Wood Truss Analysis-Modeling 1

[RFreund]

Questions on evaluating/designing a wood truss:

Assuming loads are applied as distributed loads (i.e. purlins are closely spaced or no purlins and sheathing distributes load)

Q1. Should the top chords be modeled as continuous over panel points (assuming that they are continuous) or should a "pin" be used at panel points so that the moment at the panel point is zero for the top chord.

I thought that I would get smaller (absolute value) bending moments when I modeled the truss with a continuous top chord. As opposed to modeling the top chord as simple span between panel points. However when I model the truss in Risa that is not what seems to happen. I believe what happens is that when the top chord is modeled as continuous member some the the tension or compression from the web members effects the top chord bending moments. This also gives me lower tension and compression values in my web and chord members. So my questions are:

Q2. Is the analysis wrong with continuous members? I realize that it may not be "wrong" however the moment near the base of the truss in the top chord is 3x that of the simple span and when unity is checked the member fails. So I guess a better question might be:

Q3. Can a trussed by analysed assuming the cord is simple span between panel points even if it is built with a continuous member?
Q4. What type of analysis is typical used?

Thanks

EIT

 

[cap4000]

I have the 4th edition Steel Design book by Segui and he treats the loads "between" the panel points as a fixed end like a beam-column analogy. Chapter 6 is devoted to this loading condition. Therfore moment and axial load should be designed for in your case.

 

[DaveAtkins]

I prefer to analyze any truss (wood or steel) as it is actually detailed and built. So I assume the chords are continuous.

However, I recall reading somewhere that if you assume the chords are a series of pinned end members the design will still be safe.

DaveAtkins

 

[RFreund]

Thanks for the replies.
I forgot to add that there would be compression as well.

Dave,

I thought I read that as well - that assuming a series of pinned connections would be conservative. However for some reason I get some pretty different results when I use a continuous model.

Maybe I'm just not used to seeing such strange moment diagrams...



EIT

 

[MiketheEngineer]

Why the sudden interest in wood trusses - I responded to your other post in "Wood Design..."

In the "olden" days (early 70's) we did it by hand based on all pin joints like your Q3 suggests using force diagrams (anybody remember how to do those??) and then did a simple combined axial and moment check. Sometimes we would cheat the moment by using WL^2/10 instead of 8 - assuming some continuity over the panel points. Usually anything under about 50' residential just never gave us any problems. Trusses 70' long and 8' oc got a bit scary!!

Then came computers(mid 70's) - first generation of software (I wrote some of them) used a matrix analysis to find the axial loads and deflections. Again the software then did a simple combined check.

Interspersed in this era, a Dr Suddath out of Purdue wrote a frame analysis program known as the PPSA - Purdue Plane Stress Analyzer - or something like that. Hard to use but worked well.

Next generation got more in depth - usually assuming the chords were continuous but major joints (heels, peaks, etc)were still pinned or a modified fixed joint. Note that the centerlines of a 2x12 / 2x12 truss never come close to meeting at the heel - at least inside the truss joint.

Last I knew - great proprietary studies were made to optimize the analysis in order to improve perfromance, reduce member sizes, optimize plate performance, etc. Even the exact same truss from two different mfgs will give slightly different results!!

Two of the major players in this industry are Alpine Engineered Products - a division of ITW and Mitek - a Berskshire company.

Both of those companies had some roots in St. Louis in some form or another. I worked for one of those roots many years ago. Check out their websites - quite informative.

Good Luck

 

[Splitrings]

A similar question popped up in the Modern Steel Construction magazine this month as to whether secondary stresses need to be evaulated. It recommends looking at a paper titled "Secondary Stresses in Trusses" by R.S. Nair. It is available on their website

http://www.aisc.org/epubs.

I would check the Wood Truss Plate Institute'e website at

http://www.tpinst.org

 

[woodman88]

In designing Glue-lam trusses in RISA, I checked both the pinned and continuous member (between splices) designs for the truss against a hand check of some of the panels stresses. I did the pinned vs continuous check, as it is relatively simple to do in RISA. To go belong this (as MiketheEngineer states) takes a lot of testing and research to come up with a analysis program.

Garth Dreger PE
AZ Phoenix area

 

[RFreund]

I see.

Mike - We are evaluating an existing wood truss with plywood gussets. That will most likely need to be strengthened. I have installed many wood trusses (used to be a framer) and have evaluated "pure" trusses (in school) but I wasn't too sure how actual design/evaluation should be done.

Splitrings - thanks I'll look into it.

Woodman88 - thats essentially what I have done. However when using a continuous top and bottom chord I get large moments at the panel points near the bearing ends. Atleast for the configuration I have (Howe Truss 50' long with 5' end panel widths and 4' for all interior)

EIT

 

[MiketheEngineer]

The dreaded hand made trusses.

