Truss or not a Truss? 7

[Michel60]

I've lurked around the site for a number of years now, offering my $0.02 occasionally (maybe two-bits with inflation and tariff). I now have a question for the group about a proposed truss profile for a residential project where my first reaction is a "hell no", see below. Its 45' long with a maximum depth slightly over 3'-4". To be fair to the truss designer they were asked by the client (without consulting me first, I was assuming that the area would be stick framed) to generate a budget so this is still a fictitious design right now to get to a preliminary cost. The "special trusses" are for a low-slope (<1:12) membrane roof, but the "girder trusses" support about an additional 12' tributary of a 3.5:12 slate roof.

So far I've only seen the profile summary, no calcs. But even with calcs saying it works I'm not inclined to accept the results. I can't imagine that typical truss design software can really handle this problem reliably. I know some of the regulars here have a pretty strong background in plate truss design so my questions to them are:
1) Do you see this as a legitimate truss profile (either typical or girder)?
2) If you do, can the common truss programs out there adequately design something like this?

Thanks!

 

[8thStreet]

Another question - As an EOR - i would love to spot check some truss designs and girders before i show them on my plans. Common items that i need to check are large girder trusses with loads above and scissor trusses for deflection. I dont want to get down the road to construction and then have the truss designer come back and say it cant be done.

So long intro to simple questions - are there any good programs out there to quickly run truss designs? I have reached out to mitek but they dont seem interested in selling their software to people who dont make trusses.

Thanks,

 

[phamENG]

Best luck I've had is talking with a local truss shop. Depends how busy they are and how much competition is around, but I've had luck getting preliminary truss designs with a promise to specify them as the manufacturer. It's even easier when the GC is picked early and they have a go-to supplier I can call. Some are pretty excited when engineers want their input early - seems there a few amongst us who have a tendency to produce less-than-buildable layouts, and any effort to prevent the headache on their end is welcome.

 

[KootK]

So long intro to simple questions - are there any good programs out there to quickly run truss designs?

For my EOR estimates, I just use RISA etc and limit myself to #2 SPF for the chords. The #2 SPF tends to keep the designs from being overly aggressive such that plating issues are pretty much non-existent for 2X trusses. This is a good way to keep yourself from doing silly EOR stuff with the trusses as well.

 

[ANE91]

are there any good programs out there to quickly run truss designs

I have high hopes for Paragon. Try their free module and report back!

 

[Aesur]

I certainly understand the sentiment but I'm going to take this opportunity to push back on the designer mistrust a bit.

MPCWT = Metal Place Connected Wood Truss.

One of several positions that I've held within the MPCWT industry was working for the Wood Truss Council of America where one of my duties was to travel North America and give 4 day seminars on "advanced" truss design to designer. So I know a fair bit about the capabilities of your average truss designer, how they typically spend their days, and the kind of pressures that they are exposed to in their work.

In my current, solo practice, I sometimes am the engineer that stamps the truss drawings as you suggest. And this is a role that is maligned by many on this forum (looking at you @Aesur, @phamENG, @JAE). I share people's concerns about this to an extent and am happy to dive into the ethics etc if desired. Hence the callouts. Why do I do this?? Well...

1) I feel somewhat uniquely qualified for the work and;

2) The regulatory ambiguity on this stuff has created a market opportunity that am happy to exploit. Yaaas... KootK does indeed like money.

3) I like the people in MPCWT. That's where I got my start and, in a lot of ways, they are my kind of people.

What EOR's want from the MPCWT situation is what they want from all delegated design situations: someone they can trust to engineer the specialty product.

I would argue that this is simply not possible within the MPCWT space. And that what needs to happen -- at least in the near term -- is that EOR's need to come to grips with this and recognize that:

1) To the extent that you can delegate truss engineering in the MPCWT world, you are delegating it to the software. Designs probably should roll off of the printer stamped by the plate suppliers. Seriously.

2) Most professional engineers stamping MPCWT work -- including me -- cannot be trusted to provide the kind of reliable engineering that EOR's want with respect to their delegated engineering. More on that below.

When reading what follows, keep in mind what kind of incentive structure this creates and what that means for the ability of EOR's to be able to trust delegated engineering in this space as they would wish. If there is anything that you can trust in this world, it is Economic Man responding rationally to incentives.

WHAT IT IS LIKE TO BE A BAT TRUSS DESIGNER

- Your job is to crank out work at breakneck speed.

- Your job is to minimize the cost associated with EOR effort on your jobs.

- Any modification to the schematic design of an EOR or architect is likely to be viewed as a customer service failure. This is a last resort.

- Your job is to master the manipulation of the truss design software that has been presented to you as infallible with respect to engineering.

