building it right, from scratch.. (part i – foundations)‏

many people are overwhelmed by not just the process of building a new home, but by the building itself. it’s always good to remember that in building homes, we are merely continuing an activity that we, as humans have carried out for thousands of years – it’s not rocket science, in fact it’s mainly common sense and attention to detail that will ensure your home is of superior quality.

it makes sense to start at the first stage – your land test – or in proper engineering terms, a “geotechnical investigation”. the purpose of this test is for us to establish what it is we’re actually going to be putting our foundations on or in.

few places around the world have the geological diversity of bali. in the bukit you’ve got limestone of varying densities, accumulated over millions of years of the ocean pushing its calcified deposits inward and upward. in north bali you will find pieces of land where foundations will be set upon volcanic rock that have been churned out over thousands of years, while in lower lying areas clay is common above layers of sand and rock. there are other types of subsoils and each vary significantly in the amount of load bearing resistance they offer your building foundations. some derive their load bearing capacity purely from compressive strength, while others provide friction resistance on the vertical planes of your foundations (e.g. pylons), essentially gripping them from the sides.

understanding what your land has underneath its surface is critical for your engineer to design a suitable foundation.

your builder or engineer will order the ground test – if they know the footprint of your building they may opt only to test applicable axis points (as the tests are charged per test point). if the building design hasn’t yet much substance, a wider ranging set of test points may be requested. once they get the report back, they can immediately see a number of things from each of the test points, including what is the subsoil composition at varying depths and what the load bearing resistance is at each of those depths.

this information coupled with the total load of your building will directly indicate what the appropriate foundation design will be.

for example, a geological report indicates a test point offers 15kg / cm² of load bearing capacity. that number is then divided, generally by three to apply an adequate margin of safety. that leaves your engineer with an allowable usable value of 5kg / cm².

in this example, your engineer has calculated he requires your substantial load bearing columns to support 18,000kg – this calculation comes from his actual maximum load with margins of safety applied to live and dead loads.

a reverse calculation gives us this, where x = area of footing in cm²;

18,000 = 5x

18,000 / 5 = x

x = 3,600cm²

your engineer has just calculated that on this piece of land, a concrete footing 60cm x 60cm will support 18,000kg.

this calculation is essentially repeated for each load bearing column – either manually or by software that takes variable input factors and pumps out an appropriate size.

now we have the footings – the height and weight of the building coupled with lateral loads will determine what bracing these footings need – in indonesia, this bracing goes by the dutch name, “sloof” – they are beams that connect the column risers from your footings – and their size and position is critical to the rigidity of your structure. nowadays engineers are blessed with software like etabs to assist in these calculations – the structure design is input and all the data is run against set building codes with margins of safety applied by your engineer. any structural member that the software calculates as inadequate, will flash red – and will need to be redesigned. then, earthquake simulations are run and again, any structural member that the software calculates as inadequate, will flash red and will need the attention of your engineer.

that’s a very high-level overview of building foundations – in our next piece in this series, we’ll discuss the structure of your home – walls, columns, beams, slabs, etc.