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Dynamic Analysis (Flexible Buildings) and Gust Factor G
For rigid structures (static analysis), which is by far the most common case, the gust effect factor G for the Directional Procedure is calculated by the program using ASCE 7 26.9.4 based upon the building exposure and geometry and is usually 0.85. However, determining this factor for flexible structures requires detailed knowledge of the building's harmonics. If you check the Dynamic analysis (flexible buildings) checkbox, you must do a flexible analysis using ASCE 7 26.9.5 then enter the resulting G value.
Apply Case 2 Loads at... and Eccentricity e
ASCE 7 Commentary C27.3.5 states that the 15% eccentricity at 75% for Directional method Case 2 loads in Figure 27.3-8 may not cover all cases, and that for certain buildings, 5% eccentricity at full loading is more appropriate.
Also, for flexible buildings (dynamic analysis), according to ASCE 7 27.3.5 the eccentricity to be used for the torsional load case 2 in Figure 27.3-8 is derived from a complicated formula that includes wind turbulence, natural frequency, damping, etc.
These inputs allows you to specify the percent loading and the eccentricity as percentage building width. If you calculate you must calculate eccentricity using 27.3.5 and input it for each building direction. .
The default value is 75% loading at building width x 0.15 eccentricity
It applies to both rigid and flexible diaphragms.