New rafter calc, kick the tires

Don P · May 08, 2006, 07:29:04 AM

I'm hoping to make a few calcs from what I learned last week before it slips.
Rafter in bending check, thrust at celiing joist connection and tension at lap in ceiling joist. It checks with the professors example.
http://www.ls.net/~windyhill/Calcs/Sloped%20Rafter%20Design%20for%20Bending1.htm

His example was
#2 2x8 SPF
Loads are #0 psf snow, 10 PSF dead
16"oc spacing, 133" span
8/12 pitch, Fb 1389
E 1.4, Shear 155, Nail 91

Jim_Rogers · May 08, 2006, 10:19:12 AM

Nice calculator.....
I tried it and then added 50 lbs for snow load to see what would come out....
Very interesting.....

Don, can I make one suggestion to your labeling?
Add the abbreviation to the labels. That is, for example, on the line that says: Maximum Allowable Fiber stress in Bending (PSI) add the "Fb" so it reads Maximum Allowable Fiber stress in Bending (PSI)(Fb).
This will help each user to input apples to apples, and it just seems easier to understand which line to place which value....
This, of course, is my opinion.....

Jim Rogers

Don P · May 08, 2006, 06:40:37 PM

Thanks Jim, good idea, I'll make the change.
I see I was typing too fast this morning, the professor's example used 30 psf snow, 10 psf dead load for a total of 40 psf.

Right now I've skipped the first lessons we did, joists and beams and multiply headers and girders, this calc is a combination of the next 2, design of rafters and nail connection design with rafter thrust. In his first lessons there are a couple of neat equations for figuring out the frequency of the floor, the girder, and the system, to avoid an annoying bouncy floor.

After that we had truss uplift, twisting in headers and girders, columns and bolted joint design. I'll try to put alot of that into calcs, they help me keep from making dumb math errors.

The NDS did change again :

. Its down to 2 books, they did bring in LRDF engineering but still allow ASD, Allowable Stress Design, what we've always done. They call it dual format. It does change some math, most noticably in column calcs. Be prepared to see "new" subscripts on E numbers. The old math will yield the same results, but there is a new column in the design values charts called E_min. The E_min value was derived like Fb and Fv using the 5% exclusion limit. Remember E is just an average of stiffness, great for floor or roof deflection. If I remember right there was a coefficient in the column calc before that modified E. Reducing an average by a safety margin was not the "right" way to do it for columns so they switched to the new way.

Jim_Rogers · May 09, 2006, 09:19:15 AM

DonP:
It looks good now.
The picture link might have gotten lost as it didn't show when I opened it.
You might need to check that.
Also, can you list the normal nail name/size that we see at the stores?
I have no idea what a .135 nail is, in regard to size....
I understand that this is what is in the books but we need to use these in the real world.
And real world labels will help, the layman.....

Jim Rogers

Don P · May 09, 2006, 10:28:41 PM

The picture is linking to my desktop just fine ;D

The nail chart for that section is pretty extensive and they did try to list pennyweight alongside their diameter entries. It gets pretty confusing,
the old pennyweight names have become pretty blurry, a 16d will have different diameters depending on whether it is a common, sinker, box, or gun nail. I think a popup chart with some frequently used combinations and pictures of commonly used nails might work. I took the popup for design values off this calc because you could be working from either the dimensional lumber or heavy timber charts of design values. As one rafter question progressed over the past day the design values for the same lumber increased by 15%. When the rafters got to 24" centers they are considered to be load sharing and their allowable bending load is increased 15%. A section in the program for automatic adjustments to lumber design values might be the thing for that.

Taking the nail part out might be the way to go too. Depending on the situation nails aren't the only solution to that problem. The first part of the bottom section gives thrust, then nails required to restrain that. The calc can go far beyond nails in a hurry. The question last night started with me looking up bolts to restrain thrust that was beyond what nails could be expected to hold. It might be better to give the value and let people figure the connection in another step, that can be confusing too though, I dunno for the most commonly used situations I like having the nails in there ??? .

BWG · May 10, 2006, 08:39:16 AM

Whoa! I guess I should have paid attention in math class. ???

Jim_Rogers · May 10, 2006, 08:50:05 AM

I think you should keep the nail section in.
Somehow you could/should define the nail diameters, but I don't know how or what would be best.
Probably pick one that is most common, say either nail gun nails or regular nails and go with that.
Then add a disclaimer saying "your nail size may vary."

Jim Rogers

Don P · May 11, 2006, 07:46:45 AM

BWG,
Your example was the perfect "monkey wrench". It had me flipping back and forth through the design manual referencing a bunch of tables. It showed me how tough an all in one kinda program would be.

