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Wood Truss Analysis-Modeling

Wood Truss Analysis-Modeling

Wood Truss Analysis-Modeling

(OP)
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

RE: Wood Truss Analysis-Modeling

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.

RE: Wood Truss Analysis-Modeling

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

RE: Wood Truss Analysis-Modeling

(OP)
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

RE: Wood Truss Analysis-Modeling

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

RE: Wood Truss Analysis-Modeling

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 www.aisc.org/epubs.

I would check the Wood Truss Plate Institute'e website at www.tpinst.org

RE: Wood Truss Analysis-Modeling

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

RE: Wood Truss Analysis-Modeling

(OP)
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

RE: Wood Truss Analysis-Modeling

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!

RE: Wood Truss Analysis-Modeling

(OP)
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

RE: Wood Truss Analysis-Modeling

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

RE: Wood Truss Analysis-Modeling

(OP)
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

RE: Wood Truss Analysis-Modeling

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

RE: Wood Truss Analysis-Modeling

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

RE: Wood Truss Analysis-Modeling

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

RE: Wood Truss Analysis-Modeling

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

RE: Wood Truss Analysis-Modeling

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

RE: Wood Truss Analysis-Modeling

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

RE: Wood Truss Analysis-Modeling

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

RE: Wood Truss Analysis-Modeling

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

RE: Wood Truss Analysis-Modeling

In my opinion modeling top chord panels as being pinned on both their ends will yield a workable solution.
However the model you're creating mathematically does not match the behavior of the structural element in real life.

If i had a choice id model the chord continuous over panel points with the web members considered pinned to the joints.
Most industry leading programs model wood trusses as such.


In reality there is a bit of moment transfer between chords and webs by nature of the metal plates' ability to carry some moment.
Although that's generally not included in the overall design of the frame. (having said that the Canadian code permits a 20%
decrease in the effective length of a compression web due to the fact that metal truss plates can carry some moment at the joints.  
I would not use this argument for plywood detailing of joints though).


BARetired, i agree with your point to a certain extent.  Ive worked in this industry and can tell you the problem is not with design adequacy.  

The problem is in the regulation of who is allowed to do this type of work.  Generally speaking you don't need a degree or any conceivable educational background
to design and build trusses.  All you need is a powerful computer program and basic understanding of required inputs. and an engineer to back your design.

Its scary that many truss designers Ive seen have no relation or interaction with the structure as a whole, nor do they care to.  Where EOR's view  trusses as designed solutions,
most truss fabricators view them as a commodity product.

Its scary to know that people with no education often detail trusses that are 80-100 ft.  But the factors of safety in this industry are substantial enough that the industry generally has a good track record.

 

RE: Wood Truss Analysis-Modeling

The problem I have with this shift in responsiblity is that the truss designers, in order to be competitive with others in the industry, may or may not employ certain bracing assumptions or requirements in their calculations to save materials.  

Under this scenario, it is NOT the responsibility of the EOR to either assume or identify these conditions, but it IS the responsibility of the truss designer to specify such.  It is also the responsibility of the truss designer to calculate the load that goes into the bracing.  

The conflict that is presently in the mill is "Who will do the detailing of the bracing connections?"  I would not say that structural engineers do not know how to design a wood truss.  That is not the problem.

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

RE: Wood Truss Analysis-Modeling

(OP)
Even though you have answered the questions of my OP. I would like to on Monday try to post the results of different of the analysis of pinned vs continuous top/bot chord as I have some specific questions.  Mostly due to the fact that it seems the top chord fails a unity check when modeled continuous and passes in when pinned. I will need to double check the model in the mean time.

Thanks again.

EIT

RE: Wood Truss Analysis-Modeling

As others have said, the problem is not with design adequacy.  The problem is with bracing of web members and the complete lack of understanding of bracing requirements on the part of framers.  Two typical scenarios follow:

(1)  On a pitched roof, the compression web members are not designed to carry the calculated load over their full length.  The shop drawing shows a symbol at midlength or third point signifying required brace locations.  When the trusses all have similar configuation, the framer runs a horizontal ribbon (usually a 2x4)connecting the web members with two 3" nails at each connection.  They invariably believe this constitutes adequate bracing, failing to recognize that all attached web members can buckle in the same direction.  What they must do is provide a certain amount of 'X' or 'V' bracing at reasonable intervals.

