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Span, rafter question help...

Started by jpgreen, October 16, 2006, 10:59:25 AM

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jpgreen

Thanks Don-

I just got in for the day... 8)

I'll post a pic here soon, so you can see what I'm doing...  8)
-95 Wood-Mizer LT40HD 27 Hp Kawasaki water cooled engine-

jpgreen

See if these work:





Rough drawings to give you an idea of what I'm doing..  8)
-95 Wood-Mizer LT40HD 27 Hp Kawasaki water cooled engine-

jpgreen

Oops.

Forgot to add the 6x6's running on the sides of the 26x16 room, under the end rafters..  ;D
-95 Wood-Mizer LT40HD 27 Hp Kawasaki water cooled engine-

Dana

I am doing something like this right now with rough sawn dimensional lumber. We call them lean to's around here. :)
Grass-fed beef farmer, part time sawyer

Jim_Rogers

Ok, so with the section that is 12' wide the run or the number you should use in the second line of Don_P's calculator for a Uniformly loaded beam is 72" not 24" oc.
First you figure your square footage. So it is 6' (72") x 2' (24" oc) = 12 sqft.
If you figure the load correctly taking the number of pounds per sqft (total load, which is snow load called the live load, and dead load the actual roofing materials used) round off to 100 lbs, and times the 12 sqft of space each one rafter will hold up it's 1200 pounds.

So if you run these numbers for the section with the gable roof it's 1200 pounds for the load, 72" for the span, 4" and 6" and then the values for the wood being used.

To explain what Don_P means by the "adjustment" stated above, you can take the standard book value and adjust it by certain factor values because of how it's being used.
For example if a floor joist is within 16" of another floor joist the two will share part of the overall floor load. This is called the "repetitive member factor". So there is an adjustment factor that says the book value can be increased. This adjustment factor is given as a small number that you multiply the book value by. Again for example Don has said: {take the book value and multiply it} "by 1.15 repetitive member factor." So if the book value is 875 you can multiply that by 1.15 and get a higher value to use in his calculator (1006.25).

There can be many adjustment factors that apply to the use of the beams. Understanding them and using them correctly is what structural engineers do.

We or should I say I, commonly don't use these adjustment factors, although I could, as it makes the beam or joist stronger if I don't. And usually no one complains if there building is stronger, unless it bumps the cost way up.

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

Jim_Rogers

Next you need to figure the other side of the shed "lean to" roof off the side.

Use the same values in every line in your calculator except change the span/run from 72" to  192" and adjust the weight from 1200 lbs to 3200 lbs (16'x 2' = 32sqft x 100 lbs per sqft).
I didn't run the numbers, but you may need to adjust the depth of the rafter until it passes...

I hope this has helped you to understand how to figure roof loads beam sizes....

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

jpgreen

Thanks Jim,

I originally figured it just the way you reposted above, I miss typed when I wrote the 24" OC.  That wasn't in the calculation, of course.. ::)

I used your figures and even added 30 percent in load weight and it all passes with what I'm doing.

Also want to comment on how valuable your and Don's info and help is here on this forum.  Holy cow.. where can a guy learn something like this without spending a fortune?  You've taught me an engineering class semester's worth of knowledge in one evening.

That's how I like to learn things.. ;D

I think this thread and all the others like it helps builders tremendously..   8) 8) 8)
-95 Wood-Mizer LT40HD 27 Hp Kawasaki water cooled engine-

Don P

Lets make sure. The 4x6's will not carry the 16' shed under that load, if it checked for you we need to do some more study hall  :D. I'm still seeing it as 4x10's out there, you have one heck of a snow load. The 4x6's begin to fail, at your load and spacing, at around 10' span. If it did check for you at 4x6 be sure that you entered inches not feet where appropriate.

Another rule of thumb, if the span doubles, the bending forces quadruple. The span doubles AND the load doubles, 2x2=4. Span eats up lumber fast.

The beam row under the gable roof /shed roof transition is the most heavily loaded. It will support 6' of the 12' bay and it will support 8' of the 16' shed. 6+8=14' x 8'(post to post)x 100 lbs per square foot=11,200lbs on a 96" span.

I was nibbling away at the edges of all this when I met everyone here, critical mass is a good thing  ;D Well, maybe not in N Korea  ::)
I used one of Raphael's tips today, it saved my client $500
Thanks to you all.

jpgreen

Now you're confusing me, which ain't hard to do..  :D

Under the "Simple Beam, Uniformly Loaded (Drop Down Species List) "

For the shed rafters I enter 3200 pounds total load, and 15lbs dead load for a 4x6 192" doug fir #2 beam and it passes.  I added 30 percent more weight for "Western Woods" pine for a total of 4420 lbs on the beam and it passes.

The beam row under the rafters will all have 6x8 stacked square log walls, so they'll have plenty of support. The only wall open will the outside gable row. I may yet enclose that too someday.

What am I doing wrong?   smiley_swinging_board
-95 Wood-Mizer LT40HD 27 Hp Kawasaki water cooled engine-

Left Coast Chris

Pat,

If you would like a set of stamped calculations for the building so you could permit it, come on down and lets sit down and go over your design.  If you do the drawings, it would not take long for me to do the calculations then you would be ready to go to the building department.   You could check with the building dept. to see if they are o.k. with using say #3 grade (lowest value in the UBC for structural pine) and no grading stamp.   

