12 December

Seismic Bracing Needs Revisiting

Blog DecemberRisk from poor seismic planning is significant in the construction industry. Since Christchurch, we’re all mindful of the hazards posed by poor design or construction; so seismic restraint in building services is now at the forefront of new and existing project planning. I’ve attended three meetings recently to discuss this issue. One of the meetings was requested by the main contractor, which was heartening, as it shows growing awareness of the issues and the need to be proactive in finding solutions and also managing a risk item themselves.

The building code is designed to raise the standard of construction. But NZS4219 referenced under B1 of the building code has some flaws which requires stakeholders to be proactive and to some extent self-regulating.  Here I want to outline some of the deficiencies and offer possible solutions.

Ducting is one area where the standard is light on detail. NZS4219 requires all ducting to be braced but provides no guidance on bracing centers or locations.  Both SMACNA and ASHRAE have excellent publications on seismic engineering for mechanical systems and cover these subjects in detail.

  • ASHRAE – Practical guide to Seismic Restraints (Second edition)
  • SMACNA – Seismic Restraint manual – Guidelines for mechanical systems
  • In addition, supplier provided information such as Mason Industries – Seismic Restraint Guidelines are also invaluable.

More up to date standards, such as ACSE 7-10 ( NZS4219 closely follows its predecessor  ACSE -7-05) stipulate ducts up to 0.5m², or weighting less than 146N/m for trapezed ducts, do not require bracing. With this, a reasonable approach has to be taken, so as not to create unnecessary cost with  ‘over compliance’  in  the final solution. Using the design loadings required by NZS 4219 and applying the bracing systems one of these publications can proved a cost effective solution.

The standard also requires post installed anchors to comply with the requirements of ACI 355.2 which is an American code and two companies can provide total solutions to this Mason and Vaico, both with excellent product ranges unfortunately only in imperial sizes and able to offer bracing solutions. Hilti now have metric fixings tested to this standard but if the motor size of the plant item is over 8kW then expansion anchors are not permitted.

Hanger rods are also required to comply with AS1111.1 but these are not included in this standard, which is for commercial grade bolts and studs. So an appropriate standard needs to be applied to these. I suggest threaded rods are specified to an appropriate material standard equivalent to an equal sized bolt to property class 4.6. A lot of failures in Christchurch were hanger rods

Penetration clearances are also important and where a service passes through a structure, a 25mm all round clearance needs to be provided. If this is also an acoustic or fire wall then a solution to maintain these needs to be designed.

Roof fixings – it’s no longer approximate to just install equipment on a roof without regard to its weight and fixings. NZS4219 bases its premise in transferring its loads to the structure, so the design of equipment plinths and roof platforms are also important.

In a ceiling, service coordination becomes of paramount importance and it’s a multistep process.

Designers need to be aware of all the service requirements and endeavour to coordinate these so there is sufficient clearance for all elements. But that’s not the whole picture. Three standards are at play, so in addition to NZS4219 we have to consider requirements for suspended ceilings and fire sprinklers. Suspended Ceilings come under the requirements of AS/NZS 2785. Fire sprinkler systems come under the requirements of NZS 4541.

The consulting team can only take the design to a certain level, unless the fire sprinkler systems and suspended ceilings and their respective hanger and bracing systems have been designed prior to building consent.

These items provide a significant cost risk to the main contractor, so the design team needs to be proactive in pushing for solutions early in the design and construction process. Sub-contractors should be mindful of elements like hanger types, threaded rod standards, clearances and bracing requirements so as to minimise re-work.

The bracing system is exactly that – a system. So it’s beneficial to look at the system as a whole; from the location of the hangers and bracing, to the location of other services etc. Big picture thinking will minimise the overall cost of systems. For example: a 40mm chilled water pipe doesn’t need to be braced as long as clearances are maintained to other services and this depends if these are braced or unbraced. If you can’t achieve the required clearances, then it may need bracing to be provided. So planning is the key.

On a project, I believe this is best left in the hands of a single entity; which then has to work with the engineers, contractor, and sub-contractors to provide a total solution.

Robert Mannes