Truss design misconceptions 29

[RontheRedneck]

I mentioned in another thread that I've been designing trusses most of my adult life. I started working in a truss plant in 1984, and it went from there.

Over the years I've run into a lot of misconceptions about how trusses are designed, who designs them, etc. I thought I'd take a crack at explaining the typical process.

The vast majority of truss plants do not have engineers on staff at the plant. Trusses are designed by guys like me. I only have a HS diploma. But I do have a lot of training and experience.

We buy our truss plates from a company called Alpine. They provide the truss design software that we use. They also have engineers on staff. The cost of the software and engineering support is built into the price of the plates.

Since we're in a rural area we don't get an engineers seal on probably 95% of what we do. There's no reason to.

If we do need sealed drawings, it's usually because someone is building in an area where there's a building department that requires them. Or on commercial work we sometimes have to send them to the project architect/engineer for review.

Once we have the trusses designed in our system we can send a job down electronically to Alpine. One of their engineers is assigned to our account, and that person typically reviews our stuff.

The engineer does not alter the truss designs - They're either approved or not approved. If they want something changed we get an email or phone call explaining what they want to see. We revise them and send them back down.

The engineers at Alpine never see the plans. They typically do not know where the job is going or any details about it. They only review what we send down.

So that's the basic process. If you have any questions let me know.

 

[EdStainless]

Back in the dark ages when I was in college I worked as a draftsman (pencil and ink on vellum) for a structural engineer.
The original formulas for the holding strength of steel nail plates and pressed plates were from his PhD thesis.
Needless to say we did a lot of trusses.
We had 18 - 3" ring binders with truss drawings, each one a different pitch, though the common ones had to vols.
And then specials.
Each page was actually a family of trusses, common pitch and load with different spans/OHs.
And with 40,000 truss layouts on hand we were still designing half a dozen different ones a week.
We tried to avoid working directly with residential builders, usually working with the truss fabs or other engineers/architects.
There were about 6 major mid-west truss fab shops that didn't design their own but only built to our drawings.
We had a very rigorous checklist that you had to sign off before you put any time into a drawing.
Beyond the basic load/moment/shear we did a bunch of other calcs for every case.
All done either with graphical methods or hand calculators.
I always laughed as we would have customers complain that we were too expensive and they didn't need 'that other stuff'.
But we were fast, very precise, and reliable, and they kept coming back.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[RontheRedneck]

EdStainless - Who was it that you worked with? I think I know who you mean, but can't recall his name.

I started in the truss business early enough that I used some of the pre-engineered drawings you mentioned. There were books that gave you the length of the webs for each truss design. Like at a 24' span the web might be 4' 11 5/8". For every inch you added to the span the web got 7/32 of an inch longer.


gte447f - I don't appreciate you referring to the truss design process as "sketchy". Nothing could be further from the truth.

What you said about me only designing residential work is not correct. It's a big part of our business. But that's not all that I do. We do a lot of hotels, apartment buildings, retirement homes, etc. So I do deal with architects and engineers.


Jumping back to the wonderful plans we often get - Here's another example:

This customer is building $300,000+ houses from sketches like this.


This afternoon I was thinking about a job we once did. One of the salesmen turned in an order for an addition to a commercial building. He didn't give me the plan, but wrote up what was needed. There was only one truss type.

He asked for a sealed drawing of the truss, and I provided it. The contractor, architect, and EOR all approved the drawing with no exceptions noted.

After the trusses were built the GC called in one day and told me the trusses were wrong. I told him he had approved the drawing, so they were his. He went ballistic.

I asked for and got a set of plans. The salesman who wrote the order screwed up badly. The span, overhang, pitch, and heel height were not correct. I have no clue how he came up with what he did.

The architect and EOR said they had no liability, as they only review for general conformance with the design documents.

The GC went over my head and went to the owner. After much discussion we ended up eating the job and replaced the trusses at our expense.

As a result of that - And of other similar stories - I have little faith in the review process. Doesn't seem to me like anyone really puts much effort into it. (Although there are exceptions)

 

[3DDave]

I think "sketchy" referred to the lack of a uniform process for this. It's like general contractors can be sketchy. Not all are, but there are enough that abscond with the funds after doing a half-demolition on a kitchen or the wall of a house, leaving it worse off than if they had simply stolen the money up front.

It's good to be out there showing the reasonable and responsible and under-appreciated side of the business. It seems frustrating to be lumped in with "those guys," even if "those guys" are rare. Few come back from a venomous snake bit with the attitude that most snakes are not venomous (Australia aside, where even some shoe laces pack a punch.)

I do feel your pain on dealing with the drawings and such and can only recommend, though not necessarily support by rigorous budget analysis, getting someone to take those drawings and cardboard sheets and creating a CAD model of the building and showing how your trusses will look sitting on it. I find that answering questions like "are those room dimensions the finished interior or are the walls subtracted from them" can help with fitting up to mating parts.

At least they used grid paper.

