Stuart Bradshaw (Letters, GE March 2006) drew attention to perceived conservatism in the design of working platforms for tracked plant. He rightly pointed out that the vertical stress on the subgrade is over-estimated if the contact stress on the surface of the mat is not distributed down through the mat to give a reduced contact pressure on the subgrade.
However, his assertion that the punch mechanism ignores any internal and bene. cial shear stresses in the mat is incorrect. The calculation of the resistance to punching shear considers the resistance provided by the mat and subgrade separately (see equations on page 19 of Appendix A1 of BRE 470).
It should not be forgotten that effectively the 'safety factor' adopted in BRE 470 ranges from 1.2 to 2.0, not the usual 2 to 3 seen in bearing capacity calculations.
This is because the load factors, which depend on the loading condition considered and whether or not a mat is present, are the only partial factors that are recommended to take a value other than 1.0.
It seems odd that eyebrows are now seemingly raised in piling contractors' design of. ces, given the Federation of Piling Specialists' initiative concerning piling mats and the close co-operation between BRE and FPS.
Having said that, it would be in the interests of the industry for the guidance to be reviewed. It is important that the whole of the guidance is looked at - it is not advisable to tinker with one part of the procedure, the calculation of bearing stresses, in isolation.
Pending a review, BRE, FPS and the main contractors who usually pay for the design and construction of piling mats, should be given credit for adopting this important safety initiative.
John Gannon, director, Byland Engineering