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Structural Engineer Review – Small Cabin Roof System

Started by Doug_D, July 08, 2010, 06:07:11 PM

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Doug_D

Hey all-
Getting ready to start putting the exposed beam roof on my cabin (FINALLY!) and starting to have second thoughts about spans and spacing.  I had been told to put the rafters (5x8) on 32" centers with 1x6 T&G white pine decking.  Above the decking I will be using nail base 8" SIP panels and then a simple metal roof.  The ridge beam is 6x12 (Engelmann Spruce) with a 12" peeled white pine supporting it in the center of the cabin.  The cabin foot print is 24x36.  I started to put together a google sketch up of the roof system with the idea of running it past a structural engineer (SE) just to be sure.  But before I engage one (SE) and start the meter running ($$$) I wanted to see if this sounded reasonable?

The rafters will be just a tad under 17 feet and will be cut plumb with the walls.  There will be a loft covering half of the cabin so it is my plan to use the floor joist double duty as collar ties too.  The other half of the cabin I was planning on using one maybe two peeled white pine logs to span the top of the walls as very large collar ties (24's span).  The gables will be stick built using standard 2x stuff.  I will try and post some pictures of the model tomorrow if I get a chance...

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Jim_Rogers

Tomorrow, I'll try and run the numbers and see how they look.
What is the snow load for your area?

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Doug_D

Sorry Jim.  I knew I would forget something :-) 

The last trusses I had designed used 15 psf  snow load (for the house). 

I will try to post that sketch up model tomorrow to help give you guys a visual.

Thanks for all the help!

Doug


Our adventure in Building a Log Cabin Rental Business!
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Jim_Rogers

Ok, so now we know some of the facts.

To summarize them, the structure is 24' wide and 36' long.
The ridge beam is 6x12 and is supported mid length at 18' from each end.
The rafters are 5x8 and are spaced at 32" on center.

In plan view a small part of the roof system should look something like this:



The area shaded in green is the area being held up by one rafter. It is half the distance between rafters on each side so that means it is the same as the spacing or 32". The length of the area is 12'. The reason that it is 12' is that the load, by gravity is vertical, and therefore half the span (building width 24' / 2 = 12' is called the tributary length).

This shaded area is called the tributary area, as it is the length times the width. This is the area that the "Live load" is applied to.

The live load is the snow load should it snow on the cabin.

The other loads this rafter will have to support are the dead load, actual weight of the roofing materials. And any wind load.

The dead load is figure based on actual length of the rafter. So you have told us that the rafter length is 17' and again the spacing is 32" so the deal load area is 32" x 17' or 45.33 square feet.

Figuring live load you take 32 sqft times 15 lbs per sqft and get 480 lbs.

Figuring dead load, you'd need to estimate the loads of each part of the roofing materials. Or actually weight a square foot of each and get a load.
Usually this is rounded off to a standard amount based on previously constructed buildings and it is normally around 10 lbs per sqft.
However if we looked at the actual numbers such as one sqft of t&g hopefully dry, but we'll round up and call it 3 lbs. And then the 8" thick nail base foam panel at 2.7 or round up again to 3 lbs, and last the metal roof and I don't have any information about the weight of metal roof panels at this time. So lets just say 3 lbs. So now where nearly at the usual standard of 10 lbs and to be conservative we could use this and if the weights are less then the roof is stronger.

So now we can figure the roof dead weight of 17'x 32" = 45.33 sqft x 10 lbs per sqft or 453.3 lbs. Adding that to the "live load" we now have the combined load or total load of (453.3 and 480 = 933.3 lbs)
This is the figure we'll use to calculate the rafter size and see if it will pass at this size and spacing.

Next, we go to the forum tool box and use DonP's calculator for uniformly loaded beam, as this is what a rafter is.
Selecting the beam and column calculator index we select the second calculator down that is titled "Simple beam, uniformly loaded, drop down species list" and we get a calculator that we can enter values into and we begin.

