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CROSS Safety Report

Load path and detailing issues with residential construction

Report ID: 1280 Published: 12 June 2024 Region: CROSS-AUS


Overview

A reporter observed the construction of a two storey residential structure and noted several issues with the design and detailing. The structure comprised a combination of lightweight timber framing, timber joists, and steel portal frames. The reporter was concerned that:

  • There were inadequate connections and a lack of sufficient detailing throughout the bracing members

  • There was a lack of consideration of load paths

  • The design may not have functioned as intended

Key Learning Outcomes

For Civil and Structural Engineers:

  • Consider and sketch the load paths in the structure as an integral system prior to commencement of the detail design
  • Consider the suitability of the proposed structural system and connection details (including standard details) to achieve the required serviceability, stability and strength
  • Clearly articulate on drawings or engineering sketches the connection details that are required
  • Locate cast-in anchors inside the zone of reinforcing bars as standard good practice
  • Consider any possible construction limitations and specify any necessary hold-points or requirements. This may involve temporary supports or braces, curing times and erection procedures

For contractors:

  • Assess the level of information provided to ensure it contains all connection and load-transfer details from one material type to another, and for stability during construction
  • Abide by the requirements of design engineers, including hold-points and stability considerations
  • Stop all affected construction if connection details are not provided or are unclear, until these gaps in information are resolved

Full Report

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The reporter observed the construction of a two storey residential development and noted several issues with the structural design and detailing. The reporter raised these with the design engineer, who amended their design and details accordingly. However, the reporter remains concerned that the design as originally documented demonstrated a lack of attention to detail and design considerations, and the problems with the process of quality control.

The structure involved a combination of lightweight timber framing, timber floor joists, and steel portal frames. The reporter was concerned the original design would not have functioned as intended as there were inadequate connections and a lack of adequate detailing throughout the bracing system. Although a catastrophic failure may have been unlikely, there could have been excessive damage to the structure in an Ultimate Limit State event and possibly higher-than-acceptable levels of damage in a Serviceability Limit State event.

The reporter noted the following in particular:

  • There was no consideration of load transfer from the timber floor diaphragm through to the packing on the steel portal frame. Although the timber joists were detailed, there was no continuous, robust load path to the packing
  • The spacing of bolts connecting the timber packing to the portal frame was too great. The design engineer accepted this and altered their details
  • The steel portal frame was designed as having a fixed base. However, this is very difficult to achieve without buttressing and the reporter was concerned that the connection would not achieve full fixity
  • Even if the connection could have achieved fixity, it was detailed with a weak mortar packing compound under the stanchions that may have crumbled, causing a pin joint to form, as well as introducing combined bending and shear in the anchor bolts
  • The cast-in anchor bolts were set outside the steel reinforcement in the foundation beam (i.e. in the cover concrete). The design engineer stated they had considered the ability of the concrete to transfer the load. Although the reporter acknowledged that this is possible under the relevant Standards, they did not consider this to be a robust design

It is the reporter's opinion that the design engineer should have considered in more detail the load path. Too much focus was given to the mathematics and computer modelling. The assessment of the overall structure also needed to be taken into account. The reporter suggests this could easily have been resolved by sketching the load path on the structural drawings and providing sketches with the calculations.

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.

Experience suggests that residential design is often poorly detailed, most likely as it is very much a fee-driven market with tight margins, serving clients with little or no capacity to judge the quality of the documents, and often with nobody responsible for overall control of the design. There tends to be separate design/supply/erect sub-contracts with no co-ordination resulting in problems of overall stability and trouble at interfaces.

Documentation for timber framing is typically produced by 'design' services using the relevant Standard, with minimal or no further engineering involved in the design process. In New Zealand, NZS 3604:2011 - Timber-framed building provides methods and details for the design and construction of timber-framed structures not requiring Specific Engineering Design (SED). In Australia, AS 1684 - Residential Timber Framed Construction is a four-part Australian Standard covering design criteria, building practices, tie-downs, bracing and span tables for timber framing members. By complying with this Standard, designers are Deemed-to-Satisfy the requirements of the Building Code of Australia. In practice, the Part 4 - Simplified version of AS 1684.4 and the member size/spacing tables contained therein are commonly used by building designers. These New Zealand and Australian Standards contain limitations on the size and type of building that designers must be aware of.

A clearly defined, robust load path must be established and designed, with appropriate and well-documented connections

Accordingly, residential building ‘design’ often involves a lack of appreciation of load paths and connections, particularly for lateral loads. A clearly defined, robust load path must be established and designed, with appropriate and well-documented connections. For example, with multi-level buildings it is critical for structural designers to understand that lateral loads must be resisted from the roof to the suspended floor.  This is usually by some form of shear wall action which in turn uses the suspended floor as a diaphragm to carry the loads into lateral systems, including any steel framing or shear walls, and finally to the ground. The identified load path needs to allow for collector loads associated with the maximum lateral actions and each individual element, including associated connections, checked for this corresponding load.

Critically important is displacement compatibility for lateral loads

The reporter notes the adoption of a portal frame solution, with attending issues, in this structure. Portal action in housing is often difficult to achieve. Critically important is displacement compatibility for lateral loads. This is often neglected, and damage to timber framing can be extensive before the resistance from the portal frame system becomes effective. This was noted in damage caused by the Canterbury/Christchurch, and other recent, earthquakes in New Zealand where mixed bracing systems of plasterboard-lined bracing walls and steel portal frames did not perform well. Plasterboard or ply linings, or diagonal bracing, which are relatively stiff compared to steel portal frames, are therefore recommended for timber-framed residential construction. The Building Research Association NZ (BRANZ) has published technical papers and recommendations on this topic.

The observations made in this report are likely to be widespread

Where portal frames are deemed suitable for adoption, careful consideration should be given to restraint of the frames, particularly at knee braces, and to details of restraints at bases. Full base fixity is difficult to achieve without appropriate detailing and competent site construction; where it is assumed but not achieved, deflections and member actions will exceed those calculated, exacerbating any compatibility issues.

Grouting to baseplates of portal frames should use high strength non-shrink flowable epoxy mortars. Anchorage of holding-down (HD) bolts in cover concrete is not considered good practice, especially in seismic areas. In addition, the capacity of anchor bolts in the zone of concrete cover will be severely limited by edge distance considerations, and shear loads in HD bolts are likely to be high due to portal action spreading and other lateral loads.

The observations made in this report are likely to be widespread. It is very concerning that, if these details had not been observed on site by an experienced practitioner who intervened, they may have been built-in and covered up as construction proceeded. This raises serious concerns about the level of experience of designers in this area of the construction industry, the checking and review of designs, and the quality of construction in residential structures.

Note: Reference should also be made to CROSS Safety Report 1289 - Issues in structural design of a house in a highly seismic zone which covers similar issues with the design of timber-framed residential buildings.

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