What is design corrosion allowance and how is it applied to steel structures?

• A. A factor subtracted from thickness to allow for potential cracking.
• B. An added thickness to account for future material loss due to corrosion; calculated from corrosion rate, exposure time, and criticality.
• C. A design margin based on wind load.
• D. A coating warranty period.

Answer: (B)

Design corrosion allowance is the extra thickness added to steel members to compensate for the expected loss of material from corrosion over the structure’s service life. It is not about margins for cracking, wind loads, or warranties; it specifically accounts for future thinning due to corrosion.

In practice, the allowance is calculated from the rate of corrosion and the exposure time (the planned service life before a major inspection or replacement), and it can be adjusted for how critical the member is and how often inspections will occur. The basic idea is simple: you determine how much thickness will be lost, then add that amount to the initial thickness so that, even after the planned life, the remaining thickness still meets the required minimum.

Example: if the required minimum thickness at a critical section is 5 mm after the service life, and the expected corrosion rate is 0.5 mm/year over 10 years, the design corrosion allowance would be 5 mm. You would start with an initial thickness of 10 mm so that after 10 years you’re at the required 5 mm minimum. The concept ensures structural integrity is maintained between inspections and over the expected life of the structure.