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Verulam

Padstones in blockwork In September 1981, Mr M. K. Hurst took up Mr B. A. Haseltine ’S statenlent that, on testing, a slender block will appear weaker than a stubbier block, but that in an actual wall it has the strength of the stubbier block. If this were so, Mr Hurst continued, surely there would be no need for Table 5 of CP 111 and the basic stresses of Table 3(b) could all be increased, with consequent savings in rnaterials? Now, Mr John Tanner writes: I note from letters in the September issue that, not withstanding Mr Thorley’s remark, designers, including Mr Hurst, do not understand the basis for the CP 111 ‘modification factor for shape of unit’. Mr Haseltine stated it correctly, of course, but not as clearly as necessary, as Mr Hurst’s comment shows. Table 3 stresses relate to crushing strengths proved by crushing samples similar in format to a brick. When the sample is of a more slender format, its equivalent strength is underestimated by the crushing test on the non-standard format and Table 5 should be thought of as a ‘correction factor’ to obtain the inherent standard format strength. It follows that, when the blocks are cut or laid on their side, the correction factor is just as applicable as when laid in the normal manner (hollow blocks on their side excluded, of course) since the inherent block strength is essentially unaffected by the attitude of the block. The choice of terms in the Code has made this issue as obscure as possible and I hope the above will englighten at least a few. Verulam