top/bottom chord modelling of truss 1

[JoeH78]

Hi All,

I'd like to know how you model the top and bottom chord of trusses for analysis. Normally they are single pieces which extends along the spans, but how about if I model them as divided short elements between the intersections of diagonals and vertical members. Will I underestimate something in design ?

Regards,

 

[BAretired]

If you mean you wish to consider all truss members as pin ended, that would be the way they have been designed for many years without any problems. Using that method, all truss members are assumed to be axially loaded, so you will be underestimating member moments but it is generally considered a satisfactory approximation and is permitted by some if not all codes.

BA

 

[Guastavino]

One thing to consider is that if you model it as segments your model will only recognize the unbraced length as the length between panel points. For the top chord that is likely OK as beams/purlins/roof decking all are likely present. For the bottom chord though, (if there is uplift, etc.) it might not be OK.

With that said I also agree with BAretired about pin ended. Just make sure the unbraced length is accurate.

 

[JoeH78]

@BAretired :
Yes, for many years truss systems are solved as pin-ended bar members, I even remember those days on college where we solved the truss system by Cremona and "node by node" method by hand. However with computer science improvement we are not limited to simplifications methods anymore (pin - end , rigid diaphragm etc..) instead we can model more realistic models. So, I don't use pin ended design anymore, simply use the bar element without any simplifications, if the whole geometry is truss-like it never induces large moment bendings.

In my case problem is that: If I use unsegmented bottom or top chord (as one single profile crossing across span) adadquate box profile is 300*200*6 mm (St.355), while if I use segmented top/bottom chord 100*50*5( St.355) is enough for the 15mt span with 0.5mt height of truss. So which one is correct?

@njlutzwe:
Sure there will be a uplift effect, since the wind may act in upward direction. I think that unbraced length is causing this whole wreak havoc as you pointed out. It is designed as per the EC3 which takes into account the slenderness for section stability check and which is a big if I use unsegmented part.

I'd be a better if I give more details:
Max span: 15mt
Truss height :limited to 0.5 mt due to some aesthatical reasons

Software: Robot structural V2015
Self loads : Computed by program
Live Loads : 40kg/m2
Wind Load : 64kg/m2
Snow Loads : 75kg/m2
Snow drift : 150 kg/m2 at the some locations
Temperature : 30 'C
Quake : Calculated by program (Equivalent seismic force method)

Combinations :
Full set of above load cases with all possibile directional combinations. Coefficients dead = 1.4; live = 1.6 ; snow = 1.0 ; wind = 1.0, temperature = 1.2. Wind and snow never acting together.

Regards,

 

[BAretired]

Your loads are extremely high but don't mean much without providing the spacing of the trusses.

I don't think 0.5m depth is adequate for a 15m span truss.

BA

 

[Ingenuity]

Your loads are extremely high

The loads are not that extreme...e.g.: Live Loads : 40kg/m2 is 0.4 kPa or 8.2 psf

Agree, the truss depth does seem a bit shallow for the span.

 

[BAretired]

You are right Ingenuity, the loads are quite low; I erred in converting.


BA

 

[Ingenuity]

Easily done, BA. Whilst we are both probably well versed in metric and imperial units, those damn EU guys still like kg/m² etc.

 

[JAE]

I'm not sure why the question is being posed as "so which one is correct".

The truss capacity in regards to unbraced length isn't that difficult to determine if you understand what lines of bracing (either translational or twisting) is available for the chords. It sounds like you are asking a question without really applying direct engineering fundamentals to this but rather relying on ROBOT to get you an answer. What am I missing here? Simply determine, in your engineering view, what the unbraced lengths are and determine the capacity of the chord segment between brace points - either tension or compression.

With a full segmented chord there would be bending moments in the chords so you capacity would be tempered by the flexural-axial combination checks. With pinned-pinned chords throughout you have only axial (unless there are chord forces applied between joints). In the past methods, some degree of pinning was assumed a the truss panel points - i.e. a type of inelastic behavior - that assumed the truss would behave as pinned-pinned even though it technically doesn't.

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[dhengr]

Joehigashi:
Despite the fact that we have all of today’s fancy-assed computer software, the concepts of a pinned-pinned truss analysis with superposition added to account for top chord bending should not be a lost concept. You can generally control that the bottom chords be loaded to the panel points or some symmetrical loading arrangement, to manage bottom chord bending. Trusses have been standing and carrying loads for years before the computer came along. The problem is that despite all your fancy calculating ability you guys have no concept of practical general proportions, depth/span ratios, etc. Obviously, with today’s software you can include axial loading and member bending, which we had some trouble doing with older long hand methods. The trick is to design the gross truss with a fairly simple model so as to hone in on the total design. Then you might pick a couple details which require much finer modeling because of their particular complexity. But then, it still boils down to good welding details or your whole computer model will blow up in your face at those failure points. Because we have computers today, to tell us everything, we have kinda forgotten to apply common sense and good engineering judgement.

