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]

Very skeptical of these truss designs.

I would recommend you do a simple Truss calc to verify tension/compression force in top and bottom chord = M = C*d, and then run a calc to see if you can get the top chord in compression to work with what every size is shown in the truss drawings.

Also - this is yet another reason why I am skeptical of truss designers and always ask for a stamped truss package with calculations before I review them.

 

[BAretired]

Now that I have had some time considering my options I think there are ways to avoid the most offending parts of this truss profile and move things forward for them.

1. You could make the truss symmetrical.
2. You could drop the bottom chord and reduce headroom.
3. You could reduce roof slope.
4. You could raise the ridge.
5. You could change to a steel truss.

Can't think of anything else you could do to move things forward.

 

[Michel60]

1. You could make the truss symmetrical.
2. You could drop the bottom chord and reduce headroom.
3. You could reduce roof slope.
4. You could raise the ridge.
5. You could change to a steel truss.

Can't think of anything else you could do to move things forward.

Actually use a bit of what you suggest and one more. Once I looked at what I would like for girder layout, chords/drags/collectors I could cut the span (more like 25' instead of 45') and with a little adjustment to the slope I have some assym trusses with an adequate heel height and geometry for the web elements....the rest I stick frame.

 

[KootK]

Also - this is yet another reason why I am skeptical of truss designers and always ask for a stamped truss package with calculations before I review them.

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.

 

[SWComposites]

the truss design software that has been presented to you as infallible with respect to engineering.

I suspect this is what gives people heartburn. In my industry I (and others) have heartburn over blind use of FE analysis.

 

[dik]

Your job is to...

You missed the most important one... Your job is to look after the interests of your client.

 

[BAretired]

Actually use a bit of what you suggest and one more. Once I looked at what I would like for girder layout, chords/drags/collectors I could cut the span (more like 25' instead of 45') and with a little adjustment to the slope I have some assym trusses with an adequate heel height and geometry for the web elements....the rest I stick frame.

Cutting the span to 25' is an excellent solution, but not one which ET members were at liberty to suggest. But good for you; let's hope the client or architect agrees.

Kootk's comments are interesting; perhaps a separate thread is warranted as it is an important subject.

 

[Flotsam7018]

Thanks @KootK for a peek into that world. Unfortunately now I can no longer pretend that there is some infinitesimal chance a PE out there is diligently reviewing each truss. Your depiction is more or less what I expected. True rubber stamping.

Also, c’mon - two downvotes for the honest and transparent soliloquy on the wood truss world. FFS.

 

[Michel60]

I've always assumed that trusses provide will for practical purposes be my trusses, beginning to end, never mind someone else's stamp. If anything goes wrong my middle name will still be defendant. And if I'm in court trying my best not to be, I've already lost. Much time, money, and aggravation will be spent no matter the result.

That said I do respect the work of the folks at the truss plant. While I've never had any contact with the ones stamping, I do regularly speak with the designers, the ones feeding the software. They are in my experience by and large really helpful. As a result I do my best to direct my work to them. In fact I spoke with the designer for the preliminary package that initiated this whole conversation, earlier today. (In fairness, what was provided was just for the purpose of budgeting...all they wanted to do was to get to a number. But that pricing package in now the benchmark, so that's why I came here looking for more information.) We went over what I needed and what I was thinking of changing. Once I get my part done we'll take another shot at it.

I now have another cell phone number to call with any other questions or ideas. And I do use those numbers from time to time. I've had some ridiculously complicated roofs where I was spinning in circles trying to find common lines of support to get a rational layout. A phone call and an email to some one who spends all day everyday working on one complicated roof after another has helped me in seeing options I might have missed or come up ideas I hadn't considered.

With the possible exception of a few who participate here, I'm still not likely to be relying on just the stamp that is provided to give me a warm fuzzy feeling. But I do expect I'll continue to use the resources and help from others to get me there.

 

[KootK]

Your depiction is more or less what I expected. True rubber stamping.

