Amplified first-order analysis

I have a small one story 3-D structure consisting of 3 frames. I analysed it without P-Delta and designed with the Amplified first-order elastic analysis method.  The design report of one of the columns has this warning "The amplification factor B1 for major axis is larger than 1.2, it is recommended a second order analysis". I thought the option I chose was an approximate second order analysis? Besides the B1 factor was not used to amplify the Mnt moment in the column. Mnt = 15.4 k-ft and Mr = 15.4 k-ft with B1 = 1.46 and B2 = 1.0. No lateral forces were applied to the model. Is there a technical document you can refer me to or an explanation. I have attached a copy of the model

6675.File.zip