Plywood gussets are often OK for panel point joints - but they can get real cumbersome at heels and splices - esp at the span you have. Trying to get enough nails or bolts into them w/o wrecking the chord members is quite difficult - hence one of the reasons leading to the invention of the metal truss plate.

Analyzing the plate to accept moment is rather tedious and quite subject to a lot of variables - although they will take quite a bit.

I usually just design the heel plate to handle the axial and shear forces. The plywood piece is so big and fasteners so numerous that any moment worries are probably secondary.

Good Luck!

 

[RFreund]

Mike - thanks. I was hesitant to even bring up the plywood gussets as I know that it a whole other issue.

Would you suggest screws, nails, or through bolts? Or would you un-suggest any?



EIT

 

[msquared48]

Mike:

Yea - Maxwell Diagrams. Still use them for small configurations. True pin-pin analysis.

RFreund:

As for the modeling, anything that is modeled different than constructed is just kidding yourself. With the computer program options today, as constructed is very doable.

As for the plywood gussets with home built... If it is for yourself and you can do it, it's your call. But for a client - use pre-manufactured trusses. If the spans are small, just use solid members and a stick-built configuration. KISS!

Another thought, and reading between the lines here - if you are only an EIT as your handle suggests, you cannot structurally stamp the design of such trusses for yourself, let alone others. Without the supervision of a licensed PE, I would not step into this quagmire if I were you. You could jeopardize your privilege to get licensed.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RFreund]

Mike^2 - The truss are not for me and I'm not stamping anything. I work for a small company, well actual its just me and the Boss. Which I like as I get to see different stuff everyday. He reviews my work and provides me with a lot of guidance but at the same time he is very hands off. He wants me to work through the problem and find the answers on my own as opposed to here is the problem this is the fastest way to do it now get it done. However I don't want you to think I'm cheating by using this web site or using it is as a "crutch" or am a burden to the senior members. I read just about the entire TPI manual Friday and was at work for about 15hrs. Also I get to go back to the Boss once a while and act like I came up with a good dea that he (or I) had not thought of
Also I really enjoy SE and am hungry for knowledge.
Thanks Again!

EIT

 

[BAretired]

I don't believe I have ever come across a case of a wood truss design where the assumption of pin-ended members was not conservative. If there is a case which shows otherwise, I would be interested in seeing it.

BA

 

[msquared48]

BA:

I can agree with you on the top and bottom chords, but not for the web members. Where the top and bottom chords are continuous over a web member, additional shear is thrown into the web members via the moment. This is assuming that the web member is pinned to the top and bottom chords. However, the difference is probably usually enough to ignore.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

The additional shear in the web member is, in my opinion, a minor factor but nevertheless, something to consider.

The main problem, in my experience has been that the typical shop drawings prepared by the wood industry deal with each truss as if it were an entity unto itself, i.e. that it has no relationship to neighboring members. Shop drawings indicate lateral bracing without regard to the fact that there is nothing available to which it can be braced.

The wood truss industry has not lived up to acceptable engineering standards and should be held accountable for their designs...but so far, they have not. To date, they have managed to dodge their responsibility. Until they accept responsibility, the EOR will be on the hook for anything pertaining to wood truss design.

BA

 

[woodman88]

BAretired, the Wood Truss Industry has lived up to USA acceptable engineering standards. Their designs meet the requirements of the IBC and IRC codes. I am sure if you check the Canadian codes that they meet their requirements also. If not, then please report them to the proper government officials.

Garth Dreger PE
AZ Phoenix area

 

[msquared48]

AAH...

And this gets into the flap over the wood truss manufacturers attempting to pass along the responsibility for the lateral bracing of the trusses, web members in particular, to the EOR. Lots of conflict over that issue.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

woodman88,

I have been retired for two and a half years. I no longer have examples of the kind of garbage the wood industry foists upon the engineering community. If their designs meet the requirements of the IBC and IRC codes, then all I can say is that those codes are totally and completely inadequate.

BA

 

[BAretired]

msquared48,

It is understandable that there is lots of conflict over that. In my opinion, the wood truss industry is completely out to lunch on the entire subject of responsible engineering.

I have attended at sites where the general contractor has attempted to comply with the truss shop drawings. In one case, I made more than twenty site visits before the GC was able to get it right. The truss manufacturer simply shrugged the whole thing off saying it was my responsibility to ensure that the necessary web member bracing was in place. What a load of crap!

Thank goodness I am no longer involved in engineering practice. If I were, I would avoid using light gauge wood trusses like the plague because that industry simply does not provide an acceptable service or product (in my opinion).

BA

 

[woodman88]

AAH... Actually, this is the flap over the EOR allowing a product they do not understand to be used on their projects. They should tell their clients that they are unable to properly design a building for wood trusses and conventionally frame the roof and/or floors.

Garth Dreger PE
AZ Phoenix area