Clearly I've let the pendulum swing a bit too far in that direction.

Let's say that I'm the rubber truss stamper on one of your projects. Whether or not your see value in what I do (and I'm not offended if you don't) very much depends on what you want. I would love to have me as the rubber truss stamper on one of my own EOR projects. Truly.

Based on what you see on this forum, many EOR's would want me to hand check something like 5% of the truss plates and members. Fat chance of that. I won't be checking any, ever. When I'm on my deathbed, I simply cannot allow myself to look back on my life and know that I've wasted even a single second on something so pointless as checking Mitek / Alpine output. @RontheRedneck is quite correct about the futility of doubting the software's ability to design a truss properly. The only question, really, is whether or not a conventional truss analysis/design is appropriate to begin with. Every once in a while it isn't.

What I will do for you is read your drawings carefully and, if you have asked for any special stuff, I will either:

1) Make sure that you get it (protecting you) or;

2) Throw it back in your face as an inappropriate ask to begin with (protecting the truss supplier).

Either way, this is a service to the project. In this, I view my role as that of a translator or, perhaps, concierge. One one side of the table, you've got an EOR that probably doesn't speak great truss. On the other side, you've got a truss designer that probably doesn't speak great EOR. Since I have some fluency with both languages, I can be quite effective at smoothing things out.

Some examples of stuff that you might want and I will ensure that you get:

1) Compliance with uncommon loads or serviceability requirements.

2) Squash blocking under all of your post loads shown on the layouts.

3) Proper consideration of pattern loading and LTB bracing at cantilevers which the software is not great at.

4) Diaphragmy stuff if you've decided to delegate that.

Some examples of stuff that I may throw back in your face:

A) Ridiculously onerous load callouts. My favorite is: "This huge job may or may not have some local spots where ceiling bulkheads would add to the dead load of the ceiling relative to a single layer or gypsum. Scour the architectural drawings, find them all, and add that extra dead load only where required".

B) Permanent bracing design unless my client is willing. All of the industry responsibility documents identify the EOR as being responsible for this. But many EOR's do not want to be responsible for it. It tends to be an annoying fee burner.

C) Places where you've asked my guys to do mission critical design and detailing without being specific about what that entails. Up here, I see this with balcony guard rails a lot. The EOR will show cantilevered balcony joists and a non-descript rim thing with a note that says "rim thing and connections by truss guy". Then, in some deeply buried general note, it will say "truss guy to ensure that every thing they supply is designed to all applicable code loads". So my guys are on the hook for a rim thing that can deal with guard loads and, more importantly, hangers that can transmit moment between the rim thing and the joists. We've even gotten into trouble with the rims simply not being wide enough to receive the stanchion fasteners. The truss guys typically have little chance of doing this properly. When challenged, they will tell me "The software designed for it! Look, there's 2PLF vertical load on there!".

EOR's typically want my role to be protecting them from crap truss designer work. And there is some of that. But just as often I am:

1) Protecting the truss designer from crap EOR work and;

2) Protecting the EOR from their own crap EOR work.

In short, I do feel that I provide real value to the projects of EOR's. It's just not in the form of verifying the software output.

This stuff is why I previously said that most of the engineering resides in the stamping of the layouts rather than the individual components. All there is to meaningfully check about the components is usually just the input, which is child's play.

From a liability perspective, I love it when somebody asks for the trusses to be stamped but not the layouts. That's getting paid for almost no liability at all in my book. Almost like being asked to stamp wide flange beam supply drawings to verify that the Ix/Sx match the manual or something...

 

[RontheRedneck]

KootK, I appreciate some of your comments. But I'm very busy, and don't have the time or energy to try to combat the ignorance that some show towards the truss industry. I'm out.

 

[Flotsam7018]

I would love to have me as the rubber truss stamper on one of my own EOR projects. Truly.

Same. I doubt you have been though, 90% of the truss packages I get are stamped by the same guy and his name is not KootK.

