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CROSS Safety Report

Inadequate modelling of existing building

Report ID: 1082 Published: 30 March 2023 Region: CROSS-AUS


Overview

An existing building suffered damage while being upgraded, which led to a more detailed inspection of the existing building’s condition. The report highlights the importance of conducting a thorough investigation and assessment of an existing building structure to ensure an understanding of how it will perform under the applied design loads, rather than relying on assumptions that may be unrealistic.

Key Learning Outcomes

For structural and civil engineers:

  • Conduct a thorough on site inspection to confirm whether assumptions about the structure of an existing building are accurate, prior to making an assessment of its loadbearing capability
  • Carefully consider whether it is suitable to use existing brick panels to contribute to the seismic resistance of an existing building
  • A quality assurance system that includes internal checking, or peer review, of calculations and design assumptions can help prevent safety issues from arising during the design process

Full Report

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The reporter’s work includes numerous projects involving older structures. In many cases, these structures are found not to comply fully with modern standards, design practices and construction techniques. In the reporter’s experience, many such buildings have also been modified, not always with the input of a suitably qualified builder or engineer.

One particular project comprised a framed structure with large non-loadbearing brickwork infill panels as the exterior walls. The brickwork suffered significant damage during the reconstruction. Design criteria for the structure required it to resist wind and seismic events as prescribed in current design codes.

An on site investigation found that the wall panels were not connected to the structural framing along their top edges (joints were filled with compressible filler foam), nor at their vertical edges in the corners of the building at the wall-column interfaces. Some brick panels also had door and window openings which limited their capacity to resist lateral forces.

Building assessment

An assessment of the out-of-plane capacities of the wall panels, based upon conditions of edge restraint observed on site, determined they could not sustain the design wind loading condition.

The reporter notes the panels had previously been assessed as being adequate for the purposes of the project. They are of the opinion that the previous modelling, when considering the capacity of the brickwork panels, failed to take into account the actual connections between the panels and the structural building frame. The modelling also did not appear to consider the effects of door and window penetrations which had a substantial negative effect on the structural behaviour of the wall panels.

The reporter suggests many designers fall into the trap of applying the same set of assumptions to an existing structure that they might apply to a 'clean sheet' design – in this case the assumption of full lateral support to all edges of the panels.

In conclusion, the reporter suggests adequate site investigation of brickwork panels should be conducted to confirm details of fixings, and therefore edge support conditions, for all loading conditions.

never assume anything until conditions have been fully inspected on site

This experience reaffirmed the belief within the reporter's company that previous work which states something complies with a specific standard should not be relied on when a quick visual assessment suggests otherwise. It is important, particularly when dealing with older structures, to never assume anything until conditions have been fully inspected on site.

Expert Panel Comments

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An Expert Panel comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-AUS Expert Panel page.

The reporter has drawn attention to an extremely important aspect of any refurbishment works, namely the full and detailed assessment of the condition of the existing building. When assessing any existing structure, it is critical to match design assumptions with the details and conditions of the building as determined on site.

There is no substitute for a detailed inspection by an experienced structural engineer with a good background in the relevant materials. In some cases, confirmation of whether assumptions adopted in the design are valid, through more extensive site investigation, will be appropriate. This is all the more important when the as-built documentation of the building is unavailable. This report confirms that, in the example given, assumptions were made that did not match the reality on site.

assumptions were made that did not match the reality on site

Implicit in the report is the importance of internal checking and/or peer review processes, which play an important role in helping identify erroneous assumptions in subsequent modelling performed in the design process.

When considering stability within the design process, it may be necessary to make conservative assumptions to account for any unknowns (for example, assuming no ties exist unless it can be verified there are in fact ties present).

A source of recommended design parameters for assessment of existing construction is the previously withdrawn (but still available) AS3826 – Strengthening Existing Buildings for Earthquake. However it is important to note that, in addition to its withdrawn status, this code was not gazetted in the National Construction Code. Accordingly, it has never been a deemed-to-satisfy solution in the Building Code of Australia for compliance during a refurbishment.

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