 

[Once20036]

SJI specifically weighs in on this and indicates that when the spacing between nodes along your top chord is less than a certain distance, you are "allowed" to idealize it as a short pin-pin member. When the spacing exceeds this threshold it needs to be treated as continuous.

 

[hokie66]

If the "maximum" depth is only 500, just use a 500 deep rolled beam section, if that is strong enough. Your dimensions for a fabricated truss are unrealistic.

 

[KootK]

@joehigashi: because of the large span to depth ratio of your truss, I suspect that the chords are tending to act more like a pair of stacked beams rather than the chords of an idealized truss. If this is the case, then your computer modelling has served you well: it has brought it to your attention that your truss isn't really much of a truss.

I second hokie's recommendation regarding using a solid beam section if possible, even if geometric constraints mean that beam section is just your two chords stitched together. If you decide to proceed with the truss, on the other hand, it will be prudent to use the continuous chord model rather than the segmented chord model in this instance as the segmented model is clearly not capturing the fundamental behavior of your structure.

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.

 

[SlideRuleEra]

Elegant solution, Hokie.

Joehigashi - There is an easy way to determine that no realistic truss will work without even using paper and pencil... much less computer software. Look in the Steel Joist Institute Load Tables. In my 20 year old edition, the longest span that any 508 mm bar joist is rated for is 12.2 meters. Without going any further, that is a clear indication that 15 meters with a workable 500 mm truss is "pie in the sky".

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[JoeH78]

Thank you All,

If the "maximum" depth is only 500, just use a 500 deep rolled beam section, if that is strong enough. Your dimensions for a fabricated truss are unrealistic.

That is pretty clear and smart I used castellated beam profile HEA 260-500 which satisfied, but I'm utterly disappointed on how computers can mislead us. Then the question arises when to trust or when not to trust? I also will testify with sap2000 the same project.

the longest span that any 508 mm bar joist is rated for is 12.2 meters. Without going any further, that is a clear indication that 15 meters with a workable 500 mm truss is "pie in the sky".

Is that true, regardless of applicable profile types, sizes and steel grade? Also can you shed some lights on which table is that exactly in SJI, it will be good to back it up from some tentative data.

Regards,

 

[KootK]

Is that true, regardless of applicable profile types, sizes and steel grade? Also can you shed some lights on which table is that exactly in SJI, it will be good to back it up from some tentative data.

The SJI stuff is a practical limitation, not an inviolate, hard one. You're at liberty to proportion your truss any way that you see fit. You've just got to make it work for all of the usual design and constructability criteria. The SJI tables let you know that your truss will be difficult, and possibly impractical to design and fabricate. That's all.


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.

 

[SlideRuleEra]

Joehigashi - KootK made a good summary of how the SJI data is a reasonable benchmark for a practical truss. Those trusses are optimized to be the best possible for a given size.

BAretired was the first to point out that your 15 meter span to 0.5 meter depth (30 : 1 ratio) was "warning flag" that the truss would have problems. Yes, the profile, size, and steel grade make little difference - its the geometry that dooms any reasonable chance of success (i.e. the 30 : 1 span-to-depth ratio). No matter how strong the steel, deflection is going to be outrageous. Hokie's proposal should have the very best chance of working, but even it will be a pushed to not deflect too much. A wide flange section has the advantage over a truss that, for a given height (0.5 meters) it is designed to put more steel at the best position (flanges) than any reasonable truss design can do.

The comparison that I make to the SJI info came from the 1994 "Fortieth Edition Standard Specifications, Load Tables, and Weight Tables for Steel Joists and Joist Girders". As stated, this is the most recent copy I have. Current info is available from SJI at this website:

http://steeljoist.org/publications-1

Specifically, in the 1994 book, I checked the following joists:

Metric Load Table, Open Web Steel Joists, K-Series (page 40)
Joist 20K10 (508 mm depth) Maximum Span rated in the tables: 12.192 meters

and

Metric Load Table, Longspan Steel Joists, LH-Series (page 60)
Joist 20LH10 (508 mm depth) Maximum Span rated in the table: 12.192 meters

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[JoeH78]

Hi Collegues,

One more thing related to that top bottom chord, conducting the buckling analysis in structure results in quite low critical buckling load ratios such as below 1.0 . As an overall stability check of structure is this a clear sign of that global buckling occurs? Is there any numerical limitation in US code for the global buckling analysis?

Regards,

 

[KootK]

As an overall stability check of structure is this a clear sign of that global buckling occurs? Is there any numerical limitation in US code for the global buckling analysis?

If your model is a suitable representation of reality then, yes, an applied load ratio less than one indicates a stability problem. I don't know of any explicit code limits on the elastic bifurcation buckling load ratio, assuming that is the analysis that you've run. However, it's a sure bet that the code prescribed methods that address inelastic buckling, initial imperfections, and residual stresses will be more restrictive.

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.