Do you have an idea if other truss stampers are providing that same level of general CD coordination review or if you’re an anomaly?

@RontheRedneck I’ll admit that I am quite ignorant to how the truss industry operates behind the scenes. Engineers don’t tend to like black boxes and are naturally very skeptical of them, which is how the truss industry can feel from an EOR perspective.

 

[phamENG]

What I will do for you is read your drawings carefully

Which is what makes you, @KootK, the unicorn. I rarely get point, line, and special loads accounted for on my trusses in the first pass - even if that first pass has a stamp on it.
I've taken to asking not for a stamp on the layout, but a statement on their sealed cover sheet that they've confirmed that the truss calc sheets and layout match, and that they've read the drawings and verified the loading called out there.

Stamped truss calcs around here seem to be based on the idea that it is the calc output they're checking, which is why I'm so annoyed with them. I don't really check the output - I check the input, check deflections, check reactions for my side of the bearing/uplift calc, look for warnings/errors, and then look at required bracing for my bracing design. That's it. I want the Truss Engineer to tell me that the package has been assembled and executed correctly.

I'm also annoyed with them because where I practice, they aren't supposed to stamp it unless they produced it. Maybe they have some weird contractual relationship where the truss plant hires them, and then they hire the truss plant's design group as a separate entity under their own contract? I doubt it, though. They probably just do what everyone else does and ignore that line in the regs or are ignorant about it because most of them are out of state and, from what I can tell, our "use of seal" wording is very unique (and onerous).

 

[XR250]

I was doing light gage truss design prior to Mitek coming out with their system. I basically designed and detailed the important trusses (using a 2D frame program and an Excel post-processor) and the fabricator filled in the rest of the similar ones and I sealed it. Many EOR;s thought i was going to design the entire roof diaphragm system for them and also the permanent bracing. No shear loads were ever provided. It did not help that the fabricator's salesman did not understand the difference when they bid the turn-key job of providing and installing the trusses and roof deck..

 

[KootK]

I'm also annoyed with them because where I practice, they aren't supposed to stamp it unless they produced it.

Certainly that is the case with me, in my market. I used to be a truss designer, it doesn't pay super well. Being a staff engineer at a truss shop can pay decently but, then, that has some drawbacks as a long term career choice.

I submit that your annoyance ought not be with the truss suppliers and their engineers but, rather, with the AHJ's and your own profession. A written "rule" doesn't mean squat unless it's uniformly applied and consistently enforced.

If there rules are uniformly applied and enforced, they will be followed and the cost of it will simply be passed along to the end consumer. As with the stock market, it is inconsistency and uncertainty that causes the most problems.

A very sad thing for me to bear witness to is the truss industry fighting tooth and nail to minimize what needs to be sealed by a P.Eng. Because the rules are inconsistent regionally, and even from one building official to the next, it feels to them as though engineering is, and always will be, a cost that they cannot pass along to their customers without putting themselves at a market disadvantage.

And they're completely wrong about that. The best thing that could happen to the truss industry would be for the AHJ's to just say "from now on everything gets stamped, no exceptions". And they would need to enforce that uniformly.

The cost of that wouldn't be crazy in the grand scheme of things and it wouldn't even put a perceptible dent in the amount of light frame construction that happens. But it would make it so that the truss suppliers no longer needed to waste any emotional energy trying to navigate inconsistent rules that earn them the ire of the EOR community. Just higher a staff P.Eng, charge a little more, and stop being the red haired step child of the delegated design world.

 

[KootK]

Do you have an idea if other truss stampers are providing that same level of general CD coordination review or if you’re an anomaly?

I'm afraid that I am enough of an anomaly that I have to admit that EOR skepticism is not without justification.

The largest problem that I see is not proper structural engineers doing a poor job of it but, rather, non-structural engineers doing a non-job of it. Mechanical engineers, oil and gas Civil/Structural guys that have never laid hands on a 2x4... stuff like that.

