Raft foundation under columns to SIA 262

Checks a reinforced concrete raft on the soil under a grid of columns, to SIA 262:2025 and SIA 267:2013: an elastic plate on soil springs with lift-off, bearing, overturning and uplift, bending, punching at every column, and in service the bar stresses and the settlements of the columns. Every step shows its formula and the clause it comes from.

Pro SIA 262:2025

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About this check

clauses, worked example and questions

What it checks

Model: an uncracked plate, D = E_cm · h³/(12 · (1 − ν²)) (4.1.4.1), on springs k_s = p/s from the soil layers or entered (EN 1997-1 §6.8(5)). Springs that would pull are released.
Loads on the soil: E_N,d = ΣN_d + ΣQ_d + (q_d + γ_G · g − γ_w · h_w) · A, SIA 267 clause 8.5.3.7.
Overturning and bearing, check 1: (e_x/l_x)² + (e_y/l_y)² ≤ 1/9, SIA 267 clause 8.5.3.6, equations (38) and (39), and E_N,d ≤ q_R,d · A′, clause 8.5.3.5, equation (37).
Uplift by the groundwater, check 2: γ_G,sup · W_k ≤ γ_G,inf · G_k, SIA 267 clause 13.5.1.1.1, equation (83).
Bending, check 3, at every point of the raft: m_xd + |m_xyd| ≤ m_xRd and its three counterparts for the bottom and top layers, 4.3.2.6, equations (34a) to (34d); the minimum reinforcement m_Rd ≥ m_cr in all four layers (4.3.1.2); the spacing and transverse bars (5.5.3.1, 5.5.3.2).
Punching at every column, check 4: V_d = N_d − ΔV_d (4.3.6.2.6) against V_Rd,c = k_r · τ_cd · d_v · u, 4.3.6.3.1, equation (58), with k_r from equation (59) and ψ from 4.3.6.4.2, equation (60), at level 3, r_s and m_sd taken from the plate (4.3.6.4.8).
Settlements, check 5: s_max ≤ s_adm (SIA 267 clause 8.5.4.1) and the angular distortion β = |s_i − s_j|/L ≤ β_adm between neighbouring columns (SIA 267 clause 8.5.4.4).
Bar stresses, check 6, in each layer against Table 17: at cracking, under the service load, and σ_s ≤ f_yd − 80 N/mm².

What you enter and what you get

You enter l_x and l_y, then on a plan the columns with their size, N_d and N, openings and concentrated loads, placed by clicking or as a grid. Then h, the bottom and top bars, additional bottom bars over each column, the materials, the depth D, up to three soil layers, the groundwater h_w, q_d, q_R,d, G_k,b and the uplift factors. In service: q, the load case, s_adm, β_adm and the crack requirements.

You get every check with its utilisation and location, and the punching of the most used column step by step. With the Pro plan, it prints as an A4 calculation note.

Assumptions and limits

One spring constant does not stiffen the edges as the ground does: edge moments come out somewhat low, those in the middle somewhat high (EN 1997-1 §6.8(6)).
Column settlements come from the layers under the contact pressure (Boussinesq), EN 1997-1 F.1: estimates, without creep or consolidation time.
Over the columns, the moments are the mean over the support strip b_s (equation (61)), admissible with ductile reinforcement (4.1.4.2), or the peak values.
k_e takes 0.9, 0.7 or 0.65 (4.3.6.2.5) or equation (57); the columns carry no bending.
One-way shear is covered by the punching check. For a long wall, the control section keeps only 1.5 · d_v from each corner (4.3.6.2.3, Figure 23).

Worked example

A 13.2 × 9.2 m raft, h = 650 mm, under nine 0.4 × 0.4 m columns at 6.0 m by 4.0 m with N_d from 900 to 2,600 kN, Ø16 at 200 mm at the bottom with additional Ø16 at 200 mm over each column, Ø20 at 150 mm at the top, f_ck = 30 N/mm², one soil layer with M_E,1 = 40 MN/m², h_w = 1.0 m and increased crack requirements. These are the module's own results for its worked example, computed when this page was built.

