Messing with Sketchup - my cabin idea

[Ljohnsaw]

If you are entering a dead load then remove it from the live load. You're adding it twice, I think.

Live load is your snow load only.

Dead load is all your roofing materials, and the timbers themselves.

You may need to "run" your numbers again.

When you have the calculator open, you can select "save as" and put it onto your own home computer so that you don't have to be online at the forum to use it.
For your information.

I don't have any weight values on Ponderosa pine so I'm not sure if that is correct or not. I'll assume you have done your research and that they are correct.

As far as I can see other then the double counting the dead load, your process looks right.

Jim Rogers

I went back to run the numbers again.  For "Total" load, I put in the snow + live load for the area.  Dead load is just the wood (factored up for the actual roof area - 9 purlins total in 12 feet of "run" on the rafter).  For the actual purlin calculation, I still fail on a 5x7 so I'm sticking with a 6x9.

The old numbers were:

This gives me a pass on all three.  The Section Modulus Required is 94.5 and I hit 192, the deflection is just about zero, and the section required for shear is 36.4 and I hit 96.

The new numbers:

This gives me a pass on all three.  The Section Modulus Required is 92.95 and I hit 192, the deflection is just about zero, and the section required for shear is 35.74 and I hit 96. (for an 8x12).  I do pass with a little margin with a 7x10.  It might look odd to have 6x9 purlins hanging off that so I'm leaning toward the 8x12.  Besides, with the climate change, we will only be getting more snow!

I'm still "safe" :D

Jim, by Save As, are you referring to the File menu for explorer?

[Ljohnsaw]

John:
If you use the first calculator for uniformly loaded beams, now that you know the values for grade #2 ponderosa pine, you would enter the total load on the beam.

I don't know why DonP added the dead load factor in the second calculator.

But, if you understand that the gravity load is vertical, and the dead load is in the slope of the roof plane, (which means it is higher then if the roof was level) you can add to the live load the adjusted dead load to get one number; which would be the total load. And then use the first calculator that asks for one load only. Then you won't be double dipping, and over sizing your timbers.

Jim Rogers

Jim, when I use the first calculator with the Ponderosa values, I fail on the 6x9 purlins.  The Section Modulus Input is 81 for both.  The first calculator gives me a value required of 86.6 and the second of 74.3.  If I up the depth of the purlin to 9.5, I'm good in both calcs.

[Jim_Rogers]

John:
I think you may have it right now, but I haven't personally ran your numbers.

When you have it all designed; your engineer will verify and make suggestions to your sizes.

I'm sure he is aware if the loads in your area if he's a local that has done it before.

Jim Rogers

[Jay C. White Cloud]

Hi John,

I have been following along this entire time, and I track with 90% of what Jim is trying to say, (he is faster and better at this "prelim" engineering stuff than I am.) Jim has hinted at a number of design changes to your frame, even with your extreme snow loads, I think you will be able to, with Jim's and FF guidance, come up with a workable and aesthetically pleasing design.  Jim last post about your PE is also very true. Good luck and keep asking your questions.

Regards,  jay

[Ljohnsaw]

Ok, some questions:

I'm doing princple rafter/purlins - the purlins will be 6x9 two foot on center fitted into 8x12 rafters.  I'm thinking a 6" wide x 4" tusk tenon?  Also, how do I finish the ridge?  The roof sheathing/planks will be 2" stock.

Speaking of purlins, what is the typical vertical spacing on walls and what is the typical wall plank thickness (2")?

For the top "purlin", I'm thinking upping the size to a 6x8 so I can attach bracing to it.

Thanksi

[Jay C. White Cloud]

Hi John,

For me, as a designer, I really need to know what someone likes and doesn't like, before I can give them good advice.  It gives me the parameters and lets me know if I can really be of service, because there personal style may not fit how I design.   

One thing, I have noted, is that if you went from a 12/12 to a 16/12 pitch, you could lesson the size of your materials, if you would like.  Now that is just one example of a style nuance.  Most of my work is based off the different elements of the "golden mean," like the 3-4-5 tangle.  The difference between 45 and 53 degrees isn't that much, but it does make the math different for loads and it makes your numbers for measuring much easier.  Not to mention that 16/12 is a classic pitch on traditional mountain architecture, form the Carpathians to the Niseko range of Hokkaidō.

