Roof Truss conversion into loft living space 3

[lubos1984]

I'm looking to create a loft in an attic that has a roof constructed from trusses. I've attached a sketch to show what I'm trying to do. Essentially I would like to remove the 3 chords in the hatched out section.
I'm wondering how would i go analyzing and strengthening the existing chords like the ceiling and top beam ? There are no load bearing walls underneath. For the ceiling, should i analyze it as a regular floor load since those 3 chords are removed? If i need to add floor joists can i span them just to the edges of the opening and tie them in to the existing trusses.
happy to hear your thoughts.
Thank You!

 

[JLNJ]

It looks like you want to remove diagonals, not chords. The analysis is easy. The reinforcement, very very difficult.

The load on the truss would increase significantly, probably 50% or more. All the members and connections would be suspect.

Existing wood trusses are full of unknowns in terms of the connection strengths and wood types and grades.

Your truss would become unstable. When you don’t have triangles, you are trying to transmit forces through vierendeel action, which is dicey in a retrofitted wood truss situation.

 

[Klitor]

On a 50foot span and with 2x4 elements,id be really surprised if you could do anything.

Did you try a quick structural analysy run to see the ballpark values?

 

[r13]

Do a truss analysis without the middle 3 members in the shade (middle post and two diagonals), then you will find the stress changes, and the members require strengthening.

 

[kipfoot]

I think you could get this to work if you construct new trusses (or add beams) that run perpendicular to the modified truss on each side of your new opening. The trusses could be the full depth of the vertical web and support both the monoslope portion and the gable on top. If the length of the new truss or beam is too much, you'll also need columns below.

 

[JAE]

I'm wondering how would i go analyzing and strengthening the existing chords like the ceiling and top beam ?
[red]You'd analyze the new configuration the same way you'd analyze any truss - I think you will find that the existing top chords will totally fail at the upper two corners of the hatched area.[/red]

There are no load bearing walls underneath.
[red]To get this opening, you might have to add columns as others have stated.[/red]

For the ceiling, should i analyze it as a regular floor load since those 3 chords are removed?
[red]Yes - this is what ultimately will be required as you are decimating the truss's ability to function as a truss. A full span floor joist system will be required to span across the full width of the truss unless you add supplemental beams/columns to shorten the span.[/red]

If i need to add floor joists can i span them just to the edges of the opening and tie them in to the existing trusses.
[red]No - that won't work as the truss will no longer be able to support anything anymore - it will become a bunch of framing needing other support.[/red]

 

[XR250]

Geometrically, this truss works without the center vertical or diagonal webs (think standard room truss).
That being said, the 50 foot span is going to kill you.
You need two interior, load-bearing walls or beams below.

 

[BAretired]

I disagree that the truss works geometrically without the diagonals. The diagonal web members are required to resist shear which will be present as the truss is not symmetrical. Even if it had been symmetrical, unbalanced snow would produce shear in the two shaded panels.

Additional live load in the loft will affect members and connections outside of the shaded area.

BA

 

[XR250]

You are correct - actually thought about that after posting. It turns into a linkage.
I guess in a symmetric room truss, there is enough continuity/moment capacity in the top chord to to deal with the unbalanced loading.
Curious how flat 2x4 chord floor trusses deal with unbalanced loading across the duct runs. Does not seem like two flat 2x4's have enough stiffness to vierendeel across the gap.

 

[r13]

I've ran a similar model with, and without the middle three members in the shade. Surprisingly, only the ceiling beam sees 50% increase in axial force. Please check the real model to verify.

 

[dhengr]

Lubos1984:
The original 50’ + long truss was never designed to accomplish (or really even consider) what you are thinking of doing. It is not symmetrical in elevation for starters, and for shipping purposes the lower 7.67’ and the upper 4’ were made in two pieces and then the upper 4’ more or less just sits atop the lower part, to form/complete the roof planes to the ridge. And, the vert. and two diag. members which you want to remove are significant members in making that lower truss span properly. Then, the lower horiz. chord has to be completely reconfigured to span about 17’ with floor loads, which were never contemplated in the design of the whole truss either. There are some trusses, variously called ‘bonus room’ trusses, etc., designed to accomplish what you are thinking. I don’t think I’ve ever seen one which wasn’t symmetrical, wind loads and unbalanced snow loads a tough enough to deal with. Then the middle bot. chord is a 2x8 or 2x10, on a shorter span, the two opening vert. side members at the room and their joints and the truss framing over the room are beefed up to accommodate the room opening. I doubt that what you want to do can be made to work with the truss you show, and on that span.

 

[BAretired]

I've ran a similar model with, and without the middle three members in the shade. Surprisingly, only the ceiling beam sees 50% increase in axial force. Please check the real model to verify.

The loft live load increases the truss reactions, so the top and bottom chords beyond the shaded area are affected, not to mention the end truss plates.

BA

 

[r13]

BA,

Yes, I did not consider the live load, only evaluated the existing and the proposed change. Live load will alter the chord forces.