concentrated load on old building roof
concentrated load on old building roof
(OP)
I have a project involving the need to reinforce a clear span roof joist framing for the addition of a proposed rooftop HVAC unit.
The building was constructed around the early 1900's and uses multi-wythe load bearing brick and mortar side walls with parapets projecting above roof level. Access to roof framing for inspection is very limited, however were visible in the proposed location of the HVAC unit, the roof joists are clear span rough sawn 2x6s with 23.5'span at 16" on center and project into the brick and mortar side walls. Roof decking is 1x6 planking nailed to the joists. I have all the load and dimensional data that I need from the rooftop unit and proposed location on the roof.
Local building authority uses 2006 IBC.
As expected, existing roof framing appears quite inadequate for current live (snow) and dead load requirements which are 30 and 15 psf, respectively.
Question: Should I design with consideration for current code required live and dead loads in addition to the concentrated dead loading (HVAC unit) or just the concentrated dead load?
Before I inquire with the local building official what is your experience in your locality?
The building was constructed around the early 1900's and uses multi-wythe load bearing brick and mortar side walls with parapets projecting above roof level. Access to roof framing for inspection is very limited, however were visible in the proposed location of the HVAC unit, the roof joists are clear span rough sawn 2x6s with 23.5'span at 16" on center and project into the brick and mortar side walls. Roof decking is 1x6 planking nailed to the joists. I have all the load and dimensional data that I need from the rooftop unit and proposed location on the roof.
Local building authority uses 2006 IBC.
As expected, existing roof framing appears quite inadequate for current live (snow) and dead load requirements which are 30 and 15 psf, respectively.
Question: Should I design with consideration for current code required live and dead loads in addition to the concentrated dead loading (HVAC unit) or just the concentrated dead load?
Before I inquire with the local building official what is your experience in your locality?






RE: concentrated load on old building roof
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: concentrated load on old building roof
Mark
RE: concentrated load on old building roof
But if you cut the span exactly in half - it appears to work.
I wonder if somebody removed a beam at some point in time??
RE: concentrated load on old building roof
RE: concentrated load on old building roof
Mike McCann
MMC Engineering
RE: concentrated load on old building roof
Are you sure about that? Many times these older buildings framed the roof with a sort of cobbled-together truss system where the roof rafters and the ceiling rafters formed a sort of truss system.
Per Mike above - 300% overstress doesn't sound right for older buildings. The older wood did tend to be a denser, stronger material...but not 3 times stronger.
RE: concentrated load on old building roof
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: concentrated load on old building roof
It WAS consevative but the software was up and running!!
RE: concentrated load on old building roof
Can you see any evidence of memebers having been removed (discoloration, nail holes...)? From that era, trusses were commonly hand built with 1X6 webs nailed the face of the chords.
RE: concentrated load on old building roof
I was thinking about transferring the concentrated load (HVAC unit 86" x 47" and 635 lbs, long axis perpendicular to the 2x6 joists) directly to the bottom 2x12s. Does this make sense?
RE: concentrated load on old building roof
It certainly simplifies the situation. If the existing roof framing is to be reinforced, a large area (the 84" length of the HVAC unit by the width of the roof) of lathe and plaster below the original 2x12 ceiling joists will need to be opened up for access (not to mention partial removal of the suspended ceiling). Removal of the roof membrane and decking from the top is an even less desirable option.
Of course this does not address any upgrade to the existing roof capacity, but there are not signs of unusual damage or deflection and it appears to "have stood the test of time". Anyone disagree on this approach?
RE: concentrated load on old building roof
RE: concentrated load on old building roof
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: concentrated load on old building roof
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: concentrated load on old building roof
RE: concentrated load on old building roof
Is the existing roof showing any distress, other than needing a new roof, and badly? By the way, I would strip off all the old roof prior to any reroofing.
Mike McCann
MMC Engineering
RE: concentrated load on old building roof
I would strongly recommend a 1/2" CDX plywood overlay of the existing 1X6 planking at the next re-roofing, if not sooner.
Mike McCann
MMC Engineering
RE: concentrated load on old building roof