Truss Calculators

[Alex W]

Interesting to look at the differences in the outputs depending on the pinned vs fixed peak joint.  The question is, which is "correct"?  It seems that the pinned joint reduces the stress in the top cord slightly, so treating it as a fixed joint should be the more conservative approach, correct?  The other members seem to be unaffected from what I can see.

Regarding the application of this analysis to my problem, I have a question.  As I mentioned previously, I have a pole barn that I am trying to check the trusses on.  It is a 32' span with 48" truss spacing.  The trusses are of a modified queen design, with 2x6 top cord and 2x4 bottom cord, both are MSR lumber rated at 1650fb / 1.5e.  Since the Fink is the only truss with full analysis in your program I have mostly been studying it, but obviously it doesn't match my case exactly. 

The modified queen splits the top cord into 3 pieces per side and the bottom cord into 4 pieces (compared to the Fink which splits the top cord into 2/side and 3 across the bottom).  With that in mind, does it seem reasonable to consider the case of a 24' span Fink truss?  My reasoning is that a 24' Fink has (roughly) three 8' sections of bottom cord, and the length of the bottom cord between web connections should be a large factor in the required bending strength.  My modified queen with a 32' span has (roughly) four 8' sections of bottom cord, so at least in terms of bending stresses the results should be similar.  Obviously other stresses might be different due to the difference in total loading.

Thoughts?

[Medeek]

the pinned vs. rigid has been bothering for few days now.  After reading more papers on the subject I'm of the opinion that semi rigid is the best way to go.  I'm on the road for a couple of days when I get back I will see about getting the full analysis done for the mod queen truss.  The overall span seems to control the axial loads more so than anything.  Compare fink against a double fink.  I'm trying to type on an iPad so this response is brief.

[Don_P]

I'm back in from a week on the road, going back to the SP/SYP question... I'm not sure. The only time I've really noticed SP on a gradestamp it designated Scots Pine, you'll find it in table 4F.

Southern Pine to me is SYP and contains the 4 stronest species of southern pine. The stamp Mixed Southern Pine can contain any of 7 species of southern pine, those 4 stronger species mixed with or any individual species and carries the strength design values of the weaker species.

To be honest after last year's downgrade of SYP it's all approaching the same values in many cases. If you're checking a pre 2012 truss the old values would be appropriate.

For my $.02 keep all the conditions available. When I field build a truss it's closer to pinned, a manufactured truss is closer to fixed. In wood I don't believe it is ever completely rigid.

[Medeek]

I have all three options available for the fixity of the peak joint, the user can select which one best fits their situation.

Starting to work on the modified queen truss, here is the schematic for the matrix analysis of it.  The structure stiffness matrix will be a 30 x 30 matrix (900 values), its no wonder they didn't do this sort of thing prior to our modern computers, imagine trying to calculate this by hand.



Compare this to the fink truss, which has a few less webs and hence the computations are less 21 x 21 matrix (441 values)



These schematics really say nothing about which members are pinned, semi-rigid or rigidly connected.  The stiffness matrix (k) for each member is what determines that.



In my analysis I am treating all of the webs as pinned jointed on both ends and only capable of transferring axial loads (classical truss members).  The top and bottom chords at panel points are treated as rigid connections.  The peak joint is treated either as rigid, pinnned or semi-rigid, this is user configurable.  The heel joint is treated as rigid or semi-rigid.  My reasoning and justification for these model settings is based on a number of papers I have compiled on the rigidity of joints of MPC wood trusses.  I have saved each one and will compile a reference list at some point to accompany the truss designer documentation.

These two papers especially the bottom one were quite helpful:

http://design.medeek.com/resources/truss/DOCUMENTS/Paper_124.pdf

http://design.medeek.com/resources/truss/DOCUMENTS/20103.pdf

[Medeek]

The downgrades to southern yellow pine has got me wondering.  What was the primary reason for that? 

Visiting Home Depot lately I've notice their stud piles are basically complete garbage, I wouldn't even considering framing my house/garage with any of the knotty / warped stuff they somehow call "stud" grade lumber.  As the old growth timber has all but disappeared I guess we are now building with saplings.

[Don_P]

There ya go. The cell wall of a sapling is made up of microfibrils that run somewhat diagonally to the axis of the tree, like about 30 degrees. In mature wood the MFA, microfibril angle runs 0-5 degrees to the axis. Juvenile wood has a bad internal "slope of grain" within its' cellular makeup, we actually measure slope of grain on the macro level, this is a level deeper, unseen by the naked eye. That's the biggest reason but there are more.

In the 90's the American Lumber Standards Committee, ALSC, the quasi governmental authority who oversees the third party grading system, along with the Forest Products Labs conducted an "In-Grade" audit of the lumber available... they broke a bunch of boards, a whole bunch, of the commercially available species, sizes, and grades. They then refined the strength tables, the NDS supplement of design values for 2-4" thick lumber for commonly available commercial species was then based on the actual stock at hand. The other species and dimensions, say a red oak 2x6 or a heavy timber are still based on regressions from breaking a limited number of small clear samples and mathmatically degrading and applying factors of safety. For framing lumber it is based on massive real world testing to destruction. Look at pine or SPF and compare it to red oak, you know the oak is stronger than they are stating, there's less real testing so a higher safety factor.

Part of the In Grade program also calls for periodic retesting of smaller sample groups to make sure the forest stock is not changing. About 2 years ago SYP failed that small sampling in some places with some dimensions. They increased the sampling size and saw a trend. They argued, cried, politicked and finally degraded the allowable strength values.

A good stick of SYP is as good as it ever was and I suspect we will see more of that stock go to MSR to prove it and to increase the return, this will throw more junk into the lower visual grades. What's a stud? a #3 with #1 edges, exactly where the trash goes, well except to pallets. They are refusing to say they can visually sort the trash from the higher grades, mainly because the grading rules have no teeth regarding juvenile, reaction and compression wood. I can certainly pick them out of a pile, there was a lack of will and it is coming home to roost. Don't just blame the bad lumber barons though, we drove it through demand and through environmental policy. My wife and I have planted over a million SYP's through that period, many were "supertrees" selected for rapid growth. They were to be thinned for pulp and later the sawlogs would be selected out. When we decided to offshore paper the pulp market crashed, at about the same time we had a housing boom. Wood who's best and highest use should have been to wipe our bottoms became framing lumber... and now you know, the rest of the story. We need to develop silvicultural practices that reward a landowner for quality rather than simply for production, we aren't there yet, most foresters still just see the tree rather than knowing a thing about the wood within.

PS, a little gem regarding undue praise of old growth timber, in some species, such as the oaks, a fast grown timber, within reason, is far stronger than a slow grown timber, my oaks make a stronger frame than my grandaddy's, his was more stable, I'm using his design strength values since I still get slow grown trees alongside fast grown ones.

[Medeek]

Currently working on the plate calculations.  Those will be rather lengthy but the upside is the summary is what most people will want or need, however I will show each lateral resistance, tension, shear, net section and moment check for each plated area of each joint.  I'm doing one by hand first before I code it and I've already used 10 pages for the calculations and I still have to add the heel joint.  I hope if nothing else people can use this app to at least better appreciate all of the checks that go into a simple truss.

[Medeek]

I'm back in from a week on the road, going back to the SP/SYP question... I'm not sure. The only time I've really noticed SP on a gradestamp it designated Scots Pine, you'll find it in table 4F.

Southern Pine to me is SYP and contains the 4 stronest species of southern pine. The stamp Mixed Southern Pine can contain any of 7 species of southern pine, those 4 stronger species mixed with or any individual species and carries the strength design values of the weaker species.

I was just on Simpson Strong Tie's website looking something up and I noticed they use SP to designate Southern Pine in their documentation, however it seems like most of the truss softwares use SYP, now I'm confused.

[MountainDon]

same thing.... scroll down this page just a little....

""According to the handbook Utilization of the Southern Pines, published by the USDA Forest Service, Southern Pine is defined as those species whose major range is in the United States south of the Mason-Dixon line and east of the Great Plains. There are 10 species, all "hard" pines – diploxylon (hard needled) members of the genus Pinus (see chart).

The four principal species – loblolly, shortleaf, longleaf and slash – make up 90% of the Southern Pine timber inventory and are referred to commercially as "Southern Pine" or "Southern Yellow Pine". "Mixed Southern Pine" includes the minor species of Virginia pine and Pond pine.""  .... more on the linked page....

[Medeek]

26 pages of truss plate calculations for one simple fink truss.



