Big cracks appear in the MBIE Guidelines

Why do Engineering NZ single out the MBIE Guidance as a document to refer to? Why do they not refer to Gib Guidelines when it comes to plasterboard products, or WANZ when it comes to windows, or NZ Federation of Brick and Block Layers when it comes to masonry?

The MBIE Guidance document contains many references to descriptions of damage and to repair strategies that simply do not fix a broken (structurally damaged) house. One simple example is damage to plasterboard wall linings... they say you can cosmetically repair cracks. This does not address the loss of stiffness that will always occur when plasterboard linings are damaged by extreme earthquake events.

Engineer David Townshend takes a deep dive into plaster board linings and the standard of repair for earthquake insurance claims.

Refer to the following article or download the attached document for a better reference on what loss of stiffness to a dwelling means for the homeowner, and what the insurance policy provides to repair that structural damage.

Download the PDF


Plasterboard wall linings damaged by earthquake shaking - an insurance response

By David Townshend (B Eng)

Background

1.     Plasterboard wall linings used in construction have both an aesthetic value and a functional ability.  ‘Functional ability’ is also referred to as ‘structural ability’ in this document. Whist ‘aesthetic value’ is easy to observe, ‘structural ability’ is not.

2.     Standard plasterboard wall linings provide resistance against loads (e.g. earthquake and wind) applied to building structures due to their inherent ‘structural ability’ (characteristics of stiffness and strength providing equivalent bracing units)[1].  A stiffer building will deflect less for any given load, a stronger building will resist more load before it permanently deforms. 

How an insurance contract recognises ‘earthquake damage’ to plasterboard lined wall panels

3.      An insurance contract responds to ‘earthquake damage’ which can be defined as:

‘a physical change to the building that is more than de minimis, and an impairment to its value and usefulness’[2], where the physical change is more likely than not caused by the earthquake.

(before damage could be disregarded on the grounds that it is ‘de minimis’, the effect of the physical change would need to be close to non-existent)[3]

4.     It is therefore necessary to examine the physical changes caused by the earthquake and then examine any resulting impairment to ‘structural ability’.

5.     It is likely that any combination of, fractured linings, compromised lining fasteners and compromised framing connections will exist after extreme earthquake events[4]. These physical changes, whilst sometimes observable, can also be hidden. Wall lining detachment from the structure, indicated by bowing and/or drumminess[5], may be the only observable signs of lining distress without removing trims.  It has been observed that wall linings do not show any signs of distress in some instances where bottom plates have lifted.  Occupiers observations of changes (more noise and more movement) to the building should be taken seriously and considered.

6.     The described physical changes degrade the ‘structural ability’ of the wall panel since its stiffness is considerably reduced[6].  This reduction in ‘structural ability’ results in greater displacement of the structure during successive, equal sized events.  This is an impairment to value and usefulness, since cosmetic damage and loss of amenity will occur during smaller, more frequently occurring follow-on events.  In addition, much smaller loading events will cause observable building deflection which may be noticed by building occupants.

7.     Therefore, the insurance contract recognises the described physical changes (ref. para. 5) as ‘structural damage’.

Restoration of ‘structural damage’ to plasterboard lined wall panels

8.     When reinstatement of ‘structural damage’ is contemplated by an insurance contract, the validity of a proposed remediation is determined by whether the ‘structural damage’ (degraded ‘structural ability’) will be restored to the condition provided in the contract.

9.     For full replacement insurance contracts this is most likely one of two conditions:

a.     Equivalent functionality (performance & durability) it had ‘when new’, or;

b.     Functionality (performance & durability) defined in contemporary building standards, regulations and statutes.

10.  Independent scientific testing and analysis have concluded that cosmetic repairs (repairing by raking out and re-plastering) or intermediate level repairs (inserting additional fasteners) to plasterboard linings “…will not reinstate the original house stiffness…”[7], which shows those style of repairs do not meet an insurance contract standard of ‘when new’.

11.  The independent scientific testing and analysis (referred to in para. 10) have identified only one repair method to plasterboard ‘structural damage’ that “… can be relied on to restore original house stiffness and strength, and hence avoid the serviceability problems discussed…”:

a.     Lining Repair - Overlay new linings and add wall hold down anchors. Note, this may not provide an insurance policy response in terms of aesthetics.

This conclusion is confirmed by GIB testing and analysis[8].

12.  When remediating ‘structural damage’ to plasterboard lined wall panels, it is essential to first refer to bracing plans and calculations and if none exist, they are created[9]. Manufacturers guidelines[10] should also be consulted.

Debunking the myths (or misinterpretations of an insurance response)

Myth 1. Standard plasterboard wall linings are not ‘structurally significant’ since they were not recognised as having any ‘structural ability’ when they were installed (either due to the year installed, or due to them not being recognised as ‘structural’ in plans or specifications). Anyone relying on this myth is not applying an insurance contract standard of ‘damage’ recognition, which recognises any inherent ‘structural ability’ that existed immediately prior to the damage causing event (refer para. 4).

Myth 2:  Plasterboard wall linings installed before NZ3604 existed are not ‘structurally significant’ since they do not add much bracing ability to walls compared to let in braces in the framing. Independent testing and analysis by BRANZ resulting in their calculation of bracing units for standard plasterboard (referred to in para. 2) debunks this myth.

Myth 3: Damage to plaster board lined wall panels that reduces their ‘structural ability’ is ‘minor’ or ‘not significant’.  Anyone using these terms are simply providing an opinion which does not adequately consider the insurance definition of damage (refer. para 3, ‘de minimis’).

