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
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
RE: Wood Truss Analysis-Modeling
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
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
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
I would check the Wood Truss Plate Institute'e website at www.tpinst.org
RE: Wood Truss Analysis-Modeling
Garth Dreger PE
AZ Phoenix area
RE: Wood Truss Analysis-Modeling
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
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
Would you suggest screws, nails, or through bolts? Or would you un-suggest any?
EIT
RE: Wood Truss Analysis-Modeling
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
Also I really enjoy SE and am hungry for knowledge.
Thanks Again!
EIT
RE: Wood Truss Analysis-Modeling
BA
RE: Wood Truss Analysis-Modeling
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 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
Garth Dreger PE
AZ Phoenix area
RE: Wood Truss Analysis-Modeling
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
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
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
Garth Dreger PE
AZ Phoenix area
RE: Wood Truss Analysis-Modeling
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
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
Thanks again.
EIT
RE: Wood Truss Analysis-Modeling
(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
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
RE: Wood Truss Analysis-Modeling
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
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
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
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