Wall frame collapses when structural props pulled-out of flooring
This alert highlights the importance of ensuring the structural stability of temporary propping fixed to autoclaved aerated concrete floor systems.
Published:25 January 2022
An incident occurred on a residential construction site where the external timber stud frame on the first floor collapsed and fell outwards. Inadequate structural fixing between the temporary timber props and the autoclaved aerated concrete (AAC) floor systems led to the frame collapse.
The incident placed employees and members of the public at significant risk of serious injuries.
Figure 2: Timber wall framing panel collapsed due to inadequate pull-out capacity of fixing from timber prop to AAC floor system.
During construction wall frames require temporary propping to ensure that they remain stable and in position, until the structure is braced to support itself.
Temporary propping is often constructed from members that are installed on a diagonal between the wall frame and floor structure. Propping members should be built from structural grade materials so that the capacity of the prop can be determined.
Conventional floor systems, such as timber sheet and concrete, are often used to support temporary wall props. Under normal circumstances conventional floor systems provide adequate anchor pull-out capacity to support wall frames.
However, non-conventional flooring systems, such as autoclaved aerated concrete (AAC) do not provide the same pull-out capacity as conventional floor systems.
Failing to have systems of work in place that ensures structural integrity of the load path for temporary propping of wall frames can create a risk to the health and safety of persons being struck by a wall frame collapsing.
Control the risk
When temporary propping is fixed to non-conventional floor systems, such as AAC, additional systems of work or control measures need to be developed and put in place to ensure the structural integrity (of the connection from the prop to the floor system) is maintained throughout the construction process.
Figure 3: Fixing from prop to joist system below aerated concrete (AAC) sheet. Length of screw should consider base plate, AAC sheet, and required embedment depth.
The system of work should ensure:
The load path provided is structurally adequate such as fixing through the floor sheet and into joist members. Refer to figure 3.
Where the pull-out capacity of the AAC floor system is known, the system of work should ensure that the capacity is not exceeded.
Screws being used for temporary propping should be screwed through the bracing block, through the Hebel panel and into the timber joist at a minimum depth (Approx. 40mm). Type 17 14 x150mm screw
The system of work should include a documented methodology for the installation of the props that provides details to ensure the load path is adequate. Details should include:
type of prop to be used specifying minimum size and strength grades
fixing requirements specifying type, number of, size/gauge, length for all joins (such as from the prop to the wall panel and from prop to the floor system)
any additional blocking requirements to provide anchorage from the prop to the floor joist system
Under the Occupational Health and Safety Act 2004, employers must:
so far as is reasonably practicable, provide and maintain a working environment that is safe and without risks to the health of employees and independent contractors and their employees
provide employees and independent contractors and their employees with the necessary information, instruction, training and supervision to enable them to do their work in a way that is safe and without risks to health
so far as is reasonably practicable, consult with employees, independent contractors and their employees, and health and safety representatives when identifying or assessing hazards or risks and making decisions about risk control measures
ensure, so far as is reasonably practicable, that persons other than employees are not exposed to risks to their health or safety arising from the conduct of the employer's undertaking
Employers and self-employed persons (including contractors) also have duties under the Occupational Health and Safety Regulations 2017 in relation to controlling the risks on their worksite, including ensuring high risk construction work (HRCW) is not performed unless a safe work method statement is prepared and followed.
HRCW that may involve the risk of a structural collapse includes but is not limited to, construction work involving structural alterations that require temporary support to prevent collapse.