Inputs
Input	Value	Ref.
Length of the raft in x l_x	13.2 m
Length of the raft in y l_y	9.2 m
Plan of the raft	{"o":[],"q":[],"c":[[0.6,0.6,0.4,0.4,0,900,630],[0.6,4.6,0.4,0.4,0,1500,1050],[0.6,8.6,0.4,0.4,0,900,630],[6.6,0.6,0.4,0.4,0,1500,1050],[6.6,4.6,0.4,0.4,0,2600,1820],[6.6,8.6,0.4,0.4,0,1500,1050],[12.6,0.6,0.4,0.4,0,900,630],[12.6,4.6,0.4,0.4,0,1500,1050],[12.6,8.6,0.4,0.4,0,900,630]]}
Thickness of the raft h	650 mm
Cover to the bottom bars c	50 mm	§ 5.2.2
Outer bottom layer	Bars along x
Bottom bars along x, diameter Ø_x	16 mm
Bottom bars along x, spacing s_x	200 mm
Bottom bars along y, diameter Ø_y	16 mm
Bottom bars along y, spacing s_y	200 mm
Cover to the top bars c′	40 mm
Outer top layer	Bars along x
Top bars along x, diameter Ø′_x	20 mm
Top bars along x, spacing s′_x	150 mm
Top bars along y, diameter Ø′_y	20 mm
Top bars along y, spacing s′_y	150 mm
Additional bottom bars over the columns	Bars along x and y over each column
Additional bars, diameter Ø_a	16 mm
Additional bars, spacing s_a	200 mm
Side of the zone over each column l_a	2.4 m
Characteristic compressive strength f_ck	30 N/mm²	§ 3.1.2.2.2
Mean tensile strength f_ctm	2.9 N/mm²	§ 3.1.2.2.4
Modulus of elasticity E_cm	33600 N/mm²	§ 3.1.2.3.3
Poisson's ratio ν	0.2	§ 3.1.2.3.5
Largest aggregate size D_max	32 mm	§ 4.3.3.2.1
Unit weight of the reinforced concrete γ_RC	25 kN/m³
Characteristic yield strength f_yk	500 N/mm²	§ 3.2.2.2
Modulus of elasticity E_s	205000 N/mm²	§ 3.2.2.4
Depth of the base below the ground D	1.5 m
Modulus of subgrade reaction k_s	From the soil layers (settlement analysis)	EN 1997-1 §6.8(5)
Soil layers below the base	One layer
Layer 1, compressibility from	Modulus M_E
Layer 1, compressibility modulus M_E,1	40 MN/m²	SIA 267 §1.2.1
Layer 1, effective unit weight γ_e,1	10 kN/m³
Depth that counts for the settlement	Added stress down to 20 % of the overburden	EN 1997-1 §6.6.2(6)
Groundwater above the base (optional) h_w	1.0 m	SIA 267 §13.5.1.1.1
Unit weight of the water γ_w	10 kN/m³	SIA 267 §1.2.2
Resistance factor for concrete γ_c	1.5	§ 2.4.2.6
Resistance factor for steel γ_s	1.15	§ 2.4.2.6
Duration factor η_t	1.0 — both conditions of 4.2.1.3 met	§ 4.2.1.3
Uniform load on the raft q_d	6 kN/m²
Load factor on the weight of the raft γ_G	1.35
Design bearing pressure of the soil (optional) q_R,d	400 kN/m²
Permanent load of the building on the raft G_k,b	7000 kN
Load factor on the uplift γ_G,sup	1.05	SIA 267 §13.5.1.1.1
Load factor on the stabilising weight γ_G,inf	0.9	SIA 267 §13.5.1.1.1
Moments over the columns	Mean over the support strip b_s	§ 4.1.4.2
Reduction k_e for the moment transfer k_e	Simplified values (4.3.6.2.5)	§ 4.3.6.2.4
Punching reinforcement where needed	None	§ 4.3.6.5
Uniform load on the raft q	4 kN/m²
Load case	Quasi-permanent	Table 17
Admissible settlement (optional) s_adm	30 mm	SIA 267 §8.5.4.1
Admissible angular distortion (optional) β_adm	2 mm/m	SIA 267 §8.5.4.1
Tensile strength for cracking	Mean tensile strength (4.4.1.3)	§ 4.4.1.3
Requirement level	Increased	§ 4.4.2.2.3
Nominal crack width w_nom	0.3 mm	Table 17

