What an indictment of American concrete (and steel)technology! Now I will admit that, as a consulting engineer in the UK in the mid 1960s, I did have some huge steel columns for a major steelworks in Scotland fabricated in an Irish shipyard. But that was for a major project which had to be completed within 12months of starting the design work, and I do not see a similar degree of urgency at the WTC. Perhaps some of the explanation lies in another article in your same issue, again referring to results of 9.000psi for a 12,000psi grade and, in the current article, 17,000psi results for a 14,000psi grade on this project. Such results do not suggest reliable, closely controlled concrete, so maybe they were scared of concrete columns. Many years ago in Australia we were using steel pipe columns pumped full of 100MPa (14,500psi) concrete - simple, cheap, local, - why not? Certainly they need fire protection to the essentially external steel, but so do the columns reported here. Of course to day we would happily use 150MPa concrete. At Petronas Towers, then the world's tallest,insitu concrete columns were used and concrete with a mean strength of 100MPa had a standard deviation of 3MPa(450psi). Come on America! IMPORTED steel columns! You ought to be ashamed of yourselves!

Posted By : Ken Day Posted On : 8/13/2009 6:05:56 PM

OK so the reason they used the higher strength concrete than needed is pretty obvious, if you re-read the article it tells you they cut cement demand by 2/3's so the higher strength concrete should be the same cost or less than a typical mix. So why not increase the strength? with a strong enough base it may allow them to add a few extra levels, for either more office space or maintenance floors or offer more flexibility in regards to floor loadings.

Posted By : Bede Meredith Posted On : 8/16/2009 12:44:35 AM

Bede, I don't see the relevance of your comment. My remark referred to variability of concrete and not having the confidence to use high strength concrete columns.It is not practicable to change the design of a major building like this half-way through construction, although higher EARLY strength might permit faster construction. However the cement reduction was no doubt achieved by a high pozzolan (fly ash?)content giving LOWER early strengths. So we can all have our own opinions as to what is "pretty obvious" and to whom.

Posted By : Ken Day Posted On : 9/1/2009 7:31:05 PM