If one is a proper SE and has familiarity with light frame construction, it is possible to add quite a bit of value in a short amount time. Hopefully my previous comments support that notion. However, if you simply have no clue by virtue of your non-experience, no amount of time will make much difference.

This would be a weird thing and the truss folks would hate it initially but much of this would be solved by insisting that trusses get sealed by designated SE's. Yes, truss sealing absolutely does represent a highly skilled SE fighting below their weight class. Think of it as a filter rather than a technical necessity.

 

[KootK]

But I'm very busy, and don't have the time or energy to try to combat the ignorance that some show towards the truss industry.

Just keep doing what you've been doing (please). Not in this thread, per se, but within this forum. As with most things rooted in perception, it's a slow battle for hearts and minds. And right now, the truss industry has no better ambassador here than you.

I've come to view a great many things in life as being a function of evolutionary pressure. And I feel that applies to delegated design situations as well. While the truss industry may not represent the pinnacle of rigorous delegated design, I am confident that it does represent what the market wants from delegated truss design at this moment in time.

What your average design and construction team wants, at this moment in time, is this:

1) Truss designs that do not rock the boat on projects. Make it the EOR/Architect stuff work.

2) Truss design that has an almost non-existent impact on the cost of truss supply.

3) Someone to absorb the liability of the truss design work so that the EOR doesn't have to.

It's important to recognize that the Eng-Tips community does not represent the average EOR. It's a self selected group that tends to be more rigorous / conscientious than average.

It's also important for folks to realize that this stuff isn't just a problem with wood truss work. I've have intimate knowledge of delegated design in other spaces as well, including precast. Similar issues to varying degrees. If anybody wants to hear a spicy tale, I've done nothing so ethically objectionable in delegated design as what I've done in the space that is hollow core diaphragm design.

The truss guys have it worse than other spaces because the truss supply business evolved from light frame residential where, historically, there has not been much P.Eng involvement. So they tend not to have P.Eng's on staff that can defend their interests. This tends to be different from spaces like pre-cast or OWSJ where there has been P.Eng involvement going back to the 50's at least.

 

[GregLocock]

Back to the 15:1 truss.

Here's the member force diagram for a 45x3 pin jointed truss that looks vaguely like the original, with 100 load units applies vertically to each node along the top members, reacted by 400 at each end




ie ca 10:1 load magnification in the end bay, as you'd expect, note that the first vertical in particular plays no part in this load case.

The diagonals and verticals are very lazy (that's what I was interested in).

Here's the deflected shape (the loading is shown by the lollipops).



Better hope it doesn't snow! The Cremona diagram needs to be drawn on a roll of wallpaper.

A couple of shear panels on the 3 bays on left hand end would cheer things up.

 

[KootK]

@GregLocock beautifully done.

The diagonals and verticals are very lazy (that's what I was interested in).

That's exactly what I was getting at with the low Ix_truss to Ix_chord ratio. The left hand side of the thing starts to act like a pair of beams rather than a truss. I suspect this is also why the bottom chord is larger than the top which is unusual for these things.

 

[Michel60]

Back to the 15:1 truss.

Here's the member force diagram for a 45x3 pin jointed truss that looks vaguely like the original, with 100 load units applies vertically to each node along the top members, reacted by 400 at each end

View attachment 8537
View attachment 8539

ie ca 10:1 load magnification in the end bay, as you'd expect, note that the first vertical in particular plays no part in this load case.

The diagonals and verticals are very lazy (that's what I was interested in).

Here's the deflected shape (the loading is shown by the lollipops).

View attachment 8538

Better hope it doesn't snow! The Cremona diagram needs to be drawn on a roll of wallpaper.

A couple of shear panels on the 3 bays on left hand end would cheer things up.

Thanks for the demonstration. Luckily I'm here in sunny coastal California where nothing much ever happens, until the ground shakes. But gravity is always working in the meantime.

The trusses now stop at 25' and are supported by a girder, the rest will be actual joists and not a truss doing it's best impression.