Author Topic: Truss Calculators  (Read 178489 times)

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Offline Medeek

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Truss Calculators
« on: March 12, 2013, 04:33:08 AM »
I was really hoping to get my hands on a a good truss software even if I had to shell out the big bucks.  I contacted Miitek and Alpine and neither of them are willing to sell or lease their software to an architect or engineer.  Something about trusses becoming a commodity.  Bottom line is there aren't a whole lot of decent truss calculators/designers out there.  If you want a truss design you have to get a truss manufacturer to quote you on a job and generate the engineered plans. 

So I purchased a couple of books and starting reading up on trusses and how they are manufactured and designed.  I've still got a lot to learn but just today I started into programming a basic truss estimator:

http://design.medeek.com/calculator/calculator.pl



It doesn't do a whole lot yet, just produces a picture and some dimensions but I will probably have it calculate loads and axial forces in the truss members.  I apologize that Internet Explorer does not currently work with the images, for some reason there is no native SVG support in Microsoft browsers.  I would also like it to analyze the plates and calculate those as well. 

If anyone has any thougts on what else this sort of thing should do let me know.  I'll probably being hacking away at it for the next few weeks in between other stuff so hopefully it can develop into something useful.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Medeek

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Re: Truss Calculators
« Reply #1 on: March 12, 2013, 05:35:16 PM »
Apparently there is an SVG plugin for IE8, so far it is working flawlessly:

http://download.cnet.com/Adobe-SVG-Viewer/3000-2356_4-11240.html

The truss calculator is now showing the loads.  So far I have it only setup to analyze a fink type truss,  I will also add a howe,  king and queen type trusses next.
Nathaniel P. Wilkerson, P.E.
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Offline Don_P

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Re: Truss Calculators
« Reply #2 on: March 12, 2013, 09:48:28 PM »
I haven't looked at yours yet. The software the truss mfgr's are using I believe evolved from the PPSA program, Purdue Plane Structures Analyzer. I halfheartedly attempted to find it once. Old and very obsolete. The current commercial software is capable of far more than a statically determinate truss.

But then  ;D, my attempt with kingposts... major disclaimers.
http://timbertoolbox.com/Calcs/trusswbending.htm
Right click anywhere on that page and click "view source" for the script.

I like this one
http://www.jhu.edu/~virtlab/bridge/truss.htm


Offline Medeek

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Re: Truss Calculators
« Reply #3 on: March 12, 2013, 10:29:44 PM »
I also have tried to find the PPSA3 or PPSA4 and it does not seem to exist anywhere or at least it is not available or being actively developed.  I get the feeling that the truss industry (truss plate manufacturers) has a proprietary grasp on the whole process and open source projects are not highly encouraged.  Looking at the output from some of the commercial software (ie. Mitek) your right they are running some fairly sophisticated code that checks a number of items.  I'm sure the code is incredibly complex. 

I really don't care to analyze every nail plate and size it.  Mainly all I am after is the overall shape of the truss and the general layout of its webs.  Fortunately, the Metal Plate Connected Wood Truss Handbook has helped me answer some of those questions.  For instance I knew that the panels were more or less equal in length but I didn't know that the scarf length is subtracted from the total span in the calculation of the panel lengths.

What I would like to know is if I decide to use a 6/12 truss with a 16" overhang for a 30' span, will 24" o/c spacing be fine and at what point do I need to use 2x6's for top and/or bottom chords.  Assume of course 47-50 psf total load on truss.  Then my next question as a designer is should I go with a Fink or Howe type truss?  What does one get me over the next?  Based on everything I've read it would seem the Howe truss with 4/4 panels has more internal webs and can therefore span more distance than a Fink truss. 

