Recommendations

These points aren't essential, but going through them helps you get the best results in PREFAB.

Supporting structures for covers

To ensure correct conversion of loads, make sure the covers have the correct supporting structures. As an example below: (1) wrong supporting structures on one side, (2) a missing supporting structure.

Cover with wrong supporting structures on one side and a missing supporting structure

Use the manual supporting structures option in the Cover dialogue to modify or correct a cover's supporting structures. Pay attention to the highlighted objects and the load distribution line.

Cover with corrected supporting structures

Anchorage behavior

To ensure correct behavior of anchorage (trusses), make sure to consider the settings for Compression/Tension. For non-compression, a good solution is to set it to Brittle with the limit force as $0\,\mathrm{kN}$ . Remember this is non-linear behavior, and it's only applied in load combination analysis, not load cases.

Truss dialog with Compression set to brittle and a 0 kN limit force. And Anchorage with an incorrect -6.0 kN compression force versus the corrected 0.0 kN result

Edge-connections representing the desired behavior

For the elements, it's important for the correct structural behavior to consider, in each local axis, whether:

Tension/compression – it's able to handle tension/compression in that direction
Stiffness – the stiffness represents the joint type, so the connection forces give good results

On the element below, the local axes are drawn on the top edge: y' is always perpendicular to the edge, x' is always along the edge and z' is always perpendicular to the plane of the shell.

Local axis system on an edge, and the Edge connection dialog with the local system settings

tip

Turn on the local system in the display settings as a help. If for one of the directions the edge can't handle any forces at all when there's a displacement in a specific direction, use the Detach feature.

Special conversion method for covers

For covers, it's important to know that they function using a very basic rule: any load inside a load distribution zone goes to the zone's supporting structure. This means that in a case like below, where all the surface load is placed on the right side of the cover, all the load goes to the corresponding wall.

Surface load placed on one side of a cover, all going to the corresponding wall

To avoid this, there's an option to use what's called the Special conversion method in the settings for covers.

Settings > Calculation > Cover, with special conversion for rectangle subcovers enabled

With this activated, for regular single span covers the distribution will be like a single span beam, but the resultant will be distributed along the full length of the supporting structure – hence it's recommended to split the cover in case the loading isn't full length. After the split, you'll get the result on the right.

Cover before and after splitting, showing the corrected load distribution

warning

Special conversion will be activated on ALL covers in the model, so read the special conditions, and as always pay close attention to the "Converted loads" result from the analysis to verify the distribution.