An approximate way to get the maximum load carrying capacity is to do a steel design and look at the interaction ratio. For any member, the reciprocal of that value tells you how much additional capacity that member possesses. For example, if the highest ratio for a member is 0.85, the reciprocal is 1.176, which means, the member can take about 17.6% more load.
Note that the interaction ratio is not linearly related to the applied load, so, this can only give you an approximate estimate.
Another way is to do a buckling analysis and look at the buckling factors. First, the factor would have to be greater than 1. The amount in excess of 1.0 is the additional load carrying capacity for the structure, since this analysis is based on the weakest member in the structure. The tedious aspect of this method is, you have to do the buckling analysis for every combination case, find the lowest positive buckling factor from among all the cases, and find the reserve capacity on the basis of that.
Note that the interaction ratio is not linearly related to the applied load, so, this can only give you an approximate estimate.
Another way is to do a buckling analysis and look at the buckling factors. First, the factor would have to be greater than 1. The amount in excess of 1.0 is the additional load carrying capacity for the structure, since this analysis is based on the weakest member in the structure. The tedious aspect of this method is, you have to do the buckling analysis for every combination case, find the lowest positive buckling factor from among all the cases, and find the reserve capacity on the basis of that.