Pre-Engineered Roof Trusses 1

[dcarr82775]

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?

 

[dcarr82775]

gte447f,

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

 

[JedClampett]

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.

 

[steellion]

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.

 

[OHIOMatt]

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.

 

[woodman88]

Per the 2009 IBC
"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.

 

[XR250]

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.

 

[JedClampett]

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.

 

[XR250]

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.

 

[MikeHalloran]

>>>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

 

[woodman88]

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.

 

[XR250]

Woodman88,

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.

 

[OHIOMatt]

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.

 

[XR250]

Ohiomatt;

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.

 

[OHIOMatt]

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.

 

[XR250]

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