In a framed building the load arrives at the ground in a handful of isolated pad footings. The walls between those columns have nothing underneath them. The ground beam is what they sit on.
It is the member most often built without being designed, because everybody knows the answer is 225 × 450 with four T16. This page works out why that is right, which is the part that tells you when it stops being right.
What it is for — three separate jobs
- It carries the ground floor blockwork. Walls laid on filled ground crack, because fill settles and blockwork does not bend. The beam takes the wall to the footings.
- It ties the footings together. Pads under different loads settle by different amounts. Joined by beams, the substructure settles more nearly as one thing, and the differential the frame above has to absorb is smaller.
- It is part of the tie system. BS 8110's robustness requirements ask for horizontal ties at each floor level, and the ground beams are the ones at the bottom. That is a regulatory requirement in its own right, not a by-product of the bending calculation.
It is not a foundation. A ground beam is supported by the footings at its ends; a strip footing is supported by the soil along its whole length. They look similar in a trench and are designed for opposite things. Casting a beam on fill and calling the fill a support is how a ground beam ends up spanning further than its steel was sized for.
The example
A framed two-storey house. Ground beams span 4.0 m between column bases, and carry a 225 mm rendered blockwork wall 3.0 m to the first floor. Section 225 × 450, fcu 25, fy 460, T8 links, T16 main bars. Ground floor slab is cast on compacted fill and does not bear on the beam.
Step 1 — Load
Self weight 0.225 x 0.450 x 24 = 2.43 kN/m
Gk = 14.13 kN/m
Qk = 0 the slab is on fill; nothing imposed reaches this beam
w = 1.4 Gk + 1.6 Qk = 1.4 x 14.13 = 19.8 kN/m
Qk = 0 is the whole character of this member. A ground beam carrying only walls has no imposed load at all, so 1.4Gk is the entire ultimate load, and the design is dominated by something that does not vary. It is also what makes Table 3.5 available — that table needs Qk ≤ Gk, and zero satisfies it with room to spare.
If the ground floor slab is suspended, stop and start again
A suspended ground floor — over a void, over a basement, or because the fill is too deep to trust — puts slab dead load and 1.5 to 2.0 kN/m² of imposed load onto this beam as well. That can triple w. Everything below changes, and the 225 × 450 stops being the automatic answer.
Step 2 — Moments, treating it as continuous
Ground beams run through the column stubs from one end of the building to the other. They are continuous, and continuity puts hogging moment over the columns — tension in the top face, where a simply supported design has no steel at all.
Mid, end span 0.09 FL = 0.09 x 79.2 x 4.0 = 28.5 kNm sagging
1st int. support 0.11 FL = 0.11 x 79.2 x 4.0 = 34.8 kNm hogging
Shear there 0.60 F = 0.60 x 79.2 = 47.5 kN
Design a continuous ground beam as simply supported and you get no top steel. The beam then cracks over every column, in the top face, where it is buried in the ground and nobody will ever see it. The crack does not fail the building; it opens a path straight to the steel in the wettest part of the structure.
Step 3 — Cover, and the effective depth it leaves
This member is in the ground. Cover is not the 25 mm of a first floor beam — see cover, grades and mixes:
- 50 mm where the beam is cast against blinding or in formwork in a trench
- 75 mm where concrete is cast directly against the earth, which BS 8110 requires as a minimum
= 450 - 50 - 8 - 8 = 384 mm
At 75 mm cover it would be 359 mm, and every steel area below rises about 7%. Cover is not a detail on this member; it is an input.