My problem is I'm not seeing a lot of books, literature, websites etc... out there that can help answer these sorts of questions.
Nathaniel P. Wilkerson, P.E.
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Offline Medeek

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Re: Truss Calculators
« Reply #4 on: March 14, 2013, 05:16:35 PM »
Also added the King Post and Howe type truss.  I've been studying the ASD vs. LRFD methods of analysis, I think I will probably switch back to ASD since it seems the predominant method used currently even though LRFD is supposed to eventually supplant. 
« Last Edit: March 15, 2013, 12:10:20 AM by Medeek »
Nathaniel P. Wilkerson, P.E.
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Offline Medeek

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Double Fink Truss
« Reply #5 on: March 18, 2013, 02:11:33 AM »
Added the Double Fink truss today, this one was a beast.  Tomorrow my copy of the TPI 1-2007 should show up and then I can start plucking away at the sizing of member and plates.  Just as a note, I'm using an old 2005 version of "Working Model" to independently verify the internal forces within the truss members.  It's too bad that developers of that software did not keep that product going I was a huge fan.

Nathaniel P. Wilkerson, P.E.
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Offline Medeek

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Re: Truss Calculators
« Reply #6 on: May 01, 2013, 01:21:26 AM »
Haven't had a whole lot of time to spend developing this app lately but did add a few cosmetic fixes.  Now when you select a truss type the appropriate image will be dislayed showing the general configuration of that type of truss.  Also added in a a few help images to better explain some of the terms like "overhang" and the various truss types. 

I've done the calculations and equations for deflections so I just need to code those in.  I will be using the virtual work method (unit load method) to determine the max deflection at the mid point(s) of the bottom chord of the trusses.  My goal is to provide as much of the calculations as possible in an understandable format so that the layperson can see how the engineering is done.

http://design.medeek.com/calculator/calculator.pl
Nathaniel P. Wilkerson, P.E.
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Offline Medeek

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Re: Truss Calculators
« Reply #7 on: May 30, 2013, 03:35:10 AM »
To make the app more useful I've added an on the fly CAD generator which creates a .dwg and .lsp file of the truss geometry if requested.  Note, that this is only available for the "Fink" truss, I will add it to the other configurations in the next couple of weeks.  Below is a screenshot of what a typical file looks like loaded into AutoCAD (Draftsight is the free option).

Nathaniel P. Wilkerson, P.E.
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Offline Medeek

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Re: Truss Calculators
« Reply #8 on: June 01, 2013, 01:36:52 AM »
Added some more dimensions and notes/specifications.  Current output is below:

Nathaniel P. Wilkerson, P.E.
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Offline Medeek

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Deflection Calcs
« Reply #9 on: June 07, 2013, 02:29:00 AM »
Still primarily working on the fink type truss, figured I would use this as the model truss and complete the entire analysis then once I have verified and checked it I will update all the other truss types with the same functionality.

Today I finally coded in the deflection calculations that I had worked out about a month ago.  The fink truss now gives some deflection numbers based upon pre-selected lumber specs.  Below is an example of the output:



I am really curious to see how my numbers stack up against Mitek or Alpine.  Note that the deflections given are for vertical loads and do not involve horizontal (ie. wind) loads.  My only thought is that my deflection numbers are not as conservative since I have not put in a certain amount of slip at each joint which in reality is probably what occurs and I am sure the commercial software out there has something to take this into account.
Nathaniel P. Wilkerson, P.E.
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Offline Medeek

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Moment Calculations of the Top and Bottom Chords
« Reply #10 on: June 14, 2013, 10:54:58 PM »
I've kind of hit a road block on the moment calculations.  The current spec TPI 1-2007 calls for using the matrix method in determining the moments.  However, I need to be able to run this app without doing a full blown analysis using RISA or some other FEA type product.  For now I've gone with the simplified method which is the method used in the TPI 1-1995 standard, at least I can produce a solution.  Typical result below:



If anyone has any ideas on how to do a simple matrix method analysis of a common fink truss please send me in the right direction.  My biggest unknown with this would be how to deal with the fixity of joints at panel points and heels.  I've just ordered a copy of Hibbeler's Structural Analysis to further research how best to deal with frames, trusses etc...

One thing I found really helpful was the samples provided in the previous editions of the TPI 1, its really quite disappointing to see no such example calculations in the current standard.
Nathaniel P. Wilkerson, P.E.
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Offline Don_P

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Re: Truss Calculators
« Reply #11 on: June 15, 2013, 06:50:11 PM »
You are well beyond me, interesting stuff and I hope you can figure out a simplified version. A simple, affordable or free, accurate method of evaluating frames would be quite useful and marketable...

