When designing a box you are not able to just use the ECT-value given in N/m. The ECT-test is done with a low sample and a box is much higher. Thin materials will bend out to the side and are not able to carry the full load.
The result of this effect is that almost all the strength in a box is near the bending lines. 60-90% of the compression force is generated only of the 5 cm around the edges.