Step 4 — Steel
Top, over the column
K <= K' = 0.156 PASS — singly reinforced
z = d[0.5 + sqrt(0.25 - K/0.9)] = 365 mm, capped at 0.95d = 364.8 mm
As = M / (0.87 fy z) = 34.8 x 10&sup6; / (0.87 x 460 x 364.8) = 238 mm²
Bottom, at mid-span
z = 0.95d = 364.8 mm the cap governs again
As = 28.5 x 10&sup6; / (0.87 x 460 x 364.8) = 195 mm²
As,min = 0.13% bh = 0.0013 x 225 x 450 = 132 mm²
Provide 2T16 top and 2T16 bottom = 402 mm² each face
2T12 (226 mm²) satisfies both numbers on paper. Ground beams are detailed with equal top and bottom steel carried right through anyway, and it is worth knowing why rather than copying it:
- The moment can reverse. If one footing settles more than its neighbours, the beam hogs where it used to sag. Nothing in the calculation above sees that.
- The bars are the tie steel, and a tie has to be continuous and anchored to be a tie at all.
- A buried beam is the worst place in the building to be marginal, and the cost of the extra bar is trivial next to the cost of reaching it later.
Step 5 — Shear
100As/bd = 100 x 402 / 86 400 = 0.465
vc = (0.79/1.25)(0.465)1/3(400/384)1/4 = 0.495 N/mm²
v > vc links needed
v < vc + 0.4 = 0.895 minimum links govern
Asv/sv = 0.4b / (0.87 fyv) = 0.4 x 225 / (0.87 x 250) = 0.414
sv = 100.5 / 0.414 = 243 mm (max 0.75d = 288 mm)
Provide T8 links @ 225 c/c
Step 6 — Deflection, for form's sake
fs = (2/3)(460)(195/402) = 149 N/mm²
M/bd² = 0.86
MF = 0.55 + (477 - 149)/[120(0.9 + 0.86)] = 2.10, capped at 2.0
allowable = 26 x 2.0 = 52
10.4 <= 52 PASS, by a factor of five
Which is the honest summary of this member: nothing in the structural calculation is close. What sizes a ground beam is geometry and durability — the depth needed to get below the oversite and sit on the pad, the width of the wall it carries, and the cover the ground demands.
The answer
| Item | Provided |
|---|---|
| Section | 225 × 450 mm |
| Top steel | 2T16, continuous through the columns |
| Bottom steel | 2T16, continuous |
| Links | T8 @ 225 c/c |
| Cover | 50 mm on blinding, 75 mm against earth |
The reaction has to go somewhere
Every ground beam delivers its load into the column bases at its ends, and that load lands on the footing underneath.
Ninety-one kilonewtons is not a rounding error on a small building — on the three-storey column in the load takedown it is about a sixth of the column load again. It is also the load most commonly left out, because the ground beams are drawn after the foundations have been sized.
Size the footings after the ground beams are known, or allow for them and check. A load takedown that stops at the column base and never adds the substructure is an under-estimate of the one number the foundation depends on.
Detailing, and the three things that go wrong
- Continuity is the design. Top steel must run through the column stub and lap in the span, not stop at the face. See lap and anchorage lengths — a top bar lap in grade 25 with thin cover is twice the basic length.
- Do not let it become a strip footing by accident. If the beam is cast on the bottom of the trench with no void and no blinding, on ground you have not designed as a bearing surface, you have built something that is neither a beam nor a footing. Either design it as a strip footing with a width the soil can carry, or keep it clear.
- Expansive ground lifts it. Black cotton soil in the north and the softer clays elsewhere swell when they take up water. A ground beam bedded on that can be jacked upwards hard enough to crack the wall above. The usual answer is a compressible void former under the beam, so the soil has somewhere to go.
Run the ground beam, and the column it lands on
Structura designs beams to BS 8110 — flexure, shear links and deflection, with the calc sheet, the bar bending schedule and a dimensioned detail sheet — and checks Table 3.5's own validity before using its coefficients. Beams are free to run, as many as you like. The whole-building load takedown, which carries every reaction down to the foundation for you, is the paid part.
Open Structura in your browser
On Android, get it on Google Play; on iPhone and iPad, get it on the App Store — same account as the browser.