6 . DEMOLITION OF SPECIAL STRUCTURES
Special structures are complex and / or unusual because of the nature of their construction or condition . They include :
• pre or post-tensioned construction
• pre-cast concrete panel and framed structures
• stressed skin structures ( i . e . buildings that rely on the sheeting , cladding or decking to stiffen and restrain the structural framework )
• slung structures ( for example , floors or roofs ) that are in some way suspended from a framework , supported by a structural core .
Special structures will require proper planning and care to be demolished safely . An appropriate demolition method and sequence should be selected and documented prior to the work commencing . A demolition plan and an assessment by a competent person of the proposed demolition method may assist with this process . A SWMS must be prepared where structural elements are to be demolished .
6.1 Pre and post-tensioned concrete
Pre-tensioned concrete contains tendons ( wires , strands or bars ) that have been tensioned before the concrete is placed .
Post-tensioned concrete contains tendons that have been tensioned after the concrete has hardened .
Tensioned tendons require controlled removal because the high level of potential energy stored in the tendons poses a risk to the health and safety and can cause damage to property . It is also important that structural stability is retained during and after tendon removal , prior to the final demolition of the concrete element .
The tendons can be subject to corrosion that weakens them and decreases the structural integrity of the building . Damage is not usually evident externally , even if strand breakage is extensive and conditions can vary widely even within an individual structure .
Before demolishing pre or post-tensioned concrete elements , review all available documentation on the building or structure including :
• building plans , designs and specifications to understand the type of tensioning used , the load carried , anchorage points and number of tendons
• any construction photographs to obtain information on anchorage details , the construction sequencing , and any other measures may affect moisture access .
The condition of the concrete and tendons should be considered before and during demolition , for example , by :
• conducting a visual inspection to confirm loads , obvious deviations from the original design and waterproofing details
• assessing conditions throughout the building , as well as utilising selective testing in representative areas to assess any weakening or breakage of tendons
• measuring humidity within tendon sheathing and analysing any sheathing contaminants
• removing , inspecting and testing a small number of tendons to assess their condition
• continuing to monitor tendon tension .
32 CODE OF PRACTICE | DEMOLITION WORK