Steel columns sitting on thermal blocks at braced bays
Steel columns sitting on thermal blocks at braced bays
(OP)
I am designing a freezer structure for the first time and was just informed by the freezer consultant that there needs to be a thermal block under the baseplate of each column in the freezer. Interior columns need a 6" thick block and exterior columns need a 10" thick block. This is not a big deal at columns with only gravity loads, but I also have this condition at columns that have lateral bracing tying into them. Lateral loads of around 25 kips are being applied at the base of the column, and my column baseplate is supposed to sit on a 10" thick plastic block. I'm not sure how you get this to work. You could do it through anchor bolt bending theoretically, but that gets crazy real fast. I looked at maybe doing hairpins in the slab since the columns are embedded in the slab, but AISC says the slab must be in contact with the subgrade for this to work, i.e. no underslab vapor barrier or underslab insulation. Does anyone have experience with this? How did you manage to transmit lateral loads down to the foundation?






RE: Steel columns sitting on thermal blocks at braced bays
Dik
RE: Steel columns sitting on thermal blocks at braced bays
RE: Steel columns sitting on thermal blocks at braced bays
Dik
RE: Steel columns sitting on thermal blocks at braced bays
Might have to use additional bolts. AISC, I know, frowns on anchor bolt bending though.
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RE: Steel columns sitting on thermal blocks at braced bays
Called the Shadco manufacturer and of course they punted on any engineering help although they did say that they've seen some people using long shear lugs at braced bays. Maybe that is the answer. Will need to run some numbers...
RE: Steel columns sitting on thermal blocks at braced bays
RE: Steel columns sitting on thermal blocks at braced bays
Dik
RE: Steel columns sitting on thermal blocks at braced bays
RE: Steel columns sitting on thermal blocks at braced bays
RE: Steel columns sitting on thermal blocks at braced bays
RE: Steel columns sitting on thermal blocks at braced bays
1) Use a product like the Fabreeka stuff that has published coefficient of friction values that you can have confidence in.
2) Rely on friction across the thermal pads to deal with your shear.
3) Make sure that the friction you need is available when the shear is present. That can be tough for braces that don't pick up much gravity load. One strategy is to use chevron bracing and include a drag beam just above the thermal pads. That way, you're always dealing with your shear at the base plate that's naturally in compression. Another possibility is to prestress your anchor bolts. I always hate to do that other than in a belt and suspenders kind of way or for fatigue. Quality control procedures on that still mystify me as does accounting for creep etc.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Steel columns sitting on thermal blocks at braced bays
DETstru - I think it is understood in the cold storage design industry that you can't have a perfect thermally isolated column to foundation design, because something must connect the two. Anchor bolts are a must, shear lugs are tolerated when necessary. And yes, they do provide an avenue for thermal travel. But it is much better than it would be with conventional steel to foundation design. You just don't want so much cold to migrate down to cause subgrade freezing, and you don't want enough warmth to migrate up to cause condensation. I am not worried that the addition of a 4x4 tube is enough to cause either of these to happen. But, I will wait to hear the opinion of the refrigeration consultant. They are the expert.
RE: Steel columns sitting on thermal blocks at braced bays
Is that even true of the flexural compression columns in your braced frames? Because you only need compression on those columns for the scheme to work.
I'm with DETstru on the potential ineffectiveness of the thermal break pads. With a 4x4 steel tube short circuiting the system, the pads might as well not even be there as far as I'm concerned. Some of the thermal break systems have special bushings that actually prevent the anchor bolts from coming into direct thermal contact with the columns.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Steel columns sitting on thermal blocks at braced bays
RE: Steel columns sitting on thermal blocks at braced bays
In researching the Fabreeka product I did notice that they offer bushings for the anchor bolts. I think i will go ahead and spec those, can't hurt. I'm surprised our refrigeration consultant didn't mention that such a product exists.
I understand that the 4x4 tube is counter productive, but so are anchor bolts, and everyone accepts anchor bolts passing through the thermal break. I guess I don't see a 4x4 tube adding much to the issue over just the 4 anchor bolts.
One thing I did find out was that stainless steel is 2 or 3 times more effective than carbon steel at resisting thermal flow. I wonder if stainless steel anchor bolts and shear lug would be worth the downside of having to engage in special welding procedures associated with dissimilar metals.
RE: Steel columns sitting on thermal blocks at braced bays
I'll have to think about that one for a minute. Not a bad idea. Just make the shear lug pocket bigger, cast a sleeve in the pier made out of the same material as the thermal block and grout in the shear lug within the insulated sleeve? Could work.
RE: Steel columns sitting on thermal blocks at braced bays
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Steel columns sitting on thermal blocks at braced bays
RE: Steel columns sitting on thermal blocks at braced bays
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.