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Joists & Adding Roof Mechanical Unit

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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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