Timber Rafters on a Flat roof

[Contraflexure74]

Hi,

I have a series steel portal frames with a timber joist infill. The joists are built into the rafter webs. There is a layer of 18mm plywood screw fixed to the top of the timber joists to give me diaphragm action. There is a light weight metdeck roof above this and finished with a standing aluminium seam roof clad. The roof is very light weight.

The roof is practically flat at 2 degree pitch. After doing some calculations on BS 6399 Part 2 I don't appear to have any pressure wind forces on the roof, it's all suction. Does this seem correct?

I'm trying to design the timber rafters and I was just wondering could I adopt a permissible deflection limit of 0.004 or should I limit deflection to 0.003 times the span as 2 of my spans are 5m and 6m respectively and don't want to call up ridiculous timber sizes? I was hoping to call up 44x225 (grade C16) everywhere but I might be struggling to get this to work.

Any advice welcome.

John.

 

[jayrod12]

Uplift only on a flat roof sounds right.

Depending on the finishes you can get away with .004*span (L/240) for live load deflection and .0055*span (L/180) for total. If it's brittle finishes then I would be designing to .003*span (L/360) for live and .004*span (L/240) for total.

 

[Contraflexure74]

Thanks Layrod12.

John.

 

[Contraflexure74]

Sorry again jayrod12.

I'm getting 2.4kN/m2 of wind uplift in a localised corner of my flat roof. This seems extremely high, could this be possible, does this seem correct to you?

John.

 

[jayrod12]

Feels about right. Not sure what code you're under but right at the corner that doesn't seem too out of the ordinary. It's high, but it's a small area.

 

[Contraflexure74]

Thanks Jayrod12.

John.