vierendeel truss analysis 3

[joistg]

I have to analyze warren truss with one diagonal missing somewhere in the centre of the truss.I understand that top and bottom chords will have additional moments. The required opening in the truss should quite large, so there is no room for reinforcing both top and bottom chord of the truss. Also, it is a matter of saving (there is lots of trusses like this). How can I analyze this truss? Also, should I be concerned about deflection and how can I calculate it?

 

[csd72]

I disagree that a computer analysis is required for this as it is easy to provide a good estimate of the additional deflection using similar simplifications to that used in the bending analysis.

It is simply two propped cantilevers.

My issue with computer analysis is that you always get far more information than you need, and in my experience more information means greater likelihood of mistakes.

I also disagree that it is quicker, particularly in STAAD or an FEM program.

 

[rb1957]

my issue with FE is that it's very easy to be beguiled with the very nice pictures and miss the structural issues underlying them.

it's easy enough to redistribute the shear on the cut away portion of the beam ... the upper chord will beam it into the truss beam. you might consider reinforcing the 1st diagonals either side of the missing one; there'll be some extra shear loading there, but the truss may be able to carry this extra without reinforcing.

deflections, i'd expect they wouldn't be much different from the original beam, except in the cut away portion, and it's easy enough to calc that.

 

[ToadJones]

I don't think anyone said FEM was necessary, I just think this is a good case for it.
I think most trusses are a good case for it. An experienced FEM program user could model this very quickly. I usually model, then code check by hand. So in essence I just using the FEM program to get member forces. Which, will be far more accurate (maybe needlessly so) than hand methods.

Maybe a little bit of pride interfering here?

Still no one has been willing to offer this fella insight on deflections.

 

[dik]

If you have a lot of these, then a computer model can be of great benefit. Did my first of these over 40 years ago, and by sliderule <G> and the church is still standing.

It is fairly easy and quick to analyse without a computer model. The unbalanced loading is the critical loading for this type of construction.

It is not likely that you can avoid having to design the bottom chord as a flexural member. With an unbalanced loading, the shear will be distributed in proportion to the flexural stiffness of the top and bottom chord.

Dik

 

[paddingtongreen]

I wonder what you are trying to save. You have 2L3s and you are adding an L2. Unless that 2"L is continuously connected to the 3"L, it won't do much good. Making that continuous connection may cost more than simply increasing the size of the top chord to a C5.
Material is cheap, manhours are not.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[rb1957]

which is what i said in my original post, no?

that you'd need a web splice joining the existing upper angle with the reinforcing angle. this'd imply (to me at least) that the reinforcing angle should be L3.

i'm assuming that he's got these things made and has to rework them.

 

[paddingtongreen]

I know you said it, but I needed it to suggest the using a heavier chord. You are right though, this could be an afterthought.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dhengr]

Joistg:
You say there are lots of these joists, then a thorough analysis is all the more important. I suspect deflection will hurt you if you don’t reinf. both the t&b chords, I believe others have mentioned this too. You must check existing diags. and their welded connections to existing 3x3's for the new forces. And, if they don’t check, then you have a real mess on your hands. I wonder why you wouldn’t just take 4 - 15' long, 2.5x2.5 angles, and place them toe to toe on the existing 3x3 angles, making up 4 tube shaped chord members? A little more mat’l. and labor, but once they are up there to do 2 angles to the top chord, the bot. chords go quickly, all down hand welding. And you end up with a stronger joist with acceptable deflections.

 

[dhengr]

BA..... I believe you are right. We don’t often disagree on much, but when we did a second time, I thought I better restudy my FBD on paper, and my entire thinking on the problem, rather than just imagineering it. Thanks.

BA and I were trying to offer the OP’er. a fairly clean and simple approx. method for looking at his problem. At one point in time, us older fellows learned, used and only had these methods of analysis. But, through that training and usage I think we developed an intuition and understanding of structures which is missing from the training today. And, these methods are usually our first step in looking at almost any problem. We can always make it more complex later, and more often than not someone else does that for us. As the OP’er’s. problem became more complex: i.e. different t&b chords in strength and stiffness; existing diag. members and their welded joints, rather than verts. and Tee joints; useless added verts. and a willingness to let the bot. chord develop plastic hinges and just become a tension link; I was about to write that this problem should ultimately be done with a computer program to actually hone in on member forces and moments, for the deflection, and because there were many joists involved. Many of the secondary issues can probably still be handled by engineering judgement and experience rather than FEA and VonMises stresses. ToadJ beat me to that, and then beat me with it.