Calculation: Raft and soil
Step	Value	Ref.
Area of the raftl_x·l_y minus the openings	A = 121.44 m²
Plate stiffnessE_cm·h³/(12·(1 − ν²)), uncracked	D = 800,990 kNm	§ 4.1.4.1
Weight of the raftγ_RC·h	g = 16.25 kN/m²
Effective overburden at the baseγ_e,1·D	σ′_z0 = 15.0 kN/m²
Settlement of the raft under a uniform net pressure of 100 kN/m²rigid raft (characteristic points), Σ Δσ_z·Δz/M_E down to 12.0 m	s₁₀₀ = 13.89 mm	EN 1997-1 F.1
Modulus of subgrade reaction100 kN/m²/s₁₀₀	k_s = 7.20 MN/m³	EN 1997-1 §6.8(5)
Elements of the plateBFS plate elements, finer around the columns	n_x × n_y = 25 × 19

Calculation: Section
Step	Value	Ref.
Effective depth, bottom bars along xa_s = 1,005 mm²/m	d_x = 592 mm
Effective depth, bottom bars along ya_s = 1,005 mm²/m	d_y = 576 mm
Effective depth, top bars along xa_s = 2,094 mm²/m	d′_x = 600 mm
Effective depth, top bars along ya_s = 2,094 mm²/m	d′_y = 580 mm
Additional bottom bars over the columnsin a square of 2.40 m	a_s,a = 1,005 mm²/m
Cracking moment per metrek_t·f_ctm·h²/6	m_cr = 184.2 kNm/m	§ 4.4.1.3 (99)(100)

Calculation: Loads and soil, whole raft
Step	Value	Ref.
Vertical force on the soilΣN_d + ΣQ_d + (q_d + γ_G·g − γ_w·h_w)·Athe uplift of the water is taken at its characteristic value	E_N,d = 14,378 kN	SIA 267 §8.5.3.7
Eccentricity of the resultantfrom the centre of the raft	e_x ; e_y = 0.000 ; 0.000 m
Effective area(l_x − 2·\|e_x\|)·(l_y − 2·\|e_y\|)	A′ = 121.44 m²	EN 1997-1 D.1

Calculation: Check 1 — overturning and bearing of the raft
Step	Value	Ref.
Overturning: resultant within the admissible eccentricitye_d/e_gr	η = 0.000 · Pass0 % of the capacity used	SIA 267 §8.5.3.6 (38)(39)
Bearing on the effective areaE_N,d/(q_R,d·A′)	η = 0.296 · Pass30 % of the capacity used	SIA 267 §8.5.3.5 (37)

Calculation: Check 2 — uplift by the groundwater
Step	Value	Ref.
Uplift of the waterγ_w·h_w·A	W_k = 1,214 kN
Stabilising permanent loadG_k,b + γ_RC·h·A	G_k = 8,973 kN
Upliftγ_G,sup·W_k/(γ_G,inf·G_k)	η = 0.158 · Pass16 % of the capacity used	SIA 267 §13.5.1.1.1 (83)

Calculation: Contact pressure under the design loads
Step	Value	Ref.
Largest contact pressurek_s·w, under a column	σ_d,max = 147.5 kN/m²
Part of the base lifted offsprings that would pull are released	A_lift/A = 0.0 %
Soil reaction, totalequilibrium: equals E_N,dE_N,d = 14,378 kN	ΣR = 14,378 kN

