Individual load case(s) incompatible with supports

[swmarais]

Firstly, I am new to Robot. So the following may be a rookie error...

The issue I have is the following:

The structure being analyzed is a rectangular building - steel roof trusses pinned to concrete columns on RC foundations supported by an elastic soil (uplift is permitted, i.e. no soil tension)

The following load cases are applied:
DL1 - Permanent Dead Load (incl self weight)
DL2 - Non-Permanent Dead Load (not to be taken into account under wind loading)
LL1 - Live Loading
EP1 - External wind pressure 1 - push-over wind force
Suction on roof
Pressure on LHS column
Suction on RHS column
EP2 - External wind pressure 2 - suction
Suction on roof
Suction on LHS column
Suction on RHS column
IP1 - Internal wind pressure - positive pressure
IP2 - Internal wind pressure - suction

Analysis Type:
Non-linear P-delta

Combinations (factors not shown)
DL1 + DL2 + LL1
DL1 + EP1 + IP1
DL1 + EP1 + IP2
DL1 + EP2 + IP1
DL1 + EP2 + IP2


Robot analyzes the cases indivudually, however, the uplift wind forces are incompatible with the supports and causes non-convergence.
It is not possible for the wind forces to exist without the structural dead load.

What is the correct way of dealing with this?

[gary.wyatt]

Hi
The issue is that it is often not possible to find a non linear converged solution for the wind load alone. As you correctly point out, wind load never acts in isolation, so your best solution would be to flag the wind load as an "auxiliary case" - this allows Robot to ignore the load case in question when it acts alone, but it then allows you to include it in combinations.
To set the auxiliary case, go to analysis/analysis types, highlight the loadcase you want, press "parameters" and check the auxiliary case box

Hope this helps