handmade trusses with plywood gussets 6

[Prometheus Farm]

Good Morning,

I am new to the group - thanks for the add. I had a 30' x 56' pole barn with a gambrel roof built by a local Amish contractor. I am now in the position of needed to defend (or re-mediate) the trusses they constructed using 3/4" plywood gussets that were both nailed and glued. They look rock solid to me, but the structural engineer I have been working is saying that they are "inadequate" stating that the 2 x 6 member size and gusset construction are not sufficient (to meet snow load, I assume). NYS code says, if I am understanding it correctly, is that handmade trusses are allowable if they designed to an engineers specifications. Okay fine, but just what are those specifications?? I have not been able to find any specs I can use to compare to my trusses. There was a thread last March on trusses with plywood gussets, and what I have seems to be in line with what you folks were talking about there. I am not engineer myself, but have had a fair bit of math and physics, so I can plug and chugg with the rest of them. Is anybody aware of any standardly recognized formulas or design criteria I can use?

 

[phamENG]

Prometheus,

You state you're not an engineer. This forum is for engineering professionals to discuss ideas and solve problems. If you hired a professional engineer and that engineer said it's no good, then it probably isn't. You can ask for his calculations, though unless it was agreed to up front he may not provide them. If they give them to you, he should have references if you want to track down the codes and try to decipher them. Other than that, all I can say is this: hire another engineer to get a second opinion. Express to them your desire to "peak behind the curtain" and maybe they'll let you in. I usually try to make time to explain things to clients with an aptitude and interest - and who are paying me for the time.

Coming here and insinuating that you can do our job because you can "plug and chugg with rest of them" is insulting to our profession and isn't likely to gain you much favor.

Good luck solving your issue.

 

[Ron]

Prometheus..your project is in New York and would have to be designed by a licensed professional engineer, even though in some areas agricultural buildings may have some exemption from full code compliance. Apparently the engineer who checked this was aware of the code compliances required and said that they don't work. Just because they "look rock solid" doesn't mean that they are under code-mandated loading. Trusses like this are also prone to non-ductile failure which means they will fail quickly and catastrophically without much warning.

I encourage you to follow phamENG's advice. Structural Engineering involves a lot more than "plugging and chugging" formulas. A lot more.

 

[EdStainless]

One issue that I have seen with these trusses is lack of documentation.
Lumber grades, glue types, number of nails, ans so on.
When information is lacking the prudent thing to do is assume the worst case.
This is why you may need some dialog with the engineer.
I have seen cases where a couple of extra trusses were built for destructive testing to validate the design.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[oldestguy]

Here is an answer to your question Is anybody aware of any standardly recognized formulas or design criteria I can use?
Yes, there are several ways to answer this question. All you need to do is sign up for a 4 year civil engineering course at Cornell. And of course take it. You won't get it here.

 

[Prometheus Farm]

Sorry folks, I did not mean the "plug and chugg" comment as an insult to your profession by any means. Rather than I would like to check this issue out myself before I commit a lot of money to it. I am merely looking for the criteria engineers use to determine if a truss system is adequate or not. I have had a number of experienced contractors also so that the engineer's assessment did not add up to them and that I should look for a second opinion.

 

[Prometheus Farm]

Sorry for the perceived insult - you folks are touchy. I really did not mean it that way. I have a PhD in the sciences and 25 years experience solving complex problems that involve math and physics. I think it is fair to ask why my current truss system is inadequate in a quantitative rather than qualitative sense.

 

[mikeofBelAir]

A few years ago, I needed to cut the bottom chord out of a number of roof trusses In order to install a 4 post lift in my garage. My engineer specified 2X12 rafters sistered to the existing top chords. We both "felt" that 2X12 was overkill---but his calculations consistently showed that anything less would fail. So I bought and installed what the engineer said was required. Three years later, and no issues or signs of a failing roof. I appreciate the piece of mind.

