Gravity Load Takedown Missing on Cantilever
A five-story office building has a 6 m cantilevered slab at level 5 supporting a rooftop mechanical screen. The analytical model shows the cantilever beams and the screen, but the screen's self-weight is not appearing in the column reactions at the supporting frame. The screen was modeled as a generic model family with a Shared parameter set, and its material is 'Default'. The engineer also notices that the cantilever tip deflection is 12 mm under a load case that should include the screen. Another engineer suggests simply adding a line load to the cantilever beams to compensate.
1. Diagnose why the screen load is missing from the reactions and evaluate the line-load workaround.
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The screen is a generic model family with a Default material, so it likely has no analytical properties and does not participate in load takedown. Even if it appears in the physical model, the analytical model does not see it as a load-bearing element unless it is set to be analytically modeled and assigned a material with density, or unless its load is explicitly transferred.
The line-load workaround can make the numbers look right for one load case, but it is fragile: it is manual, it will not update if the screen size or location changes, and it hides the real gap in the analytical model. It also makes the load path unauditable, because a reviewer cannot tell whether the line load represents the screen or something else.
Better approach:
- Confirm the screen's family category and whether 'Enable Analytical Model' is checked.
- Assign a real material with density, or model the screen as a loadable element and run load takedown again.
- Re-check column reactions and tip deflection.
- If the screen must stay generic, apply a documented area or line load with a clear name and note, and record the assumption.
12 mm tip deflection should be re-evaluated after the load is correctly included, not tuned to match a target.
2. How should the team verify the corrected load path before issuing?
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Verification should be independent of the modeling shortcut. Steps:
- Compare total applied gravity load in the analytical model against a hand estimate of dead plus live load for the tributary area, including the screen.
- Check that column reactions sum to the total applied load within a small tolerance; a mismatch signals a load that is not being transferred.
- Run a load-case-specific display so the screen load is visible on the cantilever, and confirm it reaches the supporting frame.
- Review the deflection under the corrected case and compare against the serviceability limit.
- Document the assumptions and the screen's load representation in the calculation record.
The judgment point is that a plausible deflection number is not proof of a correct load path. The team should reconcile totals and trace the load, not just accept the output.