Excessively high thermal loads on bracing members 3

[Edu09]

The challenge I'm facing is finding a sensible bracing member/ connection design for a 30m span steel bridge. We have an FEA model of the structure with the bracing members modelled as line beams with end releases(pinned). The thermal loading from -20 to +40 degrees celsius is applied to the whole structure apart from the bracing(due to it being completely shielded from direct sunlight). When the same thermal loads are applied to the bracing aswell the fores experienced are lower which seems odd to me.

The bracing was mainly included to provide loadpaths under an accidental collision load case but now this issue has become apparent.

The problem is the model gives a high axial force ~500kN with thermal only and up to 900kN in the worst load combination(DL + LL + Thermal).

So,
1) does this force seem unreasonable to more experienced folks here?
2) Can you see any work-arounds or reasons not to believe the model?
3) Any suggestions on where to go with this? more information needed to give advice

Cheers

Edward09

 

[WARose]

Without seeing the model, it is hard to say if it is right or not. However, it doesn't take too much temperature change to develop some pretty high forces in structures comprised of stout members.

The suggestion that comes to my mind (again without seeing it) is perhaps a model that takes into account the steel's ability to grow (and ergo relieve the force). But for a fully braced structure....that may not accomplish anything.

 

[SlideRuleEra]

Edward - I'm willing to bet that there is a problem with the model.

1. Bridges are much more flexible than most people believe.

2. The entire structure is subject to thermal expansion, whether it is in sunlight or not. How much is debatable. What was the assumed temperature differential that gave the 500kN force?

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[MacGruber22]

However, it doesn't take too much temperature change to develop some pretty high forces in structures comprised of stout members

Stout? Doesn't stout imply the opposite of slender? How does this affect amount of thermal force when it is a linear property, for all intents and purposes?

"It is imperative Cunth doesn't get his hands on those codes."

 

[Lomarandil]

SRE's point #2 is very pertinent. While members out of direct sunlight will lag in temperature and thermal expansion, they won't be totally exempt. For bridges over large bodies of water, I've seen differentials on the order of 10* or 15* C -- but certainly not as much as your full range.

Mac, thermal strain/elongation is linear, but forces are related to cross-sectional area of the members expanding/contracting. So a stout member will generate more force.

It's a long shot, but you mention pin supports. You might also consider the flexibility of any substructure to accomodate thermal movement in your primary members and reduce the force generated.

 

[MacGruber22]

Lomarandil, I answered too quickly without thinking about it. Thanks for responding.

"It is imperative Cunth doesn't get his hands on those codes."

 

[IFRs]

Are these braces capable of resisting compression or only tension?

 

[Edu09]

Firstly, thanks for all your replies.

WARose I think you are quite right, this a stout structure overall and the bracing members too do reflect this. I realise it would be easier to show you the model but where I am now that's not really practical. I do appreciate your help though!

SlideRuleEra the assumed temperature range is large: from -40 degrees Celsius thermal contraction to +40 for thermal expansion- I believe this to be very conservative.

About the heating of these members too, the explanation I was given was the one about not being exposed directly to sunlight therefore no thermal loading. But I think this could be the issue- as the whole structure expands/contracts the bracing cannot release any of this force by doing the same which is why it is so high?

Lomarandil Thanks for reiterating this. It's good to hear that because it is quite a big temperature differential and I've been thinking along similar lines. The substructure suggestion may be a bit too deep for me to go on this one, although I am interested. I'll have a think about it. Although, I meant the bracing members are modelled as pin ended- i.e. only resist axial loads.

IFRs the braces are capable of resisting both compression and tension.

One more thought; the members are going to be bolted to gusset plates in oversized holes to allow greater tolerances at construction- could this help? Is there justification for say, reducing the force by allowing extra expansion?

Thanks again chaps.

 

[SlideRuleEra]

...not being exposed directly to sunlight therefore no thermal loading.

That statement is not accurate. Thermal loading is a heat transfer (mechanical engineering) problem. The temperature of a component is determined by a combination of radiant energy (sunlight), convection (wind), and conduction (steel is really good at that). Even if the brace is in full shade, it's temperature will be essentially the same as the air around it.

I asked about the assumed "temperature differential" not the "temperature range". Lomarandil explained that 15[sup]0[/sup] C differential is a reasonable upper limit. The range is not important for thermal expansion purposes (metallurgy is another matter). For example, if the main structure is at an equilibrium temperature (a balance of radiant, convective & conductive heat transfer) of, say, +20[sup]0[/sup] C, the likely worst case is the brace (in full shade) is +5[sup]0[/sup] C.

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[Edu09]

SlideRuleEra Thanks for the reply. I understand what you're saying about the bracing temperature, it makes sense now.

I see so, in this case the differential between the main structure and the bracing elements could be too large? +40[sup]o[/sup] C. on structure and 0[sup]o[/sup] C on the bracing elements. So, perhaps applying a more reasonable temperature to the bracing would help is what I'm understanding.

Cheers

 

[Lomarandil]

SRE explained it better than I did.

Theoretically, it would be possible to account for connection slop to accomodate some thermal expansion before the braces start taking load. But, I doubt it's practical. It would require good control on the fabrication (since you could only use the oversize amount in excess of that needed for field fit-up tolerances), connections (many a snug-tight bolt actually gets tightened enough to develop some slip-critical-like clamping behavior), and would likely negate the usefulness of your braces for their intended purpose (as you'd end up with effectively zero stiffness for the first 1/x" of displacement).

 

[Edu09]

Lomarandil exactly right, the oversized holes are to allow tolerance at installation. Yes, totally makes sense. Thanks for your time and explanation, it has helped me as well.

 

[Shu Jiang PEng SE]

normally, the whole structure, not the brace only, will experience the temperature variation. In other word, the supporting ends (or at least one end) of the brace will move as temperature change, therefore, the thermal loading on the brace will much smaller.

You can also try to choose the construction time so that the installation temperature is in the middle of the temperature variation. This will significantly reduce the thermal load too.

Oversize hole is not good option for the brace may not work the way it is supposed to do.

 

[TLHS]

Have you looked at your detailing? If you can provide ways to help absorb strain, it will significantly help. When you have a bunch of bracing nodes terminating at one place, it's solid and can't move. If you have bracing nodes that allow translation, though, it allows some expansion, and you get movement instead of internal stresses.

For instance, chevron bracing allows the nodes that terminate at a beam instead of a column line to push upwards and downwards. That reduced restraint tends to significantly reduce internal thermal forces in the bracing.

You can also look at your brace bay arrangements. Are you locking sections of your structure in between brace bays instead of using expansion joints well and letting things grow and contract?

Obviously geometry is also affected by other things, so you might not be able to resolve your problems this way.

 

[Edu09]

Chaps, thanks for all your replies.

The biggest impact for me is correcting the assumption that the bracing is unloaded thermally. It was always my thought that the reduced forces this gave were due to allowing the bracing to move as well when loaded thermally.

THLS I think your point about having bracing members terminate at one node is a good one, this is the way the members are modelled terminating at either side of the same node in the model. So it probably is a modelling issue, in practice the load path is not quite like this.

SRE in his first post hit the nail on the head "I'm willing to bet that there is a problem with the model." I think this pretty much sums it up.

As others have stated, trying to allow movement at the ends of members would help but may be self defeating in terms of construction tolerances and bracing working as intended.

Thanks very much everyone.