Wednesday, 24 July 2013

A Dome, a Man, a Watering Can

A Dome, a Man, a Watering Can

This post continues the theme of monuments. The Mandela Capture site monument is one of the most remarkable sculptures in our country, a fitting artistic tribute to a remarkable person. The JD Strijdom monument on the Strijdom Square in Pretoria, in stark contrast, was without doubt one of the most monstrous, unimaginative structures ever erected here or elsewhere. It was designed as an edifice to a former president of South Africa, Johannes Gerhardus (JD) Strijdom, an avid proponent of segregation and key architect of apartheid during the 1950s.

I was sentenced to serve 16 years of my youth living in Pretoria, and wholeheartedly despised this monumental waste of space in the city  - there could have rather been a wonderful acre of living space covered in soft green, luscious grass for stressed workers to meadowtate in under leafy Acacia trees during lunch hours, with clear trout-filled artificial streams bubbling gently past, fish eagles soaring and calling above the verdant square.

Instead, there was this…

JD Strijdom monument

… A gargantuan and disembodied bust of the man connected to a plinth by a mere pinprick of a neck but protected by a massive concrete overarching dome.

The Bust

Had I joined MK (Umkhonto weSizwe)  - the armed resistance movement of the ANC fighting apartheid – I would have asked to undertake just one mission: the complete destruction of this grotesque sculpture, dome and all, and its replacement by a grassy oasis. 

Toppling the head with a judiciously placed explosive charge would have been easy. Bringing down the massive concrete dome, not so.

Or so I thought…

At 4.40 am on 31 May 2001, exactly 40 years after South Africa was declared a Republic, there was a thunderous rumble, and the whole monument simply, and completely mysteriously, collapsed; the head disappeared as if into a sinkhole, disintegrating into several pieces.
The collapsed monument

The cause of the exquisitely-timed (perhaps divine?) demolition of the JD Strijdom monument was never indubitably established.  Engineers noted that building plans for the square were never submitted and [the collapse] could most likely be attributed to a lack of maintenance on the structure.  

But recently I’ve learned the real truth, the thrilling denouement to the mysterious saga, revealed exclusively to me by a trusted source – an expert in concrete engineering, Andrew Mackellar.

Andy Mackellar personally relates: 

“For aesthetic reasons the architect wanted the concrete shell [covering and protecting the bronze head] to be as thin as possible. In order to make this structurally possible the concrete was cast with carefully located ducts through the concrete after the concrete had hardened the steel cables in the ducts were tensioned placing a compressive force on the entire concrete shell. This is a technique known as posttensioning – and is a method widely used by civil engineers. Concrete performs well in compression and the taught steel cables although placing more load on the concrete actually results in a more stable structure.

Each of the post tensioning steel cables was anchored into the concrete of the dome, while the other end came out at one of the three supporting feet. The exact numbers and locations of the cables as well as the tension that must be placed on the cables are carefully chosen by the structural engineer. After the concrete is hardened the steel cables are then tensioned using a hydraulic jack, the cable is then held in place by means of a steel wedge. So at each of the feet of the dome all the emerging cables were tensioned, wedged and covered up to hide this messy arrangement from the public eye.

Around the feet of the dome were flower beds and fountains. 

Concrete is unexpectedly a porous material. And as the years past the water from the fountains and the water from vigorous watering of the flower beds by overzealous municipal parks officials slowly seeped into the concrete of each of the dome’s feet…

… Inevitably this water then met with the ends of the posttensioning cables, where they emerged from their protective ducts to be locked off by wedges. And this is where the corrosion set in; rust slowly eating its way through the steel tendons.

As the rust eats away at the steel its cross section is reduced. The remaining steel then has to carry the tensile force. As the rust eats deeper and deeper the area of steel gets less and less. As the force on the cable remains the same the stress in the remaining steel must increase. 

Because posttension cables are stretched almost to their limit the rust didn’t have to eat very far before one by one these cables began to snap.

And thus on the morning 31 May 2001 the dome could no longer support itself and the law of gravity was fulfilled.

For many years, in the very centre of Pretoria, under the noses of the security police, enemies of the state were performing the subversive act of watering the garden.”


Postcript: Sadly, no grassy meadow luxuriantly grows nor Fish Eagle majestically cries at the Strijdom Square today. Cement still reigns.

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