Waterproofing Membrane Strength?

[Aytacoglu]

We’re currently designing a temporary enclosure structure that will be installed at car park level. Directly beneath this level is a substation located in the basement.


The existing roof slab at car park level is topped with a waterproofing membrane. The proposal is to install steel columns directly over this membrane using 300x300 mm base plates, each carrying a maximum axial load of 45 kN.


The client has raised concerns about whether this could damage the existing waterproofing layer due to the point load beneath the base plate.


Unfortunately, the original waterproofing manufacturer is no longer in business, and we’ve been unable to retrieve any technical datasheets. I’ve attached an image showing the waterproofing build-up and any available details.


My view is that:

• The applied bearing stress is relatively low.
• Even if there is some local crushing or compression of the waterproofing, the load path will still transfer directly into the concrete slab.
• The membrane is not intended to act structurally, and its potential compression shouldn't compromise the column's load-bearing capability.

However, I’d appreciate feedback on whether others have dealt with similar cases—and how you’ve justified this approach. Is there a better way to technically validate that the waterproofing won’t be significantly compromised under this loading?


Thanks in advance.

 

[Ron247]

If cars have been driving on this material, have they damaged it before? How do the wheel loads compare to the column load?

Thought: If the columns are vertical gravity load, put a neoprene pad under the baseplate to mimic car tires.

 

[MintJulep]

It's an unanswerable question. At best you could answer "Maybe". There's no possible way to convince the client that there's zero chance of damaging the waterproofing.

Also, it's not really the question that the client wants an answer to. What the client really wants to know is if you will take the liability for repairs if your installation damages the existing waterproofing system.