Calculation: Check 3 — bending, eq. (34) at every point of the raft
Step	Value	Ref.
Design compressive strength of the concreteη_fc·η_t·f_ck/γ_c	f_cd = 20.0 N/mm²	§ 2.4.2.3 (2)
Design yield strength of the steelf_yk/γ_s	f_yd = 434.8 N/mm²	§ 2.4.2.5 (4)
Bottom bars along xdemand/m_xRd	η = 0.642 · Pass64 % of the capacity used	§ 4.3.2.6 (34a)
Bottom bars along ydemand/m_yRd	η = 0.659 · Pass66 % of the capacity used	§ 4.3.2.6 (34b)
Top bars along xdemand/m′_xRd	η = 0.762 · Pass76 % of the capacity used	§ 4.3.2.6 (34c)
Top bars along ydemand/m′_yRd	η = 0.450 · Pass45 % of the capacity used	§ 4.3.2.6 (34d)
Minimum reinforcement in the four layers: no brittle failure at crackingm_cr/m_Rd,min	η = 0.746 · Pass75 % of the capacity used	§ 4.3.1.2
Detailing: spacing and transverse barsmax of the ratios	η = 0.667 · Pass67 % of the capacity used	§ 5.5.3.1, 5.5.3.2

Calculation: Check 4 — punching at every column
Step	Value	Ref.
Effective depth for punchingmean of the two bottom layers, from the top face	d_v = 584 mm	§ 4.3.6.2.1
Aggregate factor48/(16 + D_max·min(1 ; (60/f_ck)²)) ≥ 1.2	k_g = 1.200	§ 4.3.3.2.1 (37)
Column C1 (interior)V_d/V_Rd,c	η = 0.200 · Pass20 % of the capacity used	§ 4.3.6.3.1 (58)
Column C2 (interior)V_d/V_Rd,c	η = 0.391 · Pass39 % of the capacity used	§ 4.3.6.3.1 (58)
Column C3 (interior)V_d/V_Rd,c	η = 0.200 · Pass20 % of the capacity used	§ 4.3.6.3.1 (58)
Column C4 (interior)V_d/V_Rd,c	η = 0.411 · Pass41 % of the capacity used	§ 4.3.6.3.1 (58)
Column C5 (interior)V_d/V_Rd,c	η = 0.822 · Pass82 % of the capacity used	§ 4.3.6.3.1 (58)
Column C6 (interior)V_d/V_Rd,c	η = 0.406 · Pass41 % of the capacity used	§ 4.3.6.3.1 (58)
Column C7 (interior)V_d/V_Rd,c	η = 0.200 · Pass20 % of the capacity used	§ 4.3.6.3.1 (58)
Column C8 (interior)V_d/V_Rd,c	η = 0.387 · Pass39 % of the capacity used	§ 4.3.6.3.1 (58)
Column C9 (interior)V_d/V_Rd,c	η = 0.200 · Pass20 % of the capacity used	§ 4.3.6.3.1 (58)

Calculation: Punching in detail — column C5, the most used
Step	Value	Ref.
Column force	N_d = 2,600.0 kN
Net soil reaction inside the control section∫(σ − q_d − γ_G·g + γ_w·h_w) dA over 0.895 m²	ΔV_d = 81.5 kN	§ 4.3.6.2.6
Design punching forceN_d − ΔV_d	V_d = 2,518.5 kN
Control perimeterat d_v/2 from the column, straight parts ≤ 1.5·d_v from the corners	u₀ = 3,435 mm	§ 4.3.6.2.2
Reduction for the moment transferinterior column	k_e = 0.900	§ 4.3.6.2.5
Effective control perimeterk_e·u₀	u = 3,091 mm	§ 4.3.6.2.3
Distance to zero momentfrom the elastic plate, along x and y through the column	r_sx ; r_sy = 0.69 ; 0.69 m	§ 4.3.6.4.8
Width of the support strip1.5·√(r_sx·r_sy) ≤ l_min	b_s = 1.03 m	§ 4.3.6.4.6 (61)
Mean moments in the strip, bottom barsmean of m_d + \|m_xyd\| over b_s at the column faces	m_sd,x ; m_sd,y = 312.4 ; 310.9 kNm/m	§ 4.3.6.4.8
Rotation of the raft1.2·(r_s/d)·(f_yd/E_s)·(m_sd/m_Rd)^1.5, larger of x and ylevel 3: r_s and m_sd from the elastic plate	ψ = 0.0015	§ 4.3.6.4.2 (60)
Coefficient1/(0.45 + 0.18·ψ·d·k_g) ≤ 2	k_r = 1.550	§ 4.3.6.3.2 (59)
Punching resistance without punching reinforcementk_r·τ_cd·d_v·u	V_Rd,c = 3,065.2 kN	§ 4.3.6.3.1 (58)

