Question about trusses 1

I would ask for a girder truss there. Might be worth a quick call/email to your local truss designer. We have a few around here who offer free consulting at preliminary design phases (as long as you specify their trusses, of course).

It is your choice typically. I would call out a girder truss though.

You want girder trusses all day long unless there's some exotic reason not to do that. Truss suppliers do this day in and day out and the depth of the girder trusses improves their stiffness and bending resistance massively relative to beams spanning the same distance.

See the sketch below for how the truss supplier will usually want the girder positioned on the supporting stud walls. This allows the supported truss run to have the same span and heel height as the common trusses not supported by the girder. And that aid's in fabrication economy.

Thanks KootK! That's a good info to know. I have another question. Another project I have a house with crazy roof. I feel like the architect didn't really think about the benefits of trusses which essentially spanning from exterior to exterior walls. I have scissor trusses next to a standard trusses. The little triangle section in between is just a cricket. There is a bout 2.5 ft from the valley of the 2 roofs meet and the spring height. Can they do a girder truss? Or do I need to specify a beam and the trusses have to hang from the side of the beam?

KootK covered it well. In those situations I'd say we do girder trusses 99% of the time.

In your last post - Where you have essentially a double gable -

If there were bearing at the center point we would do 2 separate girder trusses. The ends would be staggered to one wasn't on top of the other.

If there is no bearing - We'd try to do one long girder truss with 2 peaks. The cricket would have to be built into the truss for it to have any hope of working. Whether it worked or not would depend on the depth we had to work with at the cricket and how much load the girder was carrying.

KootK,
Doesn't the most economical position of your girder depend on the type of overhang for the common trusses? If the underside of the overhang has a soffit, the heel height will be different from an overhang without a soffit. I often find myself looking first to see the type of overhang.

Attached is an image assuming you'd want to maintain the same heel height at the bearing plate for the common trusses (along the load bearing wall).



I'll take any opportunity to discuss truss layouts with you!

OP said:

I feel like the architect didn't really think about the benefits of trusses which essentially spanning from exterior to exterior walls.

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I agree. This has the feel of an architect who is not particularly adept at establishing roof and ceiling planes that jive with traditional, stick built framing schemes which I feel is the key to competence in this arena. Clients... am I right??

OP said:

Can they do a girder truss? Or do I need to specify a beam and the trusses have to hang from the side of the beam?

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This one would be a beam condition for me. Or, with that 2.5' depth, perhaps a flat girder truss.

I would shy away from using a girder truss that protrudes into the valley over framing for a couple of reasons:

1) It would put the compression chord of that girder up at the valley framing sheathing level. I'm skeptical that the valley sheathing would be installed in such a way that it would convincingly brace the top chord of the girder without some unconventional detailing and construction up there.

2) A girder that protrudes though the primary sheathing would interrupt the diaphragm shear load path. You could detail something fancy to move the diaphragm shear across the girder but, again, you'd be asking for something "weird" looking.

Assuming that your beam would be located above the partition wall, you may have a spatial problem with the heel of your scissor trusses at the location shown below. The girder would cut into the bearing length of the scissor which normally wants to have the bottom chord soffit touch down at the face of the stud wall. Possible solutions:

a) Talk your architect into a coffer at the bearing to allow for the requisite "flat spot". Unlikely.

b) Raise your scissor bottom chord up and then frame it back town to the ceiling with false framing, either stick built or built into the truss.



b)

StrENG007 said:

I'll take any opportunity to discuss truss layouts with you!

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Ditto. Keep it coming.

StrENG007 said:

Doesn't the most economical position of your girder depend on the type of overhang for the common trusses? If the underside of the overhang has a soffit, the heel height will be different from an overhang without a soffit.

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I disagree. Usually the trusses with and without overhangs have the same heel height. With the roof planes being a given, the thing that determines heel height is basically just the location of the end of the truss. This is why moving the girder from my suggested location to the right results in shortening the supported trusses and, thus, increasing their heel height. And that results in different bottom chord lengths and, depending on the situation, moving from:

a) A simple scarf cut joint with one plate to;

b) A joint with a slider and, possibly, as many as three plates.

As you know, repetition is key when it comes to pre-fab.

The situation is worse still if the supported trusses are scissor trusses since the girder -- moved inwards -- would cut into the bearing length of the scissor trusses. Yuckbo.

This issue of girder positioning is really why drag strut tension connectors have the form that they do despite that setup introducing load eccentricity in both the girder truss and the shear wall top plates.

KootK said:

With the roof planes being a given, the thing that determines heel height is basically just the location of the end of the truss

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I was referring to what is shown below. Most of the architects we work with like to use one of the three variations below. Bond beam elevation, overhang length, and the roof pitch are the same for all three details. Not only does this change my heel heights (between h1 and h2) but it also changes the MRF of the structure I'm designing.

Where I practice, the heel height is usually a function of the insulation required. We need R-50 minimum now, so for blown-in cellulose, we're up to 12" of insulation and a 2" air gap, requiring a 14" heel. No ifs, ands, or buts, 14" minimum. We've had bigger heels, but never smaller.

StrEng007 said:

I was referring to what is shown below. Most of the architects we work with like to use one of the three variations below.

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I get it but I don't see how that is relevant to the question at hand regarding girder positioning:

1) Yes, there are all manner of reasons for different, common truss heel heights to be used in different situations.

2) No matter the common truss heel height, the only way to maintain a consistent heel height at the girder supported trusses is to position the girder as I recommended earlier.

@StrEng007: try this on for size. It's a numerical example where I've NOT positioned the truss per my prior recommendation. I've haven't done this manually since the 90's so it's all but certain that I've messed something up.

I completely understand what you were saying KootK. I am worried how much coordination I will need with the truss manuf. If I specify a beam. First of all, the beam will be flush. So for sure the trusses have to be a bit shorter because it will hang from the side of my beam. Then we need to figure out the connection, etc. So I am trying to avoid a beam and use girder truss. The location of your girder truss totally makes sense to have as many of common trusses as possible. Thanks for the explanation.

@KootK,
OK I'm with you. That's pretty darn good for how long you haven't done it. I noticed your cut is 1 3/4". I've seen the 1/4" used elsewhere. Is that a local thing?

For a 2x4 at 6:12, Heel = Base Cut + 3.5"/cos26.56° = Base Cut + 3.913"



It's great that you can put together a digital sketch so quickly. I have to get on board with the touch screen annotations.

StrEng007 said:

I noticed your cut is 1 3/4". I've seen the 1/4" used elsewhere. Is that a local thing?

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It is both regional and temporal. When I was building the trusses in British Columbia and Ontario in the 90's, 0.25" and 1.75" were pretty common heel cuts on homes for the non-wealthy. Nowadays, with the advent of stricter energy codes in Canada, you seem mostly the kinds of heel heights that jayrod12 mentioned.