A strap footing solves a specific site problem — an exterior column sitting on a property line that won't let its own pad extend — by joining two separate pads with a stiff beam that itself never touches the soil, transferring the eccentricity moment back to the interior pad instead.
Every check above runs automatically inside the Structyze Strap Footing module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
The Example We'll Use
- Column spacing 4.5 m; pad A centroid offset 0.5 m
- Pad A 1.8 × 2.2 m; Pad B 2.1 m square
- Pad thickness 500 mm; strap 400 × 750 mm
- Column A: dead 400 kN, live 200 kN — Column B: dead 550 kN, live 280 kN
- Allowable bearing 180 kN/m²; materials N40, 500N
Working Through It
Taking moments about the interior pad's centroid fixes the exterior pad's actual reaction RA ≈ 908 kN — actually higher than its own column load (NA* = 780 kN), because the strap has to carry the couple created by pad A's offset. Both pad bearings land right at the allowable limit: 174 and 175 kN/m² against 180 kN/m², both utilisation 0.97.
Each pad is then designed as an ordinary footing against its own reaction — flexure, one-way shear and punching all checked independently, with both pads passing comfortably (utilisation 0.66 and below).
The strap itself carries the couple (RA − NA*) as a hogging moment, designed as an ordinary beam: M− ≈ 150 kN·m requiring about 587 mm² of top steel against the 511 mm² flexural minimum — calculated demand narrowly governs, covered by 3–N24 bars.
What Governs, and Why
Both pad bearings run right at the allowable limit (utilisation 0.97) — as intended, since the whole point of the strap is to force both pads toward uniform, efficient bearing pressure rather than letting the offset pad see a lopsided load. The strap's own shear check, at utilisation 0.79, is the tightest structural number in the design.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze Strap Footing module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
For Strap Footing and the rest of the full catalogue — 80+ components across RCC, Steel, Composite, Timber, Masonry and more, across 20+ design codes — buy the full Structyze desktop app or see everything it does on the Software page.
Quick FAQ
Why does the strap need to be stiff and non-bearing?
If the strap itself bore on the soil, it would pick up its own share of soil reaction and the moment-balancing mechanics that make the two pads see near-uniform pressure would break down — a compressible layer or void former under the strap is what keeps it truly non-bearing in practice.
Could an isolated footing solve the same property-line problem?
Only if the pad can be made small enough to fit without crossing the line, which usually means overstressing the soil — the strap footing exists specifically for the case where that isn't possible and the interior pad's extra capacity needs to be borrowed instead.
Final Thoughts
A strap footing is a good example of a foundation system whose governing checks are exactly where the concept says they should be — both pad bearings pinned right at the limit, confirming the load-sharing mechanism is doing its job.