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Coffer Trusses

Coffer Trusses

(OP)
Starting to take a look at coffer trusses and the first thing that jumps out at me is the peculiar way in which many of these truss types are triangulated.



I'm trying to determine if there is some method to the madness. Unfortunately, I don't have any shop drawings of these truss types and only a very few examples that I have seen online after quite some extensive searching.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Coffer Trusses

What you got there doesn't look peculiar to me. Looks pretty close to what I would have done.

I've found that the terminology varies. The names tray, trey, and coffer seem to be used almost interchangeably.

Some have the vertical ceiling height change, as in your drawing above. Others have a slope going up to the higher ceiling height.

RE: Coffer Trusses

(OP)
Tray trusses are what I call the trusses with the sloping vertical change and coffer has the vertical ceiling change. Examples of a tray trusses would be:

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Coffer Trusses

Your tray truss examples do a good job of illustrating my pet peeve about verticals. There is absolutely no reason to have verticals at every pitch break joint. But most designers put them in because it's visually appealing.

RE: Coffer Trusses

(OP)
My method of triangulating the tray trusses is based off of a number of examples I've examined by other truss designers. I agree that I can probably eliminate some of the vertical webs at the pitch breaks and potentially save some money. However at some level the additional triangulation at the pitch breaks makes sense from a structural sense since the discontinuity of the bottom chord is going to cause some higher stresses at these points. I haven't quantified this in RISA yet.

With the coffer trusses I've noticed that the vertical webs at the transition of the bottom chord typically extend all the way to the top chord. My question with web placement though is at a loss with the webs on each side of the coffer section. Where to place the panel points? and what is driving this?

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Coffer Trusses

You may need more webs in a tray truss. But there's no way you need a vertical at every joint.

In a situation like that where the TC compression is high, adding webs doesn't help much. Adding webs helps when bending forces are a problem. Which isn't the case here.

RE: Coffer Trusses

(OP)
Coffer Truss Algorithm:



This is assuming that the coffer span is centered on the truss. A non-centered coffer algorithm could get significantly more interesting and difficult.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Coffer Trusses

(OP)
Various configurations of a 40' coffer truss shown below:



The web triangulation algorithm automatically determines which configuration is appropriate based on the span between panel points. This simplifies the geometry input menu but does take some control from the user.

View model here:

https://3dwarehouse.sketchup.com/model.html?id=a9f...

I think the web placement mostly makes sense but please chime in if there is something that stands out as incorrect.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

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