Safety factor wooden roof uplift.

[serrojo]

Hey Guys!

I am reviewing a wooden structure, offered by a third party for my project. As wind on slopped roofs produces pressure(windward) and uplift(leeward), I found that the wind uplift is comparable to the self weight of the elements (uplift/self weight=96% approx), so I am worried about the roof stability and I don´t wanna give a wrong feedback, because of my ignorance.

So my question is: Is it OK? If not, what should be the safety factor between the self weight and the uplift effect, and which normative could be referred.

All opinions are welcome.

MSc. Eng. Serguei Joa
Structural Engineer
Bouygues Batiment International, Cuba.

 

[atrizzy]

Are you using a wind load factor (increase) and a dead load factor (decrease) in your load combination?
I'm not sure where you are working, but many modern codes utilize ultimate limit states design. If you're using ULS and your uplift is 96% of uplift resistance, you're good to go.

 

[jayrod12]

Around here, even if there is no net uplift, we still provide some sort of nominal tie-down connector at each end of every truss. Even the smallest tie down gives almost 1000lbs of uplift resistance.

 

[P1ENG]

It is hard to answer for Cuba. In Florida, it is a requirement of insurers to have uplift clips (hurricane clips) if you want to pay a better rate. It seems Cuba might want something similar being an island in hurricane prone waters. As far as stability and safety factor, it depends on your approach. If you use the ASD load combination 0.6D+0.6W (ASCE 7-10) or 0.6D+W (ASCE 7-05), it has the safety factor built in: 0.9D/1.5 = 0.6D. You also have safety factors in your connectors and members per NDS. If the design allows for a mechanical connector (screw or lag so pullout isn't an issue; not a clip/bracket), I would allow that in my sealed drawings. But you can't go wrong with providing a stronger load path for the wind loads. How heavy (what type of construction) is your roof that your dead is almost equal to your wind uplift?

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[serrojo]

Thank you All!!

Finally I figured out that in that extreme case, when the wind uplift is equal to the selfweight the fastener connection has to be considered working in tension regime.

best regards,


MSc. Eng. Serguei Joa
Structural Engineer
Bouygues Batiment International, Cuba.