Roof truss

[m1208]

Hi:

I am working on an old apartment building with roof slope of 12:12 (varies in some area). The roof is approximately 28' by 60'. The architect is preparing the plans at this time and I do not have the exact layout yet. The roof has several hip/valley/ridge lines and the floor joist run perpendicular to the roof rafters. I need to replace the roof rafters with the roof trusses if possible. Is there any "attic space" style truss that can meet our roof layout. Attached is a preliminary cross section of one of the roof area and the roof trusses are shown in magenta color (yellow color is existing members).


Thanks

 

[BAretired]

https://www.google.ca/search?q=atti...yeAZL0oATnhIKYBA&ved=0CCsQsAQ&biw=853&bih=505

BA

 

[SteelPE]

What's the prupose of replacing the roof rafters with a roof truss? The item titled living space is shown in yellow so is that existing or are your creating this new space?

 

[m1208]

It seems in all cases the floor joists are parallel to the top chords. In may case the floor joists are perpendicular to the top chords.

 

[m1208]

Is there a simple truss computer program online (i.e. TrusCAD) for preliminary design check of an attic style roof truss?

 

[paddingtongreen]

You must have a tension member at the base. Re-orient your floor joists. All of the solutions in BA's link need this orientation.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[msquared48]

You will not find a true truss arrangement without a bottom chord, horizontal or sloping. It needs that to function structurally.

The best thing you can do here, if you do not want to stick frame the areas where the floor joists are perpendicular to the rafters, is to raise the structure height by a foot or so in that area (you may not have to if you change the pitch of the trusses), and put the trusses on top of the existing floor framing. You will have to tear off the existing roof structure to do that. When the trusses are designed, you will also want to keep the bearing points of the new trusses as close to the original bearing points as possible to avoid changes to the stories below.



Mike McCann
MMC Engineering

 

[Triangled]

Yes, agreeing with all the above, definitely need a bottom chord, or, it is not even a truss. I could see in some cases like your attachment utilizing perhaps a kind of steel CMST style strap (Simpson nailed strap) as the bottom chord. Detailing is a little tricky, and gets a lot trickier at the hip and valley regions I would imagine, and needs to be correctly done.

 

[XR250]

Is there a subfloor on top of the joists? If so, you could make a tension attachment to that to keep the roof from spreading.