Cracking Effects on full masonry structures
Brickwork will resist cracking where it can. It will attempt to span areas that lose support because of subsided foundations or raised points. It is therefore usual to see cracking at weak points, such as openings for windows and doors.
In the event of construction settlement, cracking will usually remain unchanged after the process of settlement has eased.
With local or shear or erosion, cracking will usually continue to develop until the original cause has been remedied, or until the subsidence has completely neutralized the affected portion of footing and the structure has stabilized or other footings that remain affected.
In the case of shrink/swell effects, the brickwork in some cases return to its original position after completion of a cycle, however, it is more likely that the rotational effect will not be exactly reversed, and it is also usual that brickwork will settle in its new position and will resist the forces trying to return it to its original position. This means that in a case where swelling takes place after construction cracking occurs, the cracking is likely to at least partly to remain after the shrinking segment of the cycle is complete. Thus, each time the cycle is repeated, the likelihood is that the cracking will become wider until the sections of brickwork become virtually independent.
With repeated cycles, once the cracking is established if there is no other complication, it is normal for the incidence of cracking to stabilize, as the building has the articulation it needs to cope with the problem. This is by no means always the case, however and monitoring of cracks in walls and floors should always be treated seriously.
The upheaval caused by the growth of tree roots under footings is not simple vertical shear stress. There is a tendency for the root to also exert lateral forces that attempt to separate sections of brickwork after initial cracking has occurred.
The normal structural arrangement is that the inner leaf of brickwork in the external walls and at least some of the internal walls (depending on the roof type) comprise the load-bearing structure on which any upper floors, ceilings and the roof are supported. In these cases, it is internally visible cracking that should be the main focus of attention, however, there are a few examples of dwellings whose external leaf of masonry plays some supporting role, so this should be checked if there is any doubt, In any case, externally visible cracking is important as a guide to stresses on the structure generally, and it should also be remembered that the external walls must be capable of supporting themselves.
Effects on framed structures
Timber or steel framed buildings are less likely to exhibit cracking due to swell/shrink than masonry buildings because of their flexibility. Also, the doming/dishing effects tend to be lower because of the lower weight of walls. The main risks of framed buildings are encountered because of the isolated pier footings used under walls. Where erosion or saturation causes a footing to fall away, this can double the span that a wall can bridge. The additional stress can create cracking in wall linings, particularly where there is a weak point in the
structure caused by a door or window openings. It is, however, unlikely that the framed structure will be so stressed as to suffer serious damage without first exhibiting some or all the above symptoms for a considerable period. The same warning period should apply in the case of upheaval. It should be noted, however, that where framed buildings are supported by strip footings there is only one leaf of brickwork and therefore the externally visible walls are the supporting structure for the building. In this case, the subfloor masonry walls can be expected to behave as full brickwork walls.
Effects on brick veneer structures
Because the load-bearing structure of brick veneer building s the frame that makes up the interior leaf of the external walls plus perhaps the internal walls, depending on the type of roof, the building can be expected to behave as a framed structure, except that the external masonry will behave in a similar way to the external leaf of a full masonry structure.
Water service and drainage
Where a water service pipe, a sewer or stormwater drainage pipe is in the vicinity of a building, a water leak can cause erosion, swelling or saturation of susceptible soil. Even a minuscule leak can be enough to saturate a clay foundation. A leaking tap near a building can have the same effect. In addition, trenches containing pipes can become watercourses even though backfilled, particularly where broken rubble is used as fill. Water that runs along these trenches can be responsible for serious erosion. Interstate seepage into subfloor rea and saturation.
Pipe leakage and trench water flow also encourage tree and shrub roots to the source of water, complicating and exacerbating the problem.
Poor roof plumbing can result in large volumes of rainwater being concentrated in a small area of soil:
· Incorrect falls in roof guttering may result in overflows, as may gutters blocked with leaves etc.
· Corroded gutters or downpipes can spill water to the ground.
· Downpipes not positively connected to a proper stormwater collection system will direct a concentration of water to the soil that is directly adjacent to footings, sometimes causing large-scale problems such as erosion, saturation, and migration of water under the building.
Contact the Victorian building authority for more information.