EN 1991-1-4. Probability Factor (Cprob), 1 Year Return Period

[calc_80]

Dear comunity,

I have to ballast plate laying on a flat roof. The plate has to stay there for less than 6 months.
Unfortunately, with 2Years of probability period I get too much ballast required.
Have you calculated that with 1 year probability. How you calculate it actually Cprob for 1 Year probability? Eurocode starts from 2 Years, and the given formula there doesn`t work with 1 year.
I have seen only into the Spanish DBSE-AE norm for wind: 1 year probability --> Cprob=0.41. But how I can reach that value 0.41?

Thank you for your help!

 

[HTURKAK]

I looked this thread a few times and waited to see the responds of others .. It is not clear for me 1 Year probability..Can you show Where is the statement ( Eurocode starts from 2 Years)?..AFAIK , Eurocode uses characteristic wind velocity values having annual probabilities of exceedence of 0,02, which is equivalent to a mean return period of 50 years.

My points are;

-If you are looking for the probability that a wind velocity exceeds (r) times in (n ) years; Binom distribution will be used.( for example , characteristic n=50 year return period , annual risk p=0.02 and No exceedence in 50 years P=(1-0.02)^50=0.364 and AT LEAST ONE EXCEEDENCE P=1-0.364=0.636 .That is , the probability that 50 year return period wind velocity at least one time exceeded in 50 years is 63.6% )


- You are expected to look BS EN 1991-1-6 Table 3.1 Recommended return periods for the determination of the characteristic values of climatic actions and You will see that return period for service life less than one year is 10 years.
See note 2
NOTE 2 A minimum wind velocity during execution may be defined in the National Annex or for the individual project. The recommended basic value for durations of up to 3 months is 20 m/s in accordance with EN 1991-1-4.

- I do not know the consequences if the subject plate flies due to wind uplift. Suggest you to use your judgement.In past , i tried to follow precautionary principle for the design decisions ; When an activity , design decision has a risk to raise threats of harm to human life, health or the environment, i did not take the risk .

- I would like to remind Murphy's law : "Anything that can go wrong will go wrong."













I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[calc_80]

Thanks for your feedback.
I have another question now. Can I reduce the 20 m/s as well (the recommended basic value) with the 2 Years probability period?

Thank you!

 

[BAretired]

How much ballast is too much? And who decided that? Six months is too short a return period to be used in determining safety requirements. Plans may change.

 

[HTURKAK]

- What is the 2 Years probability period? EN1991-1-4 uses the n=50 year return period for characteristic wind velocity ,with an annual risk p=0.02

- If the duration of service less than 3 months , You are expected to use 5 YEAR PERIOD WIND and If the duration of service less than 1 year but more than 3 months You are expected to use 10 YEAR PERIOD WIND...

I have attached the relevant table below;

I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[calc_80]

NOTE 2, says that minimum wind velocity during installation is 20m/s (Is this min. speed also suject to reduction?).
If for example the duration of installation is less than 3 Months (but higher than 3 days) - 5 years return period, to reduce those 20m/s with Cprob(5 years)?

Thank you very much, I appreciate your help!

 

[HTURKAK]

- NO!! Check your National Annex or from local Meteorology dept. for the Wind with 5 year return period and use the greater one ..





I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[calc_80]

Thank you, helped me a lot!!!