On your roof purlins, you could go with the simple spacing of the purlins...or, do the high end design maneuver of harmonic progression.   take the max distance the math states is required for your purlin spacing, then apply a HP equation.  Some are hard some are easy, like: 1/a, 1/(a+d), 1/(a+2d), 1/(a+3d), etc.  The widest spacing is from the rafter plate to the first purlin, the next is closer, and so forth.  This gives the room a since of space that you are only aware of subconsciously, but you know something is happening.

Have you considered the very traditional look of "principle rafter, principle purlin.  Many Eastern European homes have this.  Then you have common rafters that create the roof plain. It is classic, beautiful, and traditional.  Plus it is much easier to build.

With 2" thick planks, you are moving into the realm of "plank house," timber framing.  You don't need purlins, others than for the Header and sill of your fenestration plan, (windows.)

Regards,  jay

[Ljohnsaw]

Hi John,

For me, as a designer, I really need to know what someone likes and doesn't like, before I can give them good advice.  It gives me the parameters and lets me know if I can really be of service, because there personal style may not fit how I design.   

One thing, I have noted, is that if you went from a 12/12 to a 16/12 pitch, you could lesson the size of your materials, if you would like.  Now that is just one example of a style nuance.  Most of my work is based off the different elements of the "golden mean," like the 3-4-5 tangle.  The difference between 45 and 53 degrees isn't that much, but it does make the math different for loads and it makes your numbers for measuring much easier.  Not to mention that 16/12 is a classic pitch on traditional mountain architecture, form the Carpathians to the Niseko range of Hokkaidō.

I've thought about steeper roof pitches but I don't like the appearance of wasted space (way up there) and the math becomes more complicated.  I'm all about keeping it simple so I don't make mistakes.

On your roof purlins, you could go with the simple spacing of the purlins...or, do the high end design maneuver of harmonic progression.   take the max distance the math states is required for your purlin spacing, then apply a HP equation.  Some are hard some are easy, like: 1/a, 1/(a+d), 1/(a+2d), 1/(a+3d), etc.  The widest spacing is from the rafter plate to the first purlin, the next is closer, and so forth.  This gives the room a since of space that you are only aware of subconsciously, but you know something is happening.

Now that sounds interesting.  I'll look into that.  If it means one extra purlin, so be it.  Since the space is getting narrower (less load), I would suspect the purlins would be getting lighter as well.  That sounds intriguing, but pretty complicated, cut wise.

Have you considered the very traditional look of "principle rafter, principle purlin.  Many Eastern European homes have this.  Then you have common rafters that create the roof plain. It is classic, beautiful, and traditional.  Plus it is much easier to build.

Googling principal purlin is not very fruitful.  I did see some images and from what I saw, you could classify them as beams supporting the rafter system mid-span or every 1/3 span and so forth?  If that is not right, can you provide a link or a picture showing what this is?

With 2" thick planks, you are moving into the realm of "plank house," timber framing.  You don't need purlins, others than for the Header and sill of your fenestration plan, (windows.)

For the walls, I'd like to go 1" boards if that is acceptable (to maximize the yield from my logs).  Drawing up my plan, there is not much room for purlins when you consider windows.  I'm thinking 2 foot from the floor is the window sill, windows being at least 4 foot tall, that only leaves 2 feet to the top of the wall.  Is a 4 foot vertical span too much for 1" boards and a wall that will have about 5" of rigid foam and vertical strapping for clapboards?  I suppose I could do "stub-purlins" to fill the space between the posts and the window frame, halfway up the window.

Regards,  jay

[Ljohnsaw]

OK, here is my drawing so far in SketchUp.  It does not have the main beam/support running the center of the cabin in the basement.  Just pretend it is there.  I have not run the calcs on the beams above and below the queen posts, yet.  The basement openings face West.

Funny how the building keeps getting bigger and bigger!  The loft area needs to cover the large bay (two bedrooms and stair case going down) and the north half of the next two bays (bath and kitchen as you head west from the bedrooms).  I put in the loft extending more than necessary just to see how it looks.

