Truss as a drag strut

Regarding the term “drag strut”. Are you using this term for “collector” ?




He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

Yes, it can be. Tell the manufacturer the magnitude and direction of the loads and specify connection of the diaphragm to the drag truss.

Based on your handle, I'm going to assume you may be referring to steel trusses.

Here is some reading for this type of scenario in regards to open web steel joists.

Jump to Section '5.13 Diaphragms and Collectors'

Some text taken from this book:



Example table for specifying loads from the Steel Joist Institute:

We're talking wood truss, right? If so, the non-delegated part that you'll want to tend to yourself is the connection of the drag truss to the sill plate or wherever it dumps its load to and, also importantly, how the truss needs to be positioned in plan to facilitate. that.

KootK said:

We're talking wood truss, right?

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Oops... are we?

StrEng007 said:

Oops... are we?

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Hard to say. You've based your guess on OP's handle which is reasonable. I based mine on what I've seen of his previous work... which is also reasonable.

Time will tell. Sometimes people pick their handles based on stuff they like rather than stuff they do and, sometimes, based on roles they had three jobs ago. I've got two former proteges with handles that include "steel" and "glass" based on their first assignments out of college. Both are managers now that haven't touched any of that stuff in a decade+.

My original handle was very nearly "CasinoMan". There was only one casino.

lol, yea not sure where I got my handle from.

This is wood truss I am talking about. I will detail the diaphragm, and the connection at the sill. It will line up with a steel moment frame in a wall, so can add strapping, etc as needed. I will provide the loading in the diaphragm, and let the truss types decide how many plies are needed.

The comments above hold true for wood trusses as well. You need to let the truss manufacturer know that a certain truss must be designed for drag strut loads in addition to all other loads applicable to the truss. I have seen cases in the past where the drag strut truss ended up as a double truss due to the additional load.

DaveAtkins

All, do you have an example of specifying this type of load for the wood truss manufacturer?

My residential work is in high velocity wind zones, and we're constantly butting heads with the truss designers. We specify roof C&C loading for wind, and super-imposed gravity. On top of that, we provide the wind load criteria (Example, 175MPH Exp. D) for the truss designer to run their own MWFRS winds. Oddly enough, we don't provide the MWFRS roof wind and have never had an issue with it. Uplift reactions per our own analysis are placed on our roof layouts (typically governed by 0.6D-0.6W, wind considered at MWFRS level wind forces).

I'm wondering how you direct the truss designer to integrate any drag loads into the wood truss design. Most of TDD's I review utilize a pseudo combination where top chords and webs are designed for a C&C level wind force (to capture local effects of cladding forces) and the behavior of the overall truss is captured through MWFRS.

When I do this for OWSJ, I'll typically follow the format of the table I provided above with the addition of another column for wind axial load.

JStructsteel said:

I will detail the diaphragm, and the connection at the sill

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I usually have to assign a number of plies to the truss in order to make my truss to building connections work. Keep in mind I'm depending on pre-engineered connectors due to the type of wind loads I'm dealing with. In your situation, how to you handle your connection when the number of plies is unknown?

StrEng007, I will design based on 1 ply, use strapping/connectors as needed, and then review at shop drawing phase for the trusses, and modify as needed.

OK, I got you.
That would kill our productivity because our building officials get all bent out of shape when truss plies, connectors, reactions, and TDD's don't immediately match up.

Sometimes they'll deny our permit or delay CC/CO (if it slipped through the cracks) or require resubmittal of plans, calculations, and you'll have to provide official response letters to the city.

Im lucky, most building departments just rubber stamp plans around here, lol

If introducing a different truss to accommodate the drag forces is a problem, perhaps sheathing the side of a typical truss would be sufficient.

Around here it is very common to callout a wood truss as a drag truss on the type of commercial/institutional projects I work on. I just note the magnitude of the demand on the plans by the truss. When I get the truss shop dwgs I can check if they actually add the load to it.

Like others have said, it's pretty common. Do it all the time.

Although I sometimes wonder if they get installed right.