Another Girder Truss Setback Question 1

[StrEng007]

Seems like no mattery how I try to position the hip girder setbacks, something is always off. What is the more important here?

1. Provide a setback that doesn't interfere with your window openings (not always possible).
2. Provide the SAME setback for all sides of the building, so this allows for similar construction of the jack trusses? This causes the issues shown below.
3. Provide unique setbacks so that jack trusses maintain 24" o.c. spacing? Seems excessive.
4. Provide unique setbacks so your line of common trusses along the entire building length maintain 24"o.c. Seems like this would help out the roofers the most, but I'm not sure.

I'm trying to be more aware of the wood structural panel sheathing layout but it seems like something is always going to be off? This was a lot easier when I just used one setback and spaced trusses however I wanted.

 

[KootK]

#2 is the sole driver in my experience unless there is a special condition being accommodated. Whatever dimension is common in your area. 8'-0", 7'-11 1/4" etc. Where attempts are made to accommodate rational sheathing, I've always seen that be a ceiling drywall thing rather than a roof thing.

 

[StrEng007]

My typical detail shows strength axis perpendicular to truss span. Long continuous panel edges are typically blocked.

I don't have a detail for what to do when the panel ends don't align over a truss. Should I? Do you?

 

[XR250]

I try to get a truss manufacturer in the mix early and let them give me a prelim. layout and work from that.

 

[Eng16080]

I like keeping things consistent, so would generally prefer option #2. Typically I don't give much consideration to having sheathing panels align perfectly.

Another thing to consider, IMO, is:
5. Set the girder truss back far enough from the end wall so that relative deflection between the wall and truss is not an issue.

I'll admit to having not considered this at times, although I think it could be important. As an example, if you had a long girder truss which could deflect 1" at midspan and it was only 48" from the end wall (which will not deflect), then there would be a deflection of L/48 between the two, which could be problematic, like for a sheetrocked ceiling.

 

[StrEng007]

Great! Thank you all for your replies.

XR250, I think it's a great idea and I'd like that option to be more available to me. I would say that most of the time we have no idea who is going to fabricate the trusses while we're in the design phase. I've tried to engage truss manufacturer's earlier on, but my experience is they usually don't want to give time away without having a contract. Keep in mind, this may not be indicative of the industry as a whole. I've read that some of you have much better response for early coordination. It could just be the area I'm in.

 

[lexpatrie]

StrEng007
1. Provide a setback that doesn't interfere with your window openings (not always possible).
2. Provide the SAME setback for all sides of the building, so this allows for similar construction of the jack trusses? This causes the issues shown below.
3. Provide unique setbacks so that jack trusses maintain 24" o.c. spacing? Seems excessive.
4. Provide unique setbacks so your line of common trusses along the entire building length maintain 24"o.c. Seems like this would help out the roofers the most, but I'm not sure.

Anybody savvy how this quote works now?

Anyway.
1 - Not a real concern. Either set it where you want it or deal with the header. Yea it's messy, every time I've tried to be "clever" avoiding it, the truss guy moves it onto the header.
2 - Only if you have the same roof slope, I guess, not a major concern of mine.
3 - I don't even follow what this means. (is this the "sheathing is the wrong direction" issue?)
4 - how does where you put the girder truss affect the typical truss spacing?

When I did these, I tried to be clever and set it someplace reasonable (to me) and to avoid landing a girder truss on a header, every time I adopted one truss suppliers spacing, the eventual truss supplier moved it. I eventually stuck a note on the roof plan that essentially said "move the girder truss and it'll cost you". That's the only way to keep them "firmly located" if you ask me, and I'm not sure it worked (this was, say, 2002, my recollection is fuzzy, but as I don't remember it well, I'm not convinced this actually worked, because you'd think my brain would hold onto some major victory, which, if it occurred, it didn't.).

The girder truss location has more to do with the roof pitch so they can get a workable depth than it has to do with anything else you've noted, if you ask me.

I tended to locate them 4', 6', or 8' from the corner. I think 4' for the average roof slope wasn't enough for the truss people, but if I picked 6' they'd put it at 8' and if I picked 8' they'd put it at 6'. Probably influenced by roof slope.

I'll just poach an image from this thread:https://www.eng-tips.com/threads/roof-framing-layout.470912/

 

[StrEng007]

lexpatrie
3 - I don't even follow what this means. (is this the "sheathing is the wrong direction" issue?)
4 - how does where you put the girder truss affect the typical truss spacing?

Following up with these to provide a better illustration. I've already come to the conclusion that 3 and 4 are less of an issue than using proper setbacks for consistency and consideration of deflections.

Either way, here is what I was referring to.


3- Sorry, maybe the way I worded it. I'll usually use a hip jack. The location where the hip jack meets the girder will square off the run of field jacks along the hip end. Depending on the setback, that can alter the ability of ALL jacks to go in at 24" o.c.


