Existing Wood Truss Analysis

[SoFloJoe]

Hi All,

I have an residential home that was built in 1968 in South Florida. There are 31ft long trusses with 2x4 at 24" o/c (see truss layout attached). There currently are 2 interior walls that the owner wants to remove from underneath, they have horizontal bracing which initially led me to believe they are load bearing. BUT after further investigation there are no connection points, one has an air gap of about 1/2" and the other is snug to furring strips (not a real structural connection). Both of these are within 10ft from the wall supports.

I used 64psf (conservative, probably can bring down to 55psf or so) load per today's criteria. I found the top and bottom chords are failing. I found the capacity to be at 29psf, assuming 1000psi DF wood. Today we normally spec Southern Pine but who knows what they used in 1968? (An actual question and figure of speech).

Could have +/-30psf been a standard load for HVHZ in the 60s/70s? I am not sure if I am being overly cautious here. Ideally we would not have to do anything and can remove the walls. But if not, then my solution would be to either add beam/support to replace 1 or 2 of the walls OR stiffen the affected trusses with additional 2x4s. I hesitate to stiffen the trusses only b/c then I now own the truss if there are any future issues with it, you all concur?

BTW, there is a jog in the structure just beyond these 3-4 trusses where the span drops from 31ft to about 25ft with trusses running the entire length with no supports.

Thanks in advance for any input!

 

[phamENG]

Considering DF can't grow in the Southeast, it's probably not DF. Southern Pine has been used for as long as people in the South have been cutting down pine trees to build buildings.

Can you add the walls in question on your elevation of the truss? Do they align with the panel points? If they do not align, then the wall is not load bearing. If they do align, then it's probably not load bearing, but deserves some closer attention to verify.

 

[SoFloJoe]

Thank you, yes that is what I thought in relation to the wood. I have always used SP but was not sure what they did in the past.

The trusses are staggered. meaning the panel points actually differ from truss to truss, mainly due to the transition (job in the floorplan). Its a hip roof. But see the link, this is more or less were the closest truss is located to the walls.

Thanks again

 

[phamENG]

Then I would say you're likely okay - the trusses were probably designed to span all the way. Especially the one with the gap - that's a sure sign that it's not a load bearing wall.

Let's try to address how the truss is "failing" in your calculation.

1) Did you try running it with Southern Pine values? When you do, make sure you're using values published for or prior to the 2012 NDS supplement as the downgrade in stress values was based on recent data and the use of the older, higher values is usually appropriate for investigating existing structures that predate the switch. I know in modern truss construction the web members are often No. 3 or stud grade, but the chords are No.2 or better.

2) Did you include an attic live load? If so, what intensity? Based on the web layout, I probably wouldn't consider more than 10psf. You really don't have room for an attic with storage there (unless the homeowner is really trying to squeeze it in, which could be the case).

 

[SoFloJoe]

Thank you

I did use the most conservative numbers since there were so many unknowns. So I went with the 1000psi bending (1500 psi compression) just to stay conservative, and as you know SP can vary from 1100psi up to 2000psi depending on the type and density. I have no idea what was standard back in 1968. But I will take your recommendation and run the numbers with SP#2, do you think they may have used #1 or #1 dense back then?

As far as loading I did a quick analysis for wind, live roof and general dead loads. I used a higher wind rating. But in general I know back then they usually used a blanket 40-45 psf for trusses so I just wanted to see if I am in the ballpark, and reverse calculating the truss I found it to be 29psf based on the combination loading. Using 40psf it passed in bending and compression but failed in the combination loading with +/- 2.1dcr.

I am leaning towards approving it with some notes/limitations. Something along the lines of:
WALLS TO BE DEMO'D. WALLS WERE DETERMINED NOT TO BE LOAD BEARING DUE TO LACK OF STRUCTURAL CONNECTION POINTS AND INITIAL ANALYSIS SHOWED THAT TRUSSES WERE LIKELY DESIGNED TO SPAN OVER THE ENTIRE AREA. LIMITATIONS: THE EXISITNG CONDITIONS OF THE TRUSSES, CONNECTIONS, ETC. WERE NOT FULLY ANALYZED. IT IS RECOMENDED FOR THE OWNER TO HIRE A SPECIALIZED TRUSS EXPERT/CONTRACTOR TO ASSESS THE CONDITIONS OF THE EXISTING STRUCTURE. FURTHERMORE THE SPACE IN THE AREAS WITH TRUSS DIRECTION RUNNING NORTH-SOUTH WAS NOT OBSERVED TO BE USED AS AN ATTIC STORAGE SPACE AND NOR SHOULD IT BE USED AS SUCH IN THE FUTURE.

Thanks again for the assistance!

 

[XR250]

Unless a panel point falls directly on the walls, it is unlikely to be load bearing.

" IT IS RECOMENDED FOR THE OWNER TO HIRE A SPECIALIZED TRUSS EXPERT/CONTRACTOR TO ASSESS THE CONDITIONS OF THE EXISTING STRUCTURE"

I feel this is too CYA. If you have to put this note on your drawings, you should probably not take the job.

 

[r13]

Point out the fact - the roof does not meeting the latest code....

 

[KootK]

1) For a building slapped together in 1968, I wouldn't read to much into the wall gaps. Nobody was doing that for altruistic reasons way back then. More likely explanations for the gap would be truss uplift and/or camber -- probably accidental camber.

2) The age of the building is probably also the best argument for it not being designed for interior load bearing. 1968 would have been well before the advent of computer aided design in the pre-fab truss industry. Back then, the "design" of the trusses would have amounted to flipping through a big binder of canned truss designs covering some common spans, pitches, and shapes. And not a one of those would have been anything but simple span. If you had a particularly conscientious fabricator, they might have slipped a panel point over an intended bearing wall without engineering it. Even that wouldn't have been so easy because the cuts on the web ends were also tabulated for standard configurations. Slipping in a random diagonal to hit a bearing would usually be an exercise in creative carpentry rather than trigonometry.

