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Pre-Engineered Roof Trusses

Pre-Engineered Roof Trusses

Pre-Engineered Roof Trusses

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?

RE: Pre-Engineered Roof Trusses

If they provide the calculations for generating the actual loading patterns used on the trusses, all based on your initial loads specified, I see no problem with briefly reviewing them for complicity of design.

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

They call out PLF type loadings and reference them to some vague load cases involving wind loads. Nothing is provided that shows how they arrive at their loads. So short of calculating everything from scratch myself there is no way of determining the validity of their loads, and frankly I do not think it is my responsibility or duty to do so. I am inclined to tell them it isn't my job and approval of the submittal specifically excludes the provisions of that note. I just wanted to see what others did.

I think the wood truss guys annoy me more than the PEMB metal building guys.

RE: Pre-Engineered Roof Trusses

Actually, they are likely justified in the request. Component and cladding pressures are required to be listed on the design documents prepared by the professional of record. I believe that this requirement is there because you, not the truss engineer are required to know the site and anything that could effect the structure.

So, to answer your question, in my opinion you are wrong.

RE: Pre-Engineered Roof Trusses

As a follow up:

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

As the EOR, I would always want to assure myself that a building for which I am responsible has been designed for the appropriate wind forces. Therefore, I would do the check.

RE: Pre-Engineered Roof Trusses

I agree with OHIOMatt and Hokie. The responsibility for interpretation of the code rests with the EOR, not the truss designer. This includes determining the magnitude of wind loads, gravity loads or any other type of load which may be involved.


RE: Pre-Engineered Roof Trusses

Thanks all.

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

The more I think about it, the requirement of Item 5 is essentially impossible to impart on the drawings since the factors vary so greatly with area. I suppose that is why I have never seen it done

RE: Pre-Engineered Roof Trusses

We always provide a simple table for component and cladding loads on our drawings, as it is required by code. Its very simple to implement. A chart with roof zone loads and wall zone loads based on SF , with the 'a' distance, is fairly easy to provide.

RE: Pre-Engineered Roof Trusses

We provide a diagram and charts based upon area and zone on every project. It is not that difficult or time consuming.

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

I understand, just saying in this part of the country I have never seen it done. To do it accurately on anything other than a simple box is anything but simple. Based on #5 I'm looking at it.

RE: Pre-Engineered Roof Trusses

Wood-truss manufacturers are notorious for trying to absolve themselves of any sort of design responsibility. Doing your own pressure calculation is getting off comparatively easy.

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

RE: Pre-Engineered Roof Trusses

What I am saying is that a 5x5 matrix is of minimal help when it comes to application. Filling out the matrix is cookbook, no more or less difficult that MWFRS loads which is not required to be specified down to the PSF by the EOR. The application of the matrix is the hard part and my reading of Item number 5 would require the later, not just a simple matrix.

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

The only effect wind loading has on Metal Plate Connected (MPC) Wood Trusses, up to a uplift reaction equal or greater than half the gravity reaction, is the possibility of additional bracing on the bottom chord or webs. This is mainly because the compression members can be plated for 50% of the compression force under some conditions. Given that the wind Cd factor is 1.6, the uplift reaction could go up to as much as 75% of the gravity reaction before any plates or lumber would change.

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 always require truss fabricators to have their own Engineer seal the submittals.
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

I agree with Dcarr82775.
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

Excel, why not provide the wind design data required by IBC 1603.1.4?

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.


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

I really don't mind giving loads to the truss designers. I do designs in Florida, and wind loads are not only high there, but they vary greatly from county to county. I'd rather know the loads they're designing for than assume that they're calculating them correctly. We routinely make up a table, defining all wind loads for zones 1, 2, 3, 4 and 5 and for each effective wind area. In Prescott, Arizona, we provide not only that (although wind loads are not that high there), but snow load diagrams, showing higher loads in drift zones, etc.
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

We provide truss load diagrams including PSF for dead loads, live loads, wind loads for delegated-design trusses. It is not out of line for the truss designer to ask you to verify their loads, in my opinion. I'm surprised they didn't go further and ask you to provide the loads, rather than just verify.

RE: Pre-Engineered Roof Trusses

Most of us have been guilty of complaining about code complexity and ambiguity. This issue is neither of these. It is clear, that the code requires the Engineer of Record to provide this information for delegated design. In no uncertain terms, it is your responsibility to provide it to the truss manufacturer.

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

Per the 2009 IBC
"107.3.4 Design professional in responsible charge. 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."
" 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

OK, I don't get it. The truss manufacturer's all use basically the same software which calculates the loads on the the truss members based on the basic loading parameters. This software has been around for 20+ years. Are we to assume that it is flawed? I agree, as EORs, we should do a rough check to make sure the reactions make sense, but we have to have some faith in the software. That is like you going back and re-chcking every element in the output of your RAM software or what ever you guys uses for high rises.

RE: Pre-Engineered Roof Trusses

I think the issue isn't a software issue, it's a responsibility issue. I suspect that there were places, possibly Florida after a hurricane, where the engineer of record pointed at the truss designer, the truss designer pointed at the EOR and no one took ownership of the design.
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

My suspicion is most of the failures of roof trusses during a hurricane are more an issue of bad design by the EOR, rather than the loads not being applied properly.
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

>>>My suspicion is most of the failures of roof trusses during a hurricane are more an issue of bad design by the EOR, rather than the loads not being applied properly.<<<

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

ExcelEngineering -

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

Excel, I am currently working on two retrofit projects both involving failures of internal truss elements. In each case, the trusses are scissors and the vertical web at the ridge line has extracted from the connection. I have analyzed the truss myself, and found that the members and their connections were undersized.

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

No, you just asked for examples. I provided three that I have dealt with in the last year.

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

I agree. That is why I always check the reactions to make sure they make sense.

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