Beam-Column Sidesway Moment Amplifier B2 (AISC 360 App. 8)

the second half of Mr = B1 Mnt + B2 Mlt. AISC 360 Appendix 8.2.2: B2 = 1 / (1 - alpha Pstory / Pe,story) >= 1, with Pe,story = RM (H L / dH), RM = 1 - 0.15 (Pmf / Pstory), alpha = 1.0 (LRFD) / 1.6 (ASD). Unlike the member-level B1, B2 is a STORY quantity: Pstory is the total gravity load on the story, Pmf the part on the moment-frame columns (0 braced), H the story shear that produces the first-order interstory drift dH, L the story height. A story carrying 2,000 kip (1,200 on the moment frame) that drifts 0.5 in under a 100 kip shear over 14 ft has RM 0.91, Pe,story 30,576 kip, B2 1.07; double the drift and B2 climbs to 1.15. A flexible (high-drift) story amplifies every column's sway moment, so the drift limit often governs. AISC flags B2 > 1.7 as too flexible for the amplified-first-order method. A design aid, not the engineer of record's stamped design.

RM = 1 - 0.15 (Pmf / Pstory); Pe,story = RM (H L / dH); alpha = 1.0 LRFD / 1.6 ASD; B2 = max(1, 1 / (1 - alpha Pstory / Pe,story)).

The AISC 360-22 Appendix 8.2.2 sidesway (P-Delta) beam-column moment amplifier B2, with the story elastic sidesway buckling strength Pe,story and the RM factor, by name.

AISC 360 is free to read at aisc.org (Specification for Structural Steel Buildings); the Appendix 8 amplified first-order provisions are public. Pstory, Pmf, the story shear, the drift, and the height are the user's own analysis results.

Estimate. AHJ and licensed professional govern.

Field names used by the API: pstory_kip, pmf_kip, h_kip, l_ft, drift_in, method, b2, pe_story_kip, rm

• Amplifier B2 = 1 / (1 - alpha Pstory / Pe,story) >= 1AISC 360-22 Eq. A-8-6
• Story buckling strength Pe,story = RM (H L / dH), RM = 1 - 0.15 (Pmf / Pstory)AISC 360-22 Eq. A-8-7 / A-8-8
• Basis Pstory is the total story gravity load; alpha = 1.0 LRFD / 1.6 ASD; dH uses the 0.8 tau_b reduced stiffnessAISC 360-22 App. 8.2.2