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.

 

[Sparweb]

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.

but then again, they don't understand these special conditions either and just rubber stamp many times as approved.

The stamping engineer is reviewing the individual trusses designs spit out by the software which we all know is meaningless

if the SSE doesn't have the structural plans (and hopefully the architectural plans) while reviewing the truss calcs, I don't think they're quite doing their job in full.

Wow
So, in other words "If I take my blinders off, I might not get paid".
I see...
I don't work in the civil/structural sector, so I'm an outsider and this looks like a load of dirty laundry to me.
Thanks for bringing up the subject Redneck. You have nothing to defend.

 

[BridgeSmith]

I'm particularly annoyed by the pompous ass who said I'm not qualified to design trusses. I know more abut truss design than most engineers do.

Apparently, you're referring to me. However, you need to read my post more carefully. I did not say you weren't qualified to "design" trusses; I said you weren't qualified to "engineer" trusses, and there is a difference. I don't doubt that you know more about truss "design" than many engineers, including myself. All I was saying is that simply by virtue of not having a PE license, you are not legally allowed to practice engineering.

If you can apply the loads to a truss and calculate the force on each and every member of the truss, with just your brain and a calculator, then you may have the skills to do engineering, but unfortunately, that would still not make you legally qualified to engineer anything.

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

 

[RontheRedneck]

Aesur - You mentioned lumber grades and hold-downs.

We often get plans calling for DF lumber. But it's not readily available in our area.
No one has ever told me that lumber grades are a problem regarding hold-downs.

 

[Brad805]

My question about training was aimed more about the software, not your specific practice. Most are reading this thread as a discussion to clarify the truss design/production practice writ large. We looked into using the Mitek 3D modelling software years ago to simplify the process of tracking all the loads to the foundation. At that time there was a lot of training required before you were able to use their software. I think this is germane to understand what goes into creating a truss designer. I still encounter many that have difficulty reading drawings, and I can see many lumber companies with hiring practices that do not attract ideal candidates. Again, that is not directed at you.

I mentioned cost conscious. We all understand why. I rarely do residential work, but use wood trusses in commercial work. The wood truss guys take it to the next level and it always causes problems. Now much of that is on the GC that does not look at the quote, nor asks questions. My steel suppliers quote us exactly what is drawn.

"...We're selling components, not an engineered system.."
This one comment highlights the fundamental problem in the wood truss world. The wood truss packages we see are very complicated and the guidance for truss design about their weak axis is limited. In all my 30years in the business I have never been provided a sealed layout plan for wood trusses. Every open web steel joist drawing package comes with sealed calcs, plans, load specs, decking plans.... Why the wood truss guys see their role as less is confusing.

 

[JStructsteel]

Ron, I think our issue is a non engineer is taking the engineered drawings, entering the design into the software (Which you may do correctly) and then an engineer stamping it based on what you entered and not looking at the engineered drawings. I think that is what this all boils down to.

In the odd case that you do make a mistake, and the engineer has stamped it, I dont see how he or she is not liable.

 

[Aesur]

@RontheRedneck - Take a look at the Simpson DSC capacities for example: Link, you will notice that for SPF/HF the capacity is approx. 15% less than that for DF/SP. This is common on all straps, holdowns, h clips, a clips, etc. This is based on testing, however a comparison of SG of the wood can give a picture of capacities, sim to designs using the NDS. DF has a SG of 0.5, lower SG's topically yield less capacities. Because of the competitiveness in the truss industry, this is something IMO the engineer of record should consider when designing their straps, however most (that I have seen at least) use the largest capacities which is for DF/SP. I'm honestly not sure of a good way to communicate this to the EOR's as it is a commonly overlooked item.

I don't know of the wood availability for trusses in my region, but wonder if it's a sourcing issue as well here. I assumed it was a cost savings issue as 90% of wood stud walls, etc.. are built with DF here, the occasional client wants to use HF to "save a few dollars" on the wood, but at the expense of adding more studs in some walls - I'm not sure this really saves them anything.

 

[JStructsteel]

Aesur, I am not sure why in your wood spec the wood species/values are not called out. If a supplier of a component cannot meet the spec, either they have to pass on supplying said component, or let the EOR know, and they evaluate with the client if a redesign is required.

This again does fall thru the cracks, because as Ron said, the low man wins the job.