When you first proposed your rafters they were 4x6. A 4x is the limit of one set of design values for "Dimension Lumber 2-4" thick". As I was trying to make a wide rafter spacing work at high snow loads it was looking like a 6x6 with a 2x6 collar on each side. The lower nail holding abiltity of cedar added another challenge, a 1" dia through bolt might work. Well, 6x6's go to a table for timbers 5x5" and larger, they reduce the design values from the previous table somewhat. Bolts go to another table and with bolts vs nails, load direction comes into play.

When the rafters tightened up to 2' centers another modification factor came into play, that 15% increase of the design values from the first dimensional lumber table.
That was the perfect pop quiz :D

I hope you can see why I'd like to make it work, for basic stuff the layman could then flip through options and come up with a code compliant, safely connected and sized roof. The devil's in the details

Right now it is working very well for typical dimensional stock on a simple roof. One thing that impressed me is that I need to pull the trigger on the gun more! I'll keep working on something for the nails.

Jim_Rogers · May 11, 2006, 08:30:39 AM

That's why engineers get paid what they do....
Each roof is different and you have to sort out all the details to make it right....
It maybe that one calculator can't do all?

Jim Rogers

Raphael · May 11, 2006, 09:29:40 AM

Somethings not right... It just told me I could replace my 5x7 spruce rafters with 2x2 EWP under a 50psf snow load and hold them in with 1.6 nails. :D

I don't think my building inspector will by that.

Don P · May 11, 2006, 11:58:03 AM

Whoa, sounds like you overbuilt ;D.
Run it again, pay close attention to units requested. If it still comes up wrong I need all your inputs, that ain't supposed to happen :-[.

Yup Jim you're right, I can't make it do everything, it should be just for simple gable roofs of various species, loadings and spans. The connections get too varied after nails I think. It does give numbers you can use elsewhere when it gets past nails.(I think I'm seeing the beauty of the truss plate, engineering wise). In your joint math, can you take that thrust and tension number and use it to design or check any joinery?
This section of the manuals is the basis for the prescriptive span tables in the codebook. Basically the math in this calculator is what is behind those tables, the main features of the calculator would be that it contains more species and gives some guidance on connections. Once you go past the prescriptive provisions of the code, its time for an engineer.

I think I can make it where you scroll through a species, size and grade list and make a selection. Then a button to indicate whether there are multiple members, snow load, ect for all the possible wood value modifiers. That will eliminate alot of the knowing to need which chart to read.

I haven't gotten into sizing the ceiling joist yet.

Let me know if I have troubles Raphael

Raphael · May 11, 2006, 03:17:41 PM

No joy in mudville :(
I reset the form, entered fresh data and still got flakey results. I reloaded the page, tired again and it was no different.
I should add that the very first time I used the calculator the results I got looked very plausible except nailing as I'd thrown in a low number for Max Nail Load. That was prior to Jim's suggestions and the addition of the nail data.

Heres my current input:

Live Load : 50 (#/sf)      Max Fiberstress (Fb) : 575 (PSI)
Dead Load : 10 (#/sf)      Modulus of Elasticity: 0.9 (million PSI)
OC Spacing: 32 (inches)   Max Horizontal Shear : 65 (PSI)
Horiz Span: 120 (inches)   Max Nail Load : 88 (lbs/nail)
RafterPitch: 10.5
RafterWidth: 2 (inches)
RafterDepth: 4 (inches)

And this is what I get back:

Max Moment (in-lbs)     : 4.39453125
Section Modulus Required : 0.04993785511363636
Section Modulus Input (in^3): 156000

Deflection (inches): 2.4481134100274725e-15
Max 1/240     : 0.0037500000000000003

Shear (PSI) : 0.0013188230227929375
Section Area Required : 3.4289398592616376
Area of Section Input (in^2): 7800

Axial Thrust at rafter to ceiling joist connection (lbs): 3.1317650714589624
Nails Required at Rafter to Ceiling Joist joint : 0.7829412678647406
Total Load per Inch on Rafter(pounds) : 43.40277777777778
Tension in midjoint of ceiling joists (lbs) : 0.5859375000000001
Nails required at Ceiling Joist lap : 0.14648437500000003
Angle of Roof from Horizontal (degrees) : 69.44395478041653

I notice the roof angle stays the same no matter what I input, at 10.5/12 pitch the angle should be ~41.3. I got that value on my initial visit. I've got one more thing to check on my end.

Don P · May 11, 2006, 03:50:00 PM

Eeek! I think I know what I did in that last change, hang on a bit :-[

Edit;
Well allrighty then, I think that got it back ... impact alignment ;D

Raphael · May 12, 2006, 11:00:46 AM

That's more like it.

BWG · May 12, 2006, 11:45:46 AM

Don P, I'm impressed,even though I don't really know how to enter in all the values. But that's my in experience, and not a problem with your program. Yes, it's hard to come up with an "all in one" program, but this one covers a bunch of variables very well....Nice work, my friend!

Raphael · May 13, 2006, 01:32:59 PM

Looks like my upper roof is good for a 45#/sf snow load, CT code calls for 30 but up here 40 is a more reasonable figure; at least for my house. The nailing data doesn't match my joinery as I have a false plate intervening between the common rafters and the floor joists.
I need to find a good Kf for that.

Don P · May 14, 2006, 08:57:57 AM

The thrust and tension equations were used in an example with nailing in our workshop and nailing is the easiest to program for so I stuck it in. There's pages of other connection details that are applicable to those numbers and he did explain the formulas for working those mathmatically rather than by consulting the tables, its heavy duty stuff for my little pea

. I personally feel those manuals should be a free download since the information is referenced in the code ... but don't get me started there.

I'm trying to figure out how to automatically enter the wood design values from a drop down list. Pick a species and grade and it'll attach the design values to that choice behind the scenes and then have the program adjust and use those numbers. Right now my computer skills aren't there. Whenever I get that figgered out it should make things easier, I want most of that right input column to go away.

Raphael · May 14, 2006, 10:26:13 AM

You need to be careful with drop down boxes to inform the client browser wether you are indexing from one or zero for the first entry otherwise you can wind up with half your visitors getting the wrong design values.
The drop down navigation box at the TF Guild site gets me with this from time to time going from work (MSIE) to home (Opera, etc).
W3C defaults to an index value of zero for the first entry but MicroSoft has split off from programming convention and starts indexing from one.
I usually make extensive use of five or six different web browsers (and a text editor) when working on any web page, it helps cut down on the supprises.

Jim_Rogers · May 15, 2006, 10:22:14 AM

We had an Excel worksheet program for sizing beams, that we used and got from the workshop I did last May or was it June.
Well anyway, one of my classmates took all the data (NDS values) and entered them onto page two, and when you find the values you wanted to use you entered the line it was on in a box and it would forward that line's values onto page one.
I don't know if that's something you can do with your program or not, but that's how he made it work, for your information.

Raphael · May 15, 2006, 01:15:18 PM

Is that the uniform load spead sheet from Northern Lights? That's one handy tool; I've been planning on getting a few Excel lessons from my wife so I can expand on it to cover point loads, rafters, etc.
I'll have to adopt that linked page for timber groups idea, I started adding to the right of those listed on page one and quickly found myself running out of monitor.

Jim_Rogers · May 15, 2006, 02:19:04 PM

No, it's not from Northern Lights.
I got it when I went to Heartwood School, in Becket, MA.
And I have modified it more to give more info.
I always try to design a spread sheet to be the size of a printed page.
either vertical or horizontal.
That way you can use the spreadsheet and then printed for including with the file....

Jim Rogers

Don P · June 19, 2006, 06:14:05 AM

I haven't gotten round to reworking the rafter calc yet :

Below is another calc, this one is for checking the vibration frequency of a floor. As we've been building longer spans, and living with more stuff, complaints have been increasing regarding bouncy floors, especially when built to code minimums. This is one more (4th) check while sizing joists and girders. It has nothing to do with strength or code, simply a serviceability check.

https://forestryforum.com/members/donp/floorfreq.htm

Don P · August 26, 2006, 10:10:51 AM

It came up in my mind again as I read the current truss thread. We bumped into it a couple of weeks ago too. When I made the simple beam calcs I made em simple. It applied total load to the deflection calc. That calculation should really only apply to the live load. I've added another line to the calc for Dead Load and it subtracts dead from live and than does the math. Since I usually use 10 psf for dead, it worked easier in my head to ask for DL instead of LL.

Read my post above though, deflection is a common complaint. Stiffer than minimum ain't a bad thing.

Anyway, please run a couple of scenarios and make sure I didn't bust nuthin :

. If it works ok I'll switch this to the old link

https://forestryforum.com/members/donp/beamclc06.htm

Max sawdust · August 27, 2006, 08:32:53 AM

Don P,
I see an error in the wood design values chart. Under Red Pine #1 beams and stringers the max fiberstress is 8750 instead of 750.

(I wished it was 8750 it would help me with my design dilemma :D)
Max

Don P · August 27, 2006, 10:31:12 AM

Must be high iron in the soil makes the #1 stronger there :D Thanks for the catch. I thought I was close to making a drop down selection box in the calc for that chart last night, the computer is not impressed this morning :D. I think I'll go stack rocks >:( ;D.

I showed one of the calcs to some building inspectors last week. My butt is still sore. Remember these are for design work only. We are in an engineer's area of expertise and they should review your final designs.