Column Grids In Apartment Buildings

The column grid on an apartment building is set by what has to fit on three different floors: the apartments above, the lobby or shops at ground level, and the car park below. When one line of columns works on all three, the columns run straight to the footings. When it doesn't, the building needs a transfer structure, and that is the most expensive floor you can build.

This is why I want to see the grid before the development application goes in. Once the DA drawings fix the floor levels, the basement depth and the core locations, the column positions have very little room left to move.

What the grid has to answer

An apartment floor wants columns inside walls. Nobody buying a two-bedroom unit wants a column in the corner of the living room, so columns follow the party walls, the corridor walls and the wet areas.

A car park wants columns between the bays. A retail floor wants them far apart and out of the shopfront. Those three wishes rarely line up on their own, and the grid is where someone has to make them agree.

The car park sets a hard rule

Parking dimensions come from AS 2890.1, and they don't bend to suit the structure. For a resident car space, the standard gives a width of 2.4 metres and a length of 5.4 metres, with a 5.8 metre aisle for 90 degree parking.

Three of those bays side by side need 7.2 metres between column faces, before any extra clearance where a space sits hard against a wall. Add a column width and a grid of a little over 8 metres starts to appear, and it was the cars that drew it.

Then the apartment planner draws a party wall at a spacing that suits a bedroom and a living room. If that spacing is a clean match for the car park, you have a good day. If it isn't, one of them has to change.

Blade columns and wall lines

Apartment columns are often long and narrow, called blade columns, so they can hide inside a wall. They suit the floor plan, but they have a weak direction, and they are checked for slenderness and buckling in that direction.

A blade column also runs in one direction only. Turn it 90 degrees at a floor where the walls change, and the load path has to get across that change somehow. Those are the small decisions that decide whether the structure stays simple.

Span and slab depth

A wider grid means longer spans, and longer spans mean a deeper or a stiffer slab. [Post-tensioning](/blog/post-tensioned-concrete-when-and-why) is the usual way to hold a thin slab over a longer span, while the flat slab version has to watch punching shear at every column.

Slab depth then feeds straight into the floor-to-floor height, and on a site with a height limit, that decides how many storeys fit. So the grid is a question about yield as well as a question about structure.

When the floors don't agree

A column that can't come down through the floor below has to land on something. That something is a transfer beam or a transfer slab, and it carries the column above across to columns in a different position.

Transfer structures are real engineering and sometimes the right call, particularly over a shop floor or a basement ramp. They are also deep and heavily reinforced, and they eat height, as set out in [transfer slabs and transfer beams](/blog/transfer-slabs-and-transfer-beams). What I don't accept is a transfer that exists because nobody sat down with the architect early enough to line up the walls.

Cores and lateral stability

Columns carry gravity load, but the core and the shear walls hold the building against wind and earthquake. The location of the lift core sets how far the floors span, how the lateral system behaves and where the car ramp can run past it.

So the core and the grid get set together. A core placed for the best apartment layout, without a look at the basement ramp, is a redesign waiting to happen.

When it should be decided

On a mixed-use job, the grid should be drawn at the same time as the first real floor plans. An hour with the architect at that stage can move a wall line by 300 millimetres, and that small move can take a transfer out of the building.

The same change after the DA is a [section 4.55 modification](/blog/section-4-55-modification-engineering-nsw) if it touches what the approved plans show. By construction documentation, the structure is locked and the options are gone. The commercial argument is in [why early engineering involvement saves money](/blog/why-early-engineering-involvement-saves-money).

How we approach it

We mark up the architect's plans for every level on top of each other, apartments, ground floor and each basement, and look for the columns that refuse to run straight. Then we talk to the architect, because most of those columns can be solved with a small move while the plans are still soft.

A column that runs straight to the footings is the simplest structure on the drawings, and it only happens when someone plans for it early.

ACSES provides structural engineering from DA stage through construction across Sydney. Talk to us about a project.

Building Relationships Beyond Structures