Property Inspection. 83A Ascot Avenue North New Brighton Christchurch STRUCTURAL REPORT

Transcription

1 Property Inspection 83A Ascot Avenue North New Brighton Christchurch STRUCTURAL REPORT March 2013

2 This document has been prepared for the benefit of Clint Marston. No liability is accepted by this company or any employee or sub-consultant of this company with respect to its use by any other person. Quality Assurance Statement and Revision History Revision Date Inspected By Prepared and Reviewed By Status Milton Hayward Project Manager Graeme Johnson Structural Lead CPEng MIPENZ Approved Viden Consulting Engineers Ltd PO Box Hornby, 8442 Christchurch New Zealand Tel: Web:

3 Contents Executive Summary Introduction Inspection Structural Form General Detailed Findings Summary... 4 Appendix A Site Location... 5 Appendix B Site Plan... 6 Appendix C Site Photos... 7

4 Executive Summary Viden Group Engineers have been engaged to conduct a structural inspection of 83A Ascot Avenue, North New Brighton, Christchurch. The purpose of the inspection and this report are to ascertain and document the extent of any structural damage caused through the recent seismic events in Canterbury, covering the period September 2010 to March A detailed visual inspection of the main structural elements of the house has been conducted, with a review of the structural condition and integrity as may be typical for a house of this age, material and construction. No review of the original design documentation, compliance with building code clauses B1/B2, or building consent has been undertaken as part of the inspection and this subsequent report. In addition, it should be noted that assessment of the baseline capacity of the structure is beyond the scope of this report. Further to this, Viden has not made any assessment of the structural stability or building safety in connection with future aftershocks or earthquakes which have the potential to damage the building. This report is limited by the restricted ability to carry out inspections due to coverings of many elements and no destructive testing has been undertaken. The report does not address defects that are not reasonably discoverable on visual inspection, including defects in inaccessible places. Visual inspection of the property at 83A Ascot Avenue has revealed signs of moderate structural damage attributable to the recent Canterbury earthquake sequence. The foundations are out of level and exceed recommended tolerances, although the general condition of the perimeter foundation suggests the foundations will respond well to re-levelling. In general, repairs should be undertaken to reinstate the house to its pre earthquake level. Provided appropriate repairs and replacement of the damaged internal bracing elements (plasterboard) are carried out in accordance with the available guidelines, it is expected that the dwelling will be safe to occupy in the interim. The original inspection and this subsequent report are carried out in accordance with the ACENZ Short Form Model Conditions for Engagement of Consulting Engineers, copies available at the following web address. ( 1. Introduction Viden Group conducted a structural inspection of the 83A Ascot Avenue property for the current owner on the 23 rd of March (Figure 1) The land was visually inspected and compared along with the Tonkin and Taylor detailed land reports and zoning maps for Christchurch. This has identified the house lying with the Green Zone Technical Category 3 which is described as being generally suitable to be repaired and rebuilt on but with moderate to significant land damage possible from future significant earthquakes (CERA and Department of Building and Housing Information Sheet October For houses in this technical category that have suffered foundation damage, site-specific geotechnical investigation and specific engineering design may be necessary for repair or rebuilding of those elements.

5 2. Inspection 2.1 Structural Form 83A Ascot Avenue is characterised as a timber framed single story residential dwelling with brick veneer cladding typical of modern house construction in New Zealand. It is assumed that this dwelling was designed and built in accordance with the governing codes of practice at the time of construction. No original engineering or building documentation has been reviewed as part of this report. 2.2 General The house is a single story residence. The gravity system consists of a corrugated profiled metal roof supported on timber trusses spanning between external and internal load-bearing walls. The ceiling is lined with plasterboard supported from timber joists spanning between external and internal load-bearing walls. The walls are timber framed, with internal plasterboard cladding and external brick cladding. The walls transmit loads vertically to the slab-on-grade concrete foundation. Residential construction of this type is typically detailed to use ceiling lining as a horizontal diaphragm, transferring lateral loads to the vertical resisting elements (walls) which transmit the load into the foundation. The lateral load transfer capacity of the walls is provided through panel action of the plasterboard lining, with the timber framing behind acting as compression and tension chords and detailed accordingly for hold-down. Founded on a concrete slab-on-grade, the house structure relies on adequate vertical bearing capacity and lateral resistance of the soil on which the pad is constructed. The ability of the concrete slab foundation to transmit lateral loads into the ground is typically a function of the vertical area of the sides of the slab or thickenings and the embedment into the soil, and friction interaction between the underside of these elements and the soil beneath. An attached internal access double garage forms part of the structure with vertical structure comprising profiled metal roofing spanning over timber framing which spans between an external timber wall and the timber framing of the house. The floor is a concrete slab-on-grade and the external walls are clad with brick. The east (rear) wall of the garage is a 150mm filled block fire wall being less than 1 meter from the boundary line. The house is defined as a Type C2 structure in table 2.1 of the DBH s Guide: Revised guidance on repairing and rebuilding houses affected by the Canterbury earthquake sequence (Dec 2012) and Appendix C (April 2012). Hereafter referred to as the DBH Guide. 2.3 Detailed Findings The section surrounding the building showed considerable evidence of liquefaction at the time of inspection, particularly at the street frontage and eastern wall of the garage as well as residual liquefaction dust on the garage floor on the south facing wall where maximum slab settlement had occurred. No ground Fissures were seen. There was evidence of movement in the driveway and footpaths. (Figure 4, Figure 5, Figure 6).

