Pre-Engineered Roof Trusses
Pre-Engineered Roof Trusses
(OP)
I have a project where the wood truss designer/manufacturer has a note on their submittal stating that the EOR of the building 'must review and verify the correct loads' are being used. This note specifically refers only to those load cases involving wind loads. Not the basic parameters such as wind speed and exposure, but the actual calculated design pressures.
In my opinion, as the EOR I tell them the basics (Design Code, Wind Speed, etc), but it is their responsibility to calculate the various pressures on individual members.
Am I wrong?
In my opinion, as the EOR I tell them the basics (Design Code, Wind Speed, etc), but it is their responsibility to calculate the various pressures on individual members.
Am I wrong?






RE: Pre-Engineered Roof Trusses
I would not agree to do this otherwise. Hopefully you have a provision in your contract for shop drawing review, which this would fall under... possibly as an extra service?
Mike McCann
MMC Engineering
RE: Pre-Engineered Roof Trusses
I think the wood truss guys annoy me more than the PEMB metal building guys.
RE: Pre-Engineered Roof Trusses
So, to answer your question, in my opinion you are wrong.
RE: Pre-Engineered Roof Trusses
1603.1.4 Wind design data. The following information related to wind loads shall be shown, regardless of whether wind loads govern the design of the lateral-force-resisting system of the building:
1. Basic wind speed (3-second gust), miles per hour (km/hr).
2. Wind importance factor, I, and occupancy category.
3. Wind exposure. Where more than one wind exposure is utilized, the wind exposure and applicable wind direction shall be indicated.
4. The applicable internal pressure coefficient.
[highlight #EDD400]5. Components and cladding. The design wind pressures in terms of psf (kN/m2) to be used for the design of exterior component and cladding materials not specifically designed by the registered design professional.
RE: Pre-Engineered Roof Trusses
RE: Pre-Engineered Roof Trusses
BA
RE: Pre-Engineered Roof Trusses
I agree, eyeball the loads which I have done, but it seems a bit much for them to claim no responsibility in calculating loads wind loads on their elements. Even PEMB people don't try that.
I have never seen drawings that call out component and cladding loads...ever.
RE: Pre-Engineered Roof Trusses
RE: Pre-Engineered Roof Trusses
RE: Pre-Engineered Roof Trusses
We do this for PEMB as well. This is delegated design and is your responsibility to provide the loading information. You are still the engineer of record, so by not showing this and then checking for compliance, you are basically abdicating your responsibility. If there is ever a problem, it will be you that is held liable if it is deemed that the loadings used were incorrect.
RE: Pre-Engineered Roof Trusses
RE: Pre-Engineered Roof Trusses
For components and cladding loads, in the past we've provided tables on the structural notes that give the c&c loads per area and per wind zone (so essentially a 5x5 matrix covering every case).
Brian C Potter, PE
http://simplesupports.wordpress.com
RE: Pre-Engineered Roof Trusses
Anyway, thanks all.
RE: Pre-Engineered Roof Trusses
The MWFRS loads are not required to be specified on the drawings down to the PSF pressures because the EOR is not typically going to be delegating this design to a Specialty Engineer. Note that #5 specifically qualifies the C&C requirement as "to be used for the design of exterior component and cladding materials not specifically designed by the registered design professional".
A simple table/matrix as has been described above will generally suffice, and is all that you are required to include in your drawings. I am surprised you state that you have never seen drawings that include C&C pressures. Because it is a code requirement for delegated design, and because many building envelope components and claddings are typically delegated to the contractor in my area, I see a table like the one described above on practically every set of structural drawings that I ever see.
Furthermore, the application of the table pressures to the components and cladding members to be designed is not necessarily difficult as you suggest. For example, one could simply and conservatively choose to use edge or corner zone pressures for effective wind area less that 10 ft^2 for all members and be done with it. This application part is none of your concern, except for the check that is being requested of you now, and frankly your checking would be much easier and quicker if you had already calculated and included the required pressures on your drawings.
