Span, rafter question help...

[jpgreen]

I'm new to construction engineering, and I'm trying to make sense of load and span calculations. I've got specs from The WWPA Online Lumber Technical Guide with my snow loads.

One question is... what would a 4x6's equivalent be in a 2x? Is it as strong as a 2x12?

I have a clear 12' span to cross for an open truck bay (12x26), that will be an extended part of my building, with a gable roof at 7 in 12" pitch. I want to use 4x6's on 24'" centers.  It will stand on 6x6 posts every 8.5'.

Using the WWPA chart for 80lb snow and 15lb live load (for my species) they show 11' 6" span for a 2x12  24" O.C., but do not list a roof pitch.

How far away am I from this with 4x6's? 

If I was to use a 2x6 ceiling joist and a 2x6 collar tie beam above on every rafter, does this substantially increase my span strength?

Or I was thinking of a 2x6 ceiling joist on each side of my 4x6 rafters (across the 12' span), then a 4x6 king post.  Is this necessary?

Sorry for all the questions, but I'm trying to get more of a grasp of this stuff.

Thanks,

Pat

[TW]

Some thought from one who learned wooden truss design last year.

A truss that consists of only rafters, not tied togeather by ceiling joists at the bottom will spread and collapse forcing the walls outwards. Yoy have compression in the rafters. The rafters are not vertical. That means that there will be a horizontal component pushing the lower ends outwards. (like standing on wet ice with splayed legs) That force has to be taken up by something, for excample a joist or a long bolt from side to side.

With a simple triangular (or kingpost truss) truss there will be both compressive and bending forces in the rafters.
You may have to reduce the compression resistance because of buckling. I do not know how that reduction is done in your construction code.
The bending resistance W is given by 1/6 x b x h x h  when b is the breadth and h the height of the beam or rafter.
You will forther have to calculate the number of nails needed at the junctions and check that it is smaller that the allowed nomber of nails on that size and shape of surface.

I cannot explain all this in detail because I do not know the words in English and to not have time spending several weeks explaining it.

Do not do this before you can do it right.

[beenthere]

For a partial answer, depth of a beam has more contribution to strength than the width. So, NO, the 4x6 is not as strong as the 2x12, all other things being equal.

Similarly, the 2x12 laying flat will not have the same strength as the same 2x12 standing on edge (for comparison).

[TW]

Sorry h means depth and b width

[solodan]

Pat 4x6's 24oc seems like it would be fine, and I imagine you are using doug fir, but I don't know what the calculations are. I hope someone will chime in on this, and is there a program or a calculater that already has these calculations set up? For what it's worth, I own  another house less than a half mile from my house where the snow load is 300lb. The roof is 4x6 doug fir, 4 ft on center, with no ridge beam, but there are 4x 6 floor joists that run to the center of the house, on the side with the loft and 8 ft on center on the other side, with posts all the way to the ridge, so not a clear span, but, 2 seperate spans way over 12 feet. The shear on the roof is 2x8 pine. the pitch is 6 in 12 on one side and 12 in 12 on the short steep side, every thing still seems fine after almost 50 years ??? I agree with TW, you need something to tie the two outside walls together. so joists tying the outside walls together would give you a small stoage area above.

One more thing to take into consideration is that wwpa makes their calculations based on the actual size, not the nominal size. That 2x12 is only a 1.5 x 11.25. Alot less wood than a true demensional 2x.

[Jim_Rogers]

One answer is to go to a site like www.beamchek.com and read the Introduction to Load Paths and Wood Design in the user resource section under publications.
Then you'll understand load paths and how to overcome them with proper design.
You may be able to download a trial version, not sure, there is a notice there about being closed for a while.

In the mean time you need to answer the question of what type of wood you'll be using.
Is it Douglas fir?
And are the full dimension or store bought?

Jim Rogers

[Jim_Rogers]

To answer solodan, the program to figure this is right here in the tool box on the left side of your screen.
Don_P's calculators.
First of all, the loads are figured as if the rafter wasn't on an angle as gravity loads straight down. So rafter length isn't counted only the run of the rafter which is 1/2 of span. He says the span is 12' so 6' or 72 inches is the span.
Next he says 80 lbs, snow load and 15 lbs live load. Well, I'll have to assume he means 15 lbs dead load, (the roof materials themselves, decking and shingles) as the 80 lbs snow load is the live load. Gives you a combined load of 95 lbs per sq ft.
I rounded it up to 100 to make the math a little easier and to figure it oversized.
So 2' oc means 1' either side of the rafter is being held up by that rafter so 2' x 6' = 12 sqft.
12 sqft x 100 lbs per sqft = 1200 lbs.
Open Don_P's calculator, for uniformly loaded beam and enter 1200 for the first number. Then the span/run in inches = 72. Next the size of the beam, again in inches (4 x 6). Now look at the chart for the values for the next three boxes.
I used Douglas fir-Larch #2 with the values of 875, 1.3 million and 85.
Click on "show results" and the answers say the beam passes on all tests......

