Rafter bracing in portal frames

[lorden]

Hi,
I am looking into the rafter bracing of a portal frame structure designed to BS5950. The structure spans 16m and has 4 x 4m bays, so is 16m square in plan. Eaves height and ridge height are 3.6m and 5.0m respectively. The three inner frames are portal frames but the two end bays are designed as braced gable frames. The two outer bays along each side are also braced. Cladding is profiled sheeting (supported on timber purlins and side rails). Horizontal wind loads at the top of the 3 x wind posts in the gable ends are transferred to the sides of the building using conventional rafter bracing.
My question is this: Under maximum vertical dead+imposed loads, the horizontal deflection at the eaves of the portal frames will be significantly greater than the horizontal deflection in the braced gable frame, which will be negligible. Using say H/200 allows 18mm of eaves deflection and consequently, any rafter bracing that is provided in the end bay will be subjected to axial forces substantially greater than those from the wind loads which it is designed to resist. How is this problem normally dealt with?
Many thanks.

 

[hokie66]

All this means is that the structure won't act exactly the way you intend it to in your design. Structures almost alway have a mind of their own. As long as the load paths are continuous, it is not a problem for some elements to take more of their share than intended, because when the overloaded element yields, the other load path is activated.

In your case, the first two internal portals will receive support from the end bay bracing, so will not deflect as much as the design says. Not a problem.

 

[csd72]

lorden,

This problem is normally ignored as it does not usually cause issues. I agree with hokie.

 

[lorden]

OK guys. Thanks for the reassurance. I can proceed with a bit more confidence now. Maybe I'll crank up the bolted connections at the ends of the rafter bracing though. Thanks again.

 

[csd72]

Have you considered stability under a fire situations? Not knowing your particular situation but that may /may not be required.

 

[valleyboy]

This is discussed in SCI P252.

The actual deflection of clad frames is much less than the deflection of the 'bare frame' in your analysis model.

Agree with the above that this may be safely neglected.

DW