P

where rj

If (jmax = oy, the compressive yield stress for the material of the strut, the above equation when solved for a gives

This is the Perry-Robertson formula required. If the material is brittle, however, and failure is likely to occur in tension, then the sign between the two square-bracketed terms becomes positive and av is the tensile yield strength.

2.7. British Standard procedure (BS 449)

With a load factor N applied, the Perry-Robertson equation becomes

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