Offline Medeek

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Re: Truss Calculators
« Reply #12 on: June 21, 2013, 12:24:37 AM »
I've just completed the tension and compression analysis for the web members for the fink truss.  It took a little longer than expected but most of that was comparing notes between the TPI, NDS and "Design of Wood Structures" text.  The rest was trying to format the equations correctly with html and finally resorting to LATEX and a third party program.  Below is part of the calculations for a 4/12 fink truss:



My next hurdle is to try and program the top and bottom chord analysis however I am having a bit of trouble making sense out of the TPI 1-2007 when it comes to calculating the effective lengths for the chords, the previous method used in TPI 1-2002 seems a whole lot simpler than the current standard, it will probably take me a few days to mull it over.  Once that is done a heel analysis and then all of the metal plates analysis and then its done.  However, so far I have not introduced any short term loading due to wind or seismic so in the future I may attempt that as well.
Nathaniel P. Wilkerson, P.E.
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Offline Don_P

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Re: Truss Calculators
« Reply #13 on: June 21, 2013, 03:27:46 AM »
That looks like the newer interaction equation for combined bending and axial loading. I never got it to run right in a program, congrats!

Here was where I got to a few years ago. the '44 NDS equation, a stripped down version of the equation;
http://www.timbertoolbox.com/Calcs/44axbend.htm

Do you have a link to the TPI standards? I have the other texts. Not sure if it matters especially with a copmputer humming but I was taught to resolve the heeljoint first, if you can restrain those forces it is usually buildable.

Offline Medeek

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Re: Truss Calculators
« Reply #14 on: June 28, 2013, 12:45:46 AM »
I have copies of the three most recent TPI standards (2007, 2002 and 1995).  The standard is always evolving as are the building codes as you know. 
I was looking at your app, pretty cool that you were working on this stuff as well.  For understanding this stuff I recommend Design of Wood Structures by Breyer.  It really helped me understand the reasons behind the NDS equations and how they are being used in the TPI 1.

For the fink truss I've just completed the member analysis, give it a shot, its pretty cool what it can actually do now.  Now I'm going to take a bit of time off  and then come back and figure out the engineering of all the connector plates (chapter. 8 in the TPI) and also add the heel joint check.
Nathaniel P. Wilkerson, P.E.
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Offline Adam Roby

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Re: Truss Calculators
« Reply #15 on: June 28, 2013, 06:27:15 PM »
Nice tool... maybe you could add a scissor truss in there.  :)


Offline Medeek

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Re: Truss Calculators
« Reply #16 on: June 29, 2013, 04:07:57 PM »
Nice tool... maybe you could add a scissor truss in there.  :)



The scissor truss and the attic truss are now both on my to-do list. 

The thing about the scissor truss is I am little unsure right now how the geometry is defined.  I will need to look at some different samples at different spans and pitches to determine the correct algorithm for calculating the geometry, the rest after that is just math and lots of checks.

In fact, if anyone has some sample scissor truss drawings (Mitek, Alpine, Truswal etc...) in PDF format if you could send them to me that would help with my research.
Nathaniel P. Wilkerson, P.E.
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Offline Medeek

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Bearing at Heel Check
« Reply #17 on: June 29, 2013, 04:12:21 PM »
Just finished adding the bearing at heel check (fink truss only) which looks at the stress caused by the reaction and checks for compression perpendicular to the grain of the bottom chord as well as a new stability check added to TPI 1-2007. 

Three types of scissor trusses I'm looking at adding are:  Howe Scissor (4/4), Mod Queen Scissor (6/4), Double Howe Scissor (6/6)



« Last Edit: July 02, 2013, 09:14:31 PM by Medeek »
Nathaniel P. Wilkerson, P.E.
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Offline Alex W

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Re: Truss Calculators
« Reply #18 on: September 17, 2013, 01:06:21 PM »
I just want to say, first of all, awesome work on the calculator.  I recently purchased a property with an existing pole barn, and have been trying to determine if the trusses are sufficient to support a ceiling and insulation.  Your calculator has proven incredibly useful in that.  I probably could have made the calculations myself by hand, but that would have involved dusting off some skills I have not used since engineering school ???, plus with my background being in mechanical rather than civil, the application of the basics to wooden trusses is something I do not have experience with.