Calculation: Contact pressure and settlements in service
Step	Value	Ref.
Vertical force on the soilΣN + ΣQ + (q + g − γ_w·h_w)·A	E_N = 9,785 kN
Largest contact pressure	σ_max = 101.0 kN/m²
Part of the base lifted off	A_lift/A = 0.0 %
Settlements of the columns on the springsplate on springs	w = 10.3 to 13.2 mm
Depth of the compressible zoneΔσ_z under the centre = 0.2·σ′_z0(z)	z_lim = 10.50 m	EN 1997-1 §6.6.2(6)
Settlements of the columns from the contact pressureΣ Δσ_z·Δz/M_E, Δσ_z of the contact pressure (Boussinesq)	s = 7.1 to 11.3 mm	EN 1997-1 F.1

Calculation: Check 5 — settlements
Step	Value	Ref.
Largest settlement of a columns_max/s_adm	η = 0.376 · Pass38 % of the capacity used	SIA 267 §8.5.4.1
Angular distortion between neighbouring columnsβ/β_adm	η = 0.349 · Pass35 % of the capacity used	SIA 267 §8.5.4.4

Calculation: Check 6 — bar stresses (Table 17)
Step	Value	Ref.
Cracking momentk_t·f_ctm·h²/6	m_cr = 184.2 kNm/m	§ 4.4.1.3 (99)
Crack limitation at cracking, bottom bars along xσ_s,r/σ_s,adm	η = 0.521 · Pass52 % of the capacity used	Table 17
Crack limitation under the quasi-permanent load, bottom bars along xσ_s/σ_s,adm	η = 0.633 · Pass63 % of the capacity used	Table 17
No yielding in service, bottom bars along xσ_s/(f_yd − 80)	η = 0.565 · Pass57 % of the capacity used	Table 17
Crack limitation at cracking, bottom bars along yσ_s,r/σ_s,adm	η = 0.536 · Pass54 % of the capacity used	Table 17
Crack limitation under the quasi-permanent load, bottom bars along yσ_s/σ_s,adm	η = 0.650 · Pass65 % of the capacity used	Table 17
No yielding in service, bottom bars along yσ_s/(f_yd − 80)	η = 0.580 · Pass58 % of the capacity used	Table 17
Crack limitation at cracking, top bars along xσ_s,r/σ_s,adm	η = 0.552 · Pass55 % of the capacity used	Table 17
Crack limitation under the quasi-permanent load, top bars along xσ_s/σ_s,adm	η = 0.840 · Pass84 % of the capacity used	Table 17
No yielding in service, top bars along xσ_s/(f_yd − 80)	η = 0.671 · Pass67 % of the capacity used	Table 17
Crack limitation at cracking, top bars along yσ_s,r/σ_s,adm	η = 0.571 · Pass57 % of the capacity used	Table 17
Crack limitation under the quasi-permanent load, top bars along yσ_s/σ_s,adm	η = 0.037 · Pass4 % of the capacity used	Table 17
No yielding in service, top bars along yσ_s/(f_yd − 80)	η = 0.029 · Pass3 % of the capacity used	Table 17

How this module is verified

Verified clause by clause against SIA 262:2025 and against independent calculations used on real projects, before release.

Questions

Why are additional bars placed over the columns?

The bottom of the raft is in tension under each column, with a sharp peak. Extra bars in a square zone avoid a heavy mesh everywhere; anchor them beyond it.

Why two sets of settlements?

The springs give the bending of the raft. The settlements from the soil layers describe the spreading in the ground better and are the ones checked.

Which plan includes this module?

The Pro plan, with its calculation note.

Norm: SIA 262:2025 · Page updated: 29 October 2026