arches

Silverton Mortar

I will check out dates as soon as I can. In the mean time, here is a closer look at the two maortars used.

And, of course, dates are pretty easy. The Landmark Trust have a page about the stables.

Flat arches and corrosion

The stables at Silverton are magnificent. They were in full view of the house and were designed to be imposing. The whole was intended to be Stucco but it never was. The portico faced the main house and has no entrance behind it. The pillars are surmounted by "safe" lintels of flagstone. Brick arches carry the main load. There is a band of "cement" mortar through the arches and about 3 courses above them.

Wrought iron cramps tie the lintels end to end. These have corroded and lifted the whole pediment about 2mm so the whole structure is supported on about 10000mm2 of iron at each pier.

Can the corrosion be arrested by cathodic protection?

Truth

Truth has been much in mind recently. A night out at Pride and Prejudice was one thread "It is a truth universally acknowledged". But truth in engineering is something that has exercised my mind for someyears without really coming to a head.

Last night I watched Andrew Graham-Dixon on The Secret of Drawing. He spoke of truth in a way that jelled some thoughts.

In court we are asked to speak "The truth, the whole truth and nothing but the truth". In engineering, such a demand would create a real problem because we can so rarely know the whole truth. What is vital, though is that we use nothing but the truth, or at least, where we have doubts, we acknowledge and respect them.

This reflects on the current debate about changes from UK to Eurocodes. there can be little doubt that many code changes are driven by developing ideas. If the established "truth" becomes known to be false it must be rejected whatever the cost, unless it is also known to be safe.

For some years now, I have been concerned about the "truth" of the models used in arch assessment. Not in the underlying statics of the 2D models thenselves, but in the way 3D behaviour is re-structured into a digestible 2D problem. There is an argument that we cannot set aside the existing flawed rules until we have something to replace them. Arch builders, as late as the mid 19th century, relied on rules of thumb. There is some evidence that those rules of thumb were more reliable than the calculations we do now. Is it not better even to make a judgement, knowing that you are doing so, than to make a calculation that is spurious.

I suspect I wil come back to this (I have to prepare a paper for IStructE for 16th Feb) but in the mean time see:

A Three Dimensional Model for Masonry Arch Bridge Behaviour

B. Harvey, A. Tomor, F. Smith,  UK

Moiré Tell Tales

Tell tales are used (typically) for measuring movement of cracks in buildings. In the old days, a glass patch was mortared over the crack and it broke if the crack moved. Then Roger Johnson developed the Avon Gard Tell Tale which allows the recording of movement but requires close observation especially for detailed measurements. The gauge shown on the linked picture allows measurement to 0.1mm but will not show direction at that high resolution.

Moiré tell tales use a special pattern to show both the scale and direction of movement to a very high resolution. They can be read by taking a photograph from many metres away and comparing the result with a test sample. They will even show dynamic movement if filmed through a video camera.

Bill now has a GB Patent (GB 2 386 189) on these gauges and should have them available for sale by the end of October 2005.

The Model here allows you to explore the effect of Moiré tell tales. (If your browser supports Java Applets. Drag the cursors to apply a specified displacement in mm.