Design Workshop - Analysis of failed arch

Following the collapse of the arch we carefully sorted through the broken pieces of the arch to attempt to form a full critique of the problems we had encountered. The following is a synopsis of what we observed:

• Not enough overlap between upper and lower brick courses allowing potential for joints to occasionally pass right through the arch. The cause of this was the rapid work method we had used, where the lower course was frequently completely obscured by plaster. Where good overlap been achieved the arch was very strong and it was difficult to retrieve bricks without breaking them.
• Small air-pockets between both courses. Again this appears to have been a product of the frenetic work method employed.
• The casting plaster was occasionally crumbly and hadn't set to a full strength.

Based on the above we held a design workshop over Skype with Michael Ramage and Peter Brunner.

Michael was incredibly useful and outlined our problems as being primarily geometrical in origin. A Catenary arch would be a far more stable form for a universally loaded arch to take, whereas a taller circular arches need some buttressing to contain the lateral load without buckling. Based on the discussion it appears as if the route forwards is the following:

• Drop the arches slightly (another 100mm or so)
• Build the side piers to the arches at the same time as the arches. This will entail keeping the formwork in place until the end of the project.
• Build the arches by placing the brick slips one by one. Use casting plaster for the lower course. This is not strictly necessary for the construction but will give valuable practice for the dome works.
• Use portland cement for the second course to save casting plaster
• Use bed-joint reinforcement for the bricks to the lower section of the piers.
• The geometry of the dome as designed should be fine - in this he mentioned that a good rule of thumb is to keep the angle from the apex of the dome to the springing point to a maximum of 50degrees.
• As we dismantled the collapsed arch we noticed that occasionally the casting plaster hadn't set hard but rather was biscuity and slightly crumbly. Michael recommended wetting the bricks before laying as this will allow for a greater adhesion between the casting plaster and the bricks, without the bricks taking too much water from the mix.
• Again in dismantling the collapsed arch we were able to pinpoint weaknesses in the pattern of overlapping between the upper and lower levels. Rae prepared new patterns for how to do this to ensure complete cover through the arch. Michael commented that running the second course perpendicular to the first would be adequate for our arches.

We will modify our existing formwork and complete forms for the remaining arches. On Friday we will have a brickwork workshop with Dermot Fullam of Dermot Paul construction to allow us to work through the weekend.