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Not so strong
Your interesting description of the timber gridshells to be built at the Doncaster Earth Centre and the Weald & Downland Museum (NCE 28 March), emphasised the desirability of using oak strakes to obtain sufficient member strength. When raised, the strakes at 45 to the axis approximate to an ellipse and I estimate that the grid 15m wide, 8m (average) height uses strakes 25m to 30m long.
Oak is a coarse timber, suitable for heavy building parts, with large knots and wild sloping grain: its radial cells form widespread weak planes, which fall apart even when drying in straight unstressed lengths, let alone (even slightly ) bent. This is why oak cleaves so well and any oak used as beams, of cross-section smaller than large scantlings, should be cleft to ensure reliable bending resistance. The smaller the cross- section, the more difficult to obtain 'reliable' oak by sawing.
The strakes could be a tough softwood such as larch of Douglas Fir, quick drying and easily laminated from 50mm by 10mm strips, to obtain long lengths of the desired thickness. Any industrial glue-laminating plan will do this.
The topic of geogrids leads me to comment on Dave Parker's final 'throw away' paragraph of his article on testing buildings in the No 2 Airship Shed at Cardington, that 'structural design is still stuck in the airship age'. The adjacent Shed No 1 was built to house the very successful airship, R100 designed by my father; its 275m long by 50m maximum diameter frame was comprised of aluminium alloy triangular section lattice trusses (all of the same section) joined by cast aluminium nodes. My father's method of restraining all movement of the gas bags with geodesic netting, gave him his inspiration of the (rigid joint) geodetic framework for the Wellington Bomber fuselage, 62 years ago. Geodesic structures are now being re-invented. I rest my case.
The Airship Heritage Trust intend to use Shed No 1 as a National Museum of Lighter than Air. Those interested should write to group captain Peter Garth, the executive secretary of the AHT, 5 Orchard Lane, Brampton, Huntington. PE18 8TF.
C L Wallis (M), 2 Fern Cottages, Little Marlow, Bucks, SL7 3SE
Your lead article by Matthew Jones on 2 April 'Sulphate attack hits MS bridges' was a useful summary of the situation. It mentioned that the consultant working for the Highways Agency were 'still puzzled' by the exact combination of conditions which led to sulphate attack. Halcrow has been encouraged by the support received from the Highways Agency in seeking to piece together the information obtained from the M5 bridges to give us the full picture.
It may be of interest to your readers to know that all concrete deterioration found by Halcrow to date has been to concrete in contact with Lias clay backfill. We have not yet found deterioration to concrete cast against undisturbed Lias clay.
It is hypothesised that this may be due to chemical changes in the clay as a result of exposure to the atmosphere and reworking as well as increased water penetration through the backfill.
Investigations are of course at an early stage, but if this proves to be a significant factor it would considerably reduce the potential scale of the problem. Clearly if deterioration is occurring to concrete piles, the underside of bases or other buried structures this will be much more difficult to detect and remedy. So far exposures of piles and cores taken through foundation bases have not revealed evidence of sulphate attack.
David Slater (M) and Geoff Hill (F), Sir William Halcrow & Partners, Llanthony Warehouse, The Docks, Gloucester GL1 2NS.
I commend both Dr Howsam's letter in NCE 19 March and the recent articles, on biocorrosion, on which he commented. Both, and subsequent letters, will help to address the 'amazing lack of awareness and understanding' to which he refers. The essential place for the knowledge is in BS 6349 part 1, Maritime Structures. If engineers designing with the standard are not alerted fully to the problem, significantly shorter structural life than expected will continue to be the result for quay owners. This is the serious matter that needs addressing. While NCE will have a wider general coverage, the structures will be designed and built with reference to BS 6349.
The standard is being revised for publication later this year. We need to make sure that validated and up to date information is available. Therefore, I have already met with various interested engineers (including representatives of British Steel who, with others, have been actively researching the subject for some years) to discuss amendments. It now appears that recent letters show some conflicting views, so I am proposing a meeting in London shortly (venue to be decided) to bring together any informed parties as a review committee, in the hopes of reaching a consensus. To this end I would welcome further contact names, amendment proposals, and confirmed reports of presence or absence of ALWC anywhere in the world. This could be the start of a major database to update the BSC survey of some years ago.
Tom Shelley (G), civil engineering manager, Port of Felixstowe, Suffolk IP11 8SY.