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Existing brick wall needling and propping

Existing brick wall needling and propping

(OP)
We are looking at a contractor's method statement for needling an existing 327mm brick wall and he is proposing 203UC needles at 1m cc. The wall is 9.6m tall above the opening to be needled. That means that the stress in the bricks from the selfweight of the wall alone is 0.22N/mm2. For historic london brick construction the CIRIA guide allows a bearing stress of 0.42N/mm2. The wall is also carrying some load from floors so we end up with a brick stress of around 0.30N/mm2. So if the wall is to be needled, the needles would need to occupy more than half the area of the wall which seems a bit much. Even if we assume we can increase the bearing stress by the local factor of 1.5 to get 0.63N/mm2 allowable we would still need 203UC needles at 400mm centres which is way more than allowed for. I asked one of the needle suppliers and they said they don't normally bother checking. They just put needles at 1m centres and it works! What do temp works engineers normally allow for masonry bearing stress on needles?

RE: Existing brick wall needling and propping

I do these fairly often and like to be < 150psi but do go up to 200psi (in the US) - that's about 1.3n/mm2 so roughly double your upper limit. I have an admittedly crude approach of making a judgement call based on visual observations of the brick masonry and typically will have the area above and around the needles repaired prior to the needling.

RE: Existing brick wall needling and propping

How wide is the opening? How far above the opening is the first supported floor?

RE: Existing brick wall needling and propping

What I think jayrod12 is getting at is that smaller heights of brick above wider openings are much more susceptible to problems. Narrow openings in tall walls are much easier to do because the arching action of the brick carries much of the load. The needling is only loaded as the masonry above fails.

RE: Existing brick wall needling and propping

You got it OBG. But I realize now, that Bookowski answered a 5 month old question. So I would assume the OP has already figured out a solution.

RE: Existing brick wall needling and propping

(OP)
Thanks for the later input. It's an ongoing issue so it's good to hear what others do. I like the idea of looking at the brick and locally repairing if necessary and using that as a basis for a higher capacity. We do quite a few of these and usually there are existing window and door openings in them so there isn't always an opportunity for a lot of arching in the brick. They are usually fairly wide openings 4-5m with a floor supported just above the opening although the floors tend to be relatively ligthweight compared to the selfweight of the brick.

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