Hi all,

I work for an Architects and I am building a spreadsheet to help determine the allowable percentage of unprotected / non-fire resisting area on a given elevation of a building. This is calculated based on the distance of said elevation to the boundary. The building regs provide the following data:

Distance to boundary (m) Max unprotected area (%)
1 8
2.5 20
5 40
7.5 60
10 80
12.5 100

In my spreadsheet I have column A (Measured distance to boundary) and column B (Allowable unprotected area). The above data represents a linear proportional relationship which I guess makes things a bit easier, but the building regs say that we can interpolate between the above figures. To calculate the allowable unprotected area (B) I can use the formula (A/12.5)x100 and this will interpolate between the above set figures. However, this formula fails when distance to boundary (A) is greater than 12.5 as it gives a percentage greater that 100%.

In reality, where the distance to boundary is greater than 12.5 I want the allowable unprotected area to remain at 100% - is this going to be possible?

Thanks in advance