Isolation Pads for Equipment in Buildings

[wannabeSE]

I am soliciting thoughts on when housekeeping pads for equipment inside buildings should be isolated from the slab on ground. These are pads that are placed on the ground and separated from the building slab with an isolation joint rather than a housekeeping pads placed on top of the slab. Is it a good practice to have isolation pads for any heavy equipment, or just to isolate vibration.

A current project for an contractor involves pads and anchorage of electrical equipment in a new building. The contract documents require isolation pads for any equipment weighing more than 150 psf. I am trying to understand the reasoning for this requirement. I don't see the benefit to isolating switchgear and transformers (up to 16 kips and 300 psf).

 

[SlideRuleEra]

wannabeSE - Isolation pads have two main purposes. As you stated, one is to isolate vibration to surrounding areas... that is, if the equipment on it does vibrate.

The other reason is to allow differential settlement of a "heavily" loaded (psf) pad compared to the surrounding "lightly" loaded slab on grade. If heavy equipment were placed on a housekeeping slab that is on top of the slab on grade, the slab on grade may be forced to try to resist the loading by acting as a spread footing. It was probably not designed for that. The result could be unwanted cracking.

In extreme cases, for very heavy equipment, the isolation pad may be pile supported while the surrounding area is a slab on grade. In that situation differential settlement may be reversed... the lightly loaded slab on grade may settle more than the heavily loaded, pile supported isolation pad.

It all has to do with the slab on grade design and the equipment loading. A "thin" slab on grade may require isolation pads for almost everything, even electrical switchgear. The "thicker" slab... maybe not as many isolation pads.

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[wannabeSE]

Thank you for the information. In this case, the slab should have adequate capacity (less than 300 pcf load and over 500 psf capacity using the PCA and WRI methods for aisles in the appendices of ACI 360, and the TM 5-809-12 equation for stationary live loads). However, I don't if these methods consider long term settlement. Sometimes, I wonder if it would be better to let the slab and equipment pad settle together when the slab has adequate strength.