RE: Pre-Engineered Roof Trusses
To check if the wood trusses bracing is designed for your building you would only need to check if the reactions (gravity and uplift) are equal or greater than your calculated required reaction for your building.
The only problem I have had doing this, is that some building plan checkers have wanted me to redesign my building for the higher uplifts shown on the truss designs. Where the plan checker refused to accept my approval of the truss designs for my building. I required the trusses to be redesigned for calculated reactions. Typically, the trusses were merely designed to show only the MWFRS uplift reactions, due to the fact that most wood truss programs will always design for MWFRS and C&C. I did check that the bracing, lumber and plates were still the same as the truss designs with the higher uplift reactions, before approving them.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Pre-Engineered Roof Trusses
I also always provide the required loads because i don't want the design skimmed down to minimums that may be below the project's established standards.
And I also always thoroughly review the submittals and they never make it through cleanly the first time through.
I can't say that I've ever seen them request the EOR to be responsible for the design, but they do always require the reviewer to confirm their design geometry and dimensions; again a good thing to do because they are not always on the same page.
I suspect that if you are applying a signature indicating that you've reviewed the submittal, some way or another you will be implicated in the event that there is a problem with the design.
RE: Pre-Engineered Roof Trusses
The EOR should provide the wind speed, exposure, importance, LL and DL, but it is the responsibility of the truss manufacturer and their software to calculate the loads on the individual members properly. If they want you to review it, they need to pay you. That being said, you should have a general idea of the up and down reactions of the trusses - so a quick glance at the truss cut sheet should tell you if they are in the ballpark.
In my experience, the software takes care of everything anyway except in some special cases where the truss engineer has to calculate things manually. Maybe they should be using a different truss company.
RE: Pre-Engineered Roof Trusses
RE: Pre-Engineered Roof Trusses
RE: Pre-Engineered Roof Trusses
As I stated before, I don't think a simple matrix is sufficient for 1603.1.4. My reading of it says for each and every single little scenario and building element you provide the actual design pressure. Practically speaking impossible. Others obviously have a different opinion. I'll stick the silly matrix on in the future.
Jed,
Of course supply the loads, but to what degree of detail. There is a PE designing these trusses so he should be capable of calculating wind loads when given the basic parameters just like everyone else. I look at all the loads in the submittal for general conformance with my specs, but the 'confirm and verify' is my issue. I told them I would be happy to perform their request, and they simply need to supply me with their wind load calculations since their cryptic output makes it hard to tell what wind condition they are applying. Their calculations would greatly speed up the review process. They refused, and instead have taken back their request. I looked anyway and they have bigger loads than I would most of the time except at building corners where they need larger loads.
Again, thanks all
RE: Pre-Engineered Roof Trusses
It's a matter of what liability the truss manufacturer is willing to accept. They're working on a tiny margin and the engineer sealing the design gets a small amount. I'm sure their insurance carriers require that language to share liability with the EOR.
You're correct in that the truss designs are usually delivered in a mixture of Navajo code with some Swahili thrown in. The best you can do is check the reactions and see if they're realistic.
After doing all that, I've had truss designers ask us to determine the controlling load case. That one annoys me.
RE: Pre-Engineered Roof Trusses
RE: Pre-Engineered Roof Trusses
Just because you don't want to, or don't think that you should have to, or don't think your being paid enough to, does not alleviate your responsibility.
RE: Pre-Engineered Roof Trusses
"107.3.4 Design professional in responsible charge.
107.3.4.1 General. ...
...The registered design professional in responsible
charge shall be responsible for reviewing and coordinating
submittal documents prepared by others, including
phased and deferred submittal items, for compatibility
with the design of the building."
and
"107.3.4.2 Deferred submittals....
...Documents for deferred submittal items shall be submitted
to the registered design professional in responsible
charge who shall review them and forward them to
the building official with a notation indicating that the
deferred submittal documents have been reviewed and
been found to be in general conformance to the design of
the building. The deferred submittal items shall not be installed
until the deferred submittal documents have
been approved by the building official."
It seems the original OP statement that "a note on their submittal stating that the EOR of the building 'must review and verify the correct loads' are being used." may exceed what is required by the 2009 IBC.
