Help determining the probability factor in wind load calculations 1

[StevenKatzeff]

Hi,

I have read most of the posts here relating to wind loading on structures, but have not read the specs from different countries-so please don't be upset if this is clear in other codes. In South Africa, the wind load code is called SANS 10161-3. For calculating the basic wind speed, the average value of 28 m/s is multiplied by a probability factor c_prob, where:

c_prob=((1-K xln(-(ln(1-p))))/(1-K x ln(-(ln(0.98)))))^0.5

Here,K is the shape parameter depending on the coefficient of variation of the extreme value distribution with a value of 0,2. p is the probability of annual exceedance for the 10 minute mean wind speed.

My question is: How do I determine K and p for a structure expected to last for 5 years?

Thanks in advance.

 

[hetgen]

Hi Steven,

p is annual probability of exceedance; for 5 years its 1/5 = 0.2. The distribution shape parameter K should be 0.2 as recommended.

Cprob for a structure expected to last for 5 years is therefore 0.855.

 

[StevenKatzeff]

Hi hetgen,

Thanks for the answer. So you are reading that sentence to mean that K has a value of 0.02, as opposed to what is the K value of variation of the extreme value distribution of 0.02. That sounds logical.

Is it common practice to factor down the wind load depending on the expected time it will stand, as opposed to 50 years? I read in another forum here on this subject that the conservative value of 0.02 should always be used.

Regards,

 

[hetgen]

Hi Steven,

Yes its common practice to reduces the wind force relative to the design life of the structure; the recommended value 0.02 is for 50 years probability of exceedance (1/50 = 0.02), but if one is designing temporary structures with short deign life p can be reduced. Similarity when checking serviceability requirement like deflection and occupant comfort due to wind induced acceleration of structure a 5-10 Year return period is normally considered.

K is a shape parameter for probability density function related to the wind data and if not specified by national annex the recommended value is 0.2.

HTH

 

[StevenKatzeff]

Thanks a mil hetgen.

While I have your ear, if my structure is a signboard with steel poles fixed into a bed of concrete (lampost), the moment due to the wind is reacted as a bearing load in the cement at the base. I know in aircraft structures, we would treat the base as a lug and check the bearing distribution at the top where it's value is the highest. Is there a similar check for concrete? A colleague suggested obtaining the bearing force on the one side and using 0.4 x F_{cy_concrete} x A_bearing as the allowable bearing force. What do you suggest?