4 - The idea here was setting the hip girder to meet the following.

 

[lexpatrie]

On an unrelated note (sort of), haven't read it yet.​

Effects of Roof Pitch and Gypsum Ceilings on the Behavior of Wood Roof Diaphragms, Kirkham, Gupta, Miller, J. Perf. Const. Fac., Feb 2015.​

I think in both of your drawings, wouldn't you tend to lay out the main sequence of trusses @ 24" o.c. and then place the various mono trusses and the hip trusses? so that 24" spacing bit in terms of layout is wagging the dog, if you would, and the placement of the girder truss is then somewhere at the end, 4', 6', 8' +/- (with the emphasis on the +/- there? That hip truss in this kind of situation would be 45° and if the little trusses get a bit off from 24" o.c. that's fine?

Sorry, can't get the bold off these first two lines.

 

[dik]

They may not, but in these environs, the main girder truss is spaced 8' from the outside wall stud. The space between is not a multiple of 2'.

 

[lexpatrie]

Incidentally that previous article appears to be available legally here:

I can't seem to get a landing page out of google, all I get is a flat out download, so that's what this link is /:

https://ir.library.oregonstate.edu/downloads/5q47rr00b

 

[RontheRedneck]

I can only tell you what I do and what my thoughts are. There are so many different points of view and ways of building hip ends that I don't believe there is any one right answer.

We typically sell trusses to framers. And framers want the trusses all at 2' O.C. so the plywood goes down more easily. So that's the first priority.

Girders over windows are usually not an issue, as we're not dealing with large reactions. If we have something like a 40' wide house with a hip girder sitting over a 16' patio door - That's different. We pay attention to things like that.

Commercial buildings are a bit different. Longer spans mean higher reactions and more issues. Sometimes girder setbacks need to be longer on low sloped roofs in order to get enough depth in the hip girder so it works. Or doesn't have excessive deflection.

An example of what I most often do: Take a 23' hip system. Half the span is obviously 12'. Start with a common truss centered on the intersection of the ridge lines. So from the end of the building it's 12' to the center of the truss.

We typically build corner sets 8' or less. So there would be one step-down truss and then the hip girder. The setback to the hip girder would be 7' 11 1/4". That means the plywood would break all across the hip jacks.

As I said before, there are many ways of doing it. That's only one.

Based on experience, I'd say the most common setback I've seen is between 7 and 8'. If you're somewhere around that things are likely to work fine.

 

[RontheRedneck]

As luck would have it, I did a layout for a 24' hip yesterday. Thought I'd post it just for the heck of it.

All dimensions are to the face of the truss.

 

[PlinkoEng]

#2 seems to be what drives the truss guys. No matter what I do they always shift. I've even had some where they went to three truss outfits and they all had them in different locations.

Tip List:
1) get the architectural and a line out for your roof plan to truss guys as early as possible. This can be tricky as it's often going to be the contractor managing subs.
2) I just got ~6' from the peak because it's the most common I see.
3) make sure you are modeling/detailing in a manor that is flexible so it's easy to change later.
4) If you are in a time crunch and you know it's going to land on a header put your full estimated load on both sides of the header but calc it with the girder at midspan. It oversizes everything but it makes it simple to go back and fix your drawing when you do get the loading back. This is also a strategy if you are working in jurisdictions where reviewers don't check the trusses against the engineering well. Also good if the contractor never gives you the truss engineering.

These tips only apply to residential.

 

[StrEng007]

Ron,
Thank you for posting this. I've got two follow up questions:

1.Where is your truss type A (1-11 1/4") being measured from? I ask this because the summation of dimensions across the width of the building is 23'-10 1/2". So the overall 24'-0" width is short by 1-1/2". I'm just trying to understand exactly how all the dimensions are accounted for.

2. Let's assume the building is 55'-0" in length. Using the value 55.0' - (2)1.9375' = 51'-1 1/2". How are 2ft o.c. trusses maintained with such an odd spacing? Where to you choose do make your adjustment (one set of trusses needs to be spaced 13 1/2" apart)?

 

[RontheRedneck]

The dimension of 1' 11 1/4" to the "A" jack is from the outside of framing to the face of the jack. Then 2' OC from there.

It's actually 24' 0" across the end. Where the dimensions meet in the center, that jack is 1.5" wide. If you look closely at the dimensions you'll see it.


I rarely try to line up trusses along the length of a building. Let's say there's a 24x30 building with the above hip at both ends. In theory the trusses would fall 2' OC. But in reality, framing is never perfect. The foundation maybe out a bit. The walls may be off a bit. They may lean in or out a hair. So somewhere along the line you end up with an odd space. I typically mark it as such on the layout.