng yes, sadly the double MC7x22.7 is L/438 in deflection from total load, I was trying to keep it under L/600 total, I’m not looking at the numbers now but I think I would need an W8x40 to keep it to L/600 total, which would stick down from the ceiling a little bit.

XR250, the advantage of the MC7 would be that it would only be 7” deep, going to a W8 makes it 8” deep

 

[XR250]

You can cut the subfloor and let it flush out on top. Done frequently around here to use 10" I-beams in 2x10 floor systems.
The fact is the beam will be lucky to ever see 20% of the design load so you could back off L/600 TL.
I usually use L/600 LL
Also, I have not done the math but something does not seem right about needing a W8x40 for a 16'-6" span.
I have a job now where I am using a W8x48 on a 20 ft. span with alot more load.

 

[jerseyshore]

Messing around with the subfloor above or holding down a beam a small amount is not something I've ever seen done nor something I think I'd ever consider. If I'm that tight on space and it has to be flush they can just put the beam right under the plywood. Don't want a squishy spot above.

DS, to answer your question, a nailer is obviously better when the subfloor gets nailed to it above, but if it's just a first floor reno, I still specify the same unless they really need that inch back.

A nice piece of wood between the top flange and the plywood above not only gives you less squeak potential, but it also allows for a little adjustability on the total height of the system. Can always shave it down or build it up to line things up. These homeowners aren't getting the top of the line steel contractors putting up their living room beams.

I also agree with XR that with bedrooms above you'll never see half of that design load. Reality is anything over a W8x10 will probably barely deflect at all.

 

[Rigger12]

So that comes back to my original question of if I was being grossly over conservative trying to keep total deflection under L/600, which some seem to be saying I am others are saying that’s what they design to.

The double MC7x22.7 would satisfy a live load deflection of L/600 (would be L/709), but it would only be L/438 for total load, but the benefit would be it would easily fit within the floor.

You’re right it would likely never see the full load since it’s a bedroom on one side and master bath on the other, and the tub and shower are on the other side of the joist span right next to the exterior load bearing wall, so it won’t take much load ever from that end either. I’m definitely just overthinking this because it’s my own house and I have to live with it.

I attached a rough current floor plan and a floor plan we’re considering just for the sake of it, the beam would go where the current wall is between the kitchen and dining room that the desk, oven and refrigerator is currently against, there is a void behind the desk where currently is the air return to the AC air handler in the basement. The beam would span from the wall where the stairs are on the left end of the dining room to the wall on the right side of the dining room where the oven currently is.

 

[XR250]

Here is one I looked at the other day. They screwed this one up though as they used a 10 1/2" tall W10x30 I am not sure how the flooring will work out!
I usually only do this with a W10x33 as it is 9 3/4" which gives me a little wiggle room.

 

[Eng16080]

I believe the various tile floor associations have recommended floor deflection limits dependent on the type of tile. You might want to look into that.

I would try to center the double MC7 beam on mid-depth of the 2x8 floor joists with there being small gaps both top and bottom to accommodate some movement of the wood without creating a noticeable bump at the steel beam. The joists are probably close to 7 1/8" deep if I had to guess (due to the wood drying/shrinking after construction).

I've seen the detail above before, with the floor sheathing cut and the beam flange flush with the top of sheathing. I would try to avoid that if possible.

This is slightly off topic, but I can never understand why having a flush/hidden beam is always so important to people, with the exception of there being an actual head height issue. It seems that this is a major concern of clients/owners on about half of projects. In the end, they end up with a vast, unbroken plane of ceiling spanning like 30 or 40 feet with no visual interest. If the drywall job isn't perfect, it's really noticeable over such a length. My ideal aesthetic would be something along the lines of the beam sticking down 6 or 8 inches, which would coincide nicely with a steel beam deep enough to likely limit deflection to 1/8" or so. This is of course subjective and just my opinion.

 

[kipfoot]

a few thoughts:
[ol 1]
[li]-sometimes its better to go with an absolute number for deflection. Keep the dl defl low because these will be actual cracks in your existing drywall upstairs. LL deflection can be a reflection of the existing stiffness of your house.[/li]
[li]-be mindful of the location of the posts. I think its' better if the posts miss the existing footings because then you don't have to justify the additional load on the existing...you just install a new footing. Hiding them in the wall like you show makes it much more difficult.[/li]
[li]-You may want to stiffen the floor in the new big space while you're at it.[/li]
[li] another idea: could you put an 18" lvl in the attic and hang your beam from that?[/li]
[li]-You didn't ask, but in my opinion a big room with a low ceiling is out of proportion and an 'open concept' kitchen/dining is not necessarily the improvement people hope for. An alternative would be making a wider cased opening that has better lines of sight from one space to the next and lets you use your dining room for eating instead of the new giant piece of stone counter[/li]
[/ol]

 

[jayrod12]

Playing off of kipfoot's recommendations for considering shortening the opening, I've had success in convincing homeowners to put a post at the end of their island to cut the beam spans in half. I then tell them to add some electrical outlets and maybe the light switches for the island lights etc to this post. Everyone always wants to charge their phones, use small appliances etc while at the island. And a single post really doesn't obstruct the view.

Just a thought to consider....

Something that hasn't been discussed, be mindful of what this will do to your main floor beam. Depending where your posts supporting that are, you may be significantly reducing load on one span, and significantly increasing it on the adjacent one. You may end up with some issues with that beam. In my experience those beams usually have a bunch of electrical or plumbing attached to the sides, or ducting running directly adjacent, making reinforcing of the existing beam difficult. Putting a post at the end of the island may solve that by keeping the similar total load values in the same beam spans.

 

[phamENG]

kipfoot brings up a good point.

While the outward features of Palladianism have certainly faded (when was the last time you saw a symmetrical house with ordered columns and dome?), there are still quite a few architects out there that lean on his ideas of proportions to plan rooms that are comfortable and pleasing. The large room you're creating with the kitchen and dining room are actually quite close to one of his favored plan proportions - the square and its diagonal (1:1.414, you have 1:1.443), but the plan dimensions are not the only important feature. You also have to look at ceiling height. He used 3 calculations to determine that: the arithmetic, geometric, and harmonic means. For all three of those, this room wants about a 19 foot ceiling height. Now, that's a bit extreme, and not entirely necessary if you'll have, say, pendant lights over the counter to help split the space. But I wouldn't want this room with anything shorter than 10ft ceilings for sure, preferably 12ft.