I'm not sure where the attachments go, but if your looking, the name is "LJohnSaw Cabin 2013-01-28.skp"

Thanks in advance for your comments.

[Jay C. White Cloud]

Hey John,

I didn't want to do this fast but I may not get a chance tomorrow, so I apologize in advance for my lack of normal detailed response.

I've thought about steeper roof pitches but I don't like the appearance of wasted space (way up there) and the math becomes more complicated.  I'm all about keeping it simple so I don't make mistakes.

The different between 10/12 and 16/12 is a lot but not between 12/12 and 16/12.  You also choose not to have the heated space go all the way to the ridge. Having a small flattened facet near the apex of you room is often seen as an interesting architectural detail.

Second, the math of a 12/12 is much more complicated than a 16/12, (which is a 3/4/5 triangle.)  The one you need a calculator for the others is empirical or geometric and dimensions of the sides of the triangle are reciprocal.  Try doing a little trig and compare the two, I think you see the simplicity of one over the other.

That sounds intriguing, but pretty complicated, cut wise.

It is about the same and maybe easier in some cases.

Googling principal purlin is not very fruitful.

Go to Google then hit "images," then enter: principal rafter and principal purlin you will see a number of different configurations. Also try cruck frame in images. This is the folk style that principal rafter and principal purlin came from.

Now your wall we need to talk about later, you have some choices that you may not have considered, like wall trusses, which you can find a lot of information on right here on the FF, just do a search.

Regards,  jay

[Ljohnsaw]

In my drawing, I have the purlins set so their top face is in line with the rafters - that is, the 6x9 purlin is set at a 45° angle.  Looking at the forces of gravity and snow load, is that orientation correct or do the purlins need to sit "vertical" so the depth can effectively handle the load that they were sized for?

For the ridge, should the top-most purlin be sized as a 9x9?  and how should I attach it without removing too much from the rafter  ridge joint?  Make the purlin be the "peg" (tusk) holding the top joint together, with it being housed a little?

[Jay C. White Cloud]

Your gonna have to draw that one for me, I can't really see it the way it's described and these sizes are all way off of what I normally see, even in Colorado. I do think from your description the purlin is in the correct orientation for your application. 

Regards,  jay

[Ljohnsaw]

Jay,

Here is the latest with the top "ridge purlin" in place.  When you say the way I described, as in my plan (at 45°) or that they need to be vertically orientated?

Thanks,

[Jay C. White Cloud]

The way you have it in the attachement is the normal orientation.  Does that help? ??? ;D

[Ljohnsaw]

Yes, thanks.  Its the engineer in me that says it should be in a "flat" orientation to get the strength as calculated by the tables/beam calculator.  Any thoughts on the ridge piece (attachment)?

[Rooster]

John,

Good job on your drawing!!

I have some comments and suggestions...

You have 6x9 purlins 24"oc...I didn't run the numbers but that spacing seems to "crowd" the purlins a bit. If you could spaced them further apart you would use less material, have fewer mortise/pockets to cut and it wouldn't look as "busy" in the roof.

What do you have holding the mid-span of the  first floor?... and is the spacing and sizes of the joist your final version?

You have two different joist patterns in the upper floor...the two shorter bays have a the joist farther apart and all seem small in comparison to the other large timbers in the frame.

Your windbraces/diagonals seem short.  Have you thought about changing them from 24"x24" to 36"x36"?  If you do decide to go larger you can offset the amount that they will intrude into the usable living space by making the frame taller (sidewall and interior posts) and raising the tie-beams.

The amount of "head room" upstairs in the loft seems short at 6'6", without any floor system.  6'8" off of the finished floor would be the same as a doorway opening...could you add some length to the tops of the sidewall posts above the tie-beams?

And the last thing I will leave open for others to comment...
Do principle rafter and common purlin systems need any wind-bracing from either end-wall? Do the multiple joints and decking hold it rigid enough?  These designs always make me nervous. :-\ That is why I like ridge beams with wind-bracing... ;)

Keep up the good work!!

Rooster

[bedway]

Im on my first cup of coffee and when i read the title of the thread i thought it said (messing with sasquach) . See what you young guys got to look forward to ;D.

