Column on Strip Footing with Local Underpinning

If your goalpost frame columns need to bear onto an existing strip foundation, but that foundation is too shallow to carry the column load adequately, the solution is to locally underpin the strip footing at the column position.

What “Local Underpinning” Means Here

Local underpinning in this context does not mean underpinning the entire building — it means deepening the strip foundation at the specific point where the column will sit, so that the increased load can be carried by deeper, more competent strata.

The underpinning section sits beneath the existing strip footing and extends it downward to the required depth. Once cured, the strip footing bears onto this new mass of concrete, which in turn bears onto the deeper ground.

The Sizing Rules

The underpinning depth and length follow these minimum requirements:

– Depth (H): The underpinning must extend to the required strip footing depth for the column load — this is calculated in the structural documents
– Length (L): L must be at least equal to H — so if you need to go 600 mm deeper, the underpinning section must be at least 600 mm long horizontally
– Minimum thickness: The underpinning concrete itself must be at least 300 mm thick (depth of the underpinning pour, not the total depth achieved)

Goalpost Ground Beam or Strap Still Required

Underpinning the strip footing addresses the vertical load path, but does not address the horizontal spreading forces from the goalpost frame. As with other column-on-strip configurations, a steel ground beam or mild steel strap between the column feet is still required to resist the outward kick of the goalpost legs under load.

This is the same requirement as for columns on standard strip footings without underpinning.

Concrete Specification

Underpinning pours use the same C35 concrete grade as standard pad foundations. Reinforcement is typically not required in the underpinning pour itself unless the dimensions are unusually large, but your structural drawings will confirm.

Where adjacent underpinning pours connect to one another, dowel bars (2 No. H20 bars, 600 mm long, 200 mm embedment each side) should be installed at joints for continuity.

Get Your Underpinning Design Confirmed

PorthouseDean designs column support arrangements including local underpinning. If your site requires this solution, get a quote so we can include the correct specification on your structural drawings.