This alert highlights the precautions that need to be taken when installing brace footings on concrete slabs at an early age.
Published:01 September 2010
This alert provides practical advice for builders, formworkers, panel erectors and erection design engineers (designers). It will also be useful for health and safety representatives.
The information and advice relates to the design of brace footings (including anchors and concrete) where concrete panel braces are to be fixed to floor slabs or independent pad footings of 150mm or greater thickness.
If the concrete footing thickness is less than 150mm thick, other safety factors must be considered by an engineer.
There is also a checklist on the reverse which should be used before each concrete pour. It will assist with your health and safety management of the site (consider incorporating it into your safe work method statement or site OHS plan).
For the last 10 years, to improve construction productivity, builders have started erecting precast concrete wall panels on floor slabs within 24 hours of pouring the slab.
Concrete strength at any time is greatly influenced by the age and method of curing. For example, after 24 hours air-cured concrete, without admixtures, may only achieve about 10 per cent of its specified characteristic strength.
The anchors used for fixing brace footings consist of a cast-in ferrule into which the brace is later bolted. When anchors are installed in concrete slabs that have not achieved the required strength, they may fail by pulling out under moderate loads. This may lead to catastrophic failure of the wall panel, resulting in death or serious crush injuries to workers.
To ensure anchors achieve the required pull-out capacity, the concrete must attain a minimum strength as determined by the designer (based on manufacturer and supplier specifications).
The required concrete strength to be achieved at erection must be specified. Unless designated otherwise, the concrete strength of the brace footing should be at least 20MPa at the time of fixing of the bracing.
Note: Projects specifically designed and documented by the designer may use a lower strength of concrete for erection of braces.
Where a specific design is undertaken for a brace footing to be fixed into concrete at low strength, the designer in conjunction with the builder, formwork company and panel erector, need to consider the following variables that may reduce safety of the designed brace footing:
how the required minimum concrete strength can be achieved prior to the erection of the panels
concrete curing method to be used onsite
ferrule placement tolerances
dislodgement/misalignment of ferrules during concrete placement
In addition, the designer needs to give consideration to:
ensuring panel braces, fixings and footings are designed to have a minimum capacity of 10kN
ensuring any modifications to ferrules, including the welding of base plates, is done in accordance with an engineer’s design and relevant Australian Standards
designing the ferrule inserts to have a minimum embedment depth of 150mm.
Note: Ferrules should also be designed and manufactured to ensure their designed failure mode is via cone failure of the concrete, not via direct pullout. This will usually require some form of enlarged base of the ferrule.
The builder, formwork company and panel erector should:
implement and maintain a documented sign-off system by a competent person on all slab and anchor details before each pour (see checklist)
verify the minimum concrete strength required before panel erection has been achieved under site conditions.