Time's up for terraces asprogressive collapse looms
A large number of end-of-terrace Georgian and Victorian terrace flank walls are heading for collapse, according to Clive Richardson of engineer R T James and Partners. Terrace flank walls are progressively leaning over out of plumb, using up their margins of safety. Richardson calls this phenomenon the 'bookend effect', which has been observed in hundreds of terraces throughout Britain, and is the subject of recent detailed studies. 'The life-expectancy of terraces suffering from the bookend effect is poor,' he says.
Terraces of three to five storeys are typically affected. Where they have large ground-floor openings, as for shops and pubs, the effect can be particularly marked. Of course, there can be other causes of leaning walls, such as poor structural integrity, eccentric loading, wind, etc. Some terraces are not affected, saved by other, compensating structural effects - which helps explain why the bookend effect itself has not been spelled out before.
The bookend effect is primarily the result of daily and annual cycles of expansion and contraction in the front and back masonry walls. Broadly, expansion creates movement outwards from the centre of a terrace, most pronounced in the pushing outwards of the terrace flank walls. During the contraction phase of the cycle the front and rear walls have insufficient tensile strength to pull the crosswalls back into place. Materials slip against one another. Debris falls into gaps, thus inhibiting recovery to the original position. Thus in each expansion-contraction cycle there is a ratcheting effect outwards from the centre.
For a combination of reasons the ratcheting effect is greater with height, for example because the greater weather exposure with height increases movement; lesser dead loads higher up constrain movement less. So terrace flank walls tend to lean outwards, as do party walls to a lesser extent.
The annual ratcheting might be of the order of only 1mm, but over a lifetime these effects accumulate. 'Walls of greater than a third of their thickness out of plumb cannot usually be economically restrained by domestic- scale structural measures,' says Richardson.
Remedies are technically possible, notably by creating movement joints where party walls meet front and rear walls, or by inserting reinforcement into the masonry bed joints. However, both these are whole-terrace solutions which are only likely to be practicable where the terrace is in single ownership.
Palliatives to increase terrace life include strengthening spine walls to act as buttresses, tying the structure together, bracing with steel framing or even erecting a new building at the end of the terrace to create a new bookend. 'Without structural intervention,' Richardson says, 'distortion will accelerate as gravity acts more and more on out-of-plumb walls and old age reduces the robustness of the fabric. Eventually, the flank walls must collapse, followed later by each party wall in turn.'
Full story, page 42