Truss Design Software 1

[medeek]

I recently starting working on this web based truss design software. I am still waiting on a copy of TPI 1-2007 to show up so I can make sure the analysis conforms to the proper ANSI standard for metal plate connected trusses. Any thoughts on what one should add into something like this and make it a top notch piece of software. The intended audience is residential designers, engineers and other design professionals.

I know I am recreating the wheel a bit here when much more complex pieces of software already commercially exist (ie. Mitek, Alpine etc...). However, as some of you may know these vendors do not allow their software into the hands of architects or engineers, they only sell or lease to truss manufacturers. I find this a bit troubling since the vast majority of residential trusses are designed with one of these softwares and it is essentially a black box to the design industry.

My intent is not to circumvent the usual process of truss design and manufacture by the truss plants but to give other design professionals a tool to help estimate, check and create different scenarios for their truss needs.

Once I get this project far enough along where its output is actually useful, I will try to package it into a full app and then make the binary and full source freely available and hopefully enlist the help of others in its continued development.

The current prototype is here:

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

 

[medeek]

The statics equations for the double fink got really long and sure enough I made a small typo in the code (a misplaced dollar sign in front of one of my variables, perl code). So for about 3 hours I scratched my head trying to figure out why my internal checks were not adding up. Finally I remembered I still had and old 2005 demo version of "Working Model" on an install disk so I mocked up the truss in that with all the correct forces and dimensions as a independent check. Helped lead me right to the typo. Also really cool to see that my equations checked out perfectly. It never fails to amaze me how some simple math and a few carefully placed vectors can model real world systems. Anyhow, the double fink is finally up, that one was fun.

 

[focuseng]

Wow, this is pretty ambitious and I would love to see it work. I'll check it out when I got more time.

I think some of the proprietary nature of the MFG software has a lot to do with the truss plates themselves and partial fixity at the joints. I could be wrong here but when I get trusses analyzed by contacts at the plants, their values always differ a bit from mine in not always subtle ways. They also put fun joints with "sliders" and "wedges" into places where they need to raise heels and such. Sometimes odd joints - not at nodes... Always changes ideal behavior.

______________
MAP

 

[medeek]

I agree it is a bit ambitious, but its amazing what one can accomplish if you just steadily pluck away at it.

My copy of ANSI/TPI 1-2007 showed up today and I've been studying chapters 6-8 specifically. What I seem to be missing though is the method used to calculate the truss deflections (LL and TL). They have some equations for floor trusses with parallel chords but I'm not seeing anything for your regular common roof truss. Wondering if anyone has any insight here and wouldn't mind sharing. I've done some reading on deflections using the energy method due to axial compression and tension but does this tell the whole story? What about bending of the bottom chords due to BCDL or BCLL, does this factor into the final deflection numbers? TPI1 also has something about allowing for additional deflection through the creep factor but this is simply applied to the dead load deflection, the question is how do they come upon that dead load deflection?

 

[medeek]

I've added an on demand CAD generator that produces the geometry in .dwg format hopefully making this app that much more useful.

Screenshot below of what a typical file looks like loaded in AutoCAD:

 

[medeek]

Added some more dimensions and notes/specifications. Current output is below:

 

[medeek]

Added some more dimensions and notes/specifications. Current output is below:

 

[medeek]

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.

 

[medeek]

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.

 

[medeek]

I've just completed the tension and compression 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.

 

[medeek]

With a little help from a text called "Design of Wood Structures" by Donald E. Breyer and a recently purchased copy of the NDS 2012, the equations in TPI 1 finally made sense. I will say this has been quite an education with regards to structural design of wood structures. Last night completed the full lumber analysis for the fink truss. Top and Bottom chords as well as all the webs. Now its time to start working on the heel joint check and all of the connector plates. Wind loads might be next but I need to research this more.

 

[medeek]

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)

 

[medeek]

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.

 

[medeek]

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.

A copy of the paper can be found here:

http://www.ejse.org/Archives/Fulltex...10v1/20103.pdf

 

[medeek]

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)

 

[medeek]

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]

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]

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, or possibly tell me is something should be subtracted since it is redundant or trivial:

1. Balanced Snow Load (S) + TCDL + BCDL
2. Unbalanced Snow Load (S[size=8pt]2[/size]) + 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.

 

[Strukter]

Medeek,

Amazing stuff - how do you find the time to do all this!

Truss companies here include load for heating/AC units.

There are checks for bearing stress perpendicualr to grain where bottom chord on support such as 2x4 wall.

Wind design for construction case where TCDL=5 psf and BCDL=5 psf, and cantilever ends are exposed,etc.

 

[johnbridge231]

medeek, vey interesting work well done!

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html