 

[ALK2415]

I'm sure there are many conducted tests/researches on these types of wooden structure [not lift to speculation]
but No one here, has point it clearly [the governing design manual/ or code of practice/ or manufacturing co guidelines]
I found these resources :

https://drive.google.com/file/d/1sZ0xElhTMCh0ryBCEQVgyYXBBiatDTc9/view?usp=sharing

https://drive.google.com/file/d/1uhlprf5f3nFYD91Y0n-G6yfdS5ud3pZs/view?usp=sharing

https://drive.google.com/file/d/0B-...=sharing&resourcekey=0-dZYyO5u-JORt--3WtlICXg

https://drive.google.com/file/d/1N1fXrc_ONccwXqwtMC0rsB2Ll3fFdq9r/view?usp=sharing

https://drive.google.com/file/d/1AMORgb85Yb7aSDeTPxXO1oWtBrYpCluc/view?usp=sharing

the ones below is for "KOOTK", i read once he was concern about timber cladding on seafronts environment

https://drive.google.com/file/d/1tJ5RhMLQBBjtwZzKxVoVHNOSpNVJ4Yih/view?usp=sharing

https://drive.google.com/file/d/1Ui8rio-S4o2U8r48996NlGgRmE10DRaQ/view?usp=sharing

https://drive.google.com/file/d/1sZ0xElhTMCh0ryBCEQVgyYXBBiatDTc9/view?usp=sharing

https://drive.google.com/file/d/1Ybt1mWZy-7Ko7XetEIaUfJPtMxXiLaZ_/view?usp=sharing

NOTE: Respect is free. Disrespect is Wrong.

 

[gte447f]

RontheRedneck, my comments were not intended to be demeaning toward you, but I stand by my characterization of the delegated design process for wood trusses as sketchy. It sounds like you perform your role within the process quite competently, but you yourself have provided us with several examples of a sketchy process including such practices as plans sketched on cardboard boxes and trusses ordered based on verbal descriptions when plans prepared by an engineer were available.

And again, I was not trying to insult or belittle you by saying that I interpreted your comments to mean that your work is primarily for residential or agricultural buildings that do not require any engineering. You told us in your OP that 95% of your truss designs do not require an engineer's stamp.

Trusses ordered by owners or contractors for projects that have not been designed by an engineer of record are not part of the delegated engineering design process with which many of the posters in this thread are familiar.

 

[JLNJ]

None of this surprises me, but the general public might be surprised when they see how the sausage is made.

As far as the "EOR is always found to be responsible" comments go, I doubt that this is the case. The EOR will be held to the standard of care.

 

[JStructsteel]

This afternoon I was thinking about a job we once did. One of the salesmen turned in an order for an addition to a commercial building. He didn't give me the plan, but wrote up what was needed. There was only one truss type.

He asked for a sealed drawing of the truss, and I provided it. The contractor, architect, and EOR all approved the drawing with no exceptions noted.

After the trusses were built the GC called in one day and told me the trusses were wrong. I told him he had approved the drawing, so they were his. He went ballistic.

I asked for and got a set of plans. The salesman who wrote the order screwed up badly. The span, overhang, pitch, and heel height were not correct. I have no clue how he came up with what he did.

The architect and EOR said they had no liability, as they only review for general conformance with the design documents.

The GC went over my head and went to the owner. After much discussion we ended up eating the job and replaced the trusses at our expense.

As a result of that - And of other similar stories - I have little faith in the review process. Doesn't seem to me like anyone really puts much effort into it. (Although there are exceptions)

Always gotta ask for a plan. Seems this case the sales guy made the mistake. How would the engineer know its wrong if he or she never sees the plans and just stamps the output? Architect approve a truss drawing-LOL! Contractor should have done a better job too, but he or she could be relying on the professionals to get it right.

 

[phamENG]

Architect approve a truss drawing-LOL!

What's funny about this? It's a team effort - I depend on the architects I work with to do a review of the truss drawings. Unfortunately, due to the constant changeability of drawings and whims of those who build the sort of houses I tend to work on, I usually don't dimension anything above the foundation in residential construction. I don't like it, but it's the only way to 'protect' myself...if I dimension the roof and the client requests a change after permit review, I won't know about it and I'll be stuck holding the bag when the trusses don't fit. It sucks, but it's reality. So my review is for the structural data - did they use the right loads? Is the spacing correct? Right adjustment factors? Does it all make sense? Yes? Good. Big note that says "Approved; reviewed for general conformance to the structural design requirements only. Architect to review and approve dimensions, elevations, pitch, overhangs, and other parameters relating to the architectural design of the building."

 

[XR250]

Unfortunately, due to the constant changeability of drawings and whims of those who build the sort of houses I tend to work on, I usually don't dimension anything above the foundation in residential construction

Same here!

 

[BridgeSmith]

I'm definitely thankful for Ron giving us this 'peek behind the curtain.

When I build my garage, I'm going to engineer and design the trusses myself, and take my stamped structural drawings to my local truss manufacturer.

