Building an Arch

[arthurk]

Can anyone explain some building terms, please?

An ancestor of mine was killed during the construction of Spa Mill in Ossett, WRY (there's a lot about the mill at https://www.ossett.net/ossett_mills.html). He was a joiner, so had presumably been involved with the centres (framework) over which the arch was built. Unfortunately when these were removed the arch collapsed; his death certificate, quoting the Coroner, indicates that the arch was made of brick.

The fullest report I have found is in the Huddersfield and Holmfirth Examiner of 3 June 1854, part of which is as follows:

An arch, six yards span and about forty-eight feet in length, was being built, of which the part built up had been backed up nearly level with the centre. There were three windows in the arch on one side. A great portion of the upper end had been without centres for several days. On Tuesday afternoon the centres were lowered. In two or three minutes afterwards the whole of the arch suddenly fell in, and covered up the two men.... The stone walls from which the arch sprung had not given way at all.

The building isn't there now, but I imagine this would have been a large entrance passing through one of the mill buildings. 'Upper end' probably refers to the ground sloping down from the road. I'm not a builder or architect, so some of the terminology means little to me; 'centres' are explained on a number of websites, but what is meant by 'the part built up had been backed up nearly level with the centre'?

[Biggles50]

Timber formers are manufactured and fitted in situ.  They are used to support the brickwork as it is laid on the former and brick courses built up.

Once completed, the formers are carefully released so that the arch supports itself.

The last brick to be fitted to complete the arch is called the Keystone and it is in the centre of the first course of the arch.

If the brickwork is not laid correctly or the ends of the arch are not supported correctly the arch will collapse

[arthurk]

Thanks - I'd found that diagram too. Any thoughts about 'the part built up had been backed up nearly level with the centre'?

[Andy J2022]

For an arch to be stable there needs to be sufficient forces acting perpendicular to each brick or stone trying to force it inwards. If these forces are not equal the bricks forming the initial lining of the arch will tend to move in the direction where there is least force. Imagine holding a balloon and squeezing it. The air pressure inside will be increased and so the balloon will bulge out where your hands are not applying pressure.
To create the necessary inward force on an arch more material (bricks or infill) has to be placed all round the half circle. This is the backing up process. In the case of this arch since there appears to have been insufficient backing up over the crown (ie the centre), when the supporting wooden form was removed the tendency of the bricks at the side to push inwards due to the weight of the backing up material behind them was not balanced out by a corresponding force pressing down on the crown, so the crown will have been pushed upwards and the sides would have caved in. You can see this visually at around 2.15 in this Youtube video: https://www.youtube.com/watch?v=ld2NYntHBMg

[arthurk]

Thanks, Andy - that was what I wanted to know, and the video helped a lot.

There was an extra bit from the inquest, where the architect said "the work appeared to be well done, but that the heavy rains during the last few days had no doubt injured the arch". So maybe it had washed some of the infill away? Or weakened some of the mortar? You'd think they might have been aware of what it might do...?

[artifis]

In 1854 it is likely that a lime mortar or part lime part cement mortar was used.  This takes longer to set especially the lime only mortar leaving it more vulnerable to damage by heavy rain than all cement mortar.

If the top of the newly built arch wasn't protected from the heavy rain then it's probable that some of the mortar in the joints across the top of the arch was washed away or sufficiently so as to jeopardise the load bearing capability of the arch.

It is also possible that the centering was struck before the mortar had achieved sufficient strength to create a stable construction.  The incomparable Fred Dibnah accidentally demonstrated this in one of his programmes where he'd demonstrated the building of a brick arch then removed the centering and sat on the top of the arch - for it to collapse to his laughter.

[arthurk]

Thanks - I know a lot more about this now than I did a couple of days ago! (But in view of what happened to my ancestor, I'm not sure I approve of the laughter...)

[artifis]

(But in view of what happened to my ancestor, I'm not sure I approve of the laughter...)

Definitely not, Fred was laughing at himself I think in embarrassment.

Building always was a very dangerous occupation, building the castles involved spiral ramps upwards supported by cantilever timbers set in holes in the stonework below, no handrails not that that would have done any good if a large block of stone being dragged up broke free. It still is a dangerous occupation especially if the site management don't follow the rules and cut corners on safety and working practices. You only have to look at the records for constructions years ago to see how many man and boys were killed/injured, around the time of your ancestor's aqccident.