Thermal Expansion in Pretensioned Rods 2

[questionator]

Hello,

Young engineer here. I have an unconditioned building that I'm reinforcing with tension rods for lateral restraint. I was going to pretension the rods to counteract tension loss from thermal effects and keep the rod engaged in the event of seismic activity/reduce punching force from accelerations. I'm finding that to do so, the pre tension in the rod would need to be almost as high as the rod capacity.

Two questions -

1) Could someone help me sanity check the force I would need to overcome with pretensioning? I used a coefficient of thermal expansion = 8.8E-6 for a 316 stainless steel, 14ft long, 1" dia. rod with 60 degree F temperature change. I am getting 12 kips of force that would need to be overcome. (This assumes an install temp around 40 deg F).

2) As someone pointed it out in this related post , the magnitude of elongation from thermal expansion is relatively small ~3/32" at each end for this temperature change, which seems incredibly small. How are pretensioned rods designed for unconditioned/exterior environments if such a small change in length creates such a large force?

Thanks!

 

[3DDave]

It is the difference in thermal expansion that adds or removes tension/compression. You need the coefficient of thermal expansion for whatever the rod is compressing.

 

[questionator]

Thanks! Just want to make sure I understand correctly. Why would the elongation of the rod itself due to thermal expansion from increased temperatures (it is uncovered in unconditioned space) not cause loss in tension? That would be great if it's the case - could you point me to some references?

 

[dvd]

For relatively short length changes, a spring, stack of belleville washers, or similar, might help to keep reasonably constant tension under fluctuating thermal conditions.

 

[3DDave]

A tension rod by itself cannot elongate and cannot have tension so asking about the rod alone makes no sense.

When a weight is hung from a cantilever beam, does the tension in the top disappear with increased temperature? No - because the compression side will also expand a similar amount.

In pure tension, you need to consider the source of compression that balances it and what the effect of temperature change is on it.

 

[questionator]

Of course, it is not in a vacuum. I disagree that the rod cannot elongate by itself. A rod sitting on the ground can grow and shrink with thermal changes. But I do agree it cannot hold tension without a support element.

The compression members are masonry which has a much smaller coefficient of thermal expansion. For this case, you can consider they do not expand/contract at all compared to the magnitude of change of the rod. If the rod ends are held in place by the masonry, we pretension the rod, and then the rod elongates due to thermal expansion and the masonry stays the same, we have lost tension. Correct?

 

[Just Some Nerd]

No offense Dave, but I feel like you're missing the mark with that response. It's easy to see what he means by the rod elongating due to thermal effects, which reduces the tension after you sum everything back together.

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Why yes, I do in fact have no idea what I'm talking about

 

[Tomfh]

How are pretensioned rods designed for unconditioned/exterior environments if such a small change in length creates such a large force?

They're not. You crank the rod up tight, and that's that.

 

[dvd]

A point to be cautious on with a 316 SS tension rod is galling and being unable to fully seat the nut. Your cut threads on a tension rod might cause problems.

There is plenty of literature out there on the topic - here is one: Review of Wear and Galling Characteristics of Stainless Steel