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Until very recently, it was extremely common for us here at Partridge to have no knowledge or involvement with a project until after it came out of Council with its DA approved.
For 95% of our projects, the architects would independently develop the DA-submission plans with their client, lodge the DA, and then contact us a few months later to sign us up for the job after the DA was approved.
Only then, once all floor levels, structural zones, and building heights had been approved and “locked in” with Council, would we as engineers then have the opportunity to analyse the structural loadpath of the building and determine the size and thickness of its structural elements. For example, we’d begin to explore the thickness of the suspended slabs; the depth of the floor joists or roof rafters; and the size of the beams that would span across large openings and support the structure above.
Occasionally, every now and then, we would encounter a nasty situation where the structural depth or thickness required for a beam or a floor would be greater than what the architect had assumed or allowed for in the DA. With the DA fixed with floor levels set and maximum roof lines or ridge heights locked in, there simply wasn’t scope to accommodate the necessary structure.
As you can imagine, this could then trigger several unfortunate outcomes – either a reduced ceiling height for the client (assuming the design was not already at the limit of 2400mm), or having to incorporate new supporting elements into the room layouts (i.e. posts or blade walls) so as to reduce the spans. Or, alternatively, it might trigger a Section 96 application, thus costing the client time and money.
Situations such as this can also result in some awkward conversations between architect and engineer: “Why can’t you get the structure to fit within the allowed floor zone?” asks the architect incredulously. “Why did you assume that a slab could span eight metres whilst supporting brickwork and another storey above it, yet only be 200mm thick?” might be the bemused reply!
This was a problem we used to encounter only rarely, but it’s something that’s becoming more and more prevalent as changing architectural trends impact the structural engineering of houses. For example:
- Large, open plan living has resulted in longer distances for suspended floors to span across.
- Increasing use of rendered or face brickwork means deflection tolerances for slabs are much less, in order to prevent the brickwork from cracking.
- The desire to conceal services and embed them into floor zones (e.g. plumbing, air-conditioning ductwork, etc) means these services are now cutting into floors and reducing the effective structural depth available.
- Folds, rebates, and recesses are being cut into floors to accommodate pelmets, retractable blinds, flush floors at wet areas and balconies, etc.
One of the biggest problems we frequently encounter is that many architects assume that 300mm is a sufficient floor zone depth – regardless of the span over the rooms, or the structure being supported. In the case of 1st Floor additions or extensions to single-storey houses, there appears to be an almost universal belief that 300mm is the appropriate depth to allow for between the Ground Floor ceiling and the First Floor flooring. The below illustration is a common detail we see on many architects’ DA submissions:
The problem, of course, is that this does not give the engineer 300mm to work with! In the case of a timber-joisted floor, the floor construction also consists of the flooring on top of the joists (say 19mm red-tongue particleboard + carpet), and then 19mm battens and a 13mm gyprock ceiling suspended underneath the joists. That adds up to a total of 60mm and so, even before we start our design and calculations, the actual depth available for any structural member is only 240mm! Assuming regular floor loads only (i.e. no loadbearing stud walls above or roof loads being imposed onto the joists), a standard 240mm x 45mm LVL joist at 450 centres can only span up to about 5.0m – and, even then, that’s an arguably bouncy floor that we wouldn’t recommend for a high-end residence. Of course, if the joists have to support any additional load (e.g. a loadbearing wall that supports the roof), or – worse still – a bathroom with the additional weight of a fibre-cement sheeted floor, a grout bed, floor tiles, and a heavy bathtub…then the design will obviously struggle.
(Of course, there are still solutions at hand…we could explore doubling up the joists, or closing up the centres, or introduce a steel beam to try and break the span down. However, these all add additional cost or introduce other difficulties that might possibly have been avoided if an appropriate allowance for the structural zone had been allowed for at the start.)
The above discussion focuses on timber construction for a joisted floor, but we encounter the same problem with suspended concrete slabs – particularly if the slab then supports walls and another floor or roof above it. Again, it’s more commonly an issue in domestic housing, where the upper floor walls never line up with the supporting walls underneath it. This is a far cry from commercial or unit-block construction, where the walls align. Consider the two scenarios:
In the first scenario above, there are no extra wall loads or transferred loads from the upper storeys landing on the slab, and so the slab can be reasonably thin. Assuming six metre spans, an overall slab thickness of 200mm is feasible.
However, in the second scenario, the weight of the brickwork (and whatever structure that brickwork is supporting) comes down in the middle of the slab’s span, and so the slab has to be stronger and stiffer. Not just to support the weight, but also so that it doesn’t deflect and crack the walls. In this case, assuming average loads, the same six metre span might require a 290mm thick slab. Add on your finishes, services zone, and the ceiling, and setdowns/folds for wet areas or external balconies, and you’ve thus exceeded the general 300mm allowance commonly assumed!
So what’s the answer here? First of all, don’t assume that 300mm is an adequate floor zone for every domestic structure! Secondly, simply have a discussion with your engineer before you lodge and lock in the DA submission. What we’re starting to see now is an increasing number of architects engage us in the pre-DA submission stage. We’re now more commonly being approached to give structural advice and to prepare preliminary sizing sketches in the early days of the architect’s design development. This way, adequate provision is made for structural depths in the DA submission, and there are no nasty surprises later in the project, when it’s then often harder to re-manoeuvre.