Fig. 1 shows a rectangular frame that racks sideways under load. Which one of the following additions would stop this movement? [1]
Question 1 Report
Fig. 1 shows a rectangular frame that racks sideways under load. Which one of the following additions would stop this movement? [1]
Answer Details
A diagonal brace that forms a triangle stops the sideways racking movement. [1 mark]
A rectangular frame can change shape into a parallelogram because its corners can rotate slightly under load. A triangle is rigid: once its side lengths are fixed, its angles cannot change without changing the length of a member. Adding a diagonal member therefore triangulates the frame and prevents it from racking. A gusset plate at a corner is also acceptable because it braces the joint and creates the same triangulating effect.
Examination reminder: for a frame that distorts sideways, look for triangulation, rather than simply making the existing vertical or horizontal members thicker.
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