[Ljohnsaw]

Rooster,
Thanks, lots of great comments - just what I'm looking for.

The headroom upstairs is an issue. I was worried about extending the post too far up past the tie beam.  But that was before I put the rafter tie in place. I should push up another foot or so but not sure how to do it in sketchup without totally screwing up my work so far!

The purlin spacing was driven by trying to keep the size down on the 12' bay. I could rework the numbers and try 30" and see if I can get that to work. I'm pretty sure 36" will be crazy big.

Yes, not shown but there will be a big beam running the length of the cabin in the middle. The floor joists are sized for the 12' span.

The spacing of the second floor joists is because of the 8 vs. 12 span and the width of the boards change as well to save wood. The 12' joists (bedroom ceiling) will not be visable from the 8' bays (great room).

I put the top beam (girt?) Between the bents as a window header and a place to put bracing. I could/should put some bracing from the mid purlin down to the queen post, you think?

Is there much gain in strengh going up from 24x24 to 30 or 36?  A triangle is a triangle. Though the larger the size, the more forgiving an error of 1/16" in your cuts. I suppose I could do curved bracing as I do have some interesting trees to choose from.

i

[Rooster]

ljohnsaw,

A lean towards larger diagonals because the smaller the "triangle" the closer the fulcrum is to the joint.  You will have a very large roof surface with a lot of wind load trying to push the building over. One 4x8 sheet of plywood in a 20 mph wind is hard to control..now multiple that by 31 times.

I am posting a different frame design (just an option for you to look at) that uses principle purlins and common rafters, but much of your frame sizes are the same... the bents are 12' apart and there is more head room both down stairs and up.  It is more of a concept with no joints drawn.

Rooster


 

[Ljohnsaw]

Roster,

So I took your idea and ran with it.  I calculated the loads and sized things.  I put up a ridge beam because of the loads.  Each 12' section of the ridge beam and principal purlins have a load of 10,000 lbs, hence the size of 8x15.  The "sill" purlin (forgive me, don't have the names down yet), only has 5,000 lb so a 8x12 works there.  The rafters are 5x9 36" OC.  I could get away with 5x8s if I care to.  The corner posts are 8x8 and the center ones 10x10.  The mid-wall posts are 8x10 because the beams running across are 10x16s to carry the load from the principal purlins (10,000 lb mid-span).  I've threw in some bracing but too tired now to finish.

I need to figure out how to add components to a named component.  Also, I'm having trouble with the layers - try turning things on and off and watch what happens!

[Jay C. White Cloud]

I have been following along, your new design with Rooster's influence is a vast improvement and has much more balance and symmetry.  There is some horizontal through brace methods you may employ to avoid the oblique bracing.  Also you have some other configurations that would also be horizontal in nature that could stiffen the frame.  Again this update looks much better. 

I am still not "feeling" the math on this.  I haven't ran numbers completely, I'm just running on experience.  If I didn't      better, I would think you had calculated everything for a flat roof?  Could you put your math for the 45 degree roof pitch compensation in an attachment with some of your other numbers.  Slopes like that usually only catch wind loads and 30% or 50% of snow loads, if memory serves. 

Thanks for sharing everything with us. You have been great about taking feedback and incorporating it into your design.  When you are done, I think you will be very happy.

Regards,  jay

[Ljohnsaw]

Jay,

Thanks for your feedback.  I am not happy with the looks of the bracing yet. I just threw some on to see how it would look. As far as the snow load, go back up in this thread (I thing I stated it). The ground snow load is 303 PSF for my area. On a 45° metal roof it drops all the way down to 116 PSF RSL (roof snow load).   I have rounded up - for instance, the weight on one principal purlin calc'd out at 9,700 or so. I rounded up to 10k. The rafters at 5x8 had a pretty good margin, but I decided a 5x9 would look better against the larger members without using too much more wood. My braces are just a stab at 3x5 - perhaps someone could suggest a rule for appropriate sizing with the other timbers?

