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Attic Trusses
2

Attic Trusses

Attic Trusses

(OP)
Has anyone else had much experience analyzing trusses (metal plate connected trusses) utilizing RISA3D?

I've pin jointed all of the web members and rigid jointed the heel and peak joints. For now I've ignored all of the load cases and just assumed one load case to get a better feel for the Axial, Shear and Moments in the various chords and webs.

Loads


Axial


Shear


Moments


A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Attic Trusses

Well... instead of fix the apex and heel I would pin them. I would fix the sloped rafters as continuous embers and pin the rest.

Mike McCann, PE, SE (WA)


RE: Attic Trusses

are you just bored…. :>

RE: Attic Trusses

(OP)
On the weekends I like to work on some of my various speadsheets/programs for analyzing trusses. Up until this point I haven't really considered attic trusses in any of my software however I've had enough instances where I did have to do the vertical load takedowns for them so I'm considering adding them in. However they are a bit more complicated than the run of the mill trusses I normally encounter and my lack of experience in dealing with them prompted me to put up this post.

I'm pretty sure I've got this thing figured out just fine but nine times out of ten if I post it here I will find something I have missed...

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Attic Trusses

Agree with Mike...the only fixity you need is to make a member continuous as it exists in the construction. Pin all the others.

RE: Attic Trusses

(OP)
I've generated a model for a truss that I've recently worked on. My load case is #4 from the ASCE 7-10. I'm am trying to match the results as closely as possible to the Mitek report shown below. Changing the joints from pinned to rigid does affect the numbers slightly. The ANSI TPI1-2007 is somewhat confusing or at least a little unclear in my opinion on how exactly to treat rigid versus pinned joints at the peak and the heel. The Hr reduction factor given in section 8.3.2.2 implies that the heel joint does have some moment transferred across it and probably should be treated as a rigid joint. The peak joint is a little less clear. I typically treat all of the web members as pinned at their ends and hence only axial members, this simplifies things slightly.

All Joints Rigid:





All Joints Pinned except for Heel Joints:






A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Attic Trusses

(OP)
My axial loads are for the most part quite close to the Mitek output. I am very curious though how they are treating the peak and heel joints as well as the web joints.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Attic Trusses

(OP)
I am pretty stoked to find out that Risa3D will do a NDS code check on each member, have never really used this feature before. A word of caution though, you need to carefully setup the unbraced length for each member in one of the spreadsheets.

My combined stress indexes are coming in a little higher than the ones generated by Mitek so not entirely sure that I have all the adjustment factors correct. For example the highest stressed web member is the ceiling web that is in compression and bending. Its CSI is .58 which is slightly larger than the WB = .47

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Attic Trusses

I believe they treat the heel and peak joints as pinned as MSquared mentioned. I don't think their software or testing is that sophisticated to do otherwise.
I have had instances where a contractor has hacked up a few of these for flues. They are really hard to fix as they are usually pretty highly stressed if the span is long.

RE: Attic Trusses

I don't see anything in the heel connection that would transfer moment.
Remember...just because we model something in a particular way does not necessarily dictate its action or response in construction.
It appears that the original truss design (assuming that's your last sketch) considered the heels to be pinned.

RE: Attic Trusses

I haven't scrutinized a version of TPI since the late 90's, when that was my thing. Back then there was a 'fictional heel modelling procedure" for heel joints to simulate the partial fixity generally expected there.

Truss heel heights are generally too high for a concentric joining of the chords and simultaneously not high enough to get a meaningful vertical web at the heel. You end up with wedge blocks, sliders, giant plates, and multie plates, all of which create significant potential for fixity,

I agree agree that the particular case illustrated in the sketches above would create something fairly close to pinned however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Attic Trusses

(OP)
Given the size of the plate at the heel I would agree that modeling should probably be pinned and not rigid. I have read a number of papers on modeling MPC trusses and I am still learning. Section 8.3.2.2 of the TPI1 doesn't make sense where the Hr decreases in value to .65 for higher pitch trusses 5:12 to 12:12 and then jumps up to 1.0 for truss with pitches greater than or equal to 12:12. I would really like to get into the head of whomever wrote the bulk of TPI1 to better understand some of the additional checks and exceptions. Most of it follows the NDS fairly closely but there are some things that make it special.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Attic Trusses

(OP)
I would not discount the complexity of the software produced by the giants like Mitek. If you dig through their user manuals, which I have done on occasion, you will find various options for adjusting fixity at the heel and pitchbreaks (peak joint etc...) including a semi-rigid option somewhere in between the pinned and rigid. They seem to suggest modeling the truss with semi-rigid joints at the chords.

