Bread & Butter...

We've been working on a fair few Airpsace projects now and this detail (typical floor / wall / roof build-ups) is arguably one of the most important details to consider - most definitely for the Structural Engineer.

I've seen many other calculations from other engineers and many will adopt a similar build-up and assign weights to each element, for the example above this is coming in around the 73kg/m2 mark for the total self-weight. Of that, only about 22kg/m2 is structural, the rest is build-ups (51kg/m2!). That's a 2:1 ratio in terms of finishes and structural elements (i.e. the finishes weigh twice as much as the structural elements carrying them).

In heavy construction, like an RC frame or a composite deck, adding additional layers or changes within the build-ups isn't likely to make such a massive difference because the self-weight of the structure is so heavy and makes up the bulk of the load so small changes aren't going to make a difference here.

Throw on changes to a lightweight structure, difficult. Throw on the added complication of having an existing structure designed for near capacity lying below and the smallest change can become critical.

It is brutal how much of an impact this can have on the existing structure below.

For example. Floor decks. Structurally they work to re-distribute lateral load and are sized based on imposed vertical point loads. Depending on the centres, roofs typically come in around the 15thk mark and floor 18thk. But, we know floor joists are lighter than the deck. So....

For arguments sake, a board weighs 1kg/m2 per mm thick (its around that figure). So an 18thk board = 18kg/m2 and 15thk = 15kg/m2. The 3kg/m2 difference doesn't sound like much, but over a site of 200m2, that's 600kg, or 0.6T.

Equally, going the other way and adding another 22thk board on within the finishes = 13.2T or 13,200kg on the same site.

When you're working with an existing structure that may have already been designed to 80% unity or worse. These small checks all add up. Get it wrong and you may be adding an extra 5-10T onto a project and having 50 angry residents call you when cracking starts to appear.

Airspace development is high risk. Get this right and you've got a good chance, get it wrong and you hit the news for another Airspace Scheme gone bad again. The whole idea of adding load onto an existing building which is already working hard scares the hell out of me, why would you make it harder for yourself?

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#refurbishment #design #london #structures #structuralengineering #construction #building #structuraldesign #structure #airspace