I always ask for truss calcs so I can review the designs, make sure they are designed to spec and something like this does not happen. Also I like to review reactions, incase there has been some value engineering done to make the truss packet cheaper/easier to build by maybe moving things around.

 

[phamENG]

In my area, truss suppliers will spec wood to wood hurricane tie connections, so I let them and check behind to make sure I agree. Since trusses are used for economy, I rarely spec a species. I go with a "whatever works" spec. Granted, I've never had to deal with SPF in a truss...all the truss manufacturers seem to prefer SP #3 to SPF. I'm guessing it's because the truss plate capacities drop when they drop to a lower SG lumber as well. (We don't have DF or HF here.)

 

[Aesur]

JStructsteel - We do specify the wood species/values on our drawings and our specs match every other structural set of plans I have seen in this region for species, etc. The problem is, no local truss manufacturer follows these requirements and other structural engineers just ignore this, hence why we have started just adapting internally to use the lower values for connections knowing that they will most likely use weaker lumber.

Luckily for us, the jurisdictions on the west coast tend to require sealed calculations for all deferred items; therefore we get an opportunity to review the calculations on every project. Many years ago (earlier in my career) I made a comment that we specify DF and the trusses needed meet minimum properties and the response was essentially that they provide the cheapest truss possible without concern for "our connections" and when I talked to my project manager I was told to just ignore it, they know about it, but the standard of care has been set and they can't just start demanding that they use the right lumber or design heavier straps, so they just ignore it. That never set well with me, hence why I design connections for the proper values knowing they will use the weaker lumber.

The truss reactions is one that annoys me, they sometimes use "non-structural" walls as bearing walls creating a short span - long span condition creating large uplifts at the end to get a cheaper truss. (this is similar to PEMB's pushing the cost of the building into the foundation to make their product look cheaper) I have had to reject way too many trusses for this reason. I also try to watch for trusses that are sitting on top of a parallel shear wall as I have seen many designs where the reactions are basically saying uplift, pressure, uplift, pressure on 1' increments with values so large that there doesn't exist a connection to resist these forces. Because of this, I typically design the wall to have a shear transfer with vertical slip to avoid this and rarely do they pickup on that as it requires looking into details (not saying all truss designers do this). Another one that is the contractors fault that annoys me is how they bear trusses on non-structural walls rather than providing the slip connection per the details.

Another local truss annoyance is all local engineers specify the truss to truss connections to be by the truss manufacturer and I would say 90% of the time they refuse to provide these connections until much back and forth. It is also common for if the truss is hanging from a beam or ledger that it is specified that the truss hanger is by the manufacturer, and more often than not, they refuse to design those as well.

 

[kissymoose]

Hey Aesur, check it out:

https://www.strongtie.com/products/go/fasteners/sdpw-deflector-screw

Maybe since it's easier to install it'll happen.

 

[Aesur]

kissymoose, thanks for sending that, I'll give it a try with some contractors and see how it goes.

 

[RontheRedneck]

Brad805 - You said your question about training was more about software than. But all the training has to go hand in hand.

We can have a guy sit and watch one of us work for a day or two, and explain what we're doing. But pretty soon you have to get the guy in front of the computer so he can find the icons for himself.

Along with the software we have to teach them to interpret the crap we get from customers. Many of our jobs have no plans, or maybe have terrible "plans". This is a good example of what we sometimes get:

So they have to learn to read "real" plans as well as crappy sketches. And crappy plans by someone who has a CAD program and thinks they know what they're doing.

They have to learn to ask the right questions when customers call in. Like if that ask for a 24' truss cantilevered 8', does that mean the building is 24' wide and the truss BC is 32' long? Or that the building is 16' wide and the BC is 24?

I always tell them there are a half millions things to learn, and they have to remember them all every time.



Aesur - I understand the nail holding capacity of lumber varies with species. I only said it has never been a problem on a job I was involved in.

You also mentioned truss companies not designing hangers for truss-to-truss connections. They're required to do that. I'll try to document that if you'll give me a day or so to find the info.

If you have a truss manufacturer using non-bearing walls as bearing that's a serious issue. I would never stand for that in anything I was involved in.


Jumping back to the sketches and what passes for "plans" for a minute - Sometimes we're the only design professional on a job. Which means we have to be the bad guy and enforce some of the things that you guys also do.

I'm not going to debate the issue of engineers vs. truss designers any more. My goal was to let you guys know what actually happens and dispel some misconceptions. I think I've done that.

