The 'portal frame analysis' approximation for multi-storey frames under horizontal load assumes inflection points at:
Choose the correct answer
Supports only (zero moment at base and top)
Mid-height of all columns AND midspan of all beams — makes each sub-frame statically determinate for lateral load analysis
Only at beam-column joints
No inflection points — full fixed frame analysis only
Correct Answer
B. Mid-height of all columns AND midspan of all beams — makes each sub-frame statically determinate for lateral load analysis
Why is this the answer?
Portal method (lateral loads): assumes inflection points (zero moment, zero rotation) at: (1) Midheight of each column; (2) Midspan of each beam. This makes the frame statically determinate for each storey. Shear distribution: interior columns carry twice the shear of exterior columns. Portal method is appropriate for low-rise frames (H/W < 4). Cantilever method for tall frames: column axial forces proportional to distance from centroid.
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