Inclination for Flat Portal Frame Decking (Steel)

[XABI14]

Hi all struc eng

How one can design a really small inclination (in the decking to direct rainwater) WITHOUT inclining the beams/rafters.

Shall one design different purlin sizes? Are there any other suitable and economical solutions?

thanks in advance

 

[ishvaaag]

Normally you would support then the secondary beams supporting the roof's preformed sheets or panels in small legs of suitable height; the fastener system will have to deal then with the small angle between the roof sheets or panels and the secondary beams supporting them.

 

[csd72]

You can use elongated purlin cleats to form the slope. We have done this in industrial sheds of various sizes.

When the purlins are at or near the top of the rafter then the cleats can be plats as normal but as they get longer you will need up to 3 inch angles to allow for the slenderness and cantilver effects (from the out of plane restraint forces).

Your fly braces will also need to be larger than usual due to the greater slenderness.

 

[Teguci]

Undersize your footings on the downslope side (smile).

If you are insulating the roof, you can provide a slope in the rigid insulation.

 

[hokie66]

The approach outlined by csd72 is common with metal roofing. In small scale structures, I have seen graduated purlin depths used.