Boss:
Still working on Town House design/project… Here is the scenario: Second floor Floor trusses will span across the width of the “houses”. Two bedroom units are 18′ total width (span) meaning a 17’4″ clear span. 3 bedroom units are 21’8″ total width (total span) meaning a 21′ clear span. We want to use the same height trusses thorough out the project. HVAC flex duct and plumbing will have to be run within the floor system.
Truss company ‘A’ bid 14″ tall floor trusses at 16″ OC. Truss company ‘B’ bid 16″ tall trusses at 19.2″ OC, which was cheaper. Both companies floor trusses are made out of 2x4s.
So I get back to truss company ‘A’ and ask them: what are your design specs, and are you sure a 14″ tall truss can (clear) span the 21′ without excessive bounce. They respond and say that the design specs are L480 and LL+DL = 55 PSF (since I speced no tile, etc). Further they say that 14″, 16″ and 18″ trusses are all the same price each, and I can save a little money if rather than going with 14″ tall @ 16″ OC I go with 16″ tall @ 19.2″ or save more still by going with 18″ tall @ 24″ OC but that their recommendation is the 14″ tall. 20″ tall trusses up the aunty. They also say that a 14″ tall floor truss can span something like 22’8″ (I forget exactly) so my span for the 3 bedroom units is well within spec. My experience tells me that when you have a pipe and a flex duct crossing each other inside of 14″ tall floor trusses it can get pretty tight.
The # of step risers is not an issue since I asked that the plans be drawn for 18″ tall floor trusses. (9′ ceilings and steps are 16 risers). (8.25″ risers are ok here but I don’t like ’em.) Subfloor is 3/4″ T&G. Another point is that the closer the trusses the harder it will be to miss trusses with plumbing drops (toilets and tubs), plus the duct chases that come from the attic to the 2nd floor system will have more trouble missing the floor trusses with tighter truss spacing. Did I leave out something important???
BTW – these are mid priced THs.
So, my Q to you is 14″ @ 16″ OC, 16″ @ 19.2 OC or 18″ @ 24″ OC, or something else? I’m not too worried about 3/4″ subfloor spanning 24″ OC.
Your thoughts are greatly appreciated.
Replies
Just as a bump, to let you know - Boss is floating in a hot tub right now.
He'll be back when he needs to come up for breath. ;)
I think on Sunday or Monday.
Remodeling Contractor just on the other side of the Glass City
Thanks for the schedule update!
Edited 1/4/2008 11:18 pm ET by Matt
I can offer my experience which is in the field Matt.
Bosshog will be able to give the hows and whys.
I have framed houses that we used 18" @ 16" OC for a similar span. Floor was fairly stiff.
Anything over 19.2" OC the subfloor bounces alot between the trusses.
I would go with the tallest truss you can get. When your framers strat running their 2x6 strongbacks there is even less room to get mechanicals through them.
Woods favorite carpenter
with open web floor trusses, I aim for 19.2" layout.
16OC is so close ther eis little elbow room for mechanicals and 24 can give too much deflection in the ply
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In all this just make sure you're paying attention to what taller trusses will be doing to the stairs.
Runnerguy
He sounds like he has planned for 18" trusses.
Unlike the GC's I have worked for, plans show 2x10's and they jump up to 18" floor trusses without updating the plans.
HO got a set of crazy stairs that cost them alot of money. Winders, landings.
It would be nice to work under a super like Matt. He plans things and makes life easier for those who do the work. I can't find that here. Woods favorite carpenter
With a 21' span, a 14" floor truss is absolutely maxed out IMHO. My suggestion would be to go with 16" deep at 19.2' O.C.
Deeper trusses with wider spacing will ALWAYS be stiffer than shallower trusses with closer spacing. It also means less labor due to less trusses to handle.
In my experience, with trusses up to 24" O.C. you can't tell where the trusses are from walking across the floor. (If you have decent plywood) So I don't think that's an issue.
Be sure to check on fire separation codes on multiple family structures. You will likely need some sort of fire separation between units within the floor system.
That's all I can think of at the moment. But hell, I'm on vacation - What do you expect ???
(-:
Thanks a bunch Boss.
At this point I'm pretty sure I'm going with 19.2 OC for the 21'8" wide buildings and 24" OC for the 18' wide buildings. Haven't quite decided if I will go with the 16" or the 18" heights, but will use the same for all. 16" will make my stairs a little better - reducing each riser by 1/4" but 18" will yield stiffer floors. For others reading this, as you see, these things always aren't so black and white.
>> In my experience, with trusses up to 24" O.C. you can't tell where the trusses are from walking across the floor. (If you have decent plywood) So I don't think that's an issue. << Mine too - I've built a number of houses like this, but I've also found that we have to be extra vigilant about gluing and fastening of floor sheathing. The fact that the top chords are 3.5" wide helps a little because you can really heap the glue on and the actual "clear span" of the sheathing is more like 20.5". The only thing bad about 24" OC is that you have virtually no room to (easily) move joists around to miss plumbing drops, etc. The truss co A (my favorite) has provided me with drawings showing that the plumbing drops and duct chases are all in the clear, so this shouldn't be an issue.
Re the fire separation the design includes a (slab) floor to "rafter top" fire separation wall that is several layers of dense glass and metal channel framed. On either side of this rated assembly is a regular 2x4 wall that supports floor trusses, etc.Muchas gracias for taking the time out of your vacation to respond. It just now occurred to me that a common thread (bond) that the pros here have is that we like our jobs enough to actually spend our spare time on it... That's what makes us so damn good!!!! ;-)
Just to kinda close the loop I ended up going with 18" tall floor trusses 19.2 OC for the 21'8" wide buildings and 24" OC for the 18' wide buildings.
Thanks again Boss.
No problem - Always glad to help.
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