New wood trusses onto steel joist flat roof

[TroyD]

Client has an existing warehouse (1970's) and looking to upgrade the exterior. See attached sketch. 12" CMU block walls, metal roof deck supported by steel bar joists. Client wishes to add pre-finished vertical steel to the exterior walls, and new mono-slope roof trusses, with 2x purlins and steel roof. I'm not sure the best way to secure the new trusses to the existing CMU wall. The existing roof joists and deck will remain. The roof material is likely rubber membrane, with rigid insulation underneath. I figure the rubber and insulation should be removed because the new trusses will crush it. The Simpson catalog has several pages of products for attaching trusses to CMU walls, but will need to penetrate thru the metal deck to do so. Does it make sense to cut out small areas of the metal deck at each truss location as needed to access the CMU wall below? I assume the top course block is a bond beam grouted solid. Any advice or suggestions is appreciated.

 

[TroyD]

Also, here is a page from the Simpson catalog showing a few possible tiedown products.

 

[XR250]

That's a tough one. Perhaps consider lots of attachments to the metal deck per truss using light gage clip angles. For that matter, they could even stick frame the roof at that point because you would not need the spanning capability of a truss. Maybe just screw 2x4 @ 24" o.c. down to the roof deck and provide 2x4 verticals at 24" o.c. and then a 2x4 top plate. Might be the cheaper solution.

 

[JAE]

Don't forget about:
1. Added lateral wind load to the overall building from the higher roof (new fascia surface) and from the sloping monoslope roof - check shear walls and the existing metal deck diaphragm for the added shear.
2. Added dead load on the existing roof joists in addition to current wind and snow loads.
3. If you remove decking at the walls to allow wall connections to the trusses you would be eliminating the deck-wall diaphragm connections.
4. Net wind uplift on the joists might be higher due to removal of dead load (roofing/insulation) and the higher roof eave level - and the higher slope angle.

 

[r13]

I think the new roof truss should offset the existing joist to allow for a better bearing.

 

[DaveAtkins]

I would elevate the new trusses a few inches above the existing roof, and let them span the 44'-0". I agree the other checks need to be made.

DaveAtkins

 

[r13]

Agree with the above, there is very little gain that come with a lot of complications, if the existing structure is to be relied upon. One note here, pay attention to roof ventilation for the concealed space.

 

[TroyD]

Thanks. I was intending for the trusses to clear span the 44' and to support the snow load (MN). Hand framing might be economical, but I don't want to load the existing joists with any additional weight. Maybe I add a double 2x bearing plate to elevate the trusses, and provide periodic bracing with some L6x4x5/16 angle pieces secured to the deck. We can demo the parapet blocks as needed to avoid conflict. Or I could replace the parapet blocks with 2x6 treated bearing plate stacked up...

 

[XR250]

TroyD,

Have you verified that wood trusses of that depth will span 44 ft. with an MN snow load?

 

[TroyD]

Here is an updated sketch with a different bearing condition I am considering. 4' o/c trusses supporting ~35 psf snow load will span the 44' no problem. Top and bottom chords will be beefier, likely 2x8/2x10. I plan to modify the truss profile to include an awning on the tall side, and incorporate a soffit for venting the attic space.

 

[EdStainless]

What angle is the front of the truss? The steel roof is sloped, and your panels will want to lay flat.
So either the front of the truss needs to be 90 + slope angle, or you need them set back enough from the face so that you can block and frame a truly vertical wall.
And how much holding strength wo you get out of fasteners into the deck?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[r13]

I share the concern of EdStainless. If you want to utilize the existing roof as lateral bracing of the bottom chord, then the connection needs to extend to the top chord of the joist. This is one of the complication I anticipated in my previous response, which is right after DaveAtkins comment/suggestion.

 

[TroyD]

Here is an updated sketch detailing an improved connection to the wall. I have not run thru the wind loadings yet, but am confident I can find a heavy-duty tiedown/holdown product that can resist the uplift forces. And I agree with you on the lateral bracing concern. The existing roof deck is only spot welded intermittently. It should be possible to bolt thru the roof joist top chord angle member (see sketch). Regarding the slope of the roof truss, the contractor can verify the roof slope and have the trusses fabricated with the slope incorporated into the end bearing elevations. The intent is for the tall truss face to be vertical. The roof currently is ~1/12 pitch.

-Troy