Deterioration and Structural Instability of Timber Stumps Supporting a Three-Split Level Dwelling

Background

A structural forensic engineering investigation was undertaken following concerns regarding movement and deterioration of the timber stump supports beneath a three-level residential dwelling.

The dwelling was constructed on a sloping site and supported on a series of timber stumps extending through the lower levels of the building. The stumps formed a significant part of the primary support system, transferring the building loads through the structure and into the supporting ground.

The property owner reported noticeable movement and deterioration to a number of the timber supports, including bowing, splitting and apparent decay, particularly toward the lower portions of the stumps.

Scope of Investigation

The investigation was undertaken to establish the apparent condition of the timber stump support system and to assess whether the observed deterioration had the potential to compromise the structural adequacy and stability of the dwelling.

The inspection included:

• visual inspection of accessible timber stumps and associated structural members;
• assessment of stump alignment and apparent deformation;
• examination of timber surfaces for splitting, cracking and deterioration;
• inspection of the lower portions of stumps for evidence of decay and moisture-related deterioration;
• assessment of connections between the stumps, bearers and associated framing;
• consideration of the load path through the supporting structure;
• identification of areas requiring further exposure or investigation; and
• assessment of the potential structural consequences of the observed deterioration.

Where portions of the stumps were concealed or inaccessible, the limitations associated with the inspection were taken into consideration.

Observed Conditions

A number of timber stumps exhibited significant deterioration.

The observed conditions included bowing and out-of-vertical alignment, longitudinal splitting of the timber, and localised deterioration and loss of section toward the lower portions of some stumps.

In several locations, the deterioration appeared to be concentrated near the base of the timber supports. This is particularly significant because deterioration at the base of a stump can progressively reduce its effective cross-sectional area and, consequently, its capacity to resist the vertical and lateral loads imposed by the building.

Some stumps also exhibited evidence consistent with prolonged exposure to moisture and environmental conditions conducive to timber decay.

The extent of deterioration varied between individual stumps. Accordingly, the condition of one stump could not necessarily be taken as representative of all timber supports throughout the dwelling.

Structural Significance

The timber stumps form part of the primary structural support system of the dwelling. Consequently, deterioration of the stumps is not merely a cosmetic or maintenance issue.

A timber stump subjected primarily to vertical compression must maintain sufficient cross-sectional area and structural integrity to safely transfer the imposed building loads to the supporting ground.

Where decay or splitting has reduced the effective section of the stump, the resulting reduction in capacity may become significant. Bowing or loss of alignment may further increase the structural demand by introducing eccentricity into the load path and potentially increasing bending stresses in a member originally intended to function predominantly as a compression support.

The situation is particularly important for a three-split-level dwelling, as the lower-level supports may carry the accumulated loads from the levels above. Consequently, deterioration to a lower-level stump can have a substantially greater structural consequence than deterioration to an isolated support carrying only a portion of a single-storey structure.

Probable Mechanism of Deterioration

Based on the observed conditions, the deterioration appeared to be associated with long-term environmental exposure and moisture ingress affecting the timber supports.

The lower portions of timber stumps are particularly vulnerable to deterioration where they are exposed to persistent moisture, poor drainage, inadequate clearance from the ground, or other conditions that prevent the timber from remaining sufficiently dry.

Progressive deterioration can result in:

1. moisture penetration into the timber;
2. degradation of the timber fibres;
3. loss of effective cross-sectional area;
4. longitudinal splitting and localised cracking;
5. reduction in compression and bending capacity;
6. progressive bowing or deformation under sustained loading; and
7. redistribution of loads to adjacent supports.

Once load redistribution occurs, the remaining stumps may be subjected to loads greater than those originally intended, potentially accelerating the deterioration or deformation of other members.

Forensic Assessment

The observed defects should therefore be considered as a structural deterioration issue rather than isolated timber defects.

Of particular concern is the combination of:

• deterioration at the base of the stumps;
• longitudinal splitting;
• bowing and loss of vertical alignment;
• potentially reduced effective cross-sectional area; and
• the cumulative building loads associated with the three-level dwelling.

The presence of one or more of these conditions does not, in isolation, establish that structural failure has occurred. However, where significant deterioration is present in primary load-bearing supports, the condition warrants detailed engineering assessment.

The investigation also considered whether movement of the dwelling has occurred as a consequence of differential settlement or deformation of individual stumps. Evidence such as changes in floor levels, distorted door and window openings, cracking to wall finishes, separation at joints and changes in the alignment of bearers and joists may provide useful evidence when reconstructing the progression of the structural movement.

Recommended Remedial Approach

The appropriate rectification methodology should be determined following a detailed assessment of the condition and capacity of the existing support system.

Depending on the extent of deterioration, remedial works may include:

• temporary propping and stabilisation;
• replacement of individual deteriorated stumps;
• installation of additional structural supports;
• local strengthening of affected bearers and connections;
• correction of drainage or moisture-related conditions;
• removal of sources of persistent moisture exposure;
• adjustment or reinstatement of affected structural framing; and
• monitoring where the available evidence indicates that immediate structural intervention is not required.

Where stump replacement is proposed, the replacement support should not simply be installed adjacent to the existing stump without consideration of the existing load path. The methodology should address how the existing building loads are safely transferred to the temporary and permanent support systems during the works.

Conclusion

The investigation identified deterioration affecting timber stumps forming part of the primary support system of a three-level dwelling.

The combination of bowing, splitting and decay, particularly toward the base of the timber supports, has the potential to reduce the structural capacity and serviceability of the support system.

The significance of the deterioration is increased by the three-level configuration of the dwelling, as lower-level supports may be required to transfer the cumulative loads from the structure above.

The observed conditions are considered consistent with lack of maintenance, unauthorised modifications to suspended floor structure, progressive deterioration of timber supports associated with prolonged exposure to moisture and environmental conditions conducive to decay. However, the precise extent of structural capacity loss should be established on a stump-by-stump basis, taking into account the remaining timber section, degree of decay, member geometry, loading, connections, ground support and the overall structural load path.

Where significant deterioration is confirmed, appropriate temporary stabilisation and engineered rectification should be undertaken to ensure that the dwelling remains adequately supported during and following the remedial works.