Awning collapse
Published November 30, 2006
Background
Recommendations
Health and safety obligations
Background
There have been two recent incidents where large awning structures have collapsed due to the failure of the anchorage systems.
In one incident during February 2006 a large awning which was approximately 50 metres long and 12 metres wide collapsed and fell to the ground. Construction of the awning had just been completed. The awning consisted of a cantilevered section that relied on several circular hollow steel sections that extended from the external wall to the top of the awning. The circular hollow sections were in turn attached to the wall by means of single anchor plates that relied on chemical and friction anchors embedded in the wall.
There were a number of factors that contributed to the incident including:
- attachment of shade sails to the awning; and
- incorrectly installed chemical and expansion anchors.
Some of the issues identified in relation to the anchorage system used included:
- incorrect embedment depths for chemical anchors relative to the anchorage system;
- inadequate amount of epoxy for chemical anchors;
- Concrete dust not removed from holes prior to the insertion of anchors; and
- use of other anchor types, with lower capacities and different characteristics, than those specified in the design specification.
Recommendations
Person installing anchors
Persons installing anchors are to be trained in the correct installation method for the anchor as specified by the manufacturer. If the installer is unable to install the anchor in this way (i.e. because re-enforcing steel prevents drilling the hole) a suitably qualified engineer should provide guidance on alternative anchorage systems to be used.
Principal Contractor
The principal contractor should implement or ensure the relevant person implements an effective Quality Assurance (QA) system to ensure anchorages comply with design specification. This includes inspection of the anchorages prior to loads being applied. The QA system may also include the need to do proof loading on a sample of anchors (this may be particularly important for chemical or friction anchors designed for “pull out” loads).
Designers
Designers should ensure that anchorage selection considers all of the following factors:
- structural adequacy;
- longevity (taking into consideration environmental issues - wind, temperature, rain); and
- any changes to the specifications of the designer should be approved by the designer or appropriately qualified engineer before proceeding.
Building Owner/Occupier
Building owners and occupiers should not make structural alterations to buildings, or attach additional architectural features unless the building designer or another suitably qualified engineer provides written certification. This includes the addition of shade sails to a structure. As local government planning conditions and approvals may apply the relevant local government authority should also be contacted.
Measures when using chemical and friction type anchors
Anchor embedment depth - relevant persons (persons in control of the workplace) must ensure that all anchor studs are embedded to the depth specified in the design specifications. These specifications must be consistent with the anchor manufacturer’s installation instructions.
Epoxy (chemical type anchors) - the quantity, type, and installation technique must be as per anchor manufacturers specifications. Also, ensure that the epoxy used is within the use by date.
Anchor specifications - only anchors specified by the designer can be used unless the designer specifies otherwise in writing.
Concrete dust in holes - the concrete dust needs to be completely removed prior to the epoxy being injected into the holes. Both chemical type anchors and friction type anchors require concrete dust to be removed from holes prior to the anchor being inserted. This is particularly important with chemical anchors because the strength of the anchor relies on an effective bond between the concrete surface and the epoxy. The presence of dust will negate this bond wherever dust is present.
Hole size- hole diameter and depth must comply with anchor manufacturers instructions. Holes must also be perpendicular to the surface unless specified otherwise by the designer.
Cyclic loading on friction anchors – This can cause the concrete at the friction point to powder from compressive stress and eventually the anchor may fail.
For the factors listed above, two typical examples of anchorage systems with less installation difficulties are:
- 'cast-in' type anchors (anchors cast into the concrete element); and
- 'through bolts' that pass through a structural element.
Health and safety obligations
Everyone has a part to play in ensuring their workplace health and safety and the workplace health and safety of others in Queensland, and the law prescribes obligations for individuals in their various roles.
More on the obligations of different roles within workplaces under the law
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