Cathedral Trusses
Cathedral Trusses
(OP)
I'm trying to quantify all of the possible triangulations of a cathedral truss given the vaulted to out-to-out span ratio of the truss. I've divided my classification into the 4/X and 6/X family where the numerator indicates the number of top chord panels of the truss. So far this is what I've come up with:


The reason for so many configurations is that as the web members approach vertical it makes more sense to have them truly vertical since scarf cutting a shallow angle is undesirable. I may be flawed in my reasoning but perhaps someone with more actual truss fabrication experience can enlighten me. These configurations will then form the basis for the logic in my software which determines the triangulation of this type of truss automatically.


The reason for so many configurations is that as the web members approach vertical it makes more sense to have them truly vertical since scarf cutting a shallow angle is undesirable. I may be flawed in my reasoning but perhaps someone with more actual truss fabrication experience can enlighten me. These configurations will then form the basis for the logic in my software which determines the triangulation of this type of truss automatically.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com





RE: Cathedral Trusses
Some software does or did put a vertical at every joint. Which makes for pretty web configurations, but IMHO is ridiculous and wasteful.
Some software packages I've used have a setting that reads something like "Make a web vertical if both ends are within XX feet XX inches of each other". Where you can fill in the value "XX" for whatever you want.
Some space the joints out evenly and don't worry about making webs vertical. Which IMHO makes the most sense.
Your comment that "...as the web members approach vertical it makes more sense to have them truly vertical since scarf cutting a shallow angle..." Is not accurate. Software packages typically have a setting for the maximum scarf length. (Setting this at 12" to 16" is common) Once a web nears vertical the program changes the joint type to avoid excessively long scarf cuts. That can be done without making a web vertical.
RE: Cathedral Trusses
With this particular truss type the webbing gets complicated because of the varying width of the vaulted section of the bottom chord. As you suggest above my primary goal is to space the joints out as evenly as possible thereby eliminating any webs or chord lengths that become too long and hence overstressed. At the two locations where there is a pitch break in the bottom chord I need to have a panel point, other than that the webs are a function of even spacing and as sensible arrangement as possible.
I suppose you are correct you don't need to scarf cut a web member that is near vertical, you just eliminate the scarf cut altogether. I've actually seen this quite a bit with raised heel trusses where the strut connecting heel joint to the first panel point is such a shallow angle to the horizontal that the scarf is eliminated.
Making the web truly vertical just seemed like an easier solution to the problem then inserting a web that is only slightly off vertical.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Cathedral Trusses
It is, up to a point. Maybe I didn't explain myself well enough.
Let's say the max TC panel length you want is 8'. In one of your examples above you have a panel length that is 7' 6". You shift it over a foot to make it vertical, and your panel length is now 8' 6". That means you have to add one more web to keep your panel lengths under 8'.
A lot of companies do that. They add a vertical at every pitch break, even if they're only 2' apart. Then they fill in the webs from there.
I don't know if you have a background in trusses, so I hope you don't mind if I explain why this bothers me.
When you manufacture trusses, the largest cost by far is lumber. It averages somewhere around 40% of the selling price of a truss. (Although that varies with the type of truss and quantity)
For every web you add to a truss, you have to pay for the lumber, pay someone to go get that board and take it to the saw, pay someone to run it through the saw, pay someone to take it to the assembly area, pay for someone to put it in place, plus pay for extra plates to attach it with.
That's why I'm so down on making verticals. Because when truss designers get in the habit of thinking webs have to be vertical, they tend to start wasting lumber.
RE: Cathedral Trusses
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