 

[oldrunner]

Prometheus:

You were correct in bringing in a structural engineer and you are also correct is questioning his advice as it is going to cost you money and time. Ask the structural engineer what it would take to modify the trusses. I have done some heavy trusses using plywood and nails in the past. (probably in the 1960's). At one time the plywood association provided fabrication drawings for various trusses and loadings with plywood gussets (pink or yellow sheets!). I believe they were developed by the University of Michigan Urbana. I have also salvaged in-place trusses by complete covering one side of the truss with plywood and adding in steel straps.

I have one article "Factory-Built Plywood Components" which was in the April, 1961 issue of Architectural Record.

From a 1960 document concerning plywood gussets - "DFPA Fabrication Specification GT-8" which is also mentioned in the US Forest Service research note FPL, Volume 76 which you can download as a free ebook.

I'm not doing consulting in New York at this time because my NY license is inactive.
Actually you cannot plug and chugg your analysis because many of the values used in the gusset designs were determined by testing.

 

[phamENG]

Prometheus - as I said, I enjoy explaining the process to clients with the aptitude and interest and, based on your stated qualifications, you more than fit the bill. Start by contacting the engineer who said they were no good and ask them some questions and further clarification. Hopefully they'll help you out and explain a few things.

If you're interested in a more-than-casual understanding, get an engineering statics textbook and read up on truss analysis methods. Then get a copy of Design of Wood Structures (currently on the 7th edition) and read about connection design. APA also has several useful white papers on designing with plywood. These will at least give you some background.

As for being touchy - there are quite a few people who come around looking for free engineering or think they can design a skyscraper because they read engineering mechanics for dummies. Just trying to keep them away.

 

[oldrunner]

Technical source:

https://catalog.hathitrust.org/Record/100146115

 

[JoshPlumSE]

Prometheus -

It's not so much your question that makes people touchy. It's that this is a common question that is a pet peeve of engineers.

A contractor (or someone with construction experience, but not an engineer) says something to the owner like, "The structure looks solid to me, I don't know why the engineer doesn't just approve it...." We have all dealt with multiple variations of this over our careers.

Keep in mind that there are different levels of safety. The structure may not be "unsafe" in that it can hold it's current weight and a decent snow load. But, it may not have an adequate factor of safety as determined by the current design codes. The engineer would be stepping out on limb by approving something that doesn't meet the current design codes or such.

There are other issues that come up as well. Does the engineer know what type of wood was used? And, were those wood members evaluated by a wood grading agency? If not, then he (or she) may have been forced to use a conservative value for the allowable stress for the wood members.

It is reasonable to ask the engineer approximately how "over stressed" they believe the trusses to be. And, if they have any idea of what type of remediation is required. It may just be adding a few more nails, but it may mean doubling up some of the existing members or more.

 

[dik]

You need to talk to the contractor that built it and determine how the trusses were designed... whether prescribed or actually analysed and designed. If there are no credentials for the design, you have to hire an engineer to do an analysis and design and provide any remediation required.

Did the engineer that looked at it give any reasons for his concern?

Dik

 

[skeletron]

You can always analyze a hand-framed truss, but the effort often doesn't match the value when comparing with as-new engineered trusses designed by a fabricator. For an engineer to be brought in and back-design an in-place truss they would need to evaluate:
-exact span distances
-material grades and dimensions of all members
-measured connection geometry

For someone to be out there measuring and documenting the design (prior to analysis) you'd be tying up a lot of $$$ before even commencing the analysis. Maybe one way you can address the cost issue with the engineer...go up there yourself in a lift and spend a day taking pictures and measuring all connections/members.

 

[Rabbit12]

Prometheus Farm says: "NYS code says, if I am understanding it correctly, is that handmade trusses are allowable if they designed to an engineers specifications. Okay fine, but just what are those specifications??"

Generally you hire an engineer before the trusses are built to provide a design (the specifications you reference). It seems like you didn't have an engineer beforehand....why is that?

