Raised concrete pads under mechanical equipment

[RabitPete]

I see mechanical equipment being installed on top of raised concrete pads all the time. What is the reason those pads are raised? Why not make them same level as the rest of the slab? The one on the 2nd picture appears to be isolated from the floor slab, while the 1st one seems to be not.

 

[WARose]

I see mechanical equipment being installed on top of raised concrete pads all the time. What is the reason those pads are raised?

Basically to protect the equipment from floods and/or floor cleaning chemicals and so forth. They are good for a few other purposes as well.

 

[hokie66]

Plinths like that are often called housekeeping pads...for housekeeping.

 

[IFRs]

in addition to the good posts above:
sometimes to improve NPSH
sometimes to provide enough mass to manage vibration
sometimes to make service and maintenance easier
sometimes to match the piping elevation or eliminate a hydraulic issues from elbows
often to get above high water expectations

 

[Ingenuity]

We did a project (just before COVID hit last year)[sup][/sup] to a 3rd floor mechanical slab that supported a large boiler for producing steam to the hotel's laundry.

Near 40 year old, unbonded PT slab. Did not have a protective membrane for the first few decades of its if life. Fast-forward...and after repeated water-softening (brine, so high Cl[sup]-[/sup] content) that overflowed (twice daily when the system flushed/cleaned itself) due to floor drains that were inadequate, and the unbonded PT slab had no chance. Repeated repair of the past 20 years (by others) were ineffective.

We had to support the operating 30,000lb boiler on temporary shoring and did full-depth slab replacement. It was easy to de-stress the PT - corrosion did that for us. It had a 7' cantilever (on the RHS of photo) that I have no idea how it stayed standing.

MORAL: provide a plinth/housekeeping slab, but also protect the supporting slab too, because mechanical rooms can be nasty areas, and some owner's neglect the required maintenance.

 

[JedClampett]

They also provide a larger perimeter for punching shear.

 

[JoelTXCive]

Aside from the structural items methods above, there can be other reasons.

[ul]
[li]If there is an electrical cabinet, you often want to get it a few inches off the ground so that incidental water doesn't flow into it while mopping; or other minor water issues.[/li]

[li]If you have a pump or motor, the contractor is able to get the floor elevation and levelness 100% correct at the equipment.[/li]

[li]If there is pipework, then the final elevation of valves and/or equipment can be adjusted using the housekeeping pad.[/li]
[/ul]

 

[Enable]

Ingenuity: What was the temp support protocol? I don't see any anchorage points so I assume you didn't hang the pad. Did you core for prop install from below and cast them into the slab / cutting base off after (or same but slide onto cast in blocks)?

CWB (W47.1) Div 1 Fabricator
Temporary Works Design

https://www.enable-inc.com/

 

[Ingenuity]

Enable:

Temporary re-shoring of the structural slabs from underside of L3 to loading dock using JASCO Hi-Lite frames:

For temporary support for the boiler I purchased some W4 stock and cut into 7" lengths and welded top and bottom 1/2" thick plate (8" slab thickness) with tapped holes to the bottom plates that matched holes centers to the DB-350 shoring posts. The W4's were sacrificial 'stubs' buried into the concrete.

Given the obstacles below (and a fully functioning electrical room), we stick-framed the slab formwork, and cut around the shoring stubs for ease of removal.

Would you believe, it. we finished the project then the hotel said, given COVID etc, they would be shutting down the hotel and fully refurbishing the mechanical room. The old boiler got replaced with a modern steam boiler at half the size and weight, on a new housekeeping slab.

Sorry OP, for the 'hijack'.

 

[bones206]

Was not expecting anything super sexy when I clicked on a thread about housekeeping pads. But this was worth the price of admission.. thanks Ingenuity for sharing Cool project...

Next time a contractor asks me why they have to pour HK pads, I can now reply, "Obviously to support the equipment during a floor replacement. Standard industry practice."

 

[RabitPete]

Any benefits of isolating those house keeping pads from the rest of the slab with expansion joints? Would isolated pad result in less sound/vibration transferred through the slab to the office space nearby? This is a slab on grade situation.

 

[Sparweb]

Thinking about standing at a lathe or milling machine all day, it can help the "human factors" to raise the machine a little. Many are made overseas, making them a bit undersized for the stature of many operators here. I can see boosting it on a concrete base. Also improves a floor that isn't level or smooth, which the machine also needs.

 

[Liam Nesson]

Big rotating equipment is usually isolated from the floor slab. If the equipment is heavy, the foundation maybe significantly bigger and you are only seeing the top of pedestal. Lighter stuff can be built into slab.

Construction sequence can also be a reason. Typically we get equipment layout and details, then we build foundations for equipment, then pour floor slab after. However some clients want the slab built before any equipment layout is complete. In this case we might install a thick slab and build pedestals off it. No isolation is provided between slab and pedestal. Not preferred, but it happens.

 

[bones206]

Isolation can also be provided by spring supports as in the first photo. Isolation devices range from rudimentary rubber pads, to sophisticated isolators such as those used for MRI machines.

I recently had to isolate a large equipment pad from the surrounding slab (just a normal isolation joint) based on the equipment’s own sensitivity to ambient vibrations in the plant. Not sure how much difference it really makes since it’s on a common subbase that can also transmit vibrations, but it was a manufacturer requirement.

Every situation is different I think.

 

[dflva]

Use a concrete pad for equipment anchorage. Anchors can be set in fresh concrete of pad using a template. Tie the concrete pad to the slab with reinforcing steel. Akin to using a conc pedestal between a footing and a steel column base plate. Also to accommodate long anchor rods a concrete pad thicker than the slab may be required.

 

[RabitPete]

I was thinking that pouring thicker equipment pads first is easier and they can also be isolated this way, and subgrade can be on the same level. But keep debating if making the pad isolated and hence much smaller and lighter would do more bad than good.
I really admire pad workmanship on the 2nd picture, look how those 45 degree chamfers meet at the corner. Anyone knows what kind of formwork was used?

 

[Enable]

Thanks Ingenuity neat little project.

But sadly I can believe it lol I just had a client do substantial repairs to multiple building columns (involving significant shoring / construction expense & massive tenant impact) only to decide they want to knock it down in less than a year

CWB (W47.1) Div 1 Fabricator
Temporary Works Design

https://www.enable-inc.com/

 

[Enable]

Rabit: Those chamfers are just 3/4" cant strip by looks of it. Can get them from any formwork or roofing supplier (see here for example). That'll go on to some formply. Not much to it other than the dude installing them knowing how to cut the cant at the corner (it definitely does take practice to make all the compound cuts).

CWB (W47.1) Div 1 Fabricator
Temporary Works Design

https://www.enable-inc.com/

 

[bones206]

It all depends on the situation, but in most typical cases pouring housekeeping pads later is my preferred route. It seems like equipment sizes and arrangements are always in flux, especially nowadays with supply chain issues. So having the flexibility to pour the pads at the last minute is a benefit. Plus it making finishing the flat work of a slab easier. My standard details show cast-in and post installed dowel options, and the contractors always go with the post installed.

 

[Ingenuity]

Anyone knows what kind of formwork was used?

Looks like MDO (medium dense overlay) formply with 1" pine chamfer strip.

If you want really nice edges and chamfer corners use steel forms and magnetic chamfer strip.

In the above photo, the base plate grouting used steel forms and magnetic chamfer, and the pedestal was used MDO ply with hand-cut pine chamfer. Most times the owner's only want to pay for wood form finish.