I used 64psf (conservative, probably can bring down to 55psf or so) load per today's criteria.

Are those heavy loads associated with downward wind?

 

[phamENG]

You did use load duration factors, correct? And when you say you used 40psf, how did you apply it? All toward the surface of the structure? If so, I'd check it with a more realistic down/up and up/up set of scenarios. I'm not sure of any condition that can result in downward pressure on both sides of the ridge for an enclosed building.

There's another thread going in the wood engineering forum about removing walls under trusses and dik (I think) made a good point - you're likely going to have some variation in the way the truss is behaving, and cracking of finishes away from the walls to be removed should be expected.

 

[SoFloJoe]

Thank you all for the comments.

Yes on second thought, after reading my "fine print" I will not likely use that verbiage. But I do want to protect myself. I will definitely add that "wall cracking" note.

Are those heavy loads associated with downward wind?

Yes primarily, I did use the ASD load combinations and that number is very conservative. But my concern is more that reverse engineering the truss I am nowhere near that number.

You did use load duration factors, correct?

Yes, used 10yrs. Even though this is a 50yr structure.

There's another thread going in the wood engineering forum about removing walls under trusses and dik (I think) made a good point

Thank you, I did review that. Since it is California I think the wind loads would be much different. But good note on the cracking.

Right now I refined the calc, and reverse calculating with Southern Pine #1 Dense I am at 36psf load capacity. I am not even sure if they labeled wood as SP1 Dense in 1968.

My gut is telling me it should be fine, but the numbers are low enough for concern and I do not want to necessarily own those trusses if we modify them.

 

[phamENG]

Yes, used 10yrs. Even though this is a 50yr structure.

If you're checking for wind, you should be using C[sub]D[/sub]=1.6. It has very little to do with the design life or age of the structure - especially when talking about wind. Load duration factors are intended to guard against creep rupture in the member. That is, they are there to prevent a wood member from sitting with too much load on it for too much time. Wood can handle pretty high loading for short periods of time. The problem is that wood undergoes both elastic and plastic deformation at the same time in different parts of the material. So those short duration, high intensity loads do more plastic damage and shorten the life of the member faster. But lower intensity loads that are there for a really long time also do some plastic damage, and then a little more, and then a little more, so and so forth until you get a creep rupture after 45 years. So the C[sub]D[/sub]=1.0 for 10 years means it is exposed to that level of stress for 10 cumulative years. How likely do you think it is that it will experience the full design wind load (for you probably roughly equivalent to a Cat 3 hurricane, possible a Cat 4 if you're far enough south) for a full 10 years. In other words, Hurricane Eta (as it hit Nicaragua, not the 4 times it hit Florida) sitting over this house from now until November 2030. Probably not going to happen. That's why 1.6 is used for wind loads on wood structures.

Then there's the material strength. Keep in mind that our lumber grades and reference design values for stresses put you in the 5th percentile. That means you have about a 95% chance of the lumber used being STRONGER than the allowable value. So if you're close but not quite, I wouldn't sweat it too much (EDIT: this is for existing structures, not new). Check out this recent thread where we discussed a similar topic, though it was more in reference to snow loading on a roof. I'm not completely on board with load reductions that Canada allows, but the material strength considerations make sense. While it's not part of the code in the US, I think there are some good lessons to learn there and at least let it inform our decision making.

 

[dik]

Can you take a close up photo of the truss lumber... southern pine and doug fir look a lot different. 1000 psi is pretty junkey doug fir... don't know what was used then, but machine stress graded lumber has been around for quite a while... could be 1500 psi...1500f was quite common back then.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[SoFloJoe]

Thank you phamENG, I took a look at that post.

dik, see attached for a picture of the setup. Those panels going from truss to truss are offset.

So taking a note from that thread that phamENG suggested, could we say that the T&G sheathing acts as a diaphragm?

 

[dik]

The early wood-late wood contrast looks like doug fir and not like southern pine that I'm aware of... much less contrast for the latter. Colour appears off a bit... could be the lighting. Can anyone else, out there, confirm this?... after that time, if doug fir, you can hardly drive a nail into it. I don't know how southern pine ages/cures.

Just looked at Southern Pine on google and some of the wood grain is more contrasty? than I recall...

The sheathing would act like a diaphragm, but I don't know to what extent... not a big house guy... maybe done a dozen or two...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[phamENG]

Looks like SP to me. I imagine DF would be tough to find in Florida.

Yes, the roof deck is certainly a diaphragm, though I'm not sure that's important? Isn't your concern with vertical loading?

 

[dik]

thanks... we can get Southern Pine here... so don't know why Doug fir not available in Florida... maybe pricey...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[XR250]

I'm in NC and all of our trusses are exclusively SYP. Have seen some SPF in modular homes, however.
Our framing lumber is either SYP or SPF. You can get DF but is it expensive AF and typically used for aesthetic purposes.

 

[phamENG]

dik- you can probably get it now, for a premium (closest climate region that sorts DF is a good 1000 miles from south Florida and receding). 1968 - I doubt they would have used it.

I'm roughly one market over from XR, and my local experience in VA is the same.

 

[SoFloJoe]

Thanks all for the input.

Conclusion to this is to "keep" a load bearing wall. I put it in quotations because we will just spec out a column beam approach at that location and call it a day. Works out architecturally too.

One less grey hair to worry about.. lol

 

[phamENG]

As long as it's not the one with an air gap. That could do more harm than good.