 

[DayRooster]

Hol’ up…someone submitted a plan on a piece of cardboard…without any dimensions…or notes on it. This can’t be real. If this is all you need to do then I’m doing like 1000% too much.

 

[phamENG]

DayRooster - if you look reallllly close, you'll see somebody did scribble some dims and notes on. Maybe with a pencil? But yeah. That's kinda bananas. I've heard tell of the rather 'loose' rules that exist in the rural mid-west...but I hadn't actually seen proof of it until now. I'm sure there are places not far from me where similar shenanigans go down, but it's no less shocking.

Granted...I did have a shop drawing submittal from a fabricator (railings, I think?) on a piece of notebook paper ripped from a spiral notebook, drawn in a way that would leave my 5 year old plenty to critique, and emailed as a picture taken on the guy's phone. So...it happens.

 

[RontheRedneck]

DayRooster - Yes, it is very real. That's the kind of thing I deal with on a daily basis.

I have a customer who draws "plans" on graph paper. Each square = 2'. I input the walls, then put roof planes on it and send him 3D images. That's what he uses for "elevations".

People used to tear pages out of plan books and build from them. Now they go to house plans websites and email me a PDF to use as a "plan". If he doesn't like how it looks I do a 2nd version. And a 3rd. I think the most I've done is 8.


I had a framer call me once to ask me to look at a truss job and see if any of the trusses could be used. A guy had taken one of those pages from a house plan book to a big box store and asked for a quote on the trusses. They sent it to our main office (not the one I worked at) and they sent a quote back.

Of course the quote was just a half-assed guess, since those plans do not have exterior dimensions on them.

The guy decides to build the house. So he goes to the BB store and tells them to order the trusses. The guy at the BB store calls our main office, gives them the quote#, and tells them it's an order.

So when the framer called me he gave me the dimensions of the actual foundation that was already in the ground. Turns out all he could use was the garage trusses. The other $5,000 or so worth of trusses were junk.

 

[JStructsteel]

Glad to see that the truss industry allows everyone to take advantage of them also Shitty drawings given to them, rework, etc). Engineers, etc are our own worst enemy when it comes to scope and rework, etc.

 

[gte447f]

I am a registered PE and SE, and I agree with most of the frustrations being stated in this thread by other engineers about the sketchy nature of the delegated design process for metal plate connected wood trusses, but I think many of the posters here are barking up the wrong tree with RontheRedneck. Ron and his cohorts are not subverting or otherwise corrupting the delegated design process; he is simply working on projects that have no engineering or other professional design requirements. He states in the OP that 95% of his truss designs do not require an engineer's stamp. I interpret that to mean that 95% of the truss designs are for single-family residential buildings under the International Residential Code or maybe agricultural buildings in rural areas. Guys and gals... news flash... houses in the U.S. are not designed by either architects or engineers, they are simply built by tradesmen who are working for a "contractor" with a pick-up truck and a cell phone.

 

[phamENG]

gte447f is spot on...which is why I'm always terrified one of those "contractors" with a pick-up truck and a cell phone will bid on the custom houses I work on...

 

[Brad805]

gte, the intent of the discussion seems to be to dispel misconceptions about the truss industry as a whole. I doubt anyone in the thread is unaware that endless projects are designed/constructed without the assistance of engineers. I have no problem if they design the trusses for those projects using prescriptive code clauses. If we are talking solely about his practice, frankly, I am not overly interested. I do get interested in the process when those same designers submit designs on our projects.

Ron, while the cardboard drawing is unique, a common joke among engineers is napkin sketches. We all deal with that when clients start the project.

I am curious to see what Simpson Strong Tie does to the industry as more and more of their truss plates are used. We all talk to SST reps, and I hope they listen to some of the concerns raised. I asked a SST rep recently and it seemed they may be working on some software to help the EOR with preliminary calcs for trusses. I would gladly spec exactly which plates to use if they will provide more confidence.

Ron, I applaud your effort here. I think it has been an enlightening peak behind the curtains into the truss design process. You clearly care, and take your job seriously. I suspect many of us with concerns are encountering suppliers that care more about pushing product out the door. That happens in all construction industries, but in my experience, the wood world has a terrible residential stink that impacts a lot of decision making.

 

[Celt83]

Ron,
Thank you for the informative post and continued discussion. Great to have you here.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)