Hello All!
Disclaimer – I am posting here only to make me smarter before I go talk to the local truss shop, not to make actual decisions (however good the advice is!).
I am in the planning stages for a 30’ x 42’ cabin in Northern MN on a remote island site (no cranes, no cement trucks). I have decided that a pressure treated wood foundation is the way to go for our walk-out basement. The 30’ side will face the lake & have the walk-out. We have designed a 1-story floor plan with a 12/12 roof with loft (gable end = 30’).
I am intentionally leaving out a bunch of the building details to stay on track, but the bottom line is this:
For a variety of plan reasons, I would like to support the first floor deck with two 42’ long beams spaced at 10’ and 20’ across the 30’ wide side. I would also like to use floor trusses, as running mechanicals through the webs seems to be the way to go.
In other words, I would like to have 30’ long floor trusses supported (in order) by wall, beam, beam, wall. I would like to support the 42’ long beams only at the mid-span (21’) with lally columns.
So my questions:
For typical floor loading (l/360):
What depth am I looking at for the 42’ long LVL beams supported only at the 21’mark?
and
What depth am I looking at for the floor trusses with support every 10’?
As this is a remote site, I am thinking that I could build the beams out of 24’ long LVLs, as a 1 ¾â€™ x 24’ long LVL is manageable by hand. I think we could also manage 30’ depending on how deep they end up..
Anyway, hoping you guys (Boss Hog?) can lend your thoughts to make me look smarter as I start to ask questions.
You guys are great!
Thanks….
Pinemarten
Edited 9/13/2004 5:08 pm ET by PINEMARTEN1
Replies
Some information is missing...what is the O.C. spacing on your floor trusses? Are there any column or bearing wall loads going to the floor? Are you designing to the standard 40 psf floor live load or are you going to use the area to store gold bars?
The floor trusses will be pretty shallow, 10' could be spanned with an 11 7/8" deep TJI or BCI joist.
The main support beam is another matter, it will probably be in the 14" to 16" (double 1 3/4" members) range...I'm just guessing without running through the calculations.
Here's my advice. Run only one, not two, floor beams down longways. Then spend your huge savings on one extra lally column. That will break your nominal 42' beam length into three spans of 14' each. I just went to TrusJoist's site and downloaded the microlam floor beam data, and you will need a 2-ply 16" beam, thus 3-1/2 x 16.
Your floor joists could readily be 2x12 sawn lumber at a great savings over I-joists or floor trusses, as your spans are only 15 feet. Your clear spans are really about 14 feet 9 inches, which puts you just over the edge for 2x10s and comfortably into 2x12s.
If you want living space below, and are thinking floor trusses because of ease of mechanical poke-throughs, first consider the 2x12s, after having some detailed conversations with your plumber and electrician on how much poking they actually need. If thinks sound outrageous, then move to I-joists, because the allowable poke sizes are larger.
If you have a monster array of mechanical trunks going on in this home on an island, then finally consider floor trusses. Absolute overkill for a span of 14/9 IMHO, but, hey, you might be Bill Gates, and need a lot of duct space in your floor.
for that matter you could loose the LVLs all together if you went with some pretty deep trusses - maybe 24" on 16" centers - or is the roof to be stick framed? IE - Will there be bearing walls on the first floor that will support the ceiling/attic floor? Is that the reason you are looking for LVL beams on 10' centers? 10' rooms are not very big... If that is the case, I like Bob Dylan's idea better... 15' floor joist/truss spans.
As for your origional Q - 21' is quite a long span for LVL beams - especially if they are supporting 2 floors and a roof.
BTW - WTFI Bob Dylan? I have a hunch...
Edited 9/13/2004 7:27 pm ET by DIRISHINME
For starters, I agree with Bob that using 2 beams is overkill. But assuming you do, the floor trusses could be pretty shallow - Just about any depth that could be built would work. Some places can manufacture 9 1/4" metal web floor trusses. Others don't build anything under 12" deep.
If you go with one beam, you could still use 12" deep floor trusses or 11 7/8" I-joists.
One caution comes to mind - Don't assume you can get all your mechanicals in a floor system that shallow. Running electrical will be easy. Plumbing supply lines aren't typically a big deal. Plumbing drain lines can be difficult. HVAC lines are almost impossible.
And as always - Check local codes for unknown surprises.
Give a man a fish; you have fed him for today. Teach a man to use the Net and he won't bother you for weeks.
DIRISHINME brings up a good point - What are you doing for roof framing?
If you're stick framing it, you may need the bearing walls. You can't bear foor loads out in the middle of a floor system, unless the floor members are specifically designed for it.
I haven't failed. I've found 10,000 ways that don't work. [Thomas Edison]
i must agree with Boss Hogg about the mech. Depending on where your bathrooms are and your discharge to your tank you could run into a pitch prob. Are you going to "A" frame the roof? A loaded ridge beam would get your load to the outside walls to avoid roof bearing inside. That beam you take out of the floor would serve as your ridge beam. Northern Mn. huh? are you a cities dweller now? Best of luck with your project, Mike
check into the classified I have some composite ballisters you may be interessted in.
Edited 9/13/2004 9:45 pm ET by mitremike
Thanks to all.
I suppose I should have included the following issue as well.
My original roof concept is stick framed 12/12 with a structural ridge beam. Actually 2 structural ridge beams with the loads being carried down through the loft deck to the first floor deck and then to the basement.
As this is a lake cabin/home, I am also trying to pick-up a little post/beam "look" as well as practical building solutions.
When I get home, I will try and post our plans, which should shed more light on the picture.
