Temperature load, comparison with ETABS

Problem. The roof slab of an open car park is solved separately with MindFEM and with ETABS under uniform cooling in winter and the temperature difference through the thickness caused by the sun in summer. The walls on the long facades hold the slab as it shortens, which produces tension in the slab and bending in the columns. Slab section forces, the wall base shear, column reactions and displacements are compared for the load cases and for two combinations.

Reference document. The reference is the same slab modelled in ETABS 20.1.0 with its own objects. The temperature loads are given in ETABS on the area objects as a uniform temperature change and a temperature gradient along local axis 3.

Numerical model. The slab is a flat plate 40 × 16 m in plan and 0.25 m thick at 3.5 m height, resting on fourteen 50/50 columns on an 8 × 8 m grid and on four walls 6 m long and 0.30 m thick at both ends of the two long facades; the column bases are fixed and the wall bases are held against translation. The concrete is C30/37 with a thermal expansion coefficient of 1 × 10⁻⁵ 1/°C, the finishes weigh 2 kPa and the mesh is made of 0.50 × 0.50 m quadrilateral elements.

Roof slab of an open car park#

Independent calculation. ETABS 20.1.0, the same structure modelled with ETABS's own objects

Where the numbers are read. The section forces are read in the marked quadrilateral elements, the vertical displacement at the marked node and the column reactions at the base of the marked columns. The wall base shear is the sum of the horizontal reactions at the base of the marked wall. The grid is in metres.

Note. TU is a uniform cooling of the slab by 25 °C, TG the top face of the slab 12 °C warmer than the bottom face. G + TU is the winter and G + TG the summer combination. The plan under the table shows where each row is read. The ETABS moment sign convention is the opposite of MindFEM's, and the table uses MindFEM's sign. The columns connect to the slab at a single node, and quantities near a column node change in both programs as the mesh is refined. From the 0.50 m to the 0.25 m mesh the base moment of the end column under G falls from 60.9 to 53.7 kNm in ETABS and from 50.4 to 40.6 kNm in MindFEM. The uplift of the panel centre under TG grows by 5.3 per cent in ETABS and by 6.6 per cent in MindFEM. The column moments under G and TG are therefore left out of the table. In TU the column moment comes from the horizontal movement of the slab and is in the table. The few per cent difference in the rows near a column node or a wall end is of the same order as the difference between the two programs' changes under this mesh refinement.

Model files#

The ETABS model that produced the results in the table can be downloaded. It opens in ETABS 20 and later with File > Import > ETABS .e2k File.

• Roof slab of an open car park (.e2k)