BS8100 gust factors for design of bracing

[eriksasha]

I'm investigating BS8100 concerning the design loads for lattice masts. I have problems with the The derivation of the ratio fq, concerning gust factors, which as stated is Fb/F. (Fb=mean bracing force and F=mean shear force)Ref:BS8100-2:1986 C.5.8

The Fb value appears negative while the F positive. I'm therefore unable to proceed with determining Gq and Kq.

I'm seeking advice from someone familiar with the BS8100.