First we enter the value for the "live load" as DonP has given us two boxes to enter the values so that we can have a more exact answer. We enter 480, and then in the next box we enter 453.3.
Next is the beam length in inches. Again as this is a rafter we use 144 for 12'.
Now we enter the width of the beam 5" and the depth of the beam 8".
In the next box we use the pull down arrow and select the species of the rafter so that the calculator will enter the values from the look up table.
We select ?

Opps were stuck until Doug_D tells us what the species is..... But to try it we could enter #2 Eastern White Pine B&S as Englemann spruce is not available.
B&S is defined as beams and stringers as the rafter is deeper then 2" of the width.
And all timbers should meet the grade 2 rule.

Then click the "show result" button.

We get three answers to the three most important questions defined above the calculator.

In fiber stress in bending it passes as we need 15 and it giving us 53.33.
In deflection it passes even in the roof value.
In shear it passes, so we are good to go.

Now for double checking, let's change the length, should I be mistaken from 144 for 12' to 204 for 17' and click "show result" again.

It still passes on all three test. But we'll note that the deflection is very large. That's point 85 more than 3/4 of an inch when fully loaded with a heavy snow load of 15 lbs.

This would be something to be concerned about if the underside of the rafters were going to have sheet-rock hung from them. It may cause the sheet rock to crack.

When we have more info of the type of wood that will be used we can adjust our entries into DonP's calculator and see what comes out.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Doug_D

WOW!  Jim, that is great.  I've got an email into the supplier to ensure the species.

Thank you so much for doing this.  I have often wondered about running these numbers on other projects. 



Our adventure in Building a Log Cabin Rental Business!
www.cabinstartup.com

GaryinMississippi

Jim,

Where is the tool box with the calculators that you mention above? 


Never mind!  I found it right in front of my nose!

Gary

Jim_Rogers

Doug:
I got an email from DonP saying he updated his calculator to include Engelmann Spruce. If that's the type of rafter then we can redo your calculations.....

Let us know when you find out.

Jim Rogers 

Edit: I just re-read DonP's email and it says that Engelman spruce is in the group labeled SPF for Spruce-Pine-Fir so that we should use that value in the pull down box.
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Doug_D

Jim-

You guys have been very patience and extremely helpful.  I plan on giving the supplier a call over lunch to double check.  The two of us have kind of been playing phone tag the past few days.

Thanks again!
Our adventure in Building a Log Cabin Rental Business!
www.cabinstartup.com

Doug_D

I was able to confirm the species to be engelmann spruce over lunch today.

Our adventure in Building a Log Cabin Rental Business!
www.cabinstartup.com

Jim_Rogers

I just re-read DonP's email and it says that Engelmann spruce is in the group labeled SPF for Spruce-Pine-Fir so that we should use that value in the pull down box.

So running the number again using this value for the rafters, same load, same spacing, same size rafter we see that it again passes for all three tests.



Doing the same double check and using the rafter actual length, to be sure, we see that again this passes on all three tests.




So, I'd say you could be good to go, but you may want to have someone else double check all this to be sure.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Doug_D

Jim thanks a ton for the help on this.

I am going to see an engineer friend of mine tonight and discuss.  She is a PE not an SE, but will point me in the right direction.  She is pretty good about telling me when I do and don't need to be concerned.

I also started tracking down SE's on Elance.com.  Pretty cool site.  I am getting the impression I can get an Illinois SE to review this for a few hundred bucks.  May just be pretty cheap insurance.

Thanks again,
Doug




Our adventure in Building a Log Cabin Rental Business!
www.cabinstartup.com

fred in montana

Nice job explaining the structural analysis!

There is the assumption that the walls are rigid and cannot bow outward such as the case of having ceiling joists. If there are no ceiling joists or other bracing (collar ties, intersecting walls...), then the wall should be checked for resistance to lateral loading.

edit: Sorry, I just saw that you do have a ridge so the walls couldn't bow out.
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