- Your job is not to be an engineer or, in reality, even an engineering technician. If you are capable of executing the method of joints, you are a unicorn.

- You exist in an obnoxious state of regulatory ambiguity where:

a) Some AHJ require nothing to be stamped.

b) Some AJH require the components but not the layout to be stamped (makes no sense as most of the "engineering" is in the layout).

c) Some AHJ require the layouts but not the components to be stamped. This is annoying because the engineers doing the stamping usually wind up getting paid much less (piece work) than they would if they stamped the components as well. So "layout only" jobs wind up being more expensive than you feel they should be.

WHAT IT IS LIKE TO BE AN ENGINEER STAMPING MPCWT WORK

- You tend to do the work in the shadows because you realize that the work is ethically ambiguous and that many of your peers disdain it. What's nice about working in the shadows? Nobody can see you there...

- Your MPCWT clients want you to spend as little time/$$ on their projects as humanly possible. Nothing that interferes with production will be perceived as adding value.

- Because modifying EOR/architect schematic design is perceived as "failure" for your MPCWT clients, it is also perceived as failure for you.

- You are very often external to the truss supplier precisely because the truss supplier does not want the liability associated with the engineering. MPCWT folks tend to be pretty good business people if they've managed to survive the insanely small margin pool in which they swim.

- You sympathize with the MPCWT industry regarding the AHJ ambiguity. The whole thing would be much cleaner if:

a) We went back to no stamping OR;

b) The software did the stamping (components not layouts) OR;

b) We skipped forward to stamping everything and just passing that cost along to customers. This, surely, is the future. Just get there already FFS.

- As a result of the above, there is great incentive for you to not provide the kind of trustable delegated engineering work that EOR's reasonably desire.

View attachment 8386

You may be surprised that I'm starting to come around to your thinking the longer I am in this industry, the more I just want to make money and find a way to retire earlier and earlier. My only big complaints with trusses as of late is all the verbiage of "EOR to verify loading was input correctly" for special loading cases or something unique on a project, of which I have no possible way of knowing if those magic numbers they show somewhere on there are input correctly as I can barely read or understand half the random numbers on their output, ok I'll give, I know those are nodes as least, but still I have no way to verify it was input correctly, that should be on the stamping engineer IMO. The big complaint lately is truss salesmen keep successfully convincing contractors to make changes that in the end cause MANY more headaches and there were reasons it wasn't done that way. The main change I have been seeing and pushing back on lately is they insist on rotating the trusses for the last bay to be perp to a girder truss and putting 2' long trusses bearing on an end wall and girder truss with 4 to 5' parapets built into it and then tell the EOR to figure out how to connect the 2' short truss with huge uplift loads to the wall and to their girder truss because their typical hangers can't handle such loading. And then you have a girder truss with 15+ kips uplift at the end per their calcs as well that now has to be dragged to a footing that doesn't exist for uplift as well as compression loads for opposite wind direction. uplift create by large parapet with short resisting moment arm on the rotated trusses. There is a reason we prefer to ledger and built the wall with integral parapet at the ends and many jurisdictions have now caught onto this bad practice and it's costing the EOR and owner money and time.

 

[Aesur]

Best luck I've had is talking with a local truss shop. Depends how busy they are and how much competition is around, but I've had luck getting preliminary truss designs with a promise to specify them as the manufacturer. It's even easier when the GC is picked early and they have a go-to supplier I can call. Some are pretty excited when engineers want their input early - seems there a few amongst us who have a tendency to produce less-than-buildable layouts, and any effort to prevent the headache on their end is welcome.

There is one engineer we work with who is amazing at doing preliminary truss designs for projects and has a side business doing truss repairs, we use and refer him often on projects that need truss repairs or strengthening.

 

[XR250]

Best luck I've had is talking with a local truss shop. Depends how busy they are and how much competition is around, but I've had luck getting preliminary truss designs with a promise to specify them as the manufacturer. It's even easier when the GC is picked early and they have a go-to supplier I can call. Some are pretty excited when engineers want their input early - seems there a few amongst us who have a tendency to produce less-than-buildable layouts, and any effort to prevent the headache on their end is welcome.

I do the same as Pham as well as running it in my 2-D frame program.

 

[Nick6781]

Very shallow of a truss. How much load is it seeing?

 

[Michel60]

Very shallow of a truss. How much load is it seeing?

For the typical truss area it's nothing too unusual with membrane roofing (PVC/TPO) and maybe a PV array it's in the 15-17psf range. The truss span has now been reduced and the "pointy" end eliminated and replaced by more conventional joist framing.

Girder trusses were also proposed with the same extreme configuration. Pending an owner decision they might have more significant roof loads with a 10'-12' tributary. Those have been swapped for PSL girders.