Design a raft foundation on soil springs

A raft (or mat) foundation is one thick concrete slab under the whole building. It is the choice when pad footings would be so large they almost touch, when the soil is soft, or when differential settlement between columns must be kept small. The Raft foundation template in Struxt3D models a raft slab on soil springs, with ground beams on the column lines and short column stubs carrying the loads from the building above.

#How a raft works

The columns push down at points; the soil pushes back up over the whole area. The raft spreads the column loads out and bends like a floor slab turned upside down: the soil pressure is the "load" and the columns are the "supports". So the raft has tension at the top in the middle of the bays, where the soil pushes it up, and tension at the bottom under and around the columns.

The soil is modelled the classic way, as a Winkler foundation: a bed of independent vertical springs. The stiffness of the bed is the modulus of subgrade reaction k_s (kN/m³): the pressure needed to push the soil down by one metre. A raft that settles 10 mm on soil with k_s = 20 000 kN/m³ carries 200 kPa of soil pressure there.

The ground beams on the column lines stiffen the raft and carry the column loads along the grid, so the slab between them spans like a two-way floor panel.

#Build the model

1. Pick the template: Foundations › Raft foundation.
2. Set the size and loads with the parameters below. The column loads are part of the size step: they are the dead and live loads each column brings down from the building.
3. Choose the materials. Concrete, with groups Ground beams and Column stubs. Choose the code and concrete grade. The raft thickness comes from the size step, not from Slabs and walls.
4. Supports and loads. There are no point supports — the raft is carried entirely by the springs, so the Bases and Pad footings rows don't appear. Turn Wind off: in this model it would only push on the 1 m column stubs, and the building's wind reaches the raft through the column loads you enter.
5. Run the analysis (F5).

#The template's parameters

Parameter	Default	Range	What it does
Column bays in X	4	1–10	Bays along X; a column at every grid point.
Bay width X	6 m	3–10 m	Column spacing in X.
Column bays in Y	3	1–8	Bays along Y.
Bay width Y	6 m	3–10 m	Column spacing in Y.
Raft thickness	0.6 m	0.3–2 m	Thickness of the raft slab.
Subgrade modulus	20 000 kN/m³	5 000–100 000	Soil spring stiffness ks.
Column dead load	800 kN	0–5000 kN	Dead load at the top of every column stub.
Column live load	300 kN	0–3000 kN	Live load at every column stub.

Every column gets the same load. Edge and corner columns usually carry less than inner ones; to model that, set the template loads for the typical column and add extra point loads (or upward corrections) at the inner columns with the Point load tool.

#Choosing the subgrade modulus

k_s is not a soil property alone: it depends on the size of the loaded area and the soil layers below. Ask the geotechnical engineer for a value for your raft size. Typical orders of magnitude:

Soil	ks (kN/m³)
Soft clay	5 000 – 15 000
Medium clay, loose sand	15 000 – 40 000
Dense sand, stiff clay	40 000 – 100 000

#What to check in the results

1. Settlement. Show Displacement with the service combination. The raft dishes down most under the inner columns. Look at both the largest settlement and the difference between neighbouring columns (differential settlement), which is what cracks buildings.
2. Soil pressure. Multiply the settlement by k_s and compare with the allowable bearing pressure from the site investigation. Struxt3D does not check the soil bearing for a raft — that comparison is yours.
3. Raft moments. Show Slab moment: peaks under the columns and along the ground beams.
4. Ground beams. Show Moment and Shear on the beams. They act like inverted continuous beams loaded by the soil.
5. Equilibrium. The total load on the supports (the springs) must equal the column loads plus the raft's own weight.

Hand check of the average soil pressure:

With the defaults: 20 columns × 1100 kN = 22 000 kN, raft 24 × 18 × 0.6 × 25 = 6 480 kN, area 432 m² → q_avg ≈ 66 kPa, and an average settlement of about 66 / 20 000 = 3.3 mm.

#Design the raft

Click Run design (F6). The raft is designed as one slab on the ground: bottom and top bar meshes from the plate moments (Wood–Armer), extra bars where the moments peak, shear and punching shear around the column stubs. The ground beams are designed as RC beams and the stubs as RC columns. Pad footings are not designed under a raft.

Show Slab reinforcement to see the required steel per metre over the raft, or Reinforcement bars for the bars in 3D. See Slab design for how these checks work.

#Common problems and fixes

What you see	Why	Fix
Punching fails at the columns	Raft too thin for the column load.	Thicker raft; punching usually sets the raft thickness.
Settlement too large	Soft soil or high pressure.	A larger raft area does not help much on Winkler springs — the pressure is what matters. Consider piles.
Very large differential settlement	A flexible raft on soft soil.	Thicker raft or deeper ground beams.

#Rules of thumb

#What this model leaves out

• Soil continuity: Winkler springs act independently, so the raft edge is not loaded by the soil next to it. Real soil settles in a dish even under a uniform load.
• Springs in tension: a raft corner that lifts is still held down by its spring.
• Long-term consolidation settlement.