Calculate if construction is strong enough

We need a calculation, which is reusable to calculate if a steel construnction is strong enough. The calculation is made for aluminium glass veranda roofs which need an extra steel or aluminium tube under the gutter. The same calculation should be made for gable roof, which gets a ridge-pole where we connect the roof. So we need to know how big the steel or aliminium tube or ridge-pool needs to be.