PyritePrinting
Bracing members and member stability
A member efficient in one bending direction still has stability questions. An effective restraint depends on more than contact with a purlin.
Watch an I-shaped model rafter move sideways and twist, then compare its movement as purlin lines are added. The demonstration makes restraint locations visible while showing that restraining one flange does not automatically restrain the other.
Authored explanation · separate from narration
An I-shaped rafter can be efficient for bending while still needing effective restraint against sideways movement and twist. Unbraced length is a member-stability concept, different from local flange or web slenderness.
Justice's post describes the bare model rafter twisting and the response changing with added purlin lines. He also records that the model has no flange braces, so restraining one flange does not demonstrate effective restraint of the other under every condition.
Use the movement to discuss the mechanism and what an assumed brace point must do. It is not an observed full-scale buckling failure, a measured design capacity or a finding that a member is compact, noncompact or slender.
A qualitative physical-model demonstration, not a measured buckling capacity, member classification, required brace spacing or full-size bracing design. The model has no flange braces; effective real restraint depends on the complete member, connection and supporting system.
Transcript checked against the recording.
Here we can see one of the primary limitations of using I-shaped built-up members.
They're really good at resisting load efficiently, but without bracing, they buckle in and out of plane. A reason why we need secondary is that we can brace that outside flange. You can rotate here, but it's restrained here. That same load doesn't cause it to buckle, or rotate. Same goes here.
If we add in this, now the bottom flange can still buckle in and out of plane, but it's restrained on the top. While this model doesn't show flange bracing, it does show how restraining the top flange prevents it from twisting.
Bring it into the conversation
What movement is the proposed restraint intended to control, and where does its force go?
No effective brace spacing, actual buckling capacity, flange-brace adequacy or member classification is prescribed.
A physical point of reference
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