The fire proofing specification is for calculate the weight of its material only, isn't it?
Yes, the understanding is correct. Using the density and thickness provided through input, the program calculates the weight and add it in the form of selfweight.
Please note, STAAD calculates the fire proofing weight only for the following sections:
For block fireproofing - I-shaped sections like those from the built-in tables (American W,S,M,HP, British UC and UB, etc.), tapered I shaped sections, single channels, angles and Tees.
For CFP-contour fireproofing - the sections are I-beam straight or tapered, angle, Tee.
I-shaped sections like those from the built-in tables (American W,S,M,HP, British UC and UB, etc.), tapered I shaped sections, angles and Tees..
Fire proofing weight is not calculated for the following section types: Pipe, tube, composite I beams with slab on top, double channel, double angle, HSS, I-beam with cover plates, prismatic, solid circle or rectangle, castellated, cold formed sections, wood, aluminum, tapered poles, etc.
More details can be found in the help file.
Also please note that fire proofing is used only in self weight calculation and it does not help in calculation wind forces considering the fireproffing thickness in a member. You need to calculate the wind load separately or you may have to magnify the generated wind load by a certain factor to approximately account for such effects.