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Attic Truss Analysis

Attic Truss Analysis

Attic Truss Analysis

(OP)
I've been working on an app. that does some quick and dirty truss analysis and estimating and I've managed to code a number of different configurations quite easily (calculating internal forces of member based on standard loading conditions, TCLL, TCDL, BCLL, BCDL). The nodes and members are easy enough to identify and the equations of equilibrium even though somewhat tedious are manageable. I've have a fink, howe, king post, queen post and even a double howe all nicely coded into this app and I've even verified my results with some third party software.

Now the problem is I've decided to attempt and attic truss, (maybe a bad idea). After mulling some free body diagrams over for a while it became apparent to me that a simple pin jointed model is a bit too simplistic for this type of truss. I'm sure if I keep plucking away at it I'll figure out the correct free body diagram and where to correctly apply moments and beam equations but my question is if somewhere out there is a sample problem in some textbook or paper where someone has already tackled a similar type of truss. I suppose this is more of a frame than a truss in some regards and that is probably the direction I will take, but an example or two would be very helpful.

RE: Attic Truss Analysis

Sounds like you need to get the help of a local structural engineer to me.

There are many types of trusses and truss configurations, especially when you figure in member continuity across the joints to realistically model the truss.

Common truss programs are RISA 2D and 3D, among others, but they are expensive to buy for only a one time use. Cheaper to use a structural engineer who has the program.

Mike McCann
MMC Engineering

RE: Attic Truss Analysis

Plate connected trusses are frequently evaluated by modeling as pinned. The plates do impart a bit of fixity, but the amount of rotation required to mobilize that partial fixity probably results from poor workmanship rather than design loads. At design load, the inherent flexibility of the wood and plates (and the teeth of the plates) probably means pinned models are fairly good representations. Obviously, things like continuous chords should be modeled as such.

I assume you are familiar with the wood truss industry publications (like TPI-1) for designing these? (http://www.tpinst.org/)

RE: Attic Truss Analysis

(OP)
Commmon trusses like the fink truss are statically determinate trusses. Turns out the attic truss does not nicely fit into the statically determinate bracket. Its more of a frame than a truss. Anyhow, I will attack it analysis later for now though I've at least got a pretty good handle of the fink truss check it out here:

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

The other common truss types I've listed do not have as in depth analysis yet, but that is in the works.

RE: Attic Truss Analysis

Looks like a nice application, but I can't seem to have it finish. Just keeps running. Is it operable still?

RE: Attic Truss Analysis

(OP)
Which truss type are you trying to analyze?

RE: Attic Truss Analysis

IT was the Fink truss I believe.

RE: Attic Truss Analysis

I tried it again. IT ran. Really nice application. I am in teh midst of checking my numbers vs yours now. Really nice.

RE: Attic Truss Analysis

(OP)
Thank-you. I've got a bunch more I want to do with it but lately its been hard finding the time. I would like to complete the plate analysis as well as calculate the moments using the matrix method instead of the old pre-1995 TPI 1 method.

RE: Attic Truss Analysis

Medeek,
Can you set up stiffness matrices in Excel?

I have an Excel workbook that unfortunately can only handle simple supported beams with cantilever both ends. I frequently need to calculate continuous beams up to 4 spans, with a capability of 3 different unform loads and 2 interior concentrated loads per span,and concentrated loads at supports and at cantilever ends. There are only 4 load cases. There is a capabilty of designing up to 60 beams per project, and my problemis keeping the .xlsx file under about 7mb.

I would compensate you for your effort, if you could quote me a fee.

RE: Attic Truss Analysis

(OP)
Email me a copy of your current excel file (assuming that it produces the required output that you would like to see in the final report). I'll take a look at it and see if it might be something I can bite off.

RE: Attic Truss Analysis

Medeek,

I consider my Excel to be proprietary - I have improved it for more than a decade.

You could just do the Excel worksheet to churn the numbers according to a stiffness matrix method, and I could integrate your work into my workbook.

I am enclosing a pdf copy of my workbook. There are very many hidden sheets you won't see. (such as capability to design up to 60 beams)

The example you see (the H1, etc sheets) does not have the capability of a cantilever at the left end, but I do need the cantilever at both ends.

RE: Attic Truss Analysis

(OP)
Can you draw me a simple diagram showing the applied point loads, distributed load, reactions (pinned or rigid) and then where the origin is with which to measure the location of all the loads or where you are measuring the location of the point loads from. This is an interesting problem. I could throw it together fast in Perl (my programming language of preference) but it would take me a little bit to figure it out in Excel seeing as I have not tried doing a matrix analysis in Excel before.

RE: Attic Truss Analysis

Medeek,

I thought all that was clear in the attachment to post @ 14:57

All 4 supports are always pinned (wood construction)

Origin - Support #1, the first one of the left

I need this in Excel, to integrate it into my workbook.

RE: Attic Truss Analysis

typo - all 5 supports

RE: Attic Truss Analysis

Don't know if the OP is aware, but trusses are no longer designed as pin joint models. Current design methodology is extraordinary complex.

Last I knew it was still allowed. But there was talk of it being eliminated as a valid method for designing wood roof trusses.

RE: Attic Truss Analysis

Ron,

All that technology could be for naught if the trusses are designed by an incompetent technician.

I recently had a project, a very complex wood framed custom home, where the plans checker noticed very high uplift forces from trusses and truss girders at certain support points.

They were so high that even the largest Simpson strap couldn't handle them.

I asked the truss designer to "pencil-whip" on those calculations, and sure enough, the uplift forces were reduced to about 25% of what they were initially.

RE: Attic Truss Analysis

(OP)
Just taking a look at your images now. Catching up after the break at work has had me drained. Looks like the uniform loads are all independent of each other and are rectangular loading. I'll need to dive back into my reference texts a bit since I can simplify the calculations without the local to global transformation matrices required. I'm also assuming all the supports are pinned and roller mounted essentially so that all axial forces are ignored.

RE: Attic Truss Analysis

medeek,

All your assumptions are correct.

RE: Attic Truss Analysis

medeek,

To clarify further - there needs to be capability only for a maximum of 2 interior locations per span where concentrated loads occur or uniform loads occur or change in intensity, and uniform loads are never ramped (trapezoidal).

If the there are 3 different uniform loads and 2 concentrated loads, then the concentrated loads have to occur where the uniform loads change intensity.

I did all this years ago to simplify the input and keep the Excel file down to a reasonable size since my workbook has a capability of 60 beam designs.

If a concentrated load occurs at a support, I don't need that in your matrix because I can add that load to the support reactions anyway.

RE: Attic Truss Analysis

(OP)
This is the sort of think I enjoy working on. I'll create the schematic and post it so you can see the basis for the matrices. You're right the concentrated loads at the supports only affect the reactions and have no impact on the matrix analysis, shear, moment or deflection of the beam. So I can just leave those out unless you would like me to add them in as an input.

RE: Attic Truss Analysis

medeek,

I can't recall how the stiffness method of analysis works. In the attached sketch, we have one span of a continuous beam - if the analysis is set up so that are a minimum number of nodes to keep matrices as small as possible, is it possible for the output to show the maximum deflection at that span even if it didn't occur at one of the nodes?

Or is it necessary to divide the beam up into 30 to 50 nodes to get the closest accurate maximum deflection?

I don't have to worry about exact moments and shears because I could figure those easily once I know the exact reactions.

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