Piggyback wood trusses - connections.

[ichibansan]

When a wood prefab roof truss must be broken down into two pieces for shipping purposes, whose responsibility is it to provide the design for the piggyback truss to base truss connection? The structural EOR, or the truss designer?

 

[JedClampett]

Assuming that the truss designer is designing the truss, he/she should locate and design the splice points.

 

[ichibansan]

Jed-

I agree 100%. Thanks for the reply. Got in a little bit of an argument with a fellow engineer in our office with regards to this question.

 

[jheidt2543]

A wood truss is generally considered a component of the building, like precast concrete or steel joist. The Designer of Record, the one responsible for the overall building design, sets the criteria that the component designer must meet. But, the actual design of the compnent is by the manufacturer's engineer.

And, if you think about this, it is the most efficient way, they design the components day in and day out - right down to the Gnat's A--!

 

[SteveBausch]

Why is the vendor not being advised of the need to section the truss?

If the vendor is expected to provide remedy for truss failure, they won't like the surprise of "Oh, we cut them in half for transport".

Be honest with the poor guy. Tell him upfront that the truss needs to be sectioned, and pay him what he asks.

I suspect this is all about extra costs. Am I correct?

 

[steve111]

At the end of the day everybodies in the soup if theres a problem and the courts will partition damages, which if your the 'Engineer of Record' I suggest will be considerable. So Shop Drawings bearing the seal of a Prof. Engineer are a good thing, dwgs. should detail all required connections with regards to the manufacturers product, permanent bracing, etc., etc., which would include your'piggy back' truss system in this case. If your still not clear on the whole thing call the engineer who designed it. No shop drawings, call the manufacturer he'll tell you what plate companies software they use, give them a call. By the way most light framed wood truss shop drawings we've seen include a statement to the effect 'It is the Project Engineers responsibility to make sure everythings Ok', so to speak.

 

[ichibansan]

posted by SteveBausch

"Why is the vendor not being advised of the need to section the truss?

If the vendor is expected to provide remedy for truss failure, they won't like the surprise of "Oh, we cut them in half for transport".

Be honest with the poor guy. Tell him upfront that the truss needs to be sectioned, and pay him what he asks."
--------------------------
Huh!?

I am the engineer of record. Splicing the truss for transport is at the truss designer's discretion. As some of you have answered as I anticipated, this splice design is ultimately his responsibility. Yes, as the structural engineer, we provide the specifications, general truss layout, and the truss profile and support conditions to which the truss designer must adhere, but the ultimate design of the trusses is left up to him. After all, as someone alluded to, a roof truss is a component. I don't tell him what size gang-nail he needs for chord to web connections, so why would I specify and design a truss splice connection?

methinks you missed the point, Steve.

 

[rlewistx]

I suggest that in the future you find out what the trucking height is and detail it as necessary. If the truss is 18 feet high then you know before issuing drawings that a piggyback is needed so you should show it on your drawings. The hard part is knowing what trucking height is. That is up to the supplier to an extent. I have gotten all kinds of variations from 12 feet to 14 feet. Some will push the limit if it is a short haul and others will not. When in doubt I show a piggyback.

I detail the connection to the base truss. Usually this is some additional hurricane anchors. This is noted on the truss diagram. The roof diaphragm will also help with the anchoring but you don’t know the length of nailing of the overlapping panels so I don’t count on it at all.

 

[steve111]

Well the point is I suspect you should be reviewing the 'components' that go into your design to see that they conform to your assumptions and if as 'rlewistx' has correctly pointed out, you have assumed something that is is physically or practically not possible, then you should review this change in you assumptions and see that they meet your 'design assumptions'. Your job, your project, your responsibility. You don't want to be determining the legal responsibility here, thats best left to lawyers and who wants to go there anyways. Besides i'm a structural engineer, i never leave it upto verbage to determine my fate, prefer to crunch numbers and learn something useful. Contact the 'Component' Engineer, you might learn something.

 

[rowe]

I just couldn't resist butting in.

The EOR is responsible for designing a product that works. That means the design can be built and put into service (I could design a monolithic precast concrete beam to span 500 ft. but no one would be able to get it from the casting yard to the construction site and there's probably no crane that could lift it into place).

I would say that the EOR is responsible if ALL of the following conditions were in fact:

1. The fabricator had no reasonable way to transport the truss as it was designed;
2. The plans, special provisions, or other contract documents did not include any statement, provision, or guidance with regard to breaking up the truss;
3. There are no industry standards, such as governing building codes, or trade associations (TPI - Truss Plate Institute) that provide sufficient guidance to allow any licensed fabricator to alter the truss design in a specific manner; and
4. The fabricator did NOT agree to provide truss design and documentation by a LICENSED AND QUALIFIED engineer.

If the fabricator DID agree to provide truss design and documentation by a LICENSED AND QUALIFIED engineer, then the fabricator is responsible for the truss "period". Furthermore, the owner (or his agent) should have received plans signed and sealed by the fabricator's engineer-in-charge.