StruCla against SCIA Engineer: pump foundation slab on elastic subsoil
A concrete slab of 2.8 × 1.5 × 0.5 m (C16/20) with free edges on elastic subsoil, loaded by its self weight, a small distributed load and the loads of a pump (a vertical load and four anchor forces). The same model was built in StruCla from the SCIA input. This is a limited comparison: the SCIA output of this slab holds the input, the six natural frequencies and (in the project report) one total reaction, but no internal forces, soil pressures or deflections. Only those values can be compared, so the bending moments of StruCla are not validated here. Project name, client, plant, licence data and author have been removed from the SCIA output.
1. Input
All input comes from the SCIA output (sheets 2 to 5) and the project report, except the anchor positions, which the output does not give (see section 4).
| SCIA Engineer | StruCla | |
|---|---|---|
| Slab | 2.8 × 1.5 × 0.5 m, C16/20 (E = 28 600 N/mm², 2500 kg/m³) | the same; C16/20 with a fixed unit weight of 25 kN/m³ (see section 3) |
| Edges | free | free |
| Subsoil | Individual subsoil, flexible: stiffness 2.0 MN/m³ vertical, C1x = C1y 0.2 MN/m³ (sheet 3) | bedding area over the whole slab: k 2000 kN/m³, kx = ky 200 kN/m³ |
| Load cases | LC1 self weight, LC2 railing, piping and grating (1.12 kN at the centre), LC7 pump loads | SW, LC2, LC7; every slab load has a load case |
| LC7 pump loads | 10 kN at the centre; vertical anchor forces +21.41, −21.41, +6.98 and −6.98 kN (sheet 5) | the same, at the four anchors |
| ULS Pump | 1.20 LC1 + 1.20 LC2 + 1.50 LC7 (sheet 3) | the same |
| SLS Pump | 1.00 LC1 + 1.00 LC2 + 1.00 LC7 | the same |
| Mass for the frequencies | total 8742.4 kg (sheet 6) | slab + an extra mass at the centre, chosen so the total is 8742.4 kg (see section 4) |
2. Natural frequencies (SCIA sheet 6), Hz
SCIA solves the free vibration with a finite element mesh (Lanczos, 10 modes). Modes 1 to 6 are the rigid body modes of the slab on the springs: translation in x and y, torsion about z, translation in z and rocking about the two horizontal axes. Modes 7 to 10 (163 to 397 Hz) are bending modes of the slab itself with no modal mass. StruCla calculates the slab as a rigid body on the bedding springs (six degrees of freedom, the springs under the underside of the slab, mass and rotational inertia of the slab plus the extra mass), so the bending modes are not part of the StruCla result.
| Mode | SCIA | StruCla, 25 kN/m³ | StruCla, 24.525 kN/m³ | Difference (24.525) |
|---|---|---|---|---|
| 1 x or y translation | 1.56 | 1.51 | 1.51 | −3.2 % |
| 2 x or y translation | 1.56 | 1.54 | 1.54 | −1.3 % |
| 3 torsion about z | 1.96 | 1.99 | 2.01 | +2.6 % |
| 4 z translation | 4.91 | 4.93 | 4.93 | +0.4 % |
| 5 rocking | 5.97 | 5.85 | 5.90 | −1.2 % |
| 6 rocking | 6.28 | 6.16 | 6.22 | −1.0 % |
Green: within 1 %. Yellow: within 3.2 %. SCIA prints the frequencies with two decimals. The modes are compared in ascending order of frequency.
3. Total reaction in SLS
| kN | Difference | |
|---|---|---|
| SCIA (resultant of the reactions Rz, SLS Pump, project report) | 62.28 | |
| StruCla, concrete 25 kN/m³: 52.50 + 1.12 + 10.00 | 63.62 | +2.2 % |
| StruCla, concrete 24.525 kN/m³ (2500 kg/m³ × 9.81): 51.50 + 1.12 + 10.00 | 62.62 | +0.5 % |
SCIA takes 2500 kg/m³ times the acceleration of gravity of 9.81 m/s², so 24.525 kN/m³. StruCla uses 25 kN/m³ for C16/20 and cannot yet set the unit weight per slab; the values with 24.525 kN/m³ were calculated with the same routines and only the unit weight changed. In StruCla the total reaction of the bedding equals the applied load. The remaining 0.34 kN in SCIA is not explained by the SCIA output.
4. What differs and what is assumed
- Extra mass. The SCIA total mass is 8742.4 kg, but the slab weighs 5250 kg. The extra 3.5 t comes from the mass group of the SCIA calculation, which the output does not describe. StruCla uses an extra mass at the centre of the top face to reach the same total (3492 kg with 24.525 kN/m³). The position of the extra mass influences the translations and the rocking modes slightly and is the most likely reason for the 3.2 % in mode 1.
- Anchor positions. The SCIA output shows the dimension lines but not the anchor coordinates. The four anchors are placed at ±0.325 m from the centre in both directions, from the lever arm of 0.325 m in the project report. The signs of the vertical anchor forces are read from sheet 5. This does not influence the frequencies, but it does influence every moment.
- Linear subsoil. SCIA's flexible subsoil is linear, so the springs can take tension where the slab lifts. StruCla has an option "bedding without tension" (lift-off); it is off in the values of section 5 so that the model is the same as SCIA's.
- Horizontal bedding. C1x and C1y are modelled as horizontal springs under the slab; the plate calculation in StruCla bends only, so they act only in the frequency calculation.
- Not part of this comparison: the horizontal anchor forces are in the SCIA load case (sheet 5), but a StruCla slab carries only vertical loads in its bending calculation.
5. StruCla results without a SCIA counterpart
Given for information only. These values are not validated by this page.
| SLS Pump | ULS Pump | |
|---|---|---|
| Total reaction of the bedding (kN) | 63.6 | 79.3 |
| Deflection, + = down (mm) | −3.5 … 18.7 | −7.2 … 26.1 |
| Subsoil pressure k·w (kPa; negative = the spring is in tension) | −7.0 … 37.3 | −14.4 … 52.2 |
| Design moments Wood-Armer, bottom x / y (kNm/m) | 10.2 / 8.4 | 15.2 / 12.7 |
| Design moments Wood-Armer, top x / y (kNm/m) | 7.7 / 8.1 | 11.6 / 12.1 |
With the unit weight of 25 kN/m³ and the linear subsoil. The slab is thick compared with its span, so it behaves almost as a rigid block: the moments do not depend on the stiffness of the subsoil (the same for 2000 and 494 kN/m³), only the deflection and the pressure do.
6. How to repeat it
Open the StruCla model in StruCla. The slab has the bedding, the load cases and the two combinations; choose the combination in the legend of the slab, calculate the slab and use "Natural frequencies" in the slab panel with the extra mass of 3391 kg (25 kN/m³) or 3492 kg (24.525 kN/m³). The model file switches "bedding without tension" on; switch it off to reproduce section 5. The load case per slab load, the combinations for slabs, the bedding without tension and the natural frequencies are recent additions to StruCla.