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Open Web Wood Trusses in Commercial Construction

Discussion in 'Welcome Forum' started by architect629, Mar 7, 2017.

  1. architect629

    architect629 Member

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    My firm does interior fit out projects in the North East. We typically work with buildings of non-combustible construction, but we've run across a project recently that is constructed with open web wood roof trusses. I haven't had the pleasure of working with this type of construction (aside from residential) in my 15+ year career. I'm being told things by contractors bidding the projects that are "typical of this type of construction", that I'm not seeing in the code.

    Here's some info about the project.
    Type 5B construction, roughly 10,000 s.f.
    Fully sprinklered, in both the attic and occupied space.
    B occupancy with a large (>10% of floor area) storage function (S-2) which requires separation by a one hour fire barrier
    Under 2012 IBC

    I've been told by contractors that we need to install a layer of drywall on the underside of the existing wood roof trusses. I believe there's a requirement in the IRC to cover floor & roof framing, but I don't see anything in the IBC. The only thing I potentially see in the IBC is the fire blocking section. Would we need to create a fire block across the underside of the entire attic?

    The contractors are also asking if they can stop the rated partitions at the underside of the roof trusses (which would have a layer of drywall on them). This is apparently "typical". Per the IBC a fire barrier is to be continuous from the slab to the underside of the sheathing. Even if you added drywall to the underside of the truss and created a one hour rated roof assembly, the fire barrier would need to extend to the sheathing. Am I missing anything?
     
  2. Francis Vineyard

    Francis Vineyard Sawhorse

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    Seems to me you could use Section 503 for nonseparated occupancies
     
  3. steveray

    steveray Sawhorse

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    Why are the partitions rated? In a sprinklered 1 story VB building, nothing is likely to be required to be rated....

    Don't ask me why as it makes no sense to me, but IBC allows "exits" to stop at a lid, but not other FB....

    4. Interior exit stairway and ramp enclosures required
    by Section 1022.2 and exit access stairway and ramp
    enclosures required by Section 1009.3 shall be permitted
    to terminate at a top enclosure complying
    with Section 713.12.
     
  4. tmurray

    tmurray Sawhorse

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    Not that it helps, but in Canada, all fire rated walls can terminate at a rated ceiling provided it has the same or greater rating.
     
  5. steveray

    steveray Sawhorse

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    With a sprinklered attic this does not scare me. What I am seeing now is we can build a 3 story VB apartment building with one exit stair with an unsprinklered attic over it. If the fire ends up in the attic (grill vs. siding and roof vents), it all falls in and no one gets out
     
  6. tmurray

    tmurray Sawhorse

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    If you have proper fire dampers and fire stopping, the only way you can have a fire would be electrical in nature. Our code also requires concealed spaces to be fire blocked into certain size areas that will prevent fire spread throughout the concealed space simultaneously. If the designer chooses not to sprinkler the concealed space per NFPA 13R, the concealed space limits get smaller to account for the increase in hazard.
     
  7. cda

    cda Sawhorse

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    NFPA 13
     
  8. steveray

    steveray Sawhorse

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    TM...I've seen it on houses where the unattended grill lights the vinyl siding on fire, up into the soffit vent, and then the attic is on fire before anyone even knows there is a problem...HOPEFULLY management can control this in apartments, but I am skeptical....
     
  9. tmurray

    tmurray Sawhorse

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    I think your skepticism is well placed. Our code recently started permitting mid rise combustible construction for residential and business and personal services occupancies. One of the requirements is for all balconies to be protected by sprinklers.
     
  10. Paul Sweet

    Paul Sweet Sawhorse

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    This has also happened at several apartments in the Richmond area.
     
  11. cda

    cda Sawhorse

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    """"""The contractors are also asking if they can stop the rated partitions at the underside of the roof trusses (which would have a layer of drywall on them). This is apparently "typical". Per the IBC a fire barrier is to be continuous from the slab to the underside of the sheathing. Even if you added drywall to the underside of the truss and created a one hour rated roof assembly, the fire barrier would need to extend to the sheathing. Am I missing anything? """"""


    Not an IBC person

    But I normally see four feet out from the rated wall, sheetrock attached to the roof deck
     

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