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Re: Joists & Adding Roof Mechanical Unit

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Disclaimer: I'm used to using pipe columns and pitch pockets for rooftop curbs. Can you afford a three-tier curb system - two rails over the panel points, perpendicular rails on top of that to support the curb rails at the proper spacing? It's more steel tonnage, but it will get you to the bearing points of the trusses without having mess around in the ceiling.

At 10:49 AM 10/29/2004 -0500, you wrote:
I have an existing roof structure where the owner wants to add a rooftop
mechanical unit.  The unit weighs about 3,000 lbs.  The existing roof
joists are 24H9.  I checked the roof joists and supporting girders and
they are fine to support the additional load.

I have a detailing question.  I know this has been done many times so I
am hoping someone will share their experience with me.  I plan to ask
that the unit be placed on a continuous curb that runs perpendicular to
the roof joists.  My question has to do with the curb bearing on the roof
joist.  First, I assume the curb will bear directly on the deck.  No roof
penetrations are required.  The roof deck is corrugated decking as the
joists are about 2 feet on center.  The curb will run parallel to the
deck flutes.  It is typical to just run a wood plate on top of the deck
to support the curb?  The curb will be a structural curb that spans
between joists.  Next, in new construction when the concentrated load is
put on a joist top chord at a place other than a panel point an angle is
welded from the top chord down to the bottom panel point.  The space
below is occupied by office space and to weld a few angle struts would be
very difficult on innocent tenants.  Is there another detail that might
not involve filed welding?  Would I be able to skip the angle strut if
the maximum possible point load from the curb were under 1,000 lbs?


Thanks for your help.

Rich

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