In a UK domestic terrace property, when a full width extension is constructed, the removed external wall is typically replaced by a moment resisting box frame, not only to support the wall above, but also to prevent racking and distortion in the party walls to the sides.
Calculating the load above is easy. But even ignoring the load above and assuming zero lateral wind pressure, there is still a need to retain the buttressing function from the original wall, so a moment resisting frame would still be needed if only from abundance of caution, to ensure the party walls are correctly braced.
But I can't find any reference to the assumptions and calculations typically used to calculate the profile of posts and beams needed in such a box frame. How's it done?
Example case
Imagine a scenario where 1st floor load can be ignored. Its a mid-terrace house in a long terrace, therefore wind pressure laterally can be ignored. So, unusually, the ground floor external cavity wall is not vertically load bearing. Instead, it merely provides a bracing function preventing lateral movement/distortion of the party walls inward.
When the ground floor cavity wall is fully removed, with no retained buttressing at the edges, a moment resisting frame is to be installed in its place. The frame's function in this slightly contrived example isn't to support the 1st floor load, but to brace the party walls against inward distortion.
(Even if the first floor load was to be supported, there should still be a standard for the minimum profile of the two posts, to ensure lateral distortion is controlled.)
Presumably there should be some standard for the calculation of reasonable lateral pressures to be resisted, based on some standard or practice guidance. This would allow a suitable 2nd (area) MoI to be calculated for the posts of the frame, as well as moments to be resisted in the event of racking, and therefore a suitable set of beams + connections for the box frame design.
Its that information I'm having difficulty locating.
Ideally as a bonus, a summary of the way its approached and/or relevant equations would also help, as raw UK and Eurocode documents are not the easiest things to digest.
Thanks
