Gable end framing -- hinge effect?

[rothbard]

I looked back through this forum and didn't find any examples of the 1 story 20x30 being built with balloon framed gable ends.

I was wondering if maybe others had found some way to mitigate the hinge effect as seen above when wind force acts upon a platform framed end?

the 20x30 single story design has an open beam ceiling.  This doesn't leave a diaphragm or support I can think of to stop the above hinge effect at the gable end other than staggering the sheathing.

Curious if I had missed something?  The plans don't call out baloon length studs here either.  I looked for any details providing hinge support.  I'm going to plan on just baloon framing the 2x6 gable end all the way to the roof I guess since those ends aren't "load bearing" as the roof is steep enough at 10:12 that the ridge board has essentially no downward force.

I found this particular video of the same consideration for a similar design

[rothbard]

Thought about this some more.  Approach looks like it can be split per publication I found that suggest ceiling diaphragm only needs to go 40% of width down the length of the building to brace the hinge.  Therefore rear gable will be platformed and front will be balloon framed.

I know no one probably cares, but if years and years from now someone else is building the single story 20x30, take note to consider altering the plans to platform frame the rear wall, braced by the attic diaphragm.  And then to frame the front cathedral gable end with balloon/rake wall from top to bottom.  

Note: not engineering advice

[Don_P]

R602.3  Studs shall be continuous from support at the sole plate to a support at the top plate to resist loads perpendicular to the wall. The support shall be a foundation or floor, ceiling or roof diaphragm or shall be designed in accordance with accepted engineering practice.

That whole section is worth studying 

[rothbard]

Thanks Don!

Yep that is exactly what I was worried about and thank you for showing the place where they point it out. 

The single story 20x30 plans has a 8' ceiling on one end so it looks like the ceiling diaphragm has it covered to platform frame the back.

On the other gable end of those plans has an open beam ceiling which means per 602.3 there I need to go the full ~16' with the studs from sole plate to the roof.  That means thankfully only one of two gables needs the full span rake.  Fortunately the design does not have that gable end as supporting a ridge beam so the studs can use the non load bearing length.

Maybe I've missed it but the single story 20x30 materials list doesn't ANY studs beyond 8'.  I might recommend that if I haven't missed something, they might make note of the need for longer studs if it is built with the open beam as designed.

Also really appreciate your prior posts going back.  They were instrumental in convincing me to do the full perimeter cmu foundation.

[Don_P]

Do confirm that you do not need to design the ridge as a supported beam.

R802.5.2 Ceiling joist and rafter connections.
Where ceiling joists run parallel to rafters and are located in the bottom third of the rafter height, they shall be installed in accordance with Figure R802.4.5 and fastened to rafters in accordance with Table R802.5.2(1). Where the ceiling joists are installed above the bottom third of the rafter height, the ridge shall be designed as a beam in accordance with Section R802.3. Where ceiling joists do not run parallel to rafters, rafters shall be tied across the structure with a rafter tie in accordance with Section R802.5.2.2, or the ridge shall be designed as a beam in accordance with Section R802.3.

[rothbard]

Roger that.  Will put the ceiling joists in the back parallel at the bottom of the rafter, and the ties in the open ceiling in the bottom 1/3 of the rafters so that the ridge does not become a beam.

[rothbard]

I completed this gable, balloon framed from a scaffold,  with the following notes.

* center was doubled
* 1 pair of symmetrical studs about 4" from center either side.
* 16" o/c elsewhere
* The gable framing is dropped by exactly the rafter height -- outriggers will go across this dropped gable from the nearest rafter to form an overhang.  I am slightly worried about a hinge effect at the outriggers but I will add brackets attached to the outriggers to counter this (pictured below)




Beyond 12' height the stud spacing was well within 12" o/c average, below 12' more like 16" o/c.

I found the easiest way to frame this with a single man is to set the tallest center studs a a doubled post, brace it from the vertical and horizontal axis, then balloon frame out from scaffolding.  The top plate is placed after a few studs then the rest filled in.  I used a string line like when you stack CMU at ~8' height tied 1.5" beyond the adjacent side walls so I could perfectly set the studs vertical.

[Don_P]

I can't say I've checked uplift this well but here is the page for exposure B from the code referenced WFCM, Wood Frame Construction Manual. In a class the professor noted that most roofs peel from the bottom corners. If you look at the wind/ sheathing loads tables in chapter 3 of the IRC those are the wind concentration areas. It's whipping around the corner and getting pinched by the slope as well. I had to comment to the room full of smart people, I'm usually facing down and hanging on by my toenails while building that bottom corner. I could see the light bulb come on in several design pro's who had only built virtually. However, personal discomfort doesn't change reality, make sure they are built well.

The hold down could be accomplished other ways, for instance a timber screw that has the needed capacity in its tables down to a plate that has sheathing connection capacity to maintain the uplift load path.

The "usual" cantilever rule applies for "L", "twice as much framing inboard as outboard".
If I'm having to slide the 1st rafter over into what would be the 2nd bay it needs to be doubled... but! put the outboard rafter in, shoot on the lookouts through from the backside then put the 2nd rafter on the inboard side and nail it to the 1st rafter well. Beyond 2' outboard or in high snow that inboard connection starts to need upside down joist hangers or some better connection.

We block between lookouts over the wall, it locks and fireblocks the assembly. Ah, looking at it that would be the "required blocking"

[rothbard]

That WFCM keeps coming in handy, much appreciate the notes.  Looks like I can derate to ~0.44x based on 8" overhang yielding more like 130 lbs-force / 16" o/c outrigger @ 110 mph, well within what simpsons connectors can handle.

Reality does have a cold way of smacking classroom pontification in the face, as Mike Tyson once said in so many words.  That probably helped solidify that knowledge in a unique way.  Curious what kind of class this was, something on structural engineering?

I am definitely blocking between the riggers in any case to reduce the chance of falling through while "hanging on by my toenails"

[Don_P]

The WFCM does have a lot of good tables and details... in a mind numbing format  Don't forget the commentary if you get stuck, sometimes it clears it up.

In most of those engineering short courses at the nearby land grant U there's one or two builders in a room full of design pro's and inspectors. Among them are also folks that see more fail examples in the field than I ever will with my head down building one at a time. I'm a fan of cross training and these folks do a good job. This is a flier from one earlier this year;
Structural Design Topics in Wood Construction | Continuing and Professional Education | Virginia Tech (vt.edu)

But also track down whatever is of interest, APA does training for engineered wood products, NFBA for post frame building, etc. Most of those have webinars on their websites as well... I've been watching some from the gypsum people on fire assemblies lately for the current project.

Thinking of a wall... stand the building up on end. Park a Volkswagen in the middle of the now horizontal wall, get out and walk around on the wall. That's pretty much gale force wind thinking. Now look at the connections holding your "floor" up. That's actually how good they should be. It is very easy on a calm sunny day to forget about the forces and direction those forces come from on a bad day.

Balance that with longevity, I really like large overhangs that keep water away from the walls too.