If you're using a store bought beam change the sizes to 3.5" by 5.5" and it still passes...

Unless he's using some other lower grade of timbers these should work.

But these numbers should be checked by someone of his local area who is totally familiar with engineering these roof systems.

Jim Rogers

[HARLEYRIDER]

jim...your the man 8)

[Don P]

Going back to TW's comments, this calc should give you the rafter thrust and ceiling joist nailing schedule. Keep the ceiling joist down on the plates or holler back  ;).
http://www.ls.net/~windyhill/Calcs/Sloped%20Rafter%20Design%20for%20Bending1.htm

I came up with 2x6 and checked the American Wood Council's Span calculator, if I did it right it's output will show here;
AWC  Spancalc

Notice rafter span is horizontal distance from ridge to wall not the entire room span, the 2x12's could be refigured. Also if you noticed in TW's post depth is favored to the square of width, or as a rule of thumb, "Deeper is cheaper".

The beams post to post under the rafter tails are supporting the real weight. I'll assume a 1' overhang for 7' rafter span x 8.5' beam span x 100 psf load = ~6000lbs uniformly loading the beams. I'm coming up with 6x10's in solid sawn dougfir or 3 plies of full dimension 2x8. Explanation of that depth difference, 2x dimensional stock carries higher design values than heavy timber since you can see more of the wood involved, there are also several adjustment factors for 2x lumber that aren't there for heavy timber.

I checked 2x6's for ceiling joists, no storage, and passed in dougfir on the awc calc.


... any of you guys up on the "method of joints" for calculating loads in truss design?

[Jim_Rogers]

... any of you guys up on the "method of joints" for calculating loads in truss design?

I'm ready to learn.....

Jim Rogers

[Don P]

I was hoping you would tow me to shore  :D

[solodan]

Jim and Don, thank you, I was definately interested in what you guys said, and I fiqured you guys would both have an answer to Pats question. Pat has not replied back so maybe he is out fallin some doug fir ;), I know he has a bunch up his way.I also found my answer in the tool box here in DonP's calc, right after I responded this morning. Sometimes I'm just thinkling out loud, but that's why I come here.

[jpgreen]

Hey Dan...

Just got in from out of town, and I skimmed though everyone's replys... and I appreciate them.  My brains running on bout' a quarter power right now, so I understand the word "pass" pretty well..  :D

The 4x6 rafters are clear, and I mean real clear with only a remote few tiny knots of either Western Pine or Sugar Pine, full 4x6, as all my lumber is my milled ruff sawn.

I'm using Doug fir on the ceiling joists, girts, and posts, and collar tie beams if needed. The WWPA specs were there lowest rated "Western Wood", which I beleive to include pine. We have an 80lb snow load requirement up here.

Keep the comments coming.  I've got to learn this stuff.......  8)

[jpgreen]

Also I didn't have enough DF to do the whole building, and I had this nice pine, so I hope I can make it work.

The pine also came from 28" diameter straight logs.

[jpgreen]

Good morning'...

After a few cups of Joe this morning, it's starting to make sense. I followed Jim's lead and did some more figuring.

The building extends off one side of the gable (referenced above) with the 4x6 rafters for about a 17' run, plate to plate.  The pitch changes to a 5/12.

I used Jim's model (if I followed it correctly) taking 2'x17' and multiplying that by 100 and came up with 3400 lbs. total load on beam as the first number in Don_P's calculator.

I entered the 24" OC, 4x6 size and No2 DF, and it passed. Tried it with "and grade" (what does "and grade" mean?) and it passed.

I thought that I was going to have to run a supporting beam under the 17' run, but it looks like I don't have to now... do I?

My dead load consists of 6/4 roof deck, 15lb roofing paper, and 28ga steel rib metal roofing.

Did I get this right?

[jpgreen]

Next,

Since I'm using pine for my rafters, I added 30% to the load for a total of 4420lbs load on beam as I have data here in various books, that show this pine about 25% less in strength compared to Doug Fir.

It still passed...  8)

Hope I'm doing this right..  :-\ :'( :-[

[TW]

What is that "method of joints"?  I am just curious.

[Don P]

Wooosh! You just blew right over me  :D
Can you post a plan view looking down from above showing existing and proposed, rafters, ridges and posts?

I must not have made the species and grade selection box very intuitive, sorry 'bout that, suggestions? What I mean for you to do there is click the arrow on where it says "and grade", a drop down box should appear, scroll down till you find the correct species and grade, click on that and it'll insert the correct design values.

The drop down on the beam calc is set up with heavy timber design values, 5x5 and larger. B+S stands for beams and stringers, one dimension is at least 2" greater than the other. P+T stands for posts and timbers, square or nearly square sections.

The drop down on the rafter thrust calc uses design values for dimensional lumber. Dimensional is a keyword that means 2-4" thick. If you plug in #2 Doug fir in the heavy timber beam calc drop down, you'll notice allowable bending fiberstress is limited to 825 psi. On the dimensional lumber drop down in the rafter thrust calc, #2 dougfir/larch is allowed 1190 psi in bending. That's the difference I was talking about in heavy vs dimensional timber.  

