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?

 

[msquared48]

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

 

[dcarr82775]

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.

 

[OHIOMatt]

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.

 

[OHIOMatt]

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.

 

[hokie66]

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.

 

[BAretired]

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.

BA

 

[dcarr82775]

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.

 

[dcarr82775]

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

 

[structSU10]

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.

 

[OHIOMatt]

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.

 

[dcarr82775]

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.

 

[briancpotter]

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

http://simplesupports.wordpress.com

 

[dcarr82775]

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.

 

[gte447f]

dcarr,
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.

 

[woodman88]

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.

 

[StructuralEd]

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.

 

[XR250]

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.

 

[gte447f]

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

 

[JedClampett]

This is codified in Florida. It's a pretty massive document, but it defines the EOR responsibilities with respect to the wood truss designers. And it says the EOR shall provide the loads to the truss designer, See 61G15, 31.003.