A little 'peek behind the curtain of the process for highway bridges at the DOT, just for comparison:

Each component, down to the bearing pads, gets engineered (calculating loads and capacities), checked by another engineer, corrections by the checking engineer are back checked by the design engineer. If the design engineer is a PE, he or she stamps the design. If not, the final design gets reviewed by the PE that stamps it. The stamp on the design is the mark of responsibility that the component is adequate for the design loads.

When the details are complete, our Principal Engineer reviews them, marks any corrections, and stamps the details, certifying that those details are in conformance with the stamped designs.

The details are then reviewed by the Assistant State Bridge Engineer, and the State Bridge Engineer, corrections made, and then they sign each sheet, signifying their approval.

Shop Plans for specified components are submitted, reviewed, and approved for construction (eventually, usually after being returned to the fabricator for corrections once or twice).

After the bridge is built, the DOT construction engineer, who oversaw the construction of the bridge, puts his or her stamp on the "as-built" plans, certifying that the bridge was built in conformance with plans (with any changes made in field marked on the plans).

If there's ever a failure, there's a chain of responsibility that can easily be traced all the way back.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[JStructsteel]

phamENG - Every Architect I work with forward the truss submittal to be to approve. I usually do the same, look at the loading, check reactions to what I think they should be. Will glance at the spans, usually not that hard to check.

Perhaps they do review for dimensions, and I am not privy to that.

 

[Brad805]

"...peek behind the curtain of the process for highway bridges at the DOT, just for comparison..."
I think the key difference in bridge work is the fees for engineering are roughly 5%-10% construction cost. Residential work attracts a whole different group of characters with varying ideas of what/when they need help from people like us. Residential fees are crap.

Rod, as for engineering your own trusses, the Mitek or Alpine software designs a perfectly fine truss in 2D. You will not find tables for any of the press plates to check this yourself since that is heavily guarded by those that paid for the testing. All you need to do for your garage is read what you are provided carefully and it will be fine. Not in a million years would I build an old school truss using plywood plates.

 

[phamENG]

You will not find tables for any of the press plates to check this yourself since that is heavily guarded by those that paid for the testing.

I disagree here, Brad. TPI-1 (the standard on which those programs are based) is available for free download and is a must read for anyone doing many buildings with MPC wood trusses, and the plates themselves - MiTek and Simpson, anyway - have been tested by the ICC Evaluation Service and have freely available ICC-ES reports with capacity tables for use in the TPI-1 equations.

That said...you'd be nuts to do it by hand unless you're doing it for fun (not sure that changes the 'nuts' thing, though...). The software, even the hands of the most novice tech, can produce a fine truss for a garage...just review it before you buy it.

 

[Brad805]

pham, I stand corrected. I have asked before, but I guess I am guilty of not searching. Thank you.

 

[phamENG]

No problem.

MiTek Truss Plate ICC-ES
SST Truss Plate ICC-ES
TPI-1

 

[BridgeSmith]

Rod, as for engineering your own trusses, the Mitek or Alpine software designs a perfectly fine truss in 2D. You will not find tables for any of the press plates to check this yourself since that is heavily guarded by those that paid for the testing. All you need to do for your garage is read what you are provided carefully and it will be fine. Not in a million years would I build an old school truss using plywood plates.

If I have them fabricated, I'll probably let them use the press plates, and just review that part. If I build them myself, I'll likely use plywood or OSB. I've actually already done the design, including member sizes, OSB plate sizes, and the number of fasteners at each connection.

That said...you'd be nuts to do it by hand unless you're doing it for fun (not sure that changes the 'nuts' thing, though...). The software, even the hands of the most novice tech, can produce a fine truss for a garage...just review it before you buy it.

Apparently, I'm nuts. I found designing the trusses by hand to be an interesting exercise, and fairly easy one. Much simpler than designing the 3-bed+drawers combination piece I built for my kids.

Building them wouldn't be difficult, either. I only need 5, since they'll be king trusses.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Brad805]

Rod, if you enjoy that work I get it. I agree it is not difficult. If you look at the cost of plywood now, and the waste, I am not sure it makes economical sense. The common garage package you find at most hardware stores can be a good deal, but some cheap out with their gable end trusses. I still prefer traditional ladder framing, but I have seen many do not want different truss types for the little packages.

 

[BridgeSmith]

Brad, my design requires no sheathing, so I'll be saving quite a bit there.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[phamENG]

Rod - if you're doing OSB/plywood plates, that's a whole different animal and I get it. That's not all that difficult and would be pretty fun. (I haven't done a meaningful wood working project since I built a standing cradle for my nephew. He starts middle school next year...) It's the press plates that make the design a headache without the proprietary software.

 

[RontheRedneck]

3DDave - You said "I think "sketchy" referred to the lack of a uniform process for this."

Nothing sketchy about it. I explained the process clearly, and it's pretty universal.

If you don't like the process, feel free to try to change it. But I suspect it would be like trying to turn the Queen Mary with an oar.

JStructsteel - You said "How would the engineer know its wrong if he or she never sees the plans and just stamps the output?"

I'm not talking about the engineer at Alpine - I'm talking about the EOR. If the contractor, architect, and EOR aren't even going to look at shop drawings, what's the point in sending them?

That's why I said I have little faith in the review system.