(2)  On a pitched roof with variable truss configurations, the problem becomes a good deal messier.  Again, the computer spits out brace locations for each truss design but the framer doesn't know what to do because the adjacent trusses have different web configurations.  He has nothing to nail his 2x4 ribbon to so he improvises, usually badly.  The EOR has to specify bracing on the fly and somehow explain it to the framers.  After they attempt to accommodate his instructions, he has to re-inspect the work and re-explain where bracing is still required.  

In the above two scenarios, there is no drawing showing where bracing is to be placed other than a symbol on the truss elevation.  Without the necessary bracing, the trusses could collapse.

BA

RE: Wood Truss Analysis-Modeling

(OP)
BA - I understand both point completely as I remember coming across both situations as a framer.

I'm going to post the analysis results (bending, axial, and shear diagrams) of three different models of the same truss.  

1. The top/bot chord is modeled as pin-pin between panel points
2. The top/bot chord is modeled as a continuous member
3. The top/bot chord is modeled as segments between panel points but it has fixed ends.
Webs are pinned for all.

After reviewing this I believe my question should really be to Risa (not sure if we renewed our license) but any input would be appreciated as to explain the results.  In particular why the bending moment is so large in the continous top chord model near bearing.  Also it appears that the results modeled as continuous are less conservative when unity is checked.  Let me know if the results are too cluttered to read.

Thanks again.

EIT

RE: Wood Truss Analysis-Modeling

Long thread--I didn't read it all; however, it looks like you're talking about dimension lumber trusses that fall into the TPI Standard category, albiet with plywood gussets and loose fasteners.  This type of truss is more properly seen as a frame, i.e. there is continuity across the joints on straight chord segments and you would not pin the chord segments at these locations.  At pitch breaks you should model the joint as a pin.  If there is a closed heel joint between top and bottom chords, there is no clear answer, and I would suggest at least considering an envelope solution to assure that you have the highest forces for design of the heel joint, and the most critical intra-chord positive moment in the top chord.  Better yet would be to model the heel (assuming again a closed heel) with a three-node setup and small fictitious members in the heel area.  The Canadian truss standard (TPIC) has prescriptions for these heel analogs, whereas the U.S. (TPI) does not. The Canadian standard was available online for free, last time I checked.   MiTek's software doesn't use fictitous members while ITW (Alpine) software does.  When you get your moments and forces of the members you can chuck them into an excel spreadsheet using the TPI formulae (which are substantially identical to NDS) for stress interaction and moment magnification.  If your analysis program gives you, say 50 sections of data, then you can easily have Excel give you unity equation results.  I have done this and produces nearly exact matches with commercial software (MiTek) results.  Then you have to design the joints, splices, heels, etc.  Make certain to check deflection.  Most plywood gusset trusses are seat-of-the-pants creations; the heel joints in particular are seldom of sufficient capacity.

RE: Wood Truss Analysis-Modeling

RFReund is the hatched area in (2) and (3) your moment diagram? Doesnt look correct, there should be a stress reversal at each joint location, where the moment will be negative.

If you give me the span and pitch i can run the model in one of the truss programs i have available, and check what the bending moment diagram of the top chord looks like.  The diagram should look like that of a multi-point bearing beam supporting a uniform load.

 

_________________
C

RE: Wood Truss Analysis-Modeling

Usually have a bit more luck with bolts - but that can still get messy.  Also depends on chord sizes.

As for using nails - I usually stay away from them on existing trusses - too much chance that you will ruin the members and/or crack the drywall.  Using a nail gun does help somewhat.

RE: Wood Truss Analysis-Modeling

RFreund,

I'm not quite clear on the difference between Condition 2 and 3, but there seems to be something wrong with the chord moment diagram in the vicinity of the end reactions for those conditions.  It is almost as if you forgot to include the end vertical members in your geometry.

You have chosen a web configuration using tension diagonals.  More common in the truss industry would be to reverse the direction of the diagonal members, i.e. compression diagonals and tension verticals.

BA

RE: Wood Truss Analysis-Modeling

(OP)
Mike - Thanks, I'm figuring bolts.
BA- The current design shown is an existing condition. All though I thought that it was a little different.

I've attached the moment diagram for case 2 and 3 shown more clearly and I have shown the geometry.  Also I try to show the difference between how I modeled case 2 (a continuous member) and case 3 (separate members that are fixed to each other).  They should yield the same results.  I believe your BA in that there is a problem with the last vertical because when I removed the vertical it did not change the moment diagram for case 2. When I remove them from case 3 it decreases the moment.
However I believe that you guys have answered all of my questions and know my problem is with understanding RISA and my model which is probably going to be difficult considering you don't have access to my model but if you have any suggestions as to what looks wrong or why that would be great.

Thanks.

EIT

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