Old steel truss design 2

[jayrod12]

Hey everyone.

I have this steel roof truss comprised of double angles for all members with a 10' bay spacing for the bottom chord and verticals at the intermediate 5' marks for the top chords. This truss was designed and constructed in the late 60's under Canadian design standards.

I've checked a few of the components and it appears as though the original designer neglected combined action for the design of the top chord members. As far as I know your top chord must be supported at max 24" in order for you to neglect combined action or you must provide sufficient argument that the roof deck can transfer the load to the panel points.

When I run the existing members under the original loading (and using the applicable codes from the day for both loading determination and strength determination) for combined action they fail. Am I missing some provision?

I've used a slenderness of 0.9L/r where L is the panel length (5') and r of the 6x4x3/8" double angles (1.93 in) and the steel is 44ksi yield so I used 24.75ksi for allowable axial stress and 26.5ksi for allowable bending stress. Do these seem correct?

Granted it is not all of the members failing this way just the odd top chord member (I haven't got to checking the webs yet).

Would you consider continuous beam conditions for the top chord when analyzing for the moment? Something like wl^2/12?

 

[CELinOttawa]

Do you care about the origin design? If you need to check this for your own purposes, apply modern codes. Having to tell a client that an existing item does not meet current code is pretty common.

Unless you need to report on the existing structure's original design, don't try. It is not easy as you bring too much baggage to the table.

If you really must try to get intl the validity of thr origin design (such as for a forensic report - been there, done that), well there are a variety of reason's that can cause this:

- Stop using the computer. The original designer didn't have one.
- Reduce the problem to simple node loading. That's likely a key point in your incompatible methods.
- Learn Graphic Statics. That's the most likely analysis tool.
- Pretend you only have the ability to know three significant digits, and occasionally guess a forth. Carry only four significant digits from every step to the next.

That's a start; if not, maybe I can help. Post the details of the truss and I'll let you know if it passes the old fashioned checks.

 

[jayrod12]

It has been all checked by hand so far. I don't trust the computer unless I know what answer to expect already.

It's more for my own volition to confirm the truss was designed correctly because the new loading I'm designing for is causing what I feel as excessive reinforcement in a few members. I.e. The total load added to the truss is in the range of 10% increase however I have members failing code checks into the 70% over range.

I can't post particulars quite this second but I'll ship you something later.

 

[CELinOttawa]

By hand, but did you do a Graphics Static analysis? That method if fairly obscure these days, but was very much the vogue for truss design right up to the late 70s. Far less work than method of sections or any long-hand analysis... But is it graphical and prone to a 20% variability in results.

I would say the original use of GS is your culprit.

 

[jayrod12]

Since I have no idea what you are talking about when you say graphical static analysis I'm gonna say no I did not use that.

Off to la googlea

 

[paddingtongreen]

I used to use graphic analysis occasionally to check my method of sections solutions but I never saw it used in the documentation of a job.

I don't see what good it does here, jayrod seems to be concerned with the roof loading along the chord, not just at the nodes.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[jayrod12]

Found a book by William Wolf M.S on graphic statics. I definately did not use that method from what I gathered quickly perusing the truss section. Method of joints is my go to method for full truss analysis. Method of sections when I only need to know a few members..

 

[BAretired]

From the description, it sounds like a parallel chord truss with a Warren configuration, which is relatively easy to check by hand methods assuming pin connected members. Usually, with that configuration, loads are applied at panel points so there is no bending to be considered. If load is applied along the chord, it seems reasonable to consider continuity in the design of the top chord.



BA

 

[Ingenuity]

- Stop using the computer. The original designer didn't have one.

jayrod12,

I suggest a slide rule, or abacus...sorry, could not resist re a previous thread.

 

[CELinOttawa]

Hey, a slide rule WOULD BE a good idea here!

You guys and your idiot boxes.... As I write this on my own idiot box. For shame!

 

[jayrod12]

you mean one of these?

 

[KootK]

I wouldn't hesitate to take advantage of chord continuity if that reflects the reality of your situation. An elegant reinforcement solution might be to turn your truss into a compound truss. I've always wanted the opportunity to do one.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jayrod12]

Thanks BA and Koot,

Parallel chord with a warren is correct. Johnny, tell the man what he has won.

It was analyzed using pinned joints. I plugged it into RISA for shits and giggles and played with the fixity of the joints. Fixing them makes matters much much worse.

 

[KootK]

You didn't fix the webs, right? Just the chord on one side of the joint to the chord on the other?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jayrod12]

This is why I don't trust computers. In my haste I fixed the webs as well.

 

[jayrod12]

And yes. It still makes things much worse.

 

[KootK]

In that case, I'm glad that you modelled the continuity. Without that, important aspects of the truss's behaviour would have been missed.

compound truss...

Compound truss...

Compound Truss...

COMPOUND Truss...

COMPOUND TRUSS...

I had a couple beer at lunch. And I love compound trusses.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CELinOttawa]

My my my... No. The concern you're even investigating was a nearly unthought of condition in the 60s. Trusses were designed as a set of two force members without combined actions. I have seen a separate check on the the bending actions of the chords separately from the truss analysis, the continuous beam condition you suggested at the end of your original post.

We worry about far more than our predecessors, and as far as I am aware, combined actions were rarely applied before the 1970s. Many steel trusses I have seen where I had access to calcs (industrial buildings for the most part) had ONLY a graphics statics analysis as the basis of their design. That was it, that was all. After that they added some robust panel detailing able to take combined shear forces and occasionally couples.

I would be very keen to hear from some of the older members on this issue. I'm always keen to learn, particularly about older methods.

 

[KootK]

I'm not following CEL. How does the fact that the original truss was designed with a disregard for continuity make it acceptable to do that now? I would hope that conservatism in other aspects of the original design might offset simplifications in the original analysis. Based on Jayrod's results, that does not seem to be the case.

@Jayrod: what have you modelled the unbraced lengths of the chords as? It's five feet or less for all buckling failure modes, yes?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jayrod12]

Kootk, It is yes.

See attached for the truss makeup.

Note that i only sketched one half of the symmetrical truss. These trusses occur at 12.5' on-centre.

My members of concern are the 4th and 5th TC1 from the end.

Please expand on your compound truss theory if you could. I don't see where in this construction I could go that route. What would you consider the simple trusses that make up the compound truss?