Modelling the Real World Effects of Roof Cladding on a Portal Frame

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Under Construction

When you introduce Roof Tie members in your model, you are simulating the action of the secondary roof system, which, while not as stiff, does help reflect real-world force paths.

In the Bracing tab, under Main bracing, set T (Tie) = Y for the roof bracing line so a proper tie member is present at the south end. This closes the roof truss, giving a load path to the braced bay and removing the mechanism.

The introduction of the transfer elements can effect the behaviour and cause a longitudinal 'thrust' to the building at eaves level, causing high axial forces within the eaves ties aka 'ring stresses', due to the stiffness of the roof in that direction with the transfer ties.

In situations like this, some users create upper and lower bound models (with and without transfer elements) to assess sensitivity, or alternatively, make design assumptions or consult the cladding manufacturer.

Stiff Decks (MasterFrame Pro)

In MasterFrame you have the ability to model a stiff deck - however, this approach also has limitations. The SCI acknowledges that roof cladding can significantly stiffen a frame, but generally advises against relying on it for the primary stability of standard portal frames.

Significant Stiffening: SCI P399 and SCI P397 state that portal frames clad in steel sheeting can deflect significantly less than the bare frame (reductions of horizontal deflection by over 50% are typical) due to the sheeting acting as a stressed skin diaphragm.
Design Recommendation: Despite this, the SCI recommends that the absolute deflection limits should be compared with the calculated deflection of the bare steel frame (taking account of base fixity).
Reasons for Caution (SCI P346): Stressed skin action is "rarely adopted in practice" for several reasons:
- Timing: The structural frame is often designed before the specific cladding system is selected.
- Fixings: Stressed skin action requires specific fixing details which must be adhered to on site. Many systems which rely on clips/friction, cannot be used for stressed skin action.
- Disruption: Rooflights and openings severely disrupt the diaphragm action.
Risk: If you rely on the cladding to stabilise the frame (i.e., to stop it falling down or deflecting excessively), you must ensure the cladding is specified as a structural element. Future removal of the cladding (e.g., for replacement) could render the building unstable.