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.

 

[RobertArnold]

Hi raligt,
In the United States, you are supposed to stamp (engineering stamp) and sign work that you are responsible for, and that has be performed under your direct supervision. We are not responsible for the truss calculations, therefore we will apply a review stamp rather than an engineering stamp. Also, as stated by a local truss technician, what we consider a truss calculation package is not the truss calculations. Instead, it is the truss drawings with notes about the applied loads and the summary of the results of the calculations, but not the calculations themselves. For instance, many times I will see a footnote for the applied drag load. The only potential way for me to confirm that the drag load has actually been applied would be to look at the summation of the horizontal reactions--it does not appear any other place in the output.

Struct1206--If you know which framer will be working on the project, you could contact them to get the truss company information.
--Robert

 

[KootK]

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

I believe it. Somehow, post Covid, I'm more of a call returner than a call picker upper myself.

What is the nature of the guidance that you seek? For basic stuff, there is some literature out there. And a few people here that are familiar with that space.

 

[Struct1206]

What is the nature of the guidance that you seek?

I've recently been doing more and more work with flat floor trusses and minimally sloped roof trusses. I can pick the typical stuff off of a chart or table but when I have something outside of the norm, I'd like some assurance that what I'm asking for is reasonable/feasible. For example, typical floor trusses or roof trusses with point loads. I currently have a project with a low sloped roof with an overhang on one side where I need to keep the depth to a minimum. I'd like some assurance that it can be designed and manufactured (efficiently?) at whatever depth I specify.

 

[AaronMcD]

We are in the process of building a house with extremely high uplift wind loads. This is in California and wind actually governs design of a one story house in E-W direction.

In a bid to save money, we switched to manufactured trusses from a custom exposed rafter system. We calculated all wind loads, located every truss (needed to align with studs and exterior rafters which are annoyingly 6'-9" O.C.). We provided diagrams and tables of wind loads at ridge, at each side, over the whole thing, load factors, minimum dead loads, maximum dead loads, live loads, collector and chord loads.

Their first pass had trusses that were not the correct length, used the wrong chord sizes as noted on plan to fit truss ties and top plate locations. Their loads were indecipherable and they appeared to use their own ridiculously tiny wind loads, ignoring ours. After a thorough (and very long) review process, we sent it back. They came back with almost correct lengths but some were 1/4" long and some were 1/8" short. They used our loads, incorrectly but conservative, as well as their own loads. It was impossible to decipher their load combinations, and I have the feeling they don't know what load combinations they used. But they looked nice and overdesigned so we left it at that.

No one has any business designing trusses if they can't input loads as specified, don't understand load combinations, factors, seismic and wind loading, etc.

http://www.hellodwell.design

 

[RobertArnold]

Hi Aaron, The same people that are attempting to interpret the engineering specifications for that single family residence are the same people who work on "projects requiring an engineer". If an engineer stamps the truss calcs, it is supposed to not only say that the computer did the calculations correctly, but the engineering stamp is supposed to say that the loading is correct, the load combinations are correct, and the truss has been designed in accordance with the applicable codes and engineering standards. Many truss companies will send the "truss calcs" you saw to their engineer with no other information for the calcs to get stamped and signed. How is the engineer supposed to verify the loads and load combinations without any additional information? Per TPI 1-2014, "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." This means that the truss designer, in these cases, needs to have an engineering license, and should be able to read the engineering specifications provided.

 

[driftLimiter]

My question to you Robert would be what the truss design professional even reviewing when he stamps it then? I'm sure he isn't checking the computer generated design calcs each time and stamping once they are convinced that the computer is correct, that is pretty much a given with the software these days, that the calcs are right (but based on user inputs).

I would think the only the the Truss engineer would really need to confirm is the user inputs, and if they don't do that then it just seems like they are a rubber stamp on the truss software.

Another design professional should be able to determine things like wind, seismic parameters, and snow loading given only the site location and risk category. It seems like if it were me doing these all the time I would come up with an automated system to generate the loadings that the 'should' be on the truss given this info and when reviewing the truss calcs if I see something starkly different then start asking questions.

 

[RobertArnold]

Per TPI 1-2014, "The Building Designer shall review the Truss Submittal Package for compatibility with the Building design." and "have been found to be in general conformance with the design of the Building." I will spot check some of the loading, check drag loads, check unusually framed areas, check for 3-point bearing, etc. I do not try to check every load, nor do I check load combinations, etc., unless I have a special reason to. (And as AaronMcD stated, "Their loads were indecipherable, and " It was impossible to decipher their load combinations.") Similarly, on a set of post-tensioned slab plans, I will check the plans for the loading notes and required hardware, but I hardly ever get a copy of the calcs to look at. Even if I do, I usually don't look at the calcs. That is the other engineer's responsibility.

 

[RontheRedneck]

Struct1206 -

Sorry I missed your post back in late June. Haven't checked in for a while.

I wouldn't mind looking at whatever you're working on. But if you're not in Illinois I may not be of much help.

I don't know a lot about hurricane design in Florida. Or seismic design in California. The company I work for may not have the same types of lumber that a manufacturer in a different part of the country might have.

So my suggestion would be to push for someone local that will help. Which I see in a post further down it sound like you already did.

If you're not getting answers by calling places, do you have a contractor on the job already? You might get better response if a customer of the truss place called and asked for assistance.

Or if they're close by, show up at their door. It's harder to ignore someone when they're standing in front of you.


If you want to send me something my email address is rsitruss at yahoo dot com. I don't mind checking it out, but can't guarantee that I can help.

 

[NateMT]

@Struct1206
We usually specify a minimum depth of 14" on flat/low slope truss profiles.