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.

 

[RontheRedneck]

Earlier in the discussion there was some mention of truss design responsibilities. If you're having insomnia, here's a rather lengthy explanation about it.

https://static1.squarespace.com/sta...57115397817/ANSI_TPI+1-2014StdONLY-WEB_WP.pdf

 

[BridgeSmith]

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

Yes, you explained it in good detail, and we understand it's fairly universally done that way in your industry. The reservations about the lack of (legally) qualified review of the trusses as a critical component of the overall structural system, remains. That's what makes the process sketchy.

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

 

[RobertArnold]

Hi,
1) Generally, the engineer is responsible for his own work, which includes selecting the appropriate loads. How is the truss engineer selecting and/or verifying the loads?
2) TPI-1 section 2.3.5.1 says, "The Truss Designer is responsible for the preparation of the Truss Design Drawings based on the Truss design criteria and requirements set forth in the Construction Documents or as otherwise set forth in writing by the Building Designer as supplied to the Truss Designer by the Contractor through the Truss Manufacturer." I.E. The truss designer is the one responsible for the design. Also per the TPI-1 definitions, "Truss Designer: Person responsible for the preparation of the Truss Design Drawings"
3) Section 2.3.5.3 says, "Where the Legal Requirements mandate a Registered Design Profession for buildings, each individual Truss Design Drawing shall bear the seal and signature of the Truss Designer." I.E. The truss designer has to apply his/her engineering stamp, therefore the truss designer must be an engineer in this case.
Therefore, when an engineer (Registered Design Professional) is required, the truss engineer is responsible to prepare the truss design and drawings based upon the criteria set forth in the construction documents. This does not say that the truss engineer is sign off on a non-engineer's design, without ever seeing the design criteria and requirement as set forth in the Construction Documents.

Also, as a tract home structural designer (PE), I do not draw each truss on my framing plans, nor do I specify the truss callout on my drawings. When I receive a truss package, someone has designed a truss system and has designed the individual trusses to go with the truss system. I have had many times where there are differences in truss directions from my framing plans and the truss layout drawings. Sometimes these revisions are fine, and sometimes I have to have them changed to match what I specified. The point that I am making, is regardless of any comments relating to designing individual trusses only, someone has designed the truss system. Also, if there is a girder truss, supporting another girder truss, supporting individual trusses, I have not personally specified the reactions of the individual trusses on the girder truss, nor the reaction of the first girder truss on the second girder truss. This has been done within the truss design process. And to continue on, any truss to truss hangers are specified in the truss package. Who has coordinated the loads and hangers within the truss system? Has that been an engineer?

As a bit of history, in the good old days, the building designer would subcontract the design of a couple field-built trusses to an engineer who specialized in truss design. There was communication between the building designer and the truss engineer about the loads on that individual truss. Now days, the entire building (house, multi-family, commercial) is roofed with a truss system. The truss manufacturer designs this system based upon the construction documents, not based on individual truss criteria. The issue seems to stem from the attitude of the good-old days single truss design, but the modern hope of supplying a truss package.

Also, easy trusses are easy. Hard to design trusses are not easy... A real life situation that I had was an emergency exit path going over the roof (with guardrails). How is the EOR supposed to explain to a non-engineering "truss designer" the difference between roof live loads, exit live loads, load duration factors, load combinations, and (being that there was a penthouse for the exit stairway supported by the roof) magnified earthquake loads and the related load combinations? I had one truss company walk away from the job because I would not accept their lack-of-proper-design.

 

[Greenalleycat]

Sounds like truss designs work similar overseas to here....

The only comment I'd make is that I have seen a LOT of crap truss designs done in even 3 years working in residential
Surprised more stuff doesn't fall over tbh

 

[RobertArnold]

Sorry, I get caught up in my reasoning that I forget my point. Going back to the original post, what RontheRedneck says about truss industry is what I see happening. Multiple times I have been directed to TPI-1 to justify some of the truss industry practices. TPI-1 2014 has slightly revised wording compared to the previous version, which makes it more clear that the truss designer is the truss engineer. (Of course, I am recognizing that the code does not require engineering all the time, and when no engineering is required, then anyone can be a truss designer.) This morning I was looking at some trusses that were emailed to me by someone with the title of "senior truss designer", who, like RontheRedneck is not an engineer, but is working on trusses for a commercial/educational facility. In this type of condition, it is the truss engineer's responsibility to determine the proper loading on the trusses, based on the entirety of the "construction documents", not just on the limited information provided on the structural plans (i.e. weights from mechanical plans, potentially information from the calculations, etc.). The current practice of the truss industry does not appear to follow their own guidelines (TPI-1). When presented with the requirements for the truss designer to be an engineer (or even just saying that you need to talk to their engineer), the typical truss industry response will be, "That's not how it's done." Personally I believe that a lot of truss manufacturers need to reconsider how the truss industry works, and make the appropriate actions to be in compliance with TPI-1. (FYI, for anyone who doesn't know, TPI-1 is the Truss Plate Institute document #1, which is often adopted to be part of the building code.)

RontheRedneck, thanks for what you do. As mentioned, an engineer is not always required. I have called upon the help of a non-engineer truss designer many times, and have lots of respect for their skill and knowledge. Also, I have noticed that the ones who give the best help often work with or have worked with a truss engineer in their office. With the combination of experience and knowledge of what an engineer is looking for, you get the best trusses available.

 

[dik]

I've always provided truss layouts, including support points and girder trusses... If I have a concern about how they will frame it, I check with my 'wood guy'... Vic's been doing this for decades.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[RontheRedneck]

RobertArnold - Not sure I understand your point.

