I'm quite confused bout lognormal distribution and normal distribution(adsbygoogle = window.adsbygoogle || []).push({});

Consider a three-span continuous beam. All supports are pinned supports. Each span has a length of l. The beam

has a modulus of elasticity E and a moment of inertia I. All spans are subjected to a uniformly distributed load W.

The maximum deflection of the beam occurs in the outer spans and is equal to

0.0069Wl^4 / (EI)

(a) Your job is to evaluate the probability pF that the deflection will exceed the code-specified limit of l/360

assuming that W, E, and I are statistically independent lognormal random variables and given the following

information:

l = 5 m (deterministic)

W has a mean value of 10 kN/m and a coefficient of variation of 0.4.

E has a mean value of kN/m2 and a coefficient of variation of 0.25.

I has a mean value of m4 and a standard deviation of m4.

I have the solution for it but i'm lost on when

Variables W E I are all lognormal distribution.

Now to solved for the mean you would do

d = 0.0069Wl^4 / (EI)

Now since you can't divide or multiply lognormal distributions you would have to ln the entire formual of d to be:

ln d = ln .0069 +lnW + 4ln L - ln E - ln I

since you're takin the natural log of a lognormal distribution it becomes a normal distribution ... Why is this ??? i'm confused on this part .

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Lognormal Distribution

Can you offer guidance or do you also need help?

**Physics Forums | Science Articles, Homework Help, Discussion**