Read this Article:

http://www.mitek-us.com/WorkArea/DownloadAsset.asp...

Being that the heel plates are usually rather large on most trusses I tend to agree with the article in paragraph two and I normally assume typical MPC trusses will have rigid heels and semi-rigid peak joints.

I would really like to get my hands on a copy of Mitek or Simpson's truss design software just to get a better idea of what goes on under the hood. It's really too bad they don't produce an engineer's or architect version of their software. I know the solar panel industry would definitely benefit from this.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Attic Trusses

Medeek:
You make mighty nice sketches and diagrams, but I’ll be damned if I’ll follow this thread. I have to scroll all the way into the next room to the right to read the right most two thirds of each line of text. It just isn’t worth the effort. Why not show your sketches at a much smaller scale (pixel width?) so that they don’t expand the entire thread width so much? When you show them as an attachment, we can bring them up in a separate window and scroll around them without screwing up the reading of the entire thread. Many times I can adjust the scale on these attachments to see details, if I wish. I must be the only person on the E-Tips forums who has this problem, since no one else seems to be bothered by this thread width issue.

RE: Attic Trusses

(OP)
Sorry about that dhengr, your screen resolution is probably a lot smaller than my 24" monitors. Also this may have something to do with your browser, not sure. I'm using google chrome, when I shrink the browser window the width of the thread adjusts automatically so that all of the text and images show up without any additional scrolling. What browser are you using?

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Attic Trusses

I would like to see their testing to back this up these assumptions of fixity. The amount of load these plates can take is suspect to begin with, in my opinion.
I have seen enough significant creep in floor trusses to know there is a problem somewhere. Never seen this in roof trusses, however. I guess the plates are not loaded as heavily due to the more favorable aspect ratio.

RE: Attic Trusses

medeek is right, dhengr. You were not the only one, facing this problem.

I was using Mozilla Firefox on a 17" screen, and was facing the same problem, when I viewed the thread for the first time today. However, opening the page in Google Chrome has solved the problem.

You may try the same, too.

Thanks dhengr, for pointing out the problem, and medeek, for the tip.



A good structural engineer is often a blessing for others.

RE: Attic Trusses

A little clarification.

First sentence in my previous post is not conveying the right sense. I actually meant that "medeek is right about the use of web browsers. Moreover, ..."

A good structural engineer is often a blessing for others.

RE: Attic Trusses

@XR250: the situation with floor trusses is often exacerbated by the fact that they often get seriously overloaded during construction.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Attic Trusses

you mean they are not intended to have 4,000 lbs of sheetrock stored on them? :>

RE: Attic Trusses

Surprisingly no! I actually wrote WTCA's tech note on construction loading. I can recall no instance of my labours ever having less impact. I believe that "failure" in a truss plate is usually tooth withdrawal. You retain considerable joint strength but joint stiffness goes to hell in a hand basket, particularly perpendicularly to the teeth.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Attic Trusses

What is the point of this post? Are you a mechanical engineer who is now dabbling in structural engineering?

I went to your website and read your bio, and not to be too harsh my man, but I am not sure what it is you are doing with this business or your career, and if it is ethical.

We are not allowed to assist non-engineers in engineering, and if you are not a qualified structural engineer, then we should not be helping you out.

Your post exposes a lack of knowledge of a system as basic as trusses, which is something you learn in Structures 1 and you should be able to solve simply by hand.

My educational background is a bachelor's of science in Mechanical Engineering from Brigham Young University ('98). I am a professionally licensed mechanical engineer in the State of Washington, as of December 2012, and in Utah, as of February 2014. I have spent the last few years studying structural engineering of wood frame structures in some depth, and the related commercial and residential codes.

