Ground beam (pier-and-beam footing)

A ground beam is a reinforced concrete beam spanning between pier footings, tying the piers together and carrying the structure above. It is the horizontal counterpart to the vertical pier footing: the pier transfers point load to deeper bearing soil, the ground beam distributes load along the wall line above. Together they form a pier-and-beam footing system, an alternative to strip footings or stiffened raft slabs on certain sites.

When pier-and-beam (with ground beams) is used in residential:

Reactive soil sites (Class M, H1, H2) where shallow strip footings would suffer differential movement. Deep piers reach below the reactive zone; ground beams span across, isolated from the shrink-swell zone.
Sloping sites where stepped strip footings would be inefficient or expensive. Piers can extend to the required bearing depth at each step.
Sites with weak surface fill over good underlying bearing. Piers bypass the fill.
Heritage retrofits where existing footings need supplementing without disturbing the building above.
Stumped subfloors historically; less common in new build today.

Construction sequence:

Pier holes excavated to engineered depth (commonly 1.5 to 5 m, set by the soil report and load).
Pier reinforcement cage placed in each hole.
Piers poured (often using a steel sleeve or formed shaft above ground).
Ground beam reinforcement placed in the trench between piers, lapped into the pier reinforcement.
Ground beam concrete poured in one continuous run (or with planned cold joints).
Sub-floor or slab built above the ground beam.

Why it’s engineer-designed (not deemed-to-comply):

AS 2870 deemed-to-comply tables cover strip and raft footings on standard reactive sites. Pier-and-beam with ground beams sits outside those tables and requires site-specific engineering.
The engineer designs the pier diameter, depth, reinforcement, and the ground beam dimensions (typically 300x400 mm or larger) based on the soil report, load calculations, and span between piers.
Each project’s ground beam detail is unique; there is no standard “ground beam”.

Common defects:

Ground beam poured before the pier reinforcement has set; the lap joint is compromised.
Pier hole sides not stabilised in soft soil; the bearing area is undermined.
Ground beam cast on disturbed fill rather than against the pier; the load path is broken.
Cold joint in the ground beam without an engineer’s design detail; potential failure plane.

Also known as: GB (drawing abbreviation); pier-tie beam; subfloor beam; capping beam (when above piers).

Category: Structure.

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