 

[SlideRuleEra]

I had a... gambrel roof built by a local Amish contractor.

...the structural engineer I have been working is saying that they are "inadequate" stating that the 2 x 6 member size and gusset construction are not sufficient (to meet snow load, I assume).

I have not been able to find any specs I can use to compare to my trusses.

If the gambrel roof was constructed in traditional braced-rafter style (what I would expect from an Amish contractor), trusses are used in a somewhat out-of-the-ordinary way (compared with how trusses are used today). See this image:

Gambrel roofs built this way fare poorly in high winds, don't know about snow load (we don't have that concern here). If winds (to modern standards) are the engineer's concern (ask him/her), chances are they are correct.

You probably won't find any modern standards for gambrel roof because they went out of common use about 80 years ago. Likely not much different for plywood gusseted trusses which are more or less gone from commercial use.

If, for general information, you want to read about (historical only) gambrel roof wind loading and compare plywood/glued gussets with more modern connectors see the following:

"Design of Barns to Withstand Wind Loads - 1938" This is on my website.

"Longtime (15 Years) Performance of Trussed Rafters With Different Connection Systems" From the USDA Forest Products Lab.

This info may at least let you have an informed conversation with the Engineer.

http://www.SlideRuleEra.net

 

[PropertyGuy67]

I'd like to tack onto SlideRuleEra's comment, and say that while plywood gussets are more or less gone from factory-produced trusses, plywood repair gussets (using structural plywood) are very much in use as field-replacements for damaged factory-applied metal nail plates.

 

[Rabbit12]

PropertyGuy67, I'd agree that plywood gussets are an acceptable method provided the are designed and constructed properly. The problem here appears that the OP is asking a 3rd party to verify his connections are ok after the fact. The engineer is saying they aren't ok.

As an engineer I'd be damned sure they work before I sign off on them too because I now own the trusses because if something happens almost certainly the owner/contractors will quickly start playing the "dumb owner/contractor" card.

 

[Prometheus Farm]

Thanks everyone for the information and insights - they have been very helpful and educational. My goal here really is just to educate myself before I meet with the engineer after the holidays. You folks have given me some homework - which is good.

Just for clarification, the information that the trusses may not be up to snuff came as a preliminary update before the holidays (and a preliminary price tag). Most definitely, I need to sit down with the engineer, as well as the builder, to better understand this. The thread I mentioned in my first post was from 2015, not last year. As I dig deeper into that thread, there appears to be some of the type of information I am looking for.

Again, so sorry if I offended anybody, but I understand your issues. I do appreciate the time and thoughtfulness of the other posts - thanks again. I will follow up just to let you all know the outcome.

 

[oldestguy]

Regardless that you may have some info from here and maybe want the builders involved, you can't get any approval without the stamp or signature of the licensed engineer. I'd move to that step early on and save some time as well as money. no point in being "educated" yourself, since it won't speed that step.

By he way, this old guy (age 91) has seen a lot of "great" contractors, not with an engineering degree. Most are firm in their ideas that work quite well at times. However, take my new neighbor, 62 year old contractor building his last house which he will live in. In the hundreds of yes he has built he regularly jets the fill inside the attached garages to "settle" it. so I gently advised against it with his 5-1/2" thick new concrete basement wall. Of course he immediately expounded on how many of these jobs were done that way. About like the many contractors you know also. So I took my time and with each visit warned against it, as friendly as I could be. As time went on I reminded him that his wife will either kill him or at least kick hum out should this cave in. Fixed in his opinion, I continued to bring it up as I walked at his job, each time bring in more consequences of a caved in wall. Low and behold, no compactors were used within 4 feet of the wall, no jets, etc. and it works great. So, with the Amish and other, non engineers with opinions, sometimes they will listen to an engineer, but it takes time and patience for the engineer. By the way with this wall thing you should see the damage when they jet AND down goes the wall and the house above it.