Steel columns sitting on thermal blocks at braced bays

[mbenjami1]

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?

 

[dik]

Not uncommon... Fabreeka makes a great product or you can use hardwood isolators. You should have an isolator somewhere in the assembly. Also need insulated slab as well as waste heat being circulated beneath the insulation. Bracing is no different than any other design... just maybe higher on the column at the base.

Dik

 

[mbenjami1]

Yes, we have 6" underslab insulation, glycol heating coils under the insulation, and they want to use a polyurethane thermal block under the columns provided by a company called Shadco. However, I still don't see the load path for a 25k lateral force when you have a 10" gap between your baseplate and the top of foundation or pier. The only way I see is if the anchor bolts resist the shear in bending. Or is the thermal block somehow attached to the foundation and the baseplate so that it can pass the shear forces through it and into the foundation?

 

[dik]

10" gap? Seems awfully thick. Look into the Fabreeka... they may be a better, less compressible, product.

Dik

 

[JAE]

We used about 4" oak blocks. Keeps the bolt bending down but still provides some thermal separation.

Might have to use additional bolts. AISC, I know, frowns on anchor bolt bending though.



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[mbenjami1]

Unfortunately, at this point I'm kind of at the mercy of the refrigeration consultant that the client hired to be their expert on this subject. They say 10" blocks from the Shadco manuf. are what we need to use, although I may push back on that 10" thickness. They like to use the polyurethane rather than oak because it doesn't absorb moisture like oak blocks could potentially. And rot, isn't a factor.

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...

 

[Once20036]

Can you provide a little more information on this, "AISC says the slab must be in contact with the subgrade for this to work". This is typically what we do - utilize the slab for the lateral loads at the based on the columns. I am not aware of any AISC restrictions on this process.

 

[dik]

rot's not an issue with most oak...

Dik

 

[mbenjami1]

The AISC Design Guide #1 - Base Plate and Anchor Rod Design, Section 3.5.5 titled Hairpins and Tie Rods says: "The friction between the floor slab and the subgrade is used in resisting the column base shear...", and it says "...a vapor barrier should not be used under the slab". I take this to mean the slab must be in contact with the subgrade for the hairpin method of shear resistance to work as expected. For my particular structure there is a slipsheet directly under the slab, then two layers of insulation, and then a vapor barrier, so it seemed to me that hairpins weren't an option.

 

[mbenjami1]

Decided to go with an HSS4x4 shear lug. In talking with the thermal block manufacturer, they said this is something they see often. I never could get to a comfort level with resolving the force through the slab since the slab sits on insulation and a vapor barrier and so there isn't much friction to work with. Hopefully, the refrigeration consultant doesn't complain.

 

[DETstru]

Just curious, how effective is all that insulation when you have anchor rods and a shear lug that create a directly path for heat to travel?

 

[KootK]

One of my first projects out of the gate was one of these. I didn't clue into the base connection issues until it was almost time to issue. Good times. Here's what I'd recommend:

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.

 

[mbenjami1]

KootK - Part of my issue is that I have net uplift on most of the columns that need to resist shear, so even a product like Fabreeka that may have published friction values, wouldn't do me any good. But, I will definitely take a look at Fabreeka to see the friction values they publish.

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.

 

[KootK]

Part of my issue is that I have net uplift on most of the columns that need to resist shear

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.

 

[dflva]

The mention of bushings made me think of something: perhaps the lug can be wrapped in thermal break material, 4 sides + bottom? Just throwing it out there

 

[mbenjami1]

I'm not quite sure I follow your comment suggesting that the shear is taken out by the compression column of the braced frame. The shear is applied at the uplift column. How does it get to the compression column in order to engage the friction of the Fabreeka thermal block?

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.

 

[mbenjami1]

dflva (Structural)21 Jul 17 15:40
The mention of bushings made me think of something: perhaps the lug can be wrapped in thermal break material, 4 sides + bottom? Just throwing it out there

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.

 

[KootK]

How does it get to the compression column in order to engage the friction of the Fabreeka thermal block?

One strategy is to use chevron bracing and include a drag beam just above the thermal pads.

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.

 

[mbenjami1]

Ah, I'm with you now. Thanks!

 

[KootK]

No sweat. It's an extra member and can't go where traffic needs to pass though but it's a decent system. In many instances, I'd expect the member to be considerably cheaper than fancy shear connection voodoo at the base.

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.