You need to consider several things:  connections of your collar ties (at joint over the wall and at ridge), connections from your beams (or ledgers) to the posts, footing size and reinforcement, lateral design (seismic and wind).   I would need to do a few details but I could rough them out and you could draw.    My time to get it to pass code would be say around 12 hours or so including the calcs and details.  We could talk cost if you want to call...... It would not be much since it is a FF project and I would have fun working with you.
Home built cantilever head, 24 HP honda mill, Case 580D, MF 135 and one Squirel Dog Jack Russel Mix -- Crickett

Raphael

Quote from: farmer77 on October 19, 2006, 10:38:53 AM
Pat,

If you would like a set of stamped calculations for the building so you could permit it, come on down and lets sit down and go over your design.
<snip>
It would not be much since it is a FF project and I would have fun working with you.

  Now there's an offer that's hard to refuse, to bad you're on the other side of the country.
Every time I talk to my engineer I get a billed $187.50/hr (1/2hr. billing)... And I passed on the chance to go to Clarkson.  ::)

  Actually I don't mind paying (as long as I have the money) he's the ideal engineer for what I'm doing and also a neighbor.  The last hour I paid for was more like three hours of socializing and 45 minutes of problem solving...  We share a property line and a pair of black bears.
... he was middle aged,
and the truth hit him like a man with no parachute.
--Godley & Creme

Stihl 066, MS 362 C-M & 24+ feet of Logosol M7 mill

Don P

JP, are you clicking on "Show Result" each time you change any input? That's the best guess I have  ???.

One possible error, the 15 lbs dead load is 15 lbs per square foot so the entry there should be 15 x 32 = 480 lbs. This is the assumed weight of the rafter, sheathing and roofing in the 2' wide section. This is deducted in the calculator in the deflection check, it is checking live load (snow load) deflection.

Farmer77, your expertise would sure be welcome  8)

Left Coast Chris

Hi Don........ Im not familiar with the program you guys are using so I could not comment on the outcome.    It appears that the connections are likely to control so as far as the three hinged arch I do a hand calc for the loading (considering allowable slope reductions for snow) then input into the Woodworks program to size the individual elements (joist and tie).  In my experience the connection over the walls where the tie laps the joist needs numerous nails or at least a couple of bolts for the bigger loads and required bolt or nail spacing can require upsizing the members.  The Woodworks program takes care of any loading condition and offers all of the major species allowable loads.  It looks at all of the loading combinations required by the UBC (Uniform Building Code) and takes care of load duration, size factor, allowable deflection, repettitive member and the such.

I have found that it is best to sit down with the owner or builder (preferably both) and flush out preferences in construction materials, hardware, connections, erection etc before attempting to do any calculations.  This way the builder is happy and we can make use of lower cost or already available specific materials. 

Pat't project is pretty straight forward and looks like fun........hope it goes well.
Home built cantilever head, 24 HP honda mill, Case 580D, MF 135 and one Squirel Dog Jack Russel Mix -- Crickett

jpgreen

THis is what I entered-

Total Load on Beam(pounds) ....4420

Dead Load on Beam (pounds) ....480

Span of Beam (inches) ....192

Width of Beam ...4

Depth of Beam ...6


Select Species  .....#2 Doug Fir B+SSS



I hit show result and it passes.  How are you getting a no pass Don?
-95 Wood-Mizer LT40HD 27 Hp Kawasaki water cooled engine-

jpgreen

Hi Chris,

We posted at the same time.  I'll PM you with some details and I appreciate the offer..  8)
-95 Wood-Mizer LT40HD 27 Hp Kawasaki water cooled engine-

Don P

I just entered the same numbers and failed in bending and deflection. It showed a 4.21" sag in the rafter (I think it was being polite), shear checked ok. I've got a glitch somewhere  :-\.

Are you running something other than Windows? If so can you try it with Windows?

I would like to have the Woodworks program, kinda spendy for an amateur. I know what you mean about the heeljoint connection, I had no idea how much force was there till I started calculating it. The calcs we are using are just engineering formulas I've put in javascript, kinda like online spreadsheets. I saw a need for something free and able to get us in the ballpark.
You'll find a thirsty audience here if you care to post more on engineering and design.  I would love to see the outcome and calculations you guys come up with if you all are comfortable with that.

jpgreen

I run a Mac Don.  Maybe that's the problem?
-95 Wood-Mizer LT40HD 27 Hp Kawasaki water cooled engine-

Don P

It must be.
I'll need to put a notice on the calcs, that's the part that worries me the most, possible bad results.

Raphael

Quote from: Don P on October 20, 2006, 12:20:56 AM
It must be.
I'll need to put a notice on the calcs, that's the part that worries me the most, possible bad results.
  The OS really shouldn't affect the outcome of the calculations unless it defaults to something other than base 10 for math.???  I suspect it's related to the web browser being use, something like my result interpretation error in Opera 3.61.
  It might be good to create a simple test to check compatibility of the client browser, you could give one or two sets of input values for the client to enter and the expected results.  The browsers output could then be checked against the expected output to warn the user of any problems.
... he was middle aged,
and the truth hit him like a man with no parachute.
--Godley & Creme

Stihl 066, MS 362 C-M & 24+ feet of Logosol M7 mill

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