[msandford]

If you're dealing with bridges or buildings then you're right of course, you don't stress anything and you just let the stress dissipate through various movements.

But thermal stresses are very small, for "normal" steel it's 13e-6 per degree C. If you figure that the temperature variations in CA aren't going to be more than say 40C (and that's probably too much) then you're looking at 520e-6 or basically 5e-4.

As far as strain goes, that's not a terribly big number at less than 0.1% especially considering that most of the stress/strain graphs will go up to 10% or more and the first 1% are usually WELL within the linear elastic region. That means that you're talking about doing perhaps only using a few ksi of the steel's strength for the thermal effects.

[rmxt]

Excuse my arcane units, but it is what I am familiar with:

ΔL=αΔtL; ΔL/L=αΔt; ϵ=ΔL/L; ϵ=αΔt; E=f/ϵ; f=ϵ/E; so f=αΔtE

ΔL = change in length due to temperature

L = restrained length

f = stress that arises due to full restraint

E = Young’s modulus

ϵ= strain

α= coefficient of linear thermal expansion

Using AASHTO numbers for structural steel:

f= 6.5x10^-6 (1/F)* 100 F * 29000 ksi = 18.85 ksi

That's a big chunk of the elastic 50 ksi range, and it's greater than a good portion of the allowable ranges from the table on page 41 of this PDF [1]. As you might already seem to know, AASHTO doesn't consider temperature loading for fatigue limit states, and in the Strength limit states it considers the force effects to be halved. (Though the displacement effects are multiplied by 1.2) Regardless, fully restraining these things at their ends doesn't seem like a good idea. Am I missing something in what you're talking about? Yes, the strain is low but fatigue generally works in terms of stresses.

[1] http://downloads.transportation.org/LRFDUS-6-Errata.pdf

[msandford]

So I guess it depends a lot on the kind of steel that you're using and what you're designing to. Do you need the full 100F range? Can you get a lower expanding steel for a nominal price increase if you're going to buy many mill run's worth? Are you going to have to build to bridge standards, or can you get away with something else? It's sort-of a bridge, but sort-of not. From a technical perspective you don't need as much safety factor since you've in total control of the vehicle load which isn't true on bridges. I don't know what the regulatory considerations on safety factor are especially since it would probably qualify for its own category since it bears little resemblance to anything that's man-rated like bridges or buildings.

You might also be able to get some nice double-whammy effects from using something like a514 since it's corrosion resistant, has a higher elastic yield, and may well have a reduced thermal expansion coefficient. If you increase the strength and decrease the expansion at the same time then instead of blowing 40% of your "budget" on thermal it might only be 10%. And since you're covering such a large range of climates you might be able to rate every 10 or 20 miles of loop based on the climatic averages in that region instead of looking at the absolute min and absolute max for the whole thing. It might add a few extra weeks of design but make things a lot more feasible.

[tcpekin]

Correct me if I'm wrong, but the linear elastic region usually ends somewhere around 0.2-5%, meaning 1% strain is not well within the linear elastic region. If you just do σ_y/E, you'll get the yield strain. Using rmxt's numbers you get 0.17% yield strain. Using [1] for 1018 steel you get 0.155% yield strain. The strains you calculated are not insignificant.

[1]http://www.matweb.com/search/datasheet.aspx?matguid=3f2ce033...

[msandford]

I guess it depends on the material you're looking at. The system designer would probably be the best person to ask. Are you going to make it out of 1018 or a36, or are you going to use something that's higher performance? Does the increased cost of the stronger steel get offset by the reduced weight?

You can get a36 for about $1/lb in small quantities. You can get corten (a588) for about $2/lb in small quantities. But the a588 has a yield strength about 50% higher than the a36, so strength-for-strength it's a pretty decent deal.

If you need 1" of a36 then you're going to need only 2/3" of a588 so it's only 33% more expensive. Combine that with the weathering properties which can reduce your maintenance intervals substantially (this is the same steel they make sea containers out of) and your lifetime cost might well be lower.

Further since your tubes now weigh less you might be able to get away with either smaller pylons or greater pylon spacing, both of which might be advantageous.

So you're right that the strain is non-trivial but that's only if you pick basically the toughest, least strong steel available. It's pretty easy to go stronger and to not lose too much toughness and gain other desirable features such as corrosion resistance, and in the process make the strain less significant relative to the elastic region.