Prestressed slab on polystyrene insulation on grade

[kbneng]

Hi everyone,

I'm working on the assessment of a big warehouse that used to be used for cold storage. From the old drawings, the slab construction is a 4-1/2" thick prestressed slab on 6" polystyrene. I know this warehouse was used for heavy storage and traffic. We're looking at placing heavy fit-out elements and equipment on the slab and I'm trying to assess its capacity.

Does anyone have any idea how this system behaves? What would be the reason for using a prestressed slab back in the day(this was designed in the 70s)? I don't think the prestressed slab can span between the foundation beams (they're too far apart), so they must be relying on the polystyrene for support.

I've attached a couple of drawings here.

Thank you.

 

[kostast88]

Apparently prestressing is to save reinforcement and reduce weight/thickness of slab on polysterene, to make the concept viable. Any new loads, need to consider the long term losses up to this point (relaxation,creep). Do you have a high water table?

Polysterene in some cases can be load bearing. But you have to find the specific product datasheet (if it still exists).

If it's very old, you may want to check in what condition the product is in. It could be beyond it's design life. "Detail of thermometer hole" in page 2 is where you could get access to check.

Nowadays they provide allowable pressure & deflection limits (for example: Link, Link). You would have to ensure you don't violate these.

 

[KootK]

The handful of times that I've encountered similar setups, the design intent was usually to attempt to create crack free slab on grade. The plans make mention of cold storage. Perhaps this was a food processing facility where there may have been a particularly stringent requirement for hygiene?

 

[kbneng]

Thank you for the replies, and sorry for not responding earlier!

@kostast88: Thank you. I haven't confirmed where the water table is, are you thinking effect from buoyancy or deterioration? It's an old product and I don't have the product sheet. How would you check it? Cut a piece and take to a lab and do a compression test? The links you sent were very helpful too by the way, thank you.

@KootK: Thank you. Yes it was a cold storage facility so I guess that makes sense that they would try to limit the cracking.

 

[dik]

Missed this... if cold storage and below 0degC, you should talk to your mech guys about circulating the waste heat beneath the insulation to keep the cold from going down.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[kostast88]

I believe buoyancy is not an issue, if it was they would provide a heave guard which generally is compressible. It wouldn't be able to support industrial/warehouse/storage loads. Moisture damage is a possibility if the building is 20-30 years old. If insulation is outside your scope and payment, I wouldn't bother with it a lot.

I am simply arguing that if there is damage somewhere and it's significant, your slab could be sagging more in this location (loss of support).

Could this affect your structural assessment? You have a thin slab, therefore tendons should be straight (central prestressing). I believe the answer is no because the slab was designed with this in mind, but you can crunch the numbers to confirm.

As dik said, talk to the mech guys to see what they can tell you about it. They know insulation better than structural engineers.