I am certainly not against the use of computer software to assist us in doing our structural analysis, in fact they are essential today. What I rail against is turning to them as the very first step in the problem development and solution, and because the designer (engineer? or computer runner?) has so little real understanding of structures that he/she has no other recourse, and then doesn’t really understand the output either, for the same reason. If they don’t start with some structural intuition, I don’t know how they make the proper input decisions, and many of the OP’s show this. I think the guys/gals who do some hand calcs., simple FBD’s and that sort of study to start using the programs, and then do hand calcs., and various quick checks of the computer output are on the right track.

These may be the final flailings of an old dog unable to learn new tricks. I actually think I understand most of the tricks, I just don’t know if I have enough years of engineering left to become proficient at the tricks. Given the use I would put them to, I really can’t afford the latest software, I can’t keep up with the rate of change, and they have become so complex to use, in trying to do everything under every possible set of conditions, that I have to relearn every time I try to use them, they are no longer intuitive to me, nor are they less time consuming. I was actually a pretty good programmer and computer user for engineering applications from the mid 60's - mid 70's, got away from it for a while and it passed me at the speed of light. I would love to have some of you young people working with me. Gosh, what I could learn from you, maybe teach you a little too.

 

[joistg]

These are new joists, they are not fabricated yet. Nothing is cheap these days, so to increase top chord size throughout is not economical. We have done this kind of joists in the past always reinforcing top and bottom chord over three panels with two angles so it looks like channel. The web members for these joists: HSS (tension) between angle T & B chords and two angles compression on the outside of the top and bottom angle chords. That is why this kind of reinforcing. I feel that reinforcing just the top chord will take care of additional moment. But I'm not sure about deflection. What method to use to calculate it?

 

[BAretired]

joistg,

To do a deflection calculation, you can use a simple frame program.

BA

 

[rb1957]

how do you connect the angles together, to make them work as one channel ?

 

[joistg]

butt weld them

 

[joistg]

I don't have frame program. I guess stress-strain analysis can not be used for deflection calcs?

 

[BAretired]

joistg,

You didn't dimension it, but it looks like the clear distance between chord angles is 2". Instead of adding angles over 3 panels on the top chord, why not place a HSS 4 x 2 rectangular frame (see attached). The corners can be mitered and stiffened with a plate.

The advantage is that you stiffen both chords without loss of height in the opening.

BA

 

[BAretired]

joistg,

In this day and age, I think you should have a frame program. Having said that, the analysis is pretty straightforward using hand methods. Personally, I would calculate the maximum deflection of the joist assuming full loading on all panels. That is probably the worst case.

BA

 

[joistg]

I'm in joist and trusses business, so all the joints are pinned. Moment frame program is not needed on a regular basis.
BAretired, if I reinforce my chords in this manner, HSS web will have to be welded to those reinforcing members. Also, adding two angles as per my sketch improve moment of inertia much better than adding HSS 4x2

 

[dhengr]

Joistg:
I see your sketch, but I don’t see how you avoid the outside double angle compression diags.(which you didn’t show) with your top chord reinf’mt. since some of those top panel points will have a compression diag. You could make those compression diags. an HSS also. Haven’t we seen the toes of diag. angles pinched in, to fit btwn. the t&b chord angles? You say you’ve done these kinds of joist before and reinf’d. both t&b chords, so what’s changed to prevent that now? Cheaper (initial mat’l. cost) isn’t always better. I’m not suggesting reinf’g. the whole top chord, three panel bays sounds doable on the top chord. But the predominant consideration may be a good stable compression member which takes some bending, not what appears to be a better bending member, but is a less stable compression member. You have to run the numbers to determine which is critical. I’d sure take a look at BA’s detail, my guess is you want a good stable compression member, except for the outstanding angle legs at the top. And, making both t&b chords strong enough so they participate equally should improve deflection and make your design problem easier, if you can detail around that cleanly, even with a bit more expensive mat’l. in places. I said I thought allowing the bottom chord to develop plastic hinges and become a tension link would work, I didn’t say I thought that would lead to the optimum joist design.

Are you an occasional OWSJ manuf’er., it sounds like you are? If so, how do you analyze your joists now, and doesn’t that program spit out deflections at the panel points? Despite my rant about reliance on computers to substitute for structural knowledge, I wouldn’t design many trusses with computer software to do it.

 

[BAretired]

joistg,

What is the magnitude of your largest middle panel shear and what is the panel length?

BA

 

[abusementpark]

I would definitely put this thing in an FE program. I could probably have a preliminary design in the time it took me to read this thread.