Residential Truss Engineering Logistics

[kmart30]

Typically when designing residential new construction (north and central Florida)I am given the architectural plans and the truss shop drawings. I design all the shear walls, headers, posts, foundations, etc. etc. I take the loads from the truss shop drawings and take care of any large point loads, design my uplift connectors, gable bracing, and anything else that needs to get taken care of. If nothing pops out at me and everything checks out I approve the shops and submit these with my structural plans. Every now and then I will catch a truss or girder floating out in space or the truss engineer using the wrong wind speed/exposure. I will bring this up and it gets taken care of.

Recently I had a project in South Florida where when I asked who had the truss shop drawings they looked at me like I had 5 heads. They told me that I was to do the truss layout and then they would submit my plans and calcs to the city for plan review/permitting. To me this sounds arse backwards especially with 180mph wind speeds and a complex residential roof layout. They told me the trusses get designed after they get a permit to build. Yes I could put together a preliminary layout and get a general idea of the loads but why would I when I know the truss engineer is the one who produces the final truss layout and design? I told them no, they got all pissy but did get me truss shops and everything went smoothly.

I called around to some colleagues and even some other truss companies I have worked with and received a mix amount of answers. It seems to me that it might be based on the area you are designing in and how the contractors/municipality works. To me doing a preliminary layout and basing your design on it asks for issues down the road and things that might get missed once the final truss engineering is complete. Any feedback on what is typically done in other areas would be much appreciated.

 

[JAE]

In our area (midwest USA) we typically lay out the general framing of the trusses way before any shop drawings are produced.

This makes sense to me because YOU are the engineer of record and are responsible for the overall structural layout, framing, system, etc.

The pre-manufactured trusses are a delegated design from you to the truss manufacturer.

Relying on truss designers to initiate shop drawings where I have no control over the wind speed, exposure, direction of framing, etc. just lends itself to problems later when you try to have the truss designer correct something design-wise or revise anything.

I've never had truss shops presented to me prior to my design set going out.

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[KootK]

I was a truss guy from 1994 to 2000 and have been EOR since. I've practiced in western Canada and the Midwestern US. I've not once heard of it being done your way. It sounds pretty great from the EOR perspective though. Better info at design time and the truss guys get stuck with rework where any is needed rather than the other way around.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

I also spent some time with the Wood Truss Council of America so I've got a pretty good feel for regional differences in the US. There are certain parts of the country where truss design is done uncommonly well. Usually that's due to extreme load requirement regulation (wind/EQ) and/or prefabrication happening at a very large scale (whole neighborhoods at a time). Florida is one of the regions where truss design is done well. That may be part of the reason that they're able to take the lead without generating a bunch of problems. You'd be nuts to try it in Montana or Alberta.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[WARose]

In what you are describing the architect must be sending the layout to a truss manufacturer as well. Seems to me like this procedure would result in wasted time by them because there would be some re-work on their end based on support changes, layout, and load info.

I've not seen it done that way (I'm in the southeast by the way).....but I haven't done much residential either.

 

[kmart30]

Ok sounds like Florida is different than most other areas. I do 80% residential and it's always been like this in my area. All the truss guys need are a set of elevations, a floor plan, and a roof plan and they can pump out a layout and shops very quickly. When I get into more higher end custom homes with really cut up roofs and other challenges then we coordinate a bit more during thier design process but most of the time they take the ball and run with it.

I do give them wind speeds, exposures, and other wind data up front most of the time and I do also get a fair amount of jobs were it's done and they get all the project design criteria right without any guidance. I guess if I was forced into doing it the other way I could see where it would give me more control over the design but I don't need all that control for the residential work I do unless there are problems.

 

[Ron]

kmart30....
Your process is backward. I practice in Florida as well.

If you are the engineer of record, YOU decide the loadings.....not the architect and not the truss designer.

It's ok for the architect to provide a preliminary truss layout for dimensional continuity; however, you are responsible for all structural decisions.

Read Florida Statute Chapter 471 and Florida Administrative Code (FAC) 61G Rules of The Board for how delegated engineering is to be done. You delegate to them.... not the other way around!

 

[XR250]

On simple roofs, I just call out "pre-engineered trusses by others" Unless there are some unconventional loadings or exposures, everyone around here designs for 20/10/10. If it is a hip roof or other roof that will require girder trusses (or drag trusses), I usually show the location of the girder trusses so I can take the point loads into account. I will also schematically, show arrows for the truss span direction and extent.

 

[kmart30]

Ok, I understand its all my responsibility but most of the residential I do is simple 1 to 2 stories square box with not very large spans. Also, when dealing with a contractor he most of the time already has truss engineering from his favorite truss supplier and gives me everything else to design. I still check all their loading, bracing, and all the other elements like any other engineer would review but how am I suppose to delegate when its already done right when its given to me?

I think KootK hit it on the nail, for most of the simple residential I do the truss guys in my area get it right most of the time which makes for less back and forward and a quicker project timeline.

 

[JAE]

As long as you have success with them responding to your corrections, etc. then your process can work. The problem comes when you, as the EOR, specify a certain loading requirement, etc. and then you find that the truss guy had it wrong but refuses to change it - or worse, the trusses are already fabricated and/or on site. Then, as Ron stated, the "you delegate to them...not the other way around" process is turned on its head.

