116 year old roof is inadiquate??

[Prestressed Guy]

I am working on a historic project that was built in 1903. I realize that for you folks in the eastern US this is just a pup, but in Washington, this is about as old at it gets. My work is on the floors and lateral to bring it back to level and up to seismic standards. I suggested that the original roof be removed and replaced with new MPC engineered trusses. the AHJ does not want to raise the roof pitch or change the profile due to the historic nature of the building.

The owner asked "Why can't I just jeep the original roof (no rot or decay, good straight grain lumber). After all, it has stood up for 116 years" The original roof is 2x8 ceiling rafters lapped with 2x6 roof rafters at 1 5/8":12 pitch. the total depth at the ridge is only 28.5". I ran a collar tie design and it says 300% of capacity.

Any thoughts.

 

[SlideRuleEra]

Haydenwse - I agree with the others that there are several load paths working to support the structure, but there is a very important one that has nothing to do with truss action. If your sketch is accurate, the bottom chord is 30' (continuous), making it a simply supported beam:

For the moment ignore, all possible truss action (a wooden king post truss with 1.6:12 slope is pretty worthless anyway, because of its' "flat" geometry).

As mentioned by you and others, quality of the lumber should be excellent... especially for a member 30' long. "Wood Engineering" by Gurfinkel gives the repetitive member allowable bending stress of Douglas Fir-Larch (North), Dense Select Structural as 2800 psi.

For a 30' beam with point load at the center I get dead load bending stress of 2660 psi, less than 2800 psi allowable.

For DL + LL, I get 7100 psi, exceeding 3220 psi allowable (includes snow duration factor). Overloaded? Yes, but Modulus of Rupture is 12,500 psi, so beam is not going to fail.

Together will all other known and unknown load paths, no surprise the roof works.

http://www.SlideRuleEra.net

 

[msquared48]

King post truss situation, yes. The rafters could be in some compression if there is any nailing at the chord to rafter connection, decreasing the compressive force in the bottom chord acting only as a beam, not considering any potential composite action of any decking present (this could be a stretch though). So the actual compressive force the ceiling joists see in bending should be less than sliderule calculated.

Mike McCann, PE, SE (WA)

 

[msquared48]

Also, to get a rough idea, you could take the area of the rafter and multiply it by the allowable bearing stress, them look at the area of the ceiling joists multiplied by the allowable tension stress, then multiply the lesser force by the centerline distance to see what the maximum allowable moment might be. Multiply that by 1.15 for repetitive member loads.

This should give a higher capacity than 2x8’s @ 24 if my thinking is correct.

Mike McCann, PE, SE (WA)

 

[Prestressed Guy]

JLNJ: the existing roof deck is 1x4 ship lap sheathing so no deck composite compression.

JAE: Or a warbird in stealth mode!

Kipfoot: This might have some merit but the structure has been empty and unheated for over a decade, so it is not as likely in the recent past.

SlideRuleEra: yes, the 2x8 ceiling joist is continuous 30’ long. All of the floor joists and the ceiling are 30’! Can’t find anything like that now. There are some areas that have interior walls that will add interior bearing points but I cannot see any perceivable differential deflection at the clear-span sections under the current dead load. The roof looks completely straight which is very surprising given that last year there were areas that had 10” of settlement. We leveled the floors and the roof came out straight.

The first-floor joists were infested with powder post beetles and had extensive amounts of end-of-joist rot. It was determined that they could not be treated in-situ plus the amount of sistering required the decision was made to remove all and replaced. They were carefully removed and stacked and are being treated for the beetles and are pure gold. The floor joists measure 2.5”x11.5” with no knots anywhere. The building has sat empty for the past 15-20 years and the rainwater from the neighboring building had been coming under the footings so the crawlspace was very wet. Fortunately the damage was limited to the first level floor structure and the structural elements and valuable stuff above was unaffected.