The figure below shows a portal frame being acted on by a downwards acting vertical load. The supports at A and E are fixed and the connections at B, C and D are rigid.

Sketch the reactions, shear force diagram and bending moment diagram generated on this portal frame by the loading and use it to answer the following question:

Which of the following best describes the magnitude and distribution of shear force along member AB and along member ED

Question

The figure below shows a portal frame being acted on by a downwards acting vertical load. The supports at A and E are fixed and the connections at B, C and D are rigid.

Sketch the reactions, shear force diagram and bending moment diagram generated on this portal frame by the loading and use it to answer the following question:

Which of the following best describes the magnitude and distribution of shear force along member AB and along member ED

BlackTom AI · Accepted Answer

Begin by identifying the key structural feature: the portal frame has vertical members AB and ED carrying vertical reactions, with no distributed vertical loads along those members themselves, and the only external load is the downward load applied at the top, transmitted through the frame joints.

Option analysis:

- Option: 'Constant along both members, negative in sign along AB and positive in sign along ED'
  This aligns with standard shear behavior in a frame member that has no distributed loads along its length. In member AB (the left column), the internal shear is constant from A to B because there is no along-member load; the shear sign is determined by the chosen sign convention (typically negative for a shear force that would cause a clockwise rotation of the joint on the left side, depending on the axis orientation). Similarly, in member ED (the right column), the internal shear remains constant from E to D for the same reason. The signs being opposite (negative on AB, positive on ED) reflect the equilibrium of the whole frame: the left column carries shear in one sense, while the right column carries shear in the opposite sense to balance the vertical reactions and the applied load at the apex. Hence, the magnitude is the same along each member (constant) and the signs are opposite due to the frame’s symmetric loading and the internal force resultants at the joints.

Notes on why other speculative descriptions would be incorrect if they appeared:
- If someone claimed the shear changes along AB or ED, that would imply a distributed load along those members or coupling effects not present in the stated problem (no along-member load on AB or ED).
- If the shear along AB and ED were stated to be of the same sign, that would contradict the equilibrium of vertical forces in the portal frame, which requires the reactions at A and E to balance the external load and the internal force distribution at the rigid joints.
- Any assertion that shear is zero along either member would neglect the vertical reactions transmitted by the fixed supports and the bending interaction at the rigid joints, which generate nonzero shear in the members under a vertical top load.

In summary, the stated description—constant magnitude along both AB and ED, with AB having negative shear and ED positive shear—matches the expected internal shear distribution for a portal frame with rigid joints and no distributed loads on the vertical members.

STRCTENG 302 Steel Quiz No 1: Basic Knowledge Mechanics and Structural Steel Review

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