You all have somewhat answered my question though, as I am shooting for avoiding posts in the middle of the living room and dining room (thus the 21' span), but that may be impossible (at least for the site we are building on). All of your comments give me something to dwell on....
(Mitremike - no, used to be a 612'er (actually 651'er) but now a 218'er)
Thanks again for your help!
Pinemarten
Personally I think you need professional help... ;-) Your idea of LVLs 10' apart spanning >20' and supporting joist trusses and 2 floor systems is quite "non-conventional".
Not necessarily an architect, but someone who draws plans for a living. Sure, the truss company will help with the structural aspects, but I think you need some conceptual help first. I know you are looking at this as just a "lake cabin" but you could easily end up spending $100k on materials alone, so $800 spent on plans would only represent a .8% expense. Think about it... Matt
Yep. As disclaimed, I am just making my self look dumb in front of you guys, instead of my local professionals....
Lets try this a different way.
Got a 30' x 42' foot rectangle.
Walk-out Basement, First floor, Loft (20'L x 30'W- half the length of the building)
12/12 roof pitch, no second story walls (rafters right on first floor top plate).
Remote site - no cranes, so no huge glu-lams.
First floor plan has ability for posts right down the middle at 11', 21.5', 32'. As would the basement.
What would you do for:
First floor support and joists.
Loft support and joists.
Roof support and rafters.
I am not Gates ($), but do want and can afford quailty in the bones.
Pinemarten
Will the "loft" space have any dormers to open it up?
Is there a fireplace (& associated chimney) or are we looking at a stove & metal pipe?
Just want to get all the elements right.
If I have the picture right, you'll have a first floor that opens toward the lake over the WOB, with the loft area over the non-lake side of the first floor--and some sort of cathedral ceiling and gable windows to take in the million-dollar view of the lake. The loft will likely have an open railed gallery over the living are facing the lake. The loft will get knee walls--will it get a flat ceiling to give a modest attic space, I wonder?
The only tricky parts about the structure I can think of would be bringing the support for the end of the ridge beam dow nthe middle of the gable with the great views. Not an insurmountable problem, just one that wants thinking about. Like BH, I think you'd be fine with TJI and full span the basement, and the loft, like as not. Barging 30-footers would likely be as much trouble as double the number of 15 footers.
Might be.Occupational hazard of my occupation not being around (sorry Bubba)
CapnMac:
Shed dormer from roof ridge over the entry and stairwell ("U" shaped stairwell set on one side w/ landing half way - no winders), so loft area is more of an "L" shape, than a pure 20' x 40' rectangle, but trying to keep things simple.
Propane fire place, as homeowners does not like wood burning on remote site.
You got the other details right on (maybe only a $250K view.... :))
No knee walls at this point, not planning on attic either.
You have described my greatest concern as well. How to support the ridge beam (not board!) without breaking up the view / living space on the first floor.
Maybe several wall to wall (30' direction) beams atop the first floor wall plate with a post supporting the ridge beam. If I space them close enough (?) it potentially could carry the roof load?
More worried about getting the ridge beam in place w/o a crane, then getting them to the site, thus the shorter lengths.....
Septic plan is approved as of today! Burned some good sized piles this weekend - very hot work.
Describing the situation in written form is good mental exercise!
As always, thanks for the comments.
Pinemarten
You got the other details right on
Not all of more than two decades in the archy & contruction was entirely wasted <g>.
Bringing the support for the ridgebeam down will probably be tied up in the window sizing and shapes. If there were an odd number of windows, which would center the middle window, you can bring the support down on either side of that middle window unit--you just need a header across at some point.
Personally, I'm not a big fan of triangular windows (the flashing & trim get odd right quick), but if that's what the customer wants . . .
I also don't like applying the ceiling to the bottom of rafters, either (you always seem to need more engineering than the space permits). I prefer to install a separate cathedral ceiling, if only to better define the insulation plane (and skip the vent vs no vent--and provide a utility space for ducts & lights & the like). Now, you are up against havign to barge materials to remote site--so that may not be an option.
The attachment is my twn minute massing model using 12" "slabs" for the floor. The WOB is shown with a 9' wall, as is the 1st floor; the loft has an 8' ceiling height, with 36" knee walls.Occupational hazard of my occupation not being around (sorry Bubba)
CapnMac:
Your drawing is pretty cool. If I every get one free moment at home (3 young kids), I will post the floor plans, but you are very close.
To All, thanks for the input and ideas - as hoped, I am already smarter than I was, and it was a pretty painless learning experience.
Once I post the plans, I hope all who chimed in will chime in again.
Thanks....
Pinemarten
12/12 roof pitch, no second story walls (rafters right on first floor top plate).
Why you need a ridge BEAM with that? Don't the floor joist sit on the plate also to act like ties?
SamT
Remote site - no cranes, so no huge glu-lams.
So what about that big ridge beam?? I totally get what you are going for and I agree that a ridge beam is probably a good idea but it would have to be pretty substantial. I have, in the past, put up a 3 ply LVL ridge beam one at a time. Glued, nailed and bolted in place. And these were long suckers but I will say it was a real ball bust.
Are you thinking that the loft floor system will act as collar ties for the roof at that point? If so then maybe you only really need a ridge beam for the section of roof over the two story great room area. So maybe one post at the balcony edge of the loft to support the ridge beam which is now only 22 ft long. This post must of course be supported under it with another post structure that goes all the way to footer though. The back half of the roof over the loft then is just traditional framing with the floor system acting as collar ties.
Good luck
Rob Kress