Foundations Part V

do mini plinths work?

Given limits of time and intellect I have been forced to boil down the doubtless very complex relationship between pressure and ground type to a few hand-wavy sums. I hope there are no actual engineers reading. Here goes:

How much does a 3.6m2 garden building weigh (roughly)?

dead weight

  • SIP panels are about 25kg/m2 ~1600kg
  • plasterboard and timber ~ 200kg
  • x9 mini-plinths ~ 150kg

live weight (people and furniture)

  • ~800kg

Grand total – 2750kg

Apart from the first item, these are all complete guesses. Luckily the engineering world has a special term for the fact that there is always some uncertainty in any estimate, and this is ‘factor of safety’. Given my lack of knowledge, I should probably use a very high factor of safety, but I have decided upon x2.

This means I should be planning for a maximum load equivalent to 5500kg. you may recall that 100kg = about 1kN so the maximum weight the foundations need to support = 5500/100 = 55 kN

We now need to know the surface area the load will be placed upon:

The mini-plinths are 450mm square and can be placed at 1.8m centres, so I would need x9 giving a total area of ~ 1.8m2

Thrilling conclusion

Therefore we now know the bearing pressure caused by the load of this building is:

55/1.8 = 30 kN/m2

.. and what’s the bearing capacity for loamy soil? Answer (see above): 25-35 kN/m2Ergo mini-plinths work and the building won’t fall down! Now this may all seem very convenient (you’ll have to believe me that I did not work back from the answer), but there are two inconvenient facts:

  1. I did not understand the maths involved in calculating the additional load caused by wind, snow and the like, so I have conveniently ignored these complications factors for this calculation.
  2. The only engineering certificate I could locate for this type of foundations contained the following caveat:”.. the imposed load per pad is no greater than 48kN and the ground bearing pressure is no less than 130kn/m2, otherwise additional dispersion measures must be undertaken on a project specific basis”

… hmm. Despite this I am going ahead with the mini-plinths and have rationalized this decision as follows:

  • Let’s assume that my original weight calculation is about right, and then I invited 3 fat fellows around to have a look inside and it was windy and it had just snowed, would it really more than double the load on the plinths? Well let’s hope not!
  • the plinths are adjustable up to 50mm in-situ, so if I am wrong and the workshop sinks I can just get a spanner out.

in other news, I nearly have all my quotes in and I started to clear the space for the building at the weekend.

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