That said, while digging through the calculations on the Fink truss to compare your allowables to the MSR lumber already in my trusses, I came across what appears to be a discrepancy.  In the bottom cord design section, you appear to use Fb=2350, and reference table 4A.  However, table 4A shows Fb=1250 for spruce-pin-fir select structural.  Also, in the top cord section, you use Fb=1500, which while much closer still doesn't match up to table 4A.   Am I missing something? 

As I said before, very impressive work, and very cool of you to make it free for all to use.  I look forward to seeing what other features you add in the future.

Offline Medeek

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Re: Truss Calculators
« Reply #19 on: September 17, 2013, 06:40:10 PM »
I just want to say, first of all, awesome work on the calculator.  I recently purchased a property with an existing pole barn, and have been trying to determine if the trusses are sufficient to support a ceiling and insulation.  Your calculator has proven incredibly useful in that.  I probably could have made the calculations myself by hand, but that would have involved dusting off some skills I have not used since engineering school ???, plus with my background being in mechanical rather than civil, the application of the basics to wooden trusses is something I do not have experience with.

That said, while digging through the calculations on the Fink truss to compare your allowables to the MSR lumber already in my trusses, I came across what appears to be a discrepancy.  In the bottom cord design section, you appear to use Fb=2350, and reference table 4A.  However, table 4A shows Fb=1250 for spruce-pin-fir select structural.  Also, in the top cord section, you use Fb=1500, which while much closer still doesn't match up to table 4A.   Am I missing something? 


As I said before, very impressive work, and very cool of you to make it free for all to use.  I look forward to seeing what other features you add in the future.

Thank you for using the application, I wasn't really sure who might find it useful, glad to see someone is finding practical uses for it.  Can you tell me exactly what span, pitch and loadings you used, as well as overhang and bearing width. 

To debug and figure this out I need to replicate your settings.  I've checked through my Table 4A values and they appear to be correct but I might be missing something or there could be some logic or typo problem in the code. 

Really appreciate your feedback.
Nathaniel P. Wilkerson, P.E.
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Offline Medeek

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Matrix Analysis
« Reply #20 on: September 17, 2013, 07:01:12 PM »
After about a good week of solid programming and scratching my head I've finally managed to add the requisite Matrix Analysis to my Truss Calculator.   Thank-you R.C. Hibbeler for your Structural Analysis text on the subject (ch. 14 - 16), if the subject had not clearly laid out in front of me I would never have figured out the numerous steps to arrive at the solutions. 

Here is an example of the output of my matrix analyzer for the Fink truss:



I've even inserted the correct code to account for the additional loading/moments if there are overhangs.  I double checked my work by modeling up identical trusses (beams and trusses members) in both Strand7 and Solidworks (COSMOS/Simulator).  My result were within 1.5% or better, so I'm really happy about that.



My only concern with my analysis is how correct my analog for the truss really is.  What I mean is that the bending moments are heavily influenced by the amount of rigidity of the joints.  Fixing the joints (where chords meet) or pinning them dramatically affects the bending moments and even the axial and shear loads to some extent.  My analog model is basically rigid at the heel and peak joints and pinned at all other web-to-chord or web-to-web joints.  This seems to approximate most closely the moments calculated using the simplified method (pre TPI-2002). 

What I also found quite interesting (and expected) is if you use a stronger type of lumber on the top chord as compared to the bottom chord.  The top chord loads increase and the bottom chord loads decrease.  The matrix analysis is almost as good as FEA.  It's really quite cool to be able to calculate something like this just using a bunch of matrices.
« Last Edit: September 17, 2013, 07:28:52 PM by Medeek »
Nathaniel P. Wilkerson, P.E.
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Offline Alex W

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Re: Truss Calculators
« Reply #21 on: September 18, 2013, 08:57:47 AM »
Thank you for using the application, I wasn't really sure who might find it useful, glad to see someone is finding practical uses for it.  Can you tell me exactly what span, pitch and loadings you used, as well as overhang and bearing width. 

To debug and figure this out I need to replicate your settings.  I've checked through my Table 4A values and they appear to be correct but I might be missing something or there could be some logic or typo problem in the code. 

Really appreciate your feedback.