It's no wonder we use computers for this sort of thing.

Now I just need to take it from this pseudo-code into Perl code with a bit of fancy logic and we've got her licked.

[Medeek]

Took a break from coding in all of the plates calculations last night and started working on the wind load cases.  In order to do this I had to crack open my brand new copy of the ASCE 7-10.  I always knew about the multiple load cases using the ASD method however as I dug deeper and studied a few truss  drawings produced by Mitek and other truss plate manufacturers it became clear that even for a simple truss they are running a number of load cases.  Here are the load cases I need to run for a simple 4/12 pitch fink truss with a 24' span and 12" overhang, at least this is what I've come up with so far, please add to this list if you are familiar with trusses and see that I am missing something:

1.  Balanced Snow Load (S) + TCDL + BCDL
2.  Unbalanced Snow Load (S2) + TCDL + BCDL
3.  Eave Loading (2Pf) + TCDL + BCDL
4.  TCLL (20psf) + TCDL + BCDL
5   BCLL (10psf) + TCDL + BCDL

and the wind Load Cases which I'm still trying to figure out, ASCE 7-10 is a bit different in this dept. from ASCE 7-5.

[Don_P]

It's been awhile since I've looked at a truss sheet, off the top one more I remember is 1/2 snow(unbalanced onto the lee side) + wind.

hpinson,
technically if it is not in the prescriptive code, engineering is required. Truss shops do have stock trusses, the 4/12 x 24' is an example of one you'll often see sitting premade in the yard. Heavy timber trusses are generally specced for the job.

When Medeek gets done with these lightweight trusses I was hoping to encourage him into heavy timber 

[Medeek]

I had someone else telling me they thought I should do a calculator like this but for heavy timber trusses not too long ago.  Is there a demand for this sort of thing, I don't see very much use of heavy timbers in most residential construction.  I suppose once I've completed my work on the light weight MPC trusses I'll probably have enough experience to attack the heavy trusses.

[ben2go]

This math is way over ma head.I have to build trusses on site for a re-roof.It's gonna hurt ma head really bad to figure out them angles,but once I gettem,All I'll have ta do is cut wood and assemble.Luckily my span is on 12'6" and no snow load with light wind loads.

[Medeek]

Its almost been a month since I have a chance to look at this stuff.  Work has made it tough to find the time to invest in this project.  I've been meaning to add all of the load cases into the app and basically have it either run the standard set of load cases or the user can select a custom single load case. 

On the same note I thought it might be useful to create an app that just generated the snow loads (truss or rafter). I put it here:

http://design.medeek.com/resources/snow.html

I just started working on it tonight so it probably won't work correctly for a couple of days until I finish up the backend programming/calcs.

What I am finding very strange though is with all of our technology and the taxes we pay you would think that somewhere the government has a website that you punch in a zip code or lat and long. and out pops the correct ground snow load for that location and elevation.  The ASCE 7 map is wholefully inadequate and mostly useless for much of the western United States. 

Compare that to the USGS website which basically has the feature now, the old paper maps are a thing of the past.  I actually had the chance to use their website/app at work since I was doing some structural engineering for a fixture being installed in California.

If anyone knows of such a service or database/website please feel free to post or chime in with ideas.

[MountainDon]

Nathan,

http://www.designcriteriabyzip.com/

$24.95 a month, free trial but you need to give them a credit card number to get the free trial. I never did

[Medeek]

I looked at their site in some detail, but did not pay the monthly fee either.  It looks to me like they are not doing much else except for using the ASCE map to determine the snow loads.  I could be wrong though.  After talking with the Utah County, Utah building dept. official about snow loads it seems that most counties have very specific snow load maps that they don't share with the public unless your submitting for a permit, I'm sure this varies by county and state. 

Still working on the snow load calculator, decided to add some nice graphics to it so it will take a bit longer to finish. 

Here is a template image for example:

[rick91351]

Here in Elmore County, Idaho if you are building in the mountains they require more of course than in the valley but where the dividing line is I have never asked or looked.  It is public info however it is not like a surprise envelope or you have to go find the Wizard of Oz.   I can't imagine why they would be in other places.  It should be codified and public.       

[Medeek]

Some of it is codified.  After some more research I found this information for Utah:

http://le.utah.gov/code/TITLE15A/htm/15A03_010700.htm

However, some jurisdictions, like Provo City, are a complete black box.

