Forty two concrete tubes packed side by side on the Cape Town waterfront, eighteen feet across and a hundred and eight feet tall.<\/p>\n
They held maize for most of a century. Then they held nothing.<\/p>\n
The walls are seven inches thick, which is thin for something that stands 187 feet.<\/p>\n
A tube that shape survives because it is round. Cut a hole in it and the geometry that was holding it up is gone.<\/p>\n
Somebody wanted a room<\/strong> in the middle of that.<\/p>\n The answer was not to take concrete out. It was to put more in<\/strong>.<\/p>\n A silo cell works in hoop tension. Grain pushes outward, the ring of concrete pulls against itself all the way around, and nothing bends.<\/p>\n The wall can stay thin because it never has to act as a beam. It only has to hold a circle.<\/p>\n Cut an opening and the circle stops being a circle. The load goes looking for another path and there is not much steel in there to find one.<\/p>\n That was the situation here. Old concrete, sparse reinforcement, and a proposal to remove the middle of the block.<\/p>\n So the sequence got inverted. Crews cast a new reinforced lining inside the tubes first, bonded to the old wall.<\/p>\n Seven inches became about sixteen inches<\/strong> of composite, and only after that did anybody start cutting anything<\/strong>.<\/p>\n The tubes are not rooms. They are shafts, closed at the bottom, with no floors and no windows.<\/p>\n Standing in one before the work you would see a circle of daylight a hundred feet overhead and dust on every surface.<\/p>\n The complex was finished in the mid 1920s and graded maize brought in from across the country.<\/p>\n It came out of service at the start of this century and then sat, too solid to fall down and too awkward to use.<\/p>\n For years the standing question was what anybody could possibly do with a block of sealed concrete pipes.<\/p>\n Every conventional answer ran into the same wall<\/strong>, which was that the interior had no usable volume<\/strong>.<\/p>\n The void in the middle was not drawn freehand. It was taken from the shape of a grain of corn and scaled up until it filled about 89 feet of height.<\/p>\n That surface was broken into thousands of coordinates and marked on the concrete physically, nail by nail, so the saw crews had a line to follow.<\/p>\n Diamond saws did the cutting, working down through walls that had been thickened for exactly this.<\/p>\n Roughly 160,000 cubic feet of concrete came out of the middle of the block.<\/p>\n The cut faces were then polished, so the exposed edges read as a smooth surface rather than a demolition scar.<\/p>\n The finished void is a negative shape<\/strong>, and everything around it is the original structure<\/strong>.<\/p>\n Demolition was the cheaper line on paper and it was never taken.<\/p>\n Part of that is heritage. The grain silo was the tallest thing<\/strong> in the city when it went up, built between 1921 and 1924 by indentured labor, and it is the landmark the waterfront grew around.<\/p>\n Part of it is arithmetic. Clearing 42 tubes of dense concrete on a working harbor front is not cheap<\/strong> either.<\/p>\n Reuse also kept the outline of the waterfront intact, which nothing built new on that footprint would have done.<\/p>\n The conversion came in around 37 million dollars and produced about 64,600 square feet of exhibition space across nine floors and roughly 80 rooms, opening in September of 2017.<\/p>\n The engineering write up of the composite sleeve approach is in the project record<\/a>, and it is the part that made the rest possible.<\/p>\n The construction sequence, including the sawing, is set out in a build account<\/a> that puts the added lining at about ten inches on its own.<\/p>\n Adding structure before subtracting it is not specific to silos.<\/p>\n It applies wherever an old element is strong in one direction only and somebody wants an opening through it.<\/p>\n The failure mode when that step is skipped is not subtle, as it was not when a backpropping failure<\/a> ran 105 feet across a slab under a fresh pour.<\/p>\n Concrete is also getting better at looking after itself, with embedded bacteria<\/a> sealing cracks that would otherwise need a crew.<\/p>\n None of that helps a structure already standing, which is why the sleeve trick still matters.<\/p>\n The lesson is dull and useful. Old concrete is not weak so much as single minded<\/strong>, and the work is teaching it a second job<\/strong>.<\/p>\n","protected":false},"excerpt":{"rendered":" Forty two concrete tubes packed side by side on the Cape Town waterfront, eighteen feet across and a hundred and eight feet tall. They held maize for most of a century. Then they held nothing. The walls are seven inches thick, which is thin for something that stands 187 feet. A tube that shape survives … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":2656,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[11],"tags":[],"class_list":["post-2606","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-construction"],"_links":{"self":[{"href":"https:\/\/www.ecoportal.net\/uk\/wp-json\/wp\/v2\/posts\/2606","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.ecoportal.net\/uk\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.ecoportal.net\/uk\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.ecoportal.net\/uk\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.ecoportal.net\/uk\/wp-json\/wp\/v2\/comments?post=2606"}],"version-history":[{"count":4,"href":"https:\/\/www.ecoportal.net\/uk\/wp-json\/wp\/v2\/posts\/2606\/revisions"}],"predecessor-version":[{"id":2657,"href":"https:\/\/www.ecoportal.net\/uk\/wp-json\/wp\/v2\/posts\/2606\/revisions\/2657"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.ecoportal.net\/uk\/wp-json\/wp\/v2\/media\/2656"}],"wp:attachment":[{"href":"https:\/\/www.ecoportal.net\/uk\/wp-json\/wp\/v2\/media?parent=2606"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ecoportal.net\/uk\/wp-json\/wp\/v2\/categories?post=2606"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ecoportal.net\/uk\/wp-json\/wp\/v2\/tags?post=2606"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}Why thin round tubes resist being opened<\/h2>\n
What the inside of the block was like<\/h2>\n
The cut itself and the numbers behind it<\/h2>\n
Why nobody just knocked it down<\/h2>\n
What the method is actually good for<\/h2>\n