Figure 1: Wire bracing to ductwork and unistrut bracing to an ACU
Over the past few years, the old Wigram Aerodrome has been transformed into a new community. In addition to around 1400 homes, a town centre with shops and a medical centre have been constructed to serve the community. BVT was engaged to provide design and construction monitoring for the seismic bracing of heating, ventilation and air conditioning (HVAC) equipment at the new medical and radiology centre.
While not critical to the structure of the building, unrestrained HVAC equipment such as ductwork, air conditioning units and heat exchangers pose a risk to both occupants of a building and to other building elements in the event of an earthquake. Therefore, it is important for safety and the continued operation of a building that HVAC equipment is properly restrained.
Lighter HVAC equipment is typically wire braced, with thin walled channel such as Unistrut used to brace heavier equipment. In the case of the Wigram Medical Centre, wire bracing was used for ductwork and smaller air conditioning units, and unistrut for heavier air conditioning units. A key point to remember with wire bracing is that flatter is better: wire at a lower angle imparts less force on hanger rods and connections. Wire bracing should be at an angle of less than 45 degrees from horizontal, and under 30 degrees is even better.
As is the case for many buildings, plenum space was at a premium with cable trays, pipework, and partition bracing all having to fit in with the HVAC and its bracing. In some locations there was not enough space to install the specified bracing, and so bracing had to be moved or redesigned.
BVT visited the site 3 times during construction. A visit at the beginning of bracing installation allowed potential problems to be identified early on so that they weren’t repeated throughout the whole installation. During the second visit, completed bracing was inspected and units with limited bracing space were checked in order to design bracing on a case by case basis.
Early identification of problems and quick design of solutions meant that delays to the installation of bracing were minimised. The design and installation of the HVAC restraint met the relevant sections of the NZ Building Code and so a PS4 – Construction Monitoring was issued after the final inspection.