6 The house has differentially settled to the south. Floor levels were taken through the house using a digital spirit level which showed slopes in the order of 0.2% east/west and 1.6% north/south (Figure 7, Figure 8, Figure 9). Further to this, a levelling survey was undertaken through the house using a gas level and the readings are shown on the floor plan (Figure 2 ). This survey showed that the house had settled in the order of 210mm from north to south. This relatively uniform settlement is likely caused by a loss of bearing capacity following the liquefaction. The settlement is predominantly towards the southern garage end of the house, clearly seen by the liquefaction line still on the external brickwork and the excavations still evident (Figure 10). The level of settlement in the slab over the house footprint inclusive of the garage falls in the order of 210mm North/South and 20-30mm east/west mm settlement over the floor plan is a performance criteria referenced in the DBH guide for which foundation re-levelling is required (DBH Guide Table 2.3). Therefore, based on current DBH Table 2.3 guide, re-levelling of the foundation is likely required as a minimum. The slab gradients actually exceed this criteria indicating a foundation rebuild may be required, as indicated by table 2.3 of the DBH guide. In its current state, it is expected that in the event of further significant seismic events, the foundation would not perform as designed and therefore incremental increases to the damage may be expected. It should also be noted that since the house is sited on land zoned as TC3, specific engineering design or direction may be required prior to repairs and pending any requirements for Building Consent. It was not possible to visually inspect the full concrete slab foundation inside the house due to floor coverings. However in the vinyl areas of the kitchen and the bathroom/toilet and ensuite, no cracks or movements were discernible in the slab. No obvious vertical or lateral movement was suggested in the carpet contour. No cracks were seen in the external perimeter concrete footing at any point where it was exposed for inspection. The exposed garage concrete slab-on-grade foundation is cracked along the central machine cut joint. (Figure 11). The garage has at least one smaller crack (Figure 12) that is in the order of 1-2mm in width. As above, the DBH guide suggests that cracks of this magnitude should be repaired. Most of the entire garage was covered in the occupant s belongings and was not able to be inspected. A reasonable quantity of liquefaction dust was observed along the inside front (south) wall on the garage. The external brick cladding is generally in acceptable structural condition. There is minor stepped cracking in the order of 1-2mm in width around window frames which are typically high stress areas (Figure 13, Figure 14). Further to this, a gap has opened up between the brick cladding and the garage door framing in the order of 10mm (Figure 15). These cracks do not significantly reduce the structural capacity of the house and can be remediated in due course in order to promote and maintain durability and weather-tightness. The verticality of the external cladding is within acceptable limits, and the cladding appeared secure with the framing behind. (Figure 16) There is significant damage to the internal plasterboard linings throughout the house particularly in the lounge, living room and kitchen areas. This damage is typical of many houses in Christchurch following the recent earthquakes and typically comprises 0.5-1mm wide cracks around high stress areas such as windows frames, door frames and plasterboard joins (Figure 17, Figure 18, Figure 19 ). These cracks do not significantly reduce the structural capacity of the house and can be remediated in due course in accordance with the recommendations found in the DBH guide.

7 The timber roof framing was visually inspected from the access hatch in hallway. The timber structural members are in acceptable condition and showed no signs of structural damage attributable to recent Canterbury earthquake events. Nail plates on the timber trusses and connections inspected were in good condition and no evidence of significant movement or permanent deformation was noted (Figure 20, Figure 21). 3. Summary The property at 83A Ascot Avenue shows evidence of structural damage as a result of the recent Canterbury earthquake sequence. There is movement and differential settlement of foundations and moderate damage to internal lining. Besides minor stepped fractures in the external brick cladding, the external veneer is in acceptable condition. Although the current guidance from the Department of Building and Housing suggest the foundation may require rebuilding, given the good condition of the concrete perimeter foundation the dwelling may respond well to re-levelling using injection techniques. As the house is sited on land zoned as TC3, specific engineering design or direction may be required to secure a Building Consent relating to foundation repairs or rebuilding. Provided appropriate repairs and replacement of the damaged internal bracing elements (plasterboard) are carried out in accordance with the available guidelines, it is expected that the dwelling will be safe to occupy in the interim.

8 Appendix A Site Location Figure 1 - Site location

9 Appendix B Site Plan Figure 2 - Site layout with floor levels (indicative only - not to scale)

10 Appendix C Site Photos Figure 3 - General view Figure 4 Lateral movement in driveway slabs.

11 Figure 5 - Movement between concrete driveway slabs up to 30mm Figure 6 Lateral movement between paving and floor slab, East wall

12 Figure 7 East/West benchtop level Figure 8 North/ South benchtop Level

13 Figure 9 North/South Kitchen floor level Figure 10 Excavation and liquefaction line at front of garage

14 Figure 11 - Crack to Central cut joint in garage floor slab

15 Figure 12 - Crack through garage slab (1-2mm) Figure 13 Stepped cracking of west elevation

16 Figure 14 Stepped cracking on east elevation Figure 15 Movement between brick veneer and garage door framing

17 Figure 16 Front wall of garage Figure 17 Stress cracking in living room wall

18 Figure 18 - Major horizontal joint failure in kitchen/living area Figure 19- Typical stress fractures in most rooms from 100mm to 600mm long

19 Figure 20 - Timber trusses show no sign of stress, all nail plates firm and undisturbed. Figure 21- Roof trusses in good condition