Probability factor on BS 6399 - 2 ANNEX D

[Gavis]

Hi all, I'm new in the forum so forgive me if I make a mistake.
I work as a structural engineer in the UK and I am struggling with the BS 6399-2 ANNEX D.
In this one the standard values of Q are shown (where Q is the annual probability required). For example, for a return period of 50years Q + 0.02, and so on...
My problem is the first value of Q. For the first value of Q there is a note: "The annual mode, corresponding to the most likely annual maximum value".
Could someone help me or explain me where the value Q = 0.632 comes from?

 

[patswfc]

Gavis, why are you using BS rather than Eurocodes?
Also why are you trying to adjust the Sp probability factor?

I dont know where the different values of Q come from, but with a Q value of 0.632 (are you thinking Q should = 1 for a 1 year return period?)then Sp=0.749.
This represents the reduction in wind speed if you are designing for a 1 year design life rather than the standard 50 years. As you have to factor the wind
speed to get wind pressures, the Sp=0.749 equates to a 1 year wind pressure of 56% of that for the 50 year wind pressure.
Sorry that I cant give any info on how the different Q values have been derived.