My inputs are:
32' span
4/12 pitch
48" spacing
5.5" bearing
12" overhang

I have of course been using the Fink truss to get a feel for things, although my exact situation is actually a Modified Queen, as shown here http://www.7dtruss.com/truss_configurations.pdf

I have played with the loading a bit, my goal being to verify that I will be OK with 30/5/0/5.  At that loading, it puts out a 2x6 top cord, SP SS, and a 2x4 bottom cord, also in SP SS, and the allowables appear to be wrong for both.  If I use the default loading of 30/10/0/10 the to cord goes up to a 2x8 HF SS, with the correct allowable, the bottom cord stays 2x4 SP SS with the incorrect allowable.  It seems that the questionable number is just in the spruce/pine SS numbers. 

Offline Medeek

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Re: Truss Calculators
« Reply #22 on: September 18, 2013, 11:12:10 AM »
My inputs are:
32' span
4/12 pitch
48" spacing
5.5" bearing
12" overhang

I have of course been using the Fink truss to get a feel for things, although my exact situation is actually a Modified Queen, as shown here http://www.7dtruss.com/truss_configurations.pdf

I have played with the loading a bit, my goal being to verify that I will be OK with 30/5/0/5.  At that loading, it puts out a 2x6 top cord, SP SS, and a 2x4 bottom cord, also in SP SS, and the allowables appear to be wrong for both.  If I use the default loading of 30/10/0/10 the to cord goes up to a 2x8 HF SS, with the correct allowable, the bottom cord stays 2x4 SP SS with the incorrect allowable.  It seems that the questionable number is just in the spruce/pine SS numbers.

With your original loading of 30/5/0/5 you will notice both top and bottom chord are SP (Southern Pine) whereas the webs are SPF (Spruce, Pine, Fir), two completely different animals.  In truth the SP values are coming from table 4B and not 4A, all other lumber type use table 4A (DF, HF, SPF).  Also two updates from the AWC for the SP values were issued this year and last year, so your print copy may be out of date:

http://www.awc.org/publications/update/2012NDS-Addendum-March2013.pdf

http://www.awc.org/publications/update/2012NDS-Addendum-March2012.pdf

I've noticed that sometimes the acronym SYP is used instead of SP.  I think Southern Yellow Pine and Southern Pine are one in the same, correct me someone if I am wrong.
Nathaniel P. Wilkerson, P.E.
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Offline Alex W

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Re: Truss Calculators
« Reply #23 on: September 18, 2013, 03:06:25 PM »
Ahh, that makes sense.  I had mus-interpreted the acronym, I was assuming SP was Spruce/Pine, since that was the closest thing I could find in Table 4A.  Thank you very much for the input.

Offline Medeek

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Re: Truss Calculators
« Reply #24 on: September 20, 2013, 02:31:24 AM »
Leaving the peak joint as a rigid connection without exploring the implications of a pinned or semi-rigid joint seem like a cop-out to me so I spent most of the day attempting to release the peak joint so that it could act as a pinned (zero moment transfer) joint.  For the web members I accomplished a similar task by altering the 6x6 stiffness (k') matrix so that it only included the axial terms, thereby eliminating any shear or moment forces, making these members axial only or simple pinned truss members.  However, for the top chord members it was not such an easy task.  I initially tried eliminating the row of the matrix that was responsible for the far end moments (pinned end), but it some became apparent that the interplay between moments and shear forces was more than I had originally thought.  I was about to accept defeat but then after spending a couple more hours digging about online I came upon a gem of a paper published in 2010 in the Electronic Journal of Structural Engineering by M. E. Kartal.  This paper outlined a couple of methods for obtaining the correct stiffness matrix for semi-rigid connections.   With this information I was then able to add in feature so that one can select whether the peak joint is rigid, semi-rigid or pinned.

I then tested it for accuracy against an identical model in Solidworks Simulator for both the pinned and rigid connection at the peak joint with near perfect results.  Unfortunately, Solidworks does not allow for adjusting the rigidity of connections between beams in its interface so I currently do not have the tools to test the accuracy of the semi-rigid model.  However it appears to present the correct trends when compared against the other two options.  If someone has a copy of ANSYS or some other reasonably high end FEA software I would be interested to see how well it will compare with third party verification.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer