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FAQ – Seismic Restraint of Non-Structural Building Elements

Q1. What is seismic restraint?

  1. Seismic restraint is reinforcement that is designed to prevent earthquakes from damaging an object.

Q2. Why is seismic restraint of non-structural building elements Important?

  1. Damage from falling objects such as ceilings, ducts, pipes and other non structural building elements poses a significant risk to human safety. Within New Zealand seismic restraint of these non structural elements has traditionally been an afterthought. This was clearly shown in the recent earthquakes in both Christchurch and Wellington in which the failure of non structural elements was significant.

Recent seismic testing conducted on the non structural elements within random buildings in New Zealand resulted in a failure rate of 95%1. Building services and other non structural elements can represent up to 70% of a building’s value 2.  As these components traditionally lack seismic restraint there is potential for significant financial loss –

  • Over 50% of the cost associated with the recent Christchurch Earthquakes 3, now estimated at over $40 billion has been attributed to non structural damages.
  • The damage caused to non structural elements in Wellingtons new (2009) BNZ building in the 2013 magnitude 6.5 quake resulted in closure of the building for 6 months and it took 15 months before the building was fully functioning again. The estimated cost for this single building was $10 million 1.

All new buildings require seismic restraint of non-structural elements to gain building consent, prevent future loss of life, injury and financial loss

.Figure 1. Earthquake damage to non-structural elements (1)

Figure 1. Earthquake damage to non-structural elements

 

Q3. What non structural elements require seismic restraint?

  1.  Engineering systems within buildings such as Ducting, Pipework, Electrical Cable Tracks, Large floor mounted components require seismic restraint. The New Zealand Standard NZS 4219 sets the seismic performance of requirements for these engineering systems and provides a means of compliance with the NZ building code.

Non structural architectural systems such as partitions and suspended ceilings also require restraint. These systems are excluded from the scope of NZS 4219, therefore to meet the requirements of the NZ building code a specific bracing design is required that is capable of withstanding an earthquake of magnitude determined in accordance with NZS 1170.5 – Earthquake actions.The way that the magnitude of earthquake acceleration is determined is discussed below in Q4.

 

    Seismic Restraint

Q4: How is an appropriate level of restraint determined?

A: There are several factors involved in determining a level of seismic restraint, these are:

  • geographic location,
  • subsoil type
  • building type. The type of building and its intended usage determine its Importance Level.  

The level of restraint required for individual elements is based on the importance level of the building, and the risk each element poses to safety within the building.  The component categories are P1-P7, with P1 requiring the most restraint, and P7 the least.  Part categories and their criteria are presented in the table below.

 

Criteria Component category
Hazard to life outside the building P1
Hazard to a crowd of greater than 100 people within the building P2
Hazard to individual life within the building P3
Component necessary for the continuing function of the evacuation and life safety systems within the building P4
Component required for operational continuity of the building P5
Component for which the consequential damage caused by failure is disproportionately great P6
All other components P7

 

The part category and Importance level sets an expected level of performance of each component under earthquake actions.

Q5. What does seismic restraint involve/ how are components braced?

  1. Once an earthquake design level has been determined based on the risk category of the system, bracing is designed that is capable of withstanding the imposed loads and deflections expected from this level of earthquake.

Seismic Restraint Suspended Ceiling

Figure 4 – Seismic restraint for suspended ceiling

 

Q6. When is seismic bracing design of non-structural elements required?

  1.  The first stage of seismic bracing design is required for building consent.  This is often carried out by individual contractors and engineering firms, with specialists in each area designing seismic bracing for their non-structural elements.  

During the base build stage of construction, large non-structural elements will be installed and braced.  Further bracing design by a seismic design specialist may be necessary at this stage due to variations from the design for consent.  

The Fit Out stage of construction often involves further design by a seismic design specialist, either due to discrepancies encountered during construction monitoring, variations from the design for consent, or most commonly, due to interactions with installed services and other bracing for non structural elements.

Q7. Who is involved in restraint of non-structural elements?

  1.  The design of seismic restraint begins at the architectural stage, consulting with structural engineers and installers to allow suitable spacing between structural and non-structural elements in the building.

Seismic design specialists then determine and design the seismic bracing requirements for individual components.  The seismic design specialists issue a Design Producer Statement (PS-1) for submission to the building consent authority, recommending an agreed level of construction monitoring during installation.

Seismic design specialists can also provide a Design Review Producer Statement (PS-2) for a design prepared by another party, certifying that the design complies with the relevant standards.

During installation, the seismic design specialist provides an agreed level of Construction Monitoring.  Upon completion of the work, the installer should provide a Construction Producer Statement (PS-3), certifying that the installation has been completed in accordance with the seismic specialist’s design.  The seismic specialist will then provide a Construction Review Producer Statement (PS-4) certifying that the installation has been completed in accordance with the design and any other relevant requirements of the building consent authority.

For additional information visit the BRANZ seismically resilient non-structural elements fact sheets: http://www.branz.co.nz/non-structural

References:

  1. Article: Buildings in Wellington and Auckland subject of fresh seismic inspections – www.stuff.co.nz/business/78992949/buildings-in-wellington-and-auckland-subject-of-secret-mbie-seismic-report
  2. Paper: Seismic performance of non-structural elements within buildings – NZSEE Review
  3. Paper: Ministry of Education Canterbury Earthquakes Impact on the Ministry of Education’s School Buildings

 

 team member
Dr. Louw Butler

PhD, MScEng, BEng
Contractor

e louw.butler@bvt.co.nz