Drag Truss?

[dabomaman]

Hello All!

I am working on a project in the mid atlantic region. It is a one story rectangular commercial office building, with prefabricated wood trusses approximately 25 ft span x 5 ft height at 2 ft spacing. The architect would like to remove all the partition walls. I am fairly confident that the walls do not serve as load bearing walls, however I am unsure whether some of the walls serve as shear walls.

1. Am I correct in assuming that the partition walls do not serve as load bearing walls? I have never seen a roof truss relying on an interior support for design.
2. There exists double trusses parallel and above the partition walls. This led me to think that some of these walls may be serving as shear walls. There exists no unique metal collector straps connecting the bottom chord to the top of the partition wall. Furthermore, the partition wall is constructed of gypsum board, and not plywood. What exactly is the purpose of the double roof truss in only the areas above the walls? I am concerned with removing the partition walls in these areas.

Any help would be greatly appreciated...Thank you!

 

[jayrod12]

Walls parallel to the trusses will not be load bearing, walls perpendicular could be, but not at the spans we're discussing.

what's the plan dimensions of the building? Do you need the partition walls as shear walls, or does the diapragm have capacity to take everything to the exterior walls? Do the exterior walls have enough capacity to provide everything you need?

 

[dabomaman]

jayrod12:

The building dimensions are approximately 25 ft x 75 ft. My understanding from other threads are that the roof diaphragm (roof plywood sheathing) can translate the lateral loads into the vertical force components resisted by the perimeter walls. I am still in the process of ensuring that the plywood diaphragm in my condition has adequate quantity of nails and connections to resist the lateral.

Often times, roof truss designs are designed by specific truss manufacturers and called out on the roof framing plan as "design by others". Therefor, drag trusses are often not included unless specifically requested by the engineer.

 

[jayrod12]

The 75 ft dimension is unlikely to work.

I would bet that the walls connected to the roof trusses running parallel are in fact shear resisting. What does the structure look like below these walls?

 

[dabomaman]

jayrod12:

It is a one story building, so below these walls, there is presumably a foundation. Perhaps a thickened footing. Have not done invasive inspection to determine boundary elements/posts at the end of the shear wall.

I believe that these walls may in fact contribute to the shear resistance of the building, but the trusses do not contain any metal connectors to the top of the wall. There exists only a 2x4 lapped to the side of the bottom chord. Very difficult to do inspection because of the insulation, head space in the attic space etc.

Lets determine a solution. Lets assume the walls are used for shear and that we are going to remove the walls regardless. What would cost efficient solution be? I imagine an LVL lapped to the side of the bottom chord, and spanning the width of the building. Create a moment frame of sorts with straps, timber lvl beam and posts. Please let me know your thoughts. Thanks.

 

[jayrod12]

I'm not a big supporter of wood moment frames. I'd be more prone to talk the architect into leave a portion or two. Whatever is required, and upgrade the connection ask you need.

That, or a moment frame. But with those dimensions, your moment frame is going to be fairly significant.

 

[DaveAtkins]

This is not a huge building. Are you sure the 2 X 4 nailed to the top of the shear wall and the side of the drag strut truss is unable to transfer the shear force? Even toe nails from bottom chord of drag strut truss to top of shear wall can transfer shear force.

DaveAtkins

 

[TheDW]

I'm late to the party, but the walls may be attached to the extra roof truss just to provide a load path for the 5psf wind load that interior walls are supposed to be designed for.

 

[J hugh]

Hi there, what happened with this ?

A bit late but curious how it worked out.

In Canada, we follow the CWC book for wood design ... I would see what is required in terms of shear walls from a design point of view ... if the exterior walls are insufficient to satisfy the calc, then either introduce plywood sheathing to some existing walls that sit on an interior foundation below, or introduce a new shear wall and footing below. or large moment frame on peirs below.

The arch has a vision for their space but it still needs to satisfy the requirements of lateral design.

as for the truss, the two ply member ... its not a girder that supports the jack trusses? no way in understanding if it is designed to resist lateral unless you find the designer or the old structural documents. maybe the guys just had an extra truss on site and didnt know what to do with it so they just slapped it on.

if the trusses span 25 ft they likely are supported on the exterior walls and no interior supports are required, however if the truss layout is complicated sometimes interior supports are required to break up the truss spans (ie beams or LB walls)

anyhow let us know how it works out

 

[MotorCity]

I think the question should not be whether or not the interior walls were DESIGNED as load bearing, but rather ARE they load bearing in their current condition?

If any truss deflection or building settlement has occurred over time, they could be loadbearing and if removed may cause damage to the finishes.