concentrated load on old building roof 1

[bldgmeister]

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?

 

[woodman88]

Per the 2006 IBC Chapter 34 Section 3403.2 If you increase the force in a member over 5% you need to redesign it under the 2006 IBC code.

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.

 

[bldgmeister]

Thanks Garth, just the reference I needed!
Mark

 

[MiketheEngineer]

A quick check shows that the whole roof is probably dangerous. Exceeds capacity in the neighborhood of 300% and deflection about 1000%

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??

 

[dhengr]

Make a 24' long frame which spans btwn. the two brick bearing walls, and is above the existing roof framing system and roofing. Then don’t touch the existing roof, and argue that it has withstood the test of time, unless it is truly in distress and dangerous, in which case you probably shouldn’t ignore it. Then all you have to justify is the existing bearing walls and the pockets in them for your new steel beams.

 

[msquared48]

Could be Mike, but I wonder if the members are either rough cut 2X6's or 4X6's from the picture. Seem larger than a 2X6 to me.

Mike McCann
MMC Engineering

 

[JAE]

I find it hard to believe that 2x6's are spanning 23.5 feet!

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.

 

[woodman88]

Based on a 16"o.c., 23.5' clear span (l=23.75'), rough sawn of 2”x6”, S=12 inches cubed and 1944 NDS Douglas Fir Inland lumber values for Fb of 2150 psi Select Structural to 1450 psi Common Structural I get the joists over stressed by 1.7 to 2.5 at a 45 psf total load. At a 35 psf total load 1.3 to 2.0 over stressed. Based on the Data Book for Civil Engineers 1945 (I have the 3rd ed 1960) deflection was only calculated for floors and/or plastered ceilings and/or if specified. I am not sure what values were used prior to 1944.

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.

 

[MiketheEngineer]

As I said, it was a real quick check using today's sized 2x6 SYP #1

It WAS consevative but the software was up and running!!

 

[hawkaz]

I had a project recently where the owner wanted more head height- so he removed the bottom chord and webs of his roof trusses, just leaving the top chord 2x6 to span about 25'. To fix this, we sistered new 2X12s each side of each 2X6.

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.

 

[bldgmeister]

It APPEARS that the roof deck 2x6s are clear span, however, view is very limited through an approx. 16" square hole in the lathe and plaster ceiling that is above a suspended ceiling (and I am already standing on the top of an 8' ladder). Above the plaster opening, one (but not all) of the 2x6s is diagonally braced to a lower 2x12 that also appears to be clear span like the top 2x6s. The lower 2x12s are also 16" on center. Much of the view around the plaster opening is blocked by metal or flex HVAC ducting, but there is no other mid-span support in the room below. I may have to do more destructive investigation to get a better picture of any other bracing between the top 2x6s and bottom 2x12s.

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?

 

[bldgmeister]

On second thought I like dhengr's suggestion of designing steel beams to span the width of the roof, bearing at each end by pocketing into the brick parapet walls.

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?

 

[JAE]

Yes - exactly as I indicated above - probably some type of carpenter's truss system.

 

[paddingtongreen]

I see dhengr beat me to it. I've used that a number of times. I like the added benefit of not having a structural penetration of the roofing. The one thing that might get in the way is framing out an opening for the ductwork, if indeed the ducts do penetrate the roof.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[paddingtongreen]

I forgot to write: the members framing the opening will carry the up-to-date loadings and could bring trouble.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[bldgmeister]

The photo included in my second post shows the existing roof deck/framing penetration that is to be re-used with the new HVAC unit. The new unit (47x84) will be positioned on the side of the penetration where the existing ductwork is visible and will make use of that existing penetration. I've attached another photo of this for clarity.

 

[msquared48]

If the existing HVAC unit is being replaced, and the new unit weighs less than the original, no modifications to the roof framing may be required. Can't say that I would recommend that approach though.

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

 

[msquared48]

Being a brick building, but not knowing the seismic requirements of your area, let alone the wind, I would be concerned with the roof diaphragm in that is is not diagonally sheathed. Not the best scenario here.

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

 

[bldgmeister]

msquared48, what you see in the photo is a long trunkline of ductwork that comes to the location in the photo from an old HVAC unit (even heavierl and larger than needed) from another roof on an adjacent addition. I aready alerted the building owner of the poor condition of both roof membranes.