RE: Attic Trusses

(OP)
@a2mfk

My original schooling was in mechanical engineering but I switched careers a couple years ago to the structural field, it has been challenging but definitely worth the ride.

I'm not sure what you mean that this post exposes my lack of knowledge of trusses. I challenge you to try and solve this particular truss by hand, the first thing you will determine is that it is an indeterminate structure and your method of joints or sections just does not work in this case, it can't be solved by hand. Matrix analysis is the only way to solve this truss.

Last year I programmed a matrix analsys program into my fledgling truss designer here:

http://design.medeek.com/calculator/calculator.pl (Fink truss only)

The results match up perfectly with my output from STAAD and RISA so I'm pretty confident I at least got that part of the structural analysis right.

I just received my copy of ANSI TPI-1 2014 in the mail yesterday and was pleasantly surprised to see that there are not many changes from the 2007 standard. My truss designer is at this point an academic exercise in trying to implement all of the necessary checks per the TPI 1 in regards to lumber and plate sizing, still very much a work in progress.

My next challenge it to try and add in a rudimentary attic truss.

I know that I am recreating the wheel when you consider that the truss giants (ie. Mitek, Eagle, Simpson, ITW) already have software that does this sort of analysis and probably a lot better than mine does. The problem is that as an engineer you will never gets your hands on their software unless you work for a truss company or are an employee of the plate manufacturers.

I am not an SE but then how many engineers that do residential structural engineering are? In the state of Washington and Utah and SE is only required for certain structures so what I do is perfectly within any legal or ethical realm.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Attic Trusses

(OP)
@a2mfk

I guess I'm the type of person it is hard to pigeon hole into one career or stereotype even though some people will try. I've had a very varied career, with businesses in the tech industry, building supply industry, trucking industry and native art industry to name a few. I'm an engineer but I also enjoy design, architecture, art, programming and artificial intelligence research.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Attic Trusses

I have the same problem as dhengr and I am using Firefox. I haven't tried Google Chrome, but I will. It may be that Eng-Tips can correct the problem. Even as I'm typing, I can't read the left part of the screen. For some reason, word wrap is not working properly with my current browser. And even when I do a Preview, I can't read a complete line without jogging back and forth. It's enough to drive me away from this thread.

BA

RE: Attic Trusses

Okay, now I'm using Google Chrome and the word wrap feature is working properly.

BA

RE: Attic Trusses

(OP)
This thread probably should have been started in TRUSS engineering forum, sorry about that.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Attic Trusses

Nothing wrong with having it in this forum. Putting it in another forum won't help with the display. In Google Chrome, the diagrams are much smaller than they were with Firefox.

BA

RE: Attic Trusses

FWIW, I am an ME who became an SE

RE: Attic Trusses

(OP)
Maybe I need to change my profile description to "Structural", about every 2-3 months I get a blast from another structural engineer on this board about why an ME has no business doing structural work. The funny thing is that one of my own local mentors started out as an ME in the aerospace industry like myself and went on to become a structural engineer.

Engineering is engineering, if you want a change in course go for it. A more drastic change in course from a mechanical discipline to computer or chemical engineering might raise a few more eyebrows but even then this is a free country, the choice and opportunities are there, if you want to take them.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Attic Trusses

Much better to learn, study and improve skills.

RE: Attic Trusses

From the posts I've read from Medeek he a better SE than most when it comes to wood design. And would mostly likely run circles around most people when it comes to analysis.

EIT
www.HowToEngineer.com

RE: Attic Trusses

@RFreund;

Are you guys having a bromance? bigsmile

RE: Attic Trusses

I qualified to take the national engineering exam and Architecture exam in the 90's with an Associates degree.In many States 10 years experience was the requirement.
In Florida an Engineer can practice in any field he is competent in as there is no " structural engineer " title, even though i think there should be. I don't know how many schools were teaching wood lateral load design in the past.
My brother has an engineering degree from Va. Tech.and is a P.E. and he did not study wood lateral design. The fact is school and passing the exam are only the beginning in any profession.

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