Thermal Expansion in Tension Rods

[phantomlegion86]

Hi everyone,

Question in regards to post installed tension rods for in an old brick building. Long story short, we have a rectangular building and the front (short) face has separated from the rest of the building. The plan is to use tension rods tying the front face of the building to the back face of the building in order to restrain it from any suction loads that are applied to it, transferring the load from the front to the back wall and into the perpendicular shear walls. The problem is that we need to place these tension rods in the attic area of the building, which is not insulated and therefore the tie rods are subject to thermal expansion and contraction. I have been scratching my head over this one for a while and can't seem to come up with a solution on how I could keep our tension rod end bearing plates "snug" against the exterior face of the walls in all different temperatures. I considered using a spring to absorb some of the thermal expansion, but can't justify that, as I don't believe it would work in resisting the suction forces being transferred through it (it would just stretch and allow the front wall to deflect I think). Anyone have any experience with this? Thanks in advanced for any input!

aem

 

[cliff234]

How long are the rods and how much expansion/contraction are you anticipating? Perhaps the change in length will be short enough such that the a little slack in the rods won't matter.

 

[phantomlegion86]

80' so about 3/4" of expansion is anticipated in the rod... so enough that I would be concerned about that front wall deflecting if not restrained... Then again that doesn't take into account any fixity at the base the front wall might have, it IS a very old building and it has been a problem for a long time so it definitely has SOME capacity.

 

[phantomlegion86]

Does anyone know about any low cost viscous dampers that could be used in a building environment like this?

 

[hokie66]

What's wrong with placing the rods below the ceiling in the conditioned space? Such things are often tolerated in old buildings. Failing that, perhaps you should design a more localized method of tieing the end walls to the side walls.

 

[cliff234]

A 3/4" total change in length equates to 3/8" at each end. If the rods are installed at the average temperature, that would equate to a +/- change in length of only 3/16" at each end. Can than change in length be tolerated at each end?

 

[azcats]

Is my mechanics of materials correct in calculating that it takes nearly 23 ksi to stretch a 80' long steel rod approximately 3/4"?

I was looking at how much force would be imparted if you pre-tensioned them, but that seems excessive.

How much force is in each rod?

 

[phantomlegion86]

the 3/4" expansion is not due to force applied, but instead the potential thermal expansion. cliff234 you bring up a very good point though... and have basically answered my question as 3/16" on either end is indeed not enough to worry me. I appreciate all of your input! Thanks

 

[Compositepro]

I think you are looking at it the wrong way. If the pretension strain is greater than the thermal strain there will not be any change in length of the rod. Only the tension will change. You want to select a rod diameter that is not too stiff. If you were to use a 6" diameter bar, for example, then you would have to think in terms of growth because the the tension stress would be very low. But if you use a 1/4" rod you can treat it as a spring and thermal expansion is not a problem.

 

[hokie66]

3/16" each end would be enough to worry me. And while I understand what Compositepro is saying, the smaller rod won't do much for you if the wall actually wants to move.

 

[hawkaz]

A rod like this may elongate when loaded (PL/AE). For an 80' span with a small diameter rod, this may turn out to be significant. Also, how are you planning on keeping the rods from sagging?

I saw this expansion on a project several years ago. A roof was sagging, so someone put a tension ring around the bottom using cable. As the roof continued to deflect (pushing the bottom outward), the cable just eliongated with it.

 

[dhengr]

I agree with Compositepro’s analysis of the situation, and I wouldn’t want 3/16" of slack or looseness at the tie-rod bearing plates either. The trick to a satisfactory design is that you install enough smaller rods so that when you subtract the thermal extension from the original stressing extension, and thus find a final stress, you still have the required forces at the right places to hold the bldg. together. Obviously, the small rods have to be in the right places, and the existing structure has to be able to take the max. concentrated loads, and properly transmit these tie forces. Could you place these rods near existing parallel walls and hide them behind crown moldings, chair moldings and the like?

 

[dhengr]

Could you embed the rods in horiz. brick joints, tension them and then repoint the joints?