pre-engineered metal trusses

[Lutfi]

How much design and information must be provided on roof plans for a pre-engineered metal trusses. Does anyone have a checklist of minimum items that are required for the truss designer to carry on his work? As EOR, I requested signed and sealed plans and calculations.

Regards

 

[jike]

Are you talking about light gage metal trusses? If so, I would provide the same info. as wood trusses.

Additionally, I make the truss designer responsible for all light gage bridging, framed openings, eave blocking, holdowns, connections, etc.

 

[Lutfi]

Yeah, light gage metal trusses.

I am being challenged as asked to do virtually the entire design. I think it is absurd.

Regards

 

[jike]

The design is treated the same as wood trusses.

Use the suggested specification as recommended by the light gage metal truss manufacturers. The one that Alpine (Trusteel) has on their website defines who's responsibility it is to design the trusses. It is the manufacturer that specifically designs the components and connections of the trusses, not the building EOR. Each manufacturer has different chord and web sections used for the design. It is the building EOR that specifies the loads for the manufacturer's truss designer to follow.

Any of the light gage metal truss manufacturers can explain the normal design procedure to your client.

 

[structuresguy]

We normally specify the connection of the truss to the structure for wood trusses, and let the truss engineer take over from there. Are you saying that for metal trusses you would require the truss engineer to design the connection of the truss to the structure?

Thanks.

 

[haynewp]

I don't think the connection should be specified until the thickness of the truss material is known. I have also seen the connection stated 'to be determined by the truss mfr'.

 

[Lutfi]

I am asking the specialty engineer to design the roof trusses by selecting the truss members, their connection, provide design for bridging, and asked them to design/select a truss hold down (I provided the forces perpendicular to the wall and parallel to the wall)all he has to determine is the wind uplift. I already designed the roof decking and the fastening pattern of the roof deck.

I provided the load criteria dead load, live load and the wind load criteria (although, I am attaching a wind pressure table and diagram on my plans for the entire roof and wall zones that are in the ASCE 7).

Client is asking my office to design fascia, trusses, their connections, bridging, etc.

 

[haynewp]

It sounds like a misunderstanding between the client and your office of what services were intended on this project. I like to make it clear when prefab trusses are first considered that this will be an add'l outside design fee not included in our scope. It's probably good to have this in the initial contract under specialty items not covered in fee.

I am sure he doesn't understand when you tell him 'usually' we do not design them. If he still insists on you doing it, then I would figure the time required and charge for it, which I am sure would be a lot more time than the specialty engineer would charge.

 

[akamel]

I have recently been working on checking the design of a wooden truss with truss plates at the connections. This was part of a roof collapse investigation. The design can be found in the TPI (Truss plate institute) specifications. It will take some time to go through the requirements and plate specification and what not. It took me some time to get going and develop my spreadsheets. But anyway it is by all means the truss manufacturer's scope of work to do the design (If they are too small, they usually subcontract the design to the big players like Jager). They have special programs that do the design and prepare the shop drawings including the size and cut angles of the members and the truss plate sizes.

Regards
Ayman

 

[connect2]

yes and with regards to speciality and pre-engineered and specifically with regards to 'metal' truss plates for light framed wood trusses each manufacturer of the plates has a slightly different plate in order to patent and each plate type has different structural capabilities and their particular software is not public domain. Slip characteristics and deflection gets you into a whole different world if your trying to figure out what they are up to. In our case the particular material code contains a section that outlines what info. must be provided to the 'pre-engineered' ( I hate that word), component manufacturer I guess to answer the initial question. Really sealed shop drawings are essential, but i find myself often looking at the stuff saying 'I hope they know what they are doing' ... i also feel there is somewhat of a conflict here... maybe a bit to pestimistic, however if you (The EOR) read their standard disclaimers its pretty clear where they feel most of the responsibility lies. Give me concrete, rebar or steel any day!

 

[SperlingPE]

1. Specify factory built trusses. Field built trusses by a competent specialty contractor with experience may be okay.
2. Provide designer with the lateral loads and show how you would like to connect the truss to the support (welding, screws, etc.). Leave the connection up to the designer because of proprietary shapes of truss members.
3. Spread trusses out to make a more economical design. Use metal roof deck or do a detail design of the wood sheathing.
4. Don't use metal trusses unless required for fire rating, span conditions, or economics (wood trusses are usually less expensive in my area; I don't know if this is true in all areas).