Most structural refurbishment projects, no matter how small, alter the load path within the existing building. It is the Engineer’s responsibility to ensure that the new load path is appropriate and adequately strong to resist the forces carried. In transferring these forces from one path to another it is inevitable that parts of the structure, both old and new, will respond differently to the change in loading. New structural members may compress or flex and old members may recover part of their original length or deflection.
Such movements are small in comparison with the scale of a building but can cause minor settlements or distortions in the newly completed structure. Their existence and cause are well documented but their degree and extent are difficult to predict with any accuracy.
Much research has been carried out by Boscardin, Cording, Burland and others into such movements and good advice is available from such publications as CIRA 580 where a scale of ‘damage’ is offered for guidance. Table 4 of CIRIA 580 is reproduced below.
Category of Damage |
Normal Degree of Severity |
Description of Typical Damage
|
1 |
Very slight |
Fine cracks which are easily treated during normal decoration. Damage generally restricted to internal wall finishes. Close inspection may reveal some cracks in external brickworks or masonry. Typical crack widths up to 1mm. |
2 |
Slight |
Cracks easily filled. Re-decoration probably required. Recurrent cracks can be masked by suitable linings. Cracks may be visible externally and some re-pointing may be required to ensure weather tightness. Doors and windows may stick slightly. Typical crack width up to 5mm. |
The actual degree of damage will depend on several factors including:-
-
The structural condition of the buildings.
-
The existing foundations and the quality of the supporting soil.
-
The quality of workmanship employed in the works.