I'm curious so looked it up. This is getting in kinda deep, feel free to skip or skim  :D
#2 Dougfir 2-4" thick, base design value is listed as 900 psi in bending
I can multiply that by 1.15 repetitive member factor, since you have 3 or more rafters that are 24" or closer together.
I can multiply that result by 1.15 duration of load factor as its a snow load and not present for most of the year. This assumes cumulative 2 month full loading in the life of the structure. 1.0 would be appropriate for 10 year cumulative full load, .9 would be appropriate for permanent full load.
I can then multiply that result by 1.3 form factor to account for the 6" depth of the lumber.
Taking all the adjustments, allowable bending force gets up to 1547 psi ... remember we started with a base design value of 900 psi. In dimensional lumber, the adjustment factors are a big deal, if in doubt keep it conservative.
As you can see I didn't take the form factor adjustment in the rafter calc. so I used Fb1190 psi. That is why for confirmation on dimensional I like to check the AWC's or other "official" calcs and tables.  

[Jim_Rogers]

I entered the 24" OC, 4x6 size and No2 DF, and it passed. Tried it with "and grade" (what does "and grade" mean?) and it passed.


Did I get this right?

No, the 24" oc is the wrong value. You need to use half the span. Half of 17' is 8'6" which is 102", run it again using this number.
Adjust depth of beam until it passes.......

Jim Rogers

[Jim_Rogers]

And/or use Don_P's value for the first number after the sizes of the timbers......

If you increase the total load to 4420 you'll need a much larger beam.......(I think, but I didn't run the numbers again)......

Jim Rogers

[Don P]

TW, I was writing while you posted,
The metod of joints is one way of analyzing loads in a truss. A Maxwell diagram is another graphical way of doing it.
I've been bumping up against that wall recently, it shows where I fell asleep in math. I was trying to calculate each panel point load on a simple 7/12, 12' wide, 100 psf fan truss. I'll try a bit longer and start another thread, it might be real short  :D.

[jpgreen]

I entered the 24" OC, 4x6 size and No2 DF, and it passed. Tried it with "and grade" (what does "and grade" mean?) and it passed.


Did I get this right?

No, the 24" oc is the wrong value. You need to use half the span. Half of 17' is 8'6" which is 102", run it again using this number.
Adjust depth of beam until it passes.......

Jim Rogers

Jim- I don't follow?

This new 17' has NO gable.  It's one striaght rafter at 5/12. So the span is the full 17'.  It's a different part of the building.  But it's still 4x6 timbers on 24" centers.

Are we talking about the same thing?

[jpgreen]

You guys move pretty fast, so it takes me awile to absorb the info..  :D

[jpgreen]

Don-

I tried all the numbers on everyone of your Doug Fir options, and they all passed. Can you ad species to your list for us westerners including our pines?

What would be the weakest species of wood on your list?

[Don P]

These numbers are for dimensional lumber, 2-4" thick, keep it simple all values will be #2 grade.
Don't forget the adjustments to Fb, none to MOE or Fv. For rafters I'd be comfortable with a 1.3x adjustment. I'll add some more species to the drop down.

species	Fb	MOE	Fv
Doug Fir (south)	850	1.2	180
Hem-Fir	850	1.3	150
SPF (south)	775	1.1	135
Redwood	925	1.2	160
WRC	700	1.0	155
Western Woods	675	1	135

SPF (south) is Balsam fir, Black, Englemann, Red, White, and Sitka Spruce, Jack Lodgepole and Red Pine.

Western Woods is Any in the dougfir, hem-fir or SPF classes plus Alpine fir, mountain hemlock, Idaho White, Ponderosa, or Sugar Pine.

There should be a popup on the manual entry beamcalc with heavy timber (>5x5") design values for western species.

OK, I think I'm following you, this is a whole different roof. Its a shed bearing on the building and posts set 17' away?

A 17'simple span with a 4x6, got my doubts, lets work it.
17'x2' wide=34sf x 90psf (80ll/10dl)=3060 lbs on each 4x6 rafter in a big snow on a 17' span.
I'm going to use the manual entry calc
I entered;
3060lbs
204" span
Fb 1190
MOE 1.2
Fv 180
I hit paydirt with a full sawn 4x10
The AWC calc was still down at a 16'3" span with a double 2x12 (3x11.25").

Don't worry I won't hit you so fast tomorrow, the rain appears to have stopped  ;D.

[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)

[jpgreen]

See if these work:





Rough drawings to give you an idea of what I'm doing..  8)

[jpgreen]

Oops.

Forgot to add the 6x6's running on the sides of the 26x16 room, under the end rafters..  ;D

[Dana]

I am doing something like this right now with rough sawn dimensional lumber. We call them lean to's around here.

[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

[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

[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)

[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

[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.

[Raphael]

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.

[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.

[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?

[jpgreen]

Hi Chris,

We posted at the same time.  I'll PM you with some details and I appreciate the offer..  8)

[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?

[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]

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.