Please keep in mind that truss designers are no different than any other group of humans. Some are dedicated to their work. Others are a complete waste of perfectly good oxygen. And everything in between.

Also please keep in mind that the engineering/design of trusses is only a small part of our jobs.

The hardest part is usually figuring out what people actually want. Second is keeping costs down. Then meeting deadlines. Making things simple enough for the plant guys to understand. Making layouts that are easy to frame. And other things I can't think of at the moment.

 

[RobertArnold]

Hi RontheRedneck,
I have three points. Point #1) There are some very good truss designers out there, who, I as the engineer, call upon for advise and recommendations. I highly suspect that you fall into that category, I would enjoy working with you. Point #2) When an truss engineer is involved in a project, he or she is responsible for the design of the trusses, including determining the loading to the truss. Any time an engineer stamps and signs a calculation and/or drawing, that engineer needs to verify the loading and the design, even if engineering is not required in that case. Also when a project requires engineering, per TPI-1 the engineer is the truss designer, in which case the engineer must have all the required information to design that truss, not just verify the calculation is correct. Point #3) Regardless of the generic statement that truss manufacturer is "designing individual trusses only", the truss manufacturers are actually designing the truss system. They are assigning loads from one truss to the other--it's not just individual trusses. This needs to be recognized and dealt with.

 

[RontheRedneck]

RobertArnold -

I respect your opinions, but disagree on a couple of points.

First is what you said about engineers designing trusses. As I said early on in this thread, for the most part engineers do not design trusses. They only seal them if requested.

What you said about trusses being a system could be argued endlessly, and I doubt anyone would change their mind. No point beating that dead horse any more.

The status quo is pretty well entrenched. Any attempt to change it would be a daunting challenge. Then it would take a lot of training and software upgrades to be able to do it, so it would not happen overnight.

Since there does not appear to be an organized effort to force change, I expect things will stay as they are for the foreseeable future.

 

[NateMT]

Hi all,

I'm wondering the (successful) methods of reporting your lateral loads applied to roof and floor trusses. Is it worth the hassle to indicate (Wind ASD) or (0.6W) etc. ? Has anyone had the pleasure of the truss designer heeding these, or is it easier to just write the value in 1.0W and tell them it's Wind.

Also curious about the application of omega... anyone know if they'll apply this if they know they've got a seismic load?.. :|

 

[RontheRedneck]

NateMT, I don't understand what exactly you're asking. If you can explain in a bit more detail I'll do my best to tell you what I know from the perspective of a truss designer.

Although we don't do seismic loading in Illinois, so I don't have experience with that.

 

[phamENG]

Nate, I've never seen a truss designer pay attention to the wind loads. Speed, exposure, etc.; yes. Their software figures out the wind loads and automatically applies them. Nobody is manually entering those.

Don't know about seismic - never had to worry about that on a truss before.

 

[RobertArnold]

Hi,
This is one of the issues that I have with the current method that the industry works. The EOR talks with engineering lingo, which is supposed to be used for the truss design. The person that is doing the truss design does not speak engineering lingo. The question currently asked is about factored or unfactored load--it's a basic and easy question--for those who have the experience, training, and the lingo. Now consider that I have had to specify Em (Magnified Earthquake loads) on a truss, and also exit path floor live loads on a truss. How much confidence should I have that they are being handled properly?

 

[phamENG]

How much confidence should I have that they are being handled properly?

That depends on your ability to review the profiles and associated calcs. They are a bit difficult to decipher, but the truss guys around here are usually happy to help and explain things if there are questions.

 

[jayrod12]

The EOR talks with engineering lingo, which is supposed to be used for the truss design. The person that is doing the truss design does not speak engineering lingo.

In my experience, this is not generally the case. Where I practice, many of the truss designers went to the college level structural technologist program, and therefore are more than proficient talking engineering lingo. They just usually don't because it's a bit pretentious and they're used to talking through things with contractor.

 

[RontheRedneck]

I got a screen cap from our truss design program. This gives you an idea what options are available to us. Just thought some of you might be interested.

I always try to use the settings called for on the plans. But I do sometimes miss something. Everyone seems to be in a huge rush for everything, and we're always pushed for time.

And honestly, I see it as largely a waste of time. Wind design mostly affects hold-downs. And they're rarely installed correctly if they get installed at all.

It also sometimes means more CLBs on the webs. Those are also not always installed, and rarely installed correctly.

I'm more concerned about making sure the correct trusses get to the jobsite. People don't always know what they want. Plans are not always drawn clearly. And customers often change their minds. So IMHO there are more important things to worry about.

 

[Struct1206]

RontheRedneck - You said way back in the conversation that you wish more people would call and get preliminary input on designs. I have had several projects recently (and one in the works) that I wanted some input on. Is there a good way to get in touch with you; or can you advise on where to find other truss designers that are capable of providing preliminary truss design assistance?

 

[phamENG]

Struct1206 - look back at past projects in your firm. Who did the trusses? That's who you need to call. The shops around here offer it as a "free" service - though there is at least a tacit understanding that you'll specify their company in your specs and/or drawings. Reach out to the truss designers and ask if it's something they provide.

Go through somebody with no shot at bidding the package, and you'll likely have to pay through the nose.

 

[Struct1206]

Thanks phamENG. I tried for a while today with minimal success. Believe it or not, nobody answers the phone anymore.

 

[raligt]

We always have to review the truss layouts and calculations here in Scotland as part of the SER scheme. Doesn't seem right to me to effectively rubber stamp truss drawings when you have not seen any calculations for them.