It really comes down to the levels of government:

National
State
County
City

each level has potential for input and that is what makes it complicated.

[Medeek]

Finally finished up the snow load calculator.  Moved it to this page:

http://design.medeek.com/resources/snow/snow_calculator.pl

Give it a whirl and let me know what you think. 

[Medeek]

Small bug in the html formatting fixed this morning.  Thank-you to the two people who spotted that for me.  Let me know if you spot anything else.  I think its working correctly now.

[John Raabe]

Medeek:

Nifty project.

There is a submit button at the bottom of the page. What is a person sent to at that point and what is done with their information?

Looks like there is also a request for donations? We have a no advertising policy at this forum. Just want to check to see that this is not a promotion. 

[Don_P]

I'll donate praise, well done and I hope you'll keep posting these calcs as you work on them. I don't see a problem with requesting donations and don't consider this advertising or a promotion, just a guy like us hoping for some offset on this sizeable effort. We refer people to outside sources of good information frequently, we get to be in on the development phase on this one 

It did white screen on me last night but I assumed it was on my end, it seems to be working fine today.
I noticed one typo on this page;
http://design.medeek.com/resources/snow.html
Under I_s in lower font, structur(e)s

On both that and the calc solution page I can't follow the drift equations.
....Well wait a minute, on my slow connection the calc solution page has now become legible, on the resource page I'm still seeing this coding;
γ = 0.13pg + 14 (snow density)
\({h_d} = .43\sqrt[3]{{{l_u}}} \sqrt[4]{{{p_g} + 10}} - 1.5\) (drift height)
\(l_d = \frac{8}{3} h_d \sqrt{S}\) (width of drift surcharge)
\({p_d} = {h_d}\gamma /\sqrt S \) (drift surcharge snow load)

To be honest having both formats might make it easier to work through if I try it longhand and seeing the coding might be just from my speed.

Even a slope factor adjustment for top chord Dead Load, I've usually just tried to remember to bump it up by a few lbs when the roof gets steep, nice job.

[Medeek]

I really did not intend to right this tool.  However, after looking at my truss calculator I realized that I was only giving one load case and eventually it should include all load cases to make it a more worthwhile project. 

I will eventually incorporate the snow load calculations into the truss calculator now that I've got it more or less wrapped up.

My next  project will be a wind load calculator that will determine the wind loading (MWFRS and C&C) for a residential type building with the ultimate goal to incorporate these loads into the truss calculator as well.

[Medeek]

I'll donate praise, well done and I hope you'll keep posting these calcs as you work on them. I don't see a problem with requesting donations and don't consider this advertising or a promotion, just a guy like us hoping for some offset on this sizeable effort. We refer people to outside sources of good information frequently, we get to be in on the development phase on this one 

It did white screen on me last night but I assumed it was on my end, it seems to be working fine today.
I noticed one typo on this page;
http://design.medeek.com/resources/snow.html
Under I_s in lower font, structur(e)s

On both that and the calc solution page I can't follow the drift equations.
....Well wait a minute, on my slow connection the calc solution page has now become legible, on the resource page I'm still seeing this coding;
γ = 0.13pg + 14 (snow density)
\({h_d} = .43\sqrt[3]{{{l_u}}} \sqrt[4]{{{p_g} + 10}} - 1.5\) (drift height)
\(l_d = \frac{8}{3} h_d \sqrt{S}\) (width of drift surcharge)
\({p_d} = {h_d}\gamma /\sqrt S \) (drift surcharge snow load)

To be honest having both formats might make it easier to work through if I try it longhand and seeing the coding might be just from my speed.

Even a slope factor adjustment for top chord Dead Load, I've usually just tried to remember to bump it up by a few lbs when the roof gets steep, nice job.

Fixed the structur(e)s typo, thanks for spotting that.  Its hard to catch everything but eventually they work their way to the top.

I'm using MathJax for the equations since there is no native mathematic engine built into most browsers.  However, as you suggest, I've also notice with a slow connection the rendering of the Latex code into the correct symbols takes a while to load.  I guess its back to the drawing board on this one.  The other solution is to simply use static gif images for mathematical statements, however that prevents the actual displaying of numerical values within the statements, which is nice to have.