Metal Truss Designs

[JAE]

I was checking a set of plans from another firm and noted that they basically gave the applicable code, some overall load parameters (LL per code, ground snow = 30, wind - 90 mph, exp B, etc.) and then totally left the design of the roof (a metal truss system) up to the metal truss manufacturer. They included lateral design and their specs said something to the effect of: "Perform a complete lateral analysis of the roof, diagonal braces and/or diaphragms may be utilized".

This seems a bit over the top to me. A metal truss manufacturer designs and fabricates a single component of the structure. The design of the lateral force resisting system should be by the EOR, right? Or am I missing something here. Is it OK to delegate the design of the lateral forces through the diaphragm to the truss designer? As long as you review their calculations and approve them is it OK to do this? It just seems that the engineer was delegating away control of his/her design and was not really giving the truss mfr. enough info to design the lateral system.

 

[FalsePrecision]

It sounds like a "cop-out" to me. Is this firm a regular, responsible companey that you have dealt with before?
I am not sure if you are functionning as the official plans checker on this.
Perhaps your response would be to put a comment "EOR to provide complete lateral anlysis, including statement of any drag loads on roof trusses", and the metal truss mfr would provide callculation for drag truss load in their submital.
Sorry for all the deliberate typos... just trying to see if I can avoid all the annoying hyperrlnks <grins>

 

[Lutfi]

The following is direct quote from Florida’s Laws and Rules. It is permissible for the engineer of record (EOR) to delegate the responsibility of the design of certain pre-engineered components or systems. However, he is the sole responsibly entity because he is the EOR.
“61G15-31.001 General Responsibility.
The engineer of record for a structure is responsible for all structural aspects of the design of the structure including the design of all of the structure's systems and components. As noted herein the engineer of record for a structure may delegate responsibility for the design of a system or component part of the structure to a qualified delegated engineer. In either case the structural documents shall address, as a minimum, the items noted in the following subsections covering specific structural systems or components. Both the engineer of record for the structure and the delegated engineer, if utilized, shall comply with the requirements of the general responsibility rules, and with the requirements of the more specific structural responsibility rules contained herein.”

I always delegate the responsibility of designing pre-engineered metal and wood trusses but not the diaphragm. There are many truss suppliers who design them based on proprietary process and methodology. I must say that almost always, I have to reject submittal on first go round because due to lack of the truss fabricators understanding of wind loads. The second common issue that I see is they do not use the specified wood species in the design. Most of them use specialized software. It is my suspicion that the programs are run by technicians and the results are “rubber stamped” by their PE. The vendor software spits out programmed bottom chord and top chord loads and not necessarily those specified by the EOR. . It is scary promises. I say this because when I get calls back; it is hardly ever by the truss engineer. Most of the time it is a sales man and or drafter!!! Therefore, I say to all EORs, beware because it is you project and sole responsibility.

My two cents worth.



Lutfi

http://www.cdeco.com

 

[haynewp]

What about pre-engineered metal buildings? We have ended up the EOR on these before and all we did was a couple of footings. The stability of the entire building was up to the manufacturer. Isn't this saying that being the EOR I am delegating the entire building design to the metal building mfr (except for the footings)?

 

[JAE]

haynewp - I thought of that, but it seems to me that the difference is, at least in my own perception, that the metal building companies have engineering staffs that have been trained and/or developed software to actually do all the lateral designs....after all, its their own whole system that they are selling.

For the truss manufacturers, who are a difference set of companies, I just wonder how many of them have personnel who can get into the code, know what to do with a seismic R value, know how to design diaphragm deck to truss connections, how to follow load paths, etc.

 

[haynewp]

I agree that I would never do this myself as EOR, but if someone from the truss mfr is willing to seal the trusses and the diaphragm design I don't think it is a rules violation. Ethically I think the EOR should check that the diaphragm design was done properly knowing it was likely done by someone inexperienced at it, and I am sure he will be in as much trouble as the truss mfr if the diaphragm fails, rules or not.

For trusses I think with the mfr's seal plus it being the standard that a specialty item like this is designed by the mfr would hold a little more water (in terms of EOR liability)in court if a failure occurred. But it may be interpreted that the EOR is still responsible for the stability of that item as well, but what EOR is going through and verifying all the truss mfr's calculations for every truss (or steel joist) on a job?

 

[JedClampett]

I suspect that this is a case of an engineer being pressured to turn out a design for X dollars per sheet or building, when he needs two times X. One way to do this is to delegate the design to the contractors supplier, in this case the metal truss guy.
The funny thing is that the owner doesn't really save anything. Someone has to do the design. I guess the contractor might mess up his bid the first time he sees this and forget to put in an engineers cost. But they'll quickly learn. Plus hopefully the design engineer will have to review the design, which is another cost.

 

[Lutfi]

JedClampet,

Your point is right on targtet. Pay me now or pay me later!

Lutfi

http://www.cdeco.com

 

[jhall]

I agree, this is definitely a cop-out from the EOR. Who would be responsible for the design of the connection between the diaphragm and the lateral system (walls). The EOR should be responsible for the lateral transfer. And in the case listed above, wind may have been the controlling factor. Now what if seismic is the controlling factor, there would have to be a lot more information provided before the diaphragm could be designed. I understand having the truss mfr. design for gravity and wind forces acting on a truss, but the EOR record should be responsible for the rest of the structure.