Risk Analysis and Management: Fragility versus Loads and Fire Explosion Safety Issues

1. A propane tank is located 100 m away from the nearest buildings.  The fragility to explosion can be expressed by

p = 0 for I < 0.7 MPa • s
p = aI for 0.7 < I < 1.3 MPa • s
p = 1 for I > 1.3 MPa • s

Accounting for various release and ignition scenarios, including uncertainties, the estimated probability distribution of impulses resulting from such explosions is given by the following set of values:

I (MPa • s) 0.1 0.3 0.5 0.7 0.9 1.2
pload 0.06 0.1 0.4 0.3 0.1 0.04

Supposing that you can improve the building so that you can affect the threshold of damage, increasing it above 0.7 MPa • s, and supposing the high end at 1.3 MPa • s remains the same, and so is the linear in-between behavior:

(a) Where would you place the lower threshold if you wanted the failure probability not  to exceed 5%?
(b) What would you do if your answer in (a) was the best you could do to improve the fragility, and you wanted a failure probability of less than 0.1% (moral certainty for no failures)?