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[dik]

As with Ron and JAE... never heard of it done the manner described. For gable or hip type roofs and variants, I generally locate 'rough' truss layout including any girder trusses, etc. This establishes locations of point loads to be carried downwards.

Dik

 

[kmart30]

"The right way"

Step 1: Architect/owner/contractor hires you and gives you the plans to do the structural engineering.
Step 2: You do all the engineering and provide structural plans including a truss layout and all your other standard sheets.
Step 3: You then finalize the design and give the owner/architect/contractor all your plans, calculations, invoice, ect.

When do the final shop drawings get reviewed and approved/disapproved by the EOR in this process? Is it after step 3, during, or before? For most of you this might be elementary but I am trying to get some perspective(constructive criticism) from other engineers in different areas. If anything maybe there are some changes I can make with my clients and/or the truss designers to make the process easier or more efficient.

 

[00Z]

kmart30, I am in California and I often will require the truss calc's prior to starting my design. I know several other engineers in my area who do the same. I have done it both ways but it definitely saves me time to have the trusses prior to starting my design and I prefer having the truss calc's. I have not had to much trouble with the truss company using the wrong loading, however I have many times assumed a girder truss at a hip roof will be placed 6 feet back only to get the truss calc's and find that the girder truss is actually set back 8 feet instead or visa=versa. Another typical issue is have a girder truss with interior bearing points. If I don't have the truss package I am left to model that girder truss as a multi-span beam and design for the reactions accordingly and then I have to go back and verify the loading. If you can get the trusses ahead of time, I would definitely go that route.

The one residential exception is apartment projects. I often find that the building department will accept the trusses as a deffered submittal on this type of project and that the framer actually contracts directly with the truss co. In this case I do prepare my design and develop a truss layout. I then have to go back and review and approve the truss submittal.

 

[JAE]

I once investigated a set of trusses that collapsed in a snowstorm where the ground snow was at 40 psf and the trusses that failed were designed for only 20 psf reducible live load.

When I researched into why this was so - the truss manufacturer stated that they just ordered trusses per the owner's sketch and provided them - that it wasn't their job to verify loading, etc.
The owner wasn't a design engineer and didn't have a clue about snow loading criteria.

So I'm very suspect about truss manufacturer's providing accurate load/snow/wind/seismic input into their little canned programs and then some guy from a distant land signs and seals them.
I suspect in this collapse case the guy running the truss software wasn't well versed in engineering.




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[00Z]

JAE, that is a valid concern. The EOR needs to review the trusses regardless of whether the trusses are prepared before or after the engineers design. I did not mean to imply that loading errors do not occur.

 

[ACE58]

I've been doing wood framed commercial buildings here in Ohio for number of years - mostly nursing homes, doctor's offices, etc. My structural drawings include roof truss framing plans showing my interpretation of the best way to frame the building, with girder trusses & concentrated loads located. I also include all of the design parameters & "truss profiles". These profiles are basically elevations of the different trusses reqd. The trusses are shown as an outline & locate where the trusses may bear (load bearing walls, truss girders, etc.) & where ductwork may need to run through the web area. Typically, the construction documents are sent for State plan review before I have received the detail sheets from the truss designer/fabricator (registered in the state of the project). The Plans Examiner writes this down as an item to submit before final approval will be awarded. The truss designer sends me his drawings & I review them for design parameters, lengths, etc. & note any corrections. He makes any reqd. changes & resubmits his drawings to me (with an engineer's seal), which are then forwarded to the Plans Examiner & we are awarded full approval. When I started laying out truss framing plans as an EIT (many moons ago), the truss designer would most times change the framing to be more economical & I would have to relocate some bearing points, etc. I've now gotten so familiar with truss framing that the designer rarely changes my plans (sometimes he even compliments me on the way I frame a difficult area to make it simpler). We have a very open dialog & call each other from time to time to ask questions about framing options & projects that aren't common to the both of us. The same holds true for light gage metal truss framing.

 

[kmart30]

OOZ, thank you for the feedback and explaining your process and exceptions.

JAE, that is something I have seen too. Not to the point of collapse but to were the loadings got missed because the contractor/owner was the one who ordered and paid for the trusses. Trusses were installed already and the owner wanted me to just "stamp it" for a permit. I said have a nice day and good luck finding a stamp for this one. People have no clue sometimes; this only happened once but still a huge mess up.

ACE58, you made a good point about when you were an EIT and how the truss guys would change your layouts for a more economical design. Like you said, you started to get better at your layouts over the years but why would I base my final design on my own interpretation or assumptions of how it needs to be laid out? Even if I do base my design on my own assumed layout how do I know a plan reviewer, contractor, inspector or whoever is going to catch a potential discrepancy from my assumptions and the final truss drawings? Obviously coordination and communication are the key but that doesn't always happen in the extremely fast paced residential market.

My process for this whole truss situation is that I will not finalize my drawings/calcs and sign and seal until I have the final truss shop drawings. We are the ones who are ultimately responsible for the structure right Even if its not my fault that something got missed I would be the first one the attorney calls. I would rather be the last one to review the final truss shop drawings instead of a plan reviewer, inspector, contractor, ect.