Your review stamp for deferred submittal should clearly state that you are only responsible for "compatibility with the design of the building." and/or "found to be in general conformance to the design of the building."
So you could sent them back requiring payment for the review they are requesting and/or reject the submittal because such a review may not be per the code requirements.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Pre-Engineered Roof Trusses
RE: Pre-Engineered Roof Trusses
I'm not naive enogh to think that a truss or joist designer is going to individually look at my wind loads and enter them into their magic programs. But that's what they're committed to. My building, my rules, my loads. And I'm going to check them against that, the best I can.
RE: Pre-Engineered Roof Trusses
Not enough hold-down force, not enough bottom chord bracing, not designing web bracing properly, not balloon framing gables etc.
I typically do a rough check of up and down reactions on some of the common trusses and the girder trusses to see if they are within my hand calcs. Usually the uplifts on the truss cut sheets are higher than I have designed for.
There ain't no way I am checking each truss for loads unless I am getting paid extra.
I can't imagine even trying to justify that to an owner.
RE: Pre-Engineered Roof Trusses
I worked on making temporary repairs to dozens of houses after Andrew. I would assert that it all has to do with nails holding the sheathing down. E.g., the EOR specifies say 10 nails, the contractor saves money by using only 6, and three of them miss the framing entirely.
I don't know for a fact how many extra nails are in Habitat houses, but not one of them lost a shingle to Andrew, so it must be a bunch. The volunteers who worked on re-sheathing with me just kept hammering nails until they were told to stop, or until they ran out of nails.
Mike Halloran
Pembroke Pines, FL, USA
RE: Pre-Engineered Roof Trusses
The problem is that the truss engineer typically never sees the build plans. The information that they get is input from the truss manufacturing company. The truss engineer only signs the designs for what is shown on the design. This is why the cost for the truss drawing seals are so cheep.
So it is the EOR responsibility to check that the designs meet the "for compatibility with the design of the building." and/or "found to be in general conformance to the design of the building."
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Pre-Engineered Roof Trusses
I don't know if that is an issue or not. The truss software is pretty darn sophisticated and likely cannot provide accurate truss profiles unless the building is well understood by the designer. I used to design truss bracing for light gage metal trusses and there were times that the truss engineer had to hand calc some areas that the software could not handle - typically eave beams and jack trusses on hip roof where the overhangs were large.
I would like someone to cite an example of a failure in a wood trussed roof that was actually caused by a problem in the truss itself and not an issue with construction or overall building design by the EOR.
RE: Pre-Engineered Roof Trusses
A third retrofit project that we recently completed, showed compression buckling of web members under minimal snow loading. They were far too slender for the their length. The truss drawings did not provide any indication of addition bridging or bracing at these members.
You give the impression that truss design is somehow infallible.
RE: Pre-Engineered Roof Trusses
What software was used by the scissors truss manufacturer and when was it constructed? The design of a vertical tension web and its connection is about as basic as it gets. If modern software can't get that right, there is about 10,000,000 structures that are going to fail. Sometimes the wrong plates are installed - I have seen this. No math is going to fix that. I have seen alot of older metal plate connected trusses that have undersized members and plates, but I think they were designed by seat of the pants.
Did you contact engineer who sealed the truss drawings to inquire about the lack of web bracing.
I am not saying the software is infallible, but it is pretty darn proven. Are you implying we should re-calc each and every truss that the computer spits out?
I will say that I do check that the loading was applied properly for snow drifts and drag trusses.
RE: Pre-Engineered Roof Trusses
I did not imply that the software was the issue. The design process is. Entering data into a program and getting results are only a part of the process. So, yes there needs to be some sort of check of the results to verify expected outcomes are achieved. If something appears amiss, you find out if it was a data entry issue, misunderstanding of the results, etc.
When I use software to aid in design, I have an expectation of what the results will be. If there is a significant variation between what was expected and what was reported, I try to find out why.
The truss program is only as good as the person using it.
RE: Pre-Engineered Roof Trusses