As far as wind load, I have no clue what do do there. For additional bracing and design look, I'm thinking of running the roof planks at an angle - either 30 or 45 degrees. What do you all think?  How do I calculate the thickness of the roof planks?  I'm just assuming 2x stock, T&G.  I don't see anything in the toolbox for that. The rafters are at 36" spacing. I could go to 48" with the same size rafters but that just seems too far for my comfort.

[danreed76]

I should push up another foot or so but not sure how to do it in sketchup without totally screwing up my work so far!

Whenever I want to try something new with a model without losing or potentially corrupting my work, I "save as" another file name describing the changes I'm making (i.e. working model, extended posts, 48 inch rafter spacing, etc).  This ensures that I can always go back to the master copy without having potentially lost any progress.  Also, I always keep each subsequent revision as a separate file (named rev 1, rev 2, etc) until i arrive at the final product I am happy with, so I can always go back to a previous revision and try something new.  It also allows you to open more than one instance of sketchup and compare models side by side as well as copy and paste objects or groups from one model to the next.

Dan

[Rooster]

John,

I hope you are having as much fun designing and engineering this building as we are watching you learn through the process...thanks for including us.

You are really starting to dial this "pig" in!!

So, I want to play "devil's advocate" for just a second and have you re-visit the snow load requirements.  The roof systems keeps growing even to the point it looks like it is even outgrowing itself...Eh? :D

I like the addition of the ridge beam, but it is only supporting the top 25% of both sides of the roof...right?

The purlin will take more of the roof load (50%), with the top-plate taking the last 25%.

Your rafters will only have to be big enough to span the distance from the peak to the purlin or the purlin to the top-plate.

I still would encourage you to go longer on your diagonals from your posts to the underside of your tie-beams. (wind-load stiffener)

The posts that support your ridge beam are now in your living space...ugh!
Here is a picture of a simple truss that you might be able use to carry the ridge beam without the additional posts.

Good job!

Rooster



 

[Ljohnsaw]

Rooster,
Yes, it is getting big but lots of fun.  I understand your concern about the posts in the living space, but, there will only be one...

Starting from the east end (the openings in the foundation are on the west end), there are two bedrooms and that middle post will be the ends of walls dividing up that space. The next post west will be part of the dividing wall making up the kitchen/bath area on the north side of the building.  It will only be the next post west that will be in the living area that will be ell shaped. It will help deliniate the space of the "dinning room" in this open plan. The second floor is just a loft area at this time so the posts up there don't bother me. It will be easy enough to incoporate into wall in the future if I want.

Yes, I totally understand the loading. While the ridge beam is "only" taking on 25% of each side of the roof, that still amounts to a 10,000 lb on each 12 foot beam. A scary amount of load!
I can go through the exersice and post my steps again on this roof design.  I would hope that someone could point out an obvious error in my logic and everything could shrink!  Looking at other cabins in the area gives me some validation in my calcs - they are built very stout.

[Ljohnsaw]

OK all,

Here is my latest - haven't had a lot of time with work getting in the way and all.  : I laid out my first floor floor joists.  My calc says 3x12 36" OC will hold themselves, 2" flooring and 40 PSF live load.  I put them at 35" so I would get a joist either side of the center posts.  Should I bother with that or just run the pattern at 35" all the way down the cabin?  I'd have to put some blocking at the posts to hold the ends of the flooring.  I'd save 2 or 3 joists doing even spacing.

I did a little messing around with the bracing and you can see where I'm thinking based on the first bent.  I upped to 36" where I could but I need some ideas about the principal purlin bracing going across the cabin (short dimension).  I have a lot of them but thinking I could do with less.  The cross beams in the center 3 bents are 10" wide so I suppose I could do a crossing brace.  That way I could lengthen them and put the brace into the outside purlins instead of the outside post - good idea or not?

I need a little help with SketchUp - When I put some of the braces in, I sometimes forgot to open the bent "component" so some of the braces are on their own.  I have layers so turn that on and play with removing each layer.  Is there an easy way to add items into an existing component?  Also, I was watching a YouTube tutorial but the guy was using shortcut keys so I'm not sure what all he was doing.  There is something about selecting an element from a component and making a unique copy - one not associated with the component.  Anyone know how to do that?  For instance, I design a brace in my end bent (component) and want to include it in my middle three bents, which are component clones.