Myth 4: MBIE Guidance[11], describes structural damage and repair methods for structural damage that meet an insurance contract standard.    It does not[12]. The methods provided in that document reinstate lost structural ability to undefined conditions (refer cosmetic and intermediate level repairs described in para. 10).  Furthermore, one of the lead authors of that document, a chartered professional engineer, has confirmed that “there was never any intent to meet either the EQC Act or the private insurance policy” during the creation of that document.  

Myth 5: Any degraded structural ability does not need to be restored if there is low probability of the building collapsing during Ultimate Limit State (ULS - 1 in 200 year) design event.  This could be true since the NZ Building Act 2004 only prevents buildings from being used if they are unsafe or unsanitary[13].  However, this approach ignores the existence of an insurance contract which recognises the degradation of ‘structural ability’ and demands that it is restored (refer para. 3).

Summary

In the absence of an insurance contract, a NZ Building Act 2004 approach to remediation could be taken, which does not require structural repairs to be performed if the house is not unsafe or unsanitary (even if the house is structurally damaged). In that case building owners can dictate the level of repair they wish to undertake and can choose to perform cosmetic repairs or any intermediate level repairs (where not all the degraded structural ability is restored, like MBIE Guidance method of additional fasteners for example). However, since it has been proven those intermediate level repairs are little more than cosmetic and do not restore the structural performance to ‘when new’, it is likely that the occupants will notice movement of the dwelling during future smaller wind or earthquake events and there will be greater displacement of the structure for the same sized follow-on events.  This will result in more cost over the lifetime of the building to maintain the same finish (to repair re-cracking) and amenity, than if the ‘structural ability’ were fully reinstated.

An insurance contract standard of response overcomes those shortcomings by recognising the reduction in structural performance and allowing to reinstate it fully to the insurance contract standard. 

It is essential to engage appropriately qualified and experienced experts who are instructed to recognise ‘structural damage’ as an insurance policy does and who are instructed to propose method(s) to restore that ‘structural damage’ to the standard provided in the insurance contract.

In the case of plasterboard lined walls, where any ‘structural ability’ has more likely than not been degraded by the earthquake causing ‘structural damage’, then the degraded structural ability needs to be restored to the policy contract standard. Since cosmetic repairs do not restore the structural aspects to ‘when new’ condition, where linings are ‘structurally damaged’, to meet the insurance policy standard, they will either need to be overlaid with new linings, or replaced.  

Of course, a homeowner could accept a lower level of repair that doesn’t fully reinstate the degraded structural ability, but in doing so, they will also be accepting higher ongoing maintenance costs for the life of the building.

 Author: David Townshend (B Eng)

[1] BRANZ STUDY REPORT SR305 (2013), ‘Bracing ratings for non-proprietary bracing walls’, table 8, S J Thurston. https://www.branz.co.nz/cms_show_download.php?id=31b8c08d32ef01130fbedfdecba17afd56b2efeb

[2] He v EQC, CIV-2013-409-1775 [2017], NZHC 2136, [67]. https://forms.justice.govt.nz/search/Documents/pdf/jdo/31/alfresco/service/api/node/content/workspace/SpacesStore/ce0cd226-1dcd-45cf-846a-14635b0aedfa/ce0cd226-1dcd-45cf-846a-14635b0aedfa.pdf

[3] WYNN WILLIAMS(January 2011), ‘THE DE MINIMIS PRINCIPLE’, AMANDA DOUGLAS, PARTNER. http://www.wynnwilliams.co.nz/wynnwilliams/media/Articles/De-Minimis-Principle-rebranded.pdf

[4] Gib Technical Report (2011), ‘Post-Earthquake Performance of Sheet Bracing Systems’, pages 8-11, Richard Hunt BE PhD. https://www.gib.co.nz/assets/Uploads/Post-EQ-performance11.pdf

[5] BRANZ STUDY REPORT SR 265 (2012), ‘Effect and Remediation of the Loss of Building Lateral Stiffness Caused by Earthquake Loading’, Appendix E, page 58, S J Thurston.

[6]Gib Technical Report (2011), ‘Post-Earthquake Performance of Sheet Bracing Systems’, page 5, figure 5, Richard Hunt BE PhD. https://www.gib.co.nz/assets/Uploads/Post-EQ-performance11.pdf

[7] BRANZ STUDY REPORT SR 265 (2012), ‘Effect and Remediation of the Loss of Building Lateral Stiffness Caused by Earthquake Loading’, Conclusions, page 19, S J Thurston. https://www.branz.co.nz/cms_show_download.php?id=f8defa92a13c106a1ae843a704bd6abd43a9260b

[8] Gib Technical Report (2011), ‘Post-Earthquake Performance of Sheet Bracing Systems’, page 20, Summary, Richard Hunt BE PhD. https://www.gib.co.nz/assets/Uploads/Post-EQ-performance11.pdf

[9] GIB Information Bulletin (November 2011), ‘Damage assessment for residential structures following earthquakes’ https://www.gib.co.nz/assets/Uploads/Damage-assessmentNov21.pdf

[10] GIB Information Bulletin (November 2011), ‘Guidelines for repairing GIB plasterboard linings in wind or earthquake damaged properties,  https://www.gib.co.nz/assets/Uploads/Guidelines-to-repairingNov11.pdf

[11] MBIE Guidance (1 December 2012, including updates since), ‘Repairing and rebuilding houses affected by the Canterbury earthquakes’. https://www.building.govt.nz/building-code-compliance/canterbury-rebuild/repairing-and-rebuilding-houses-affected-by-the-canterbury-earthquakes/

[12] MBIE letter to insurers (MBIE Update January 2016), which states: “The guidance provides technical solutions that comply with the Building Act and Building Code. It is not a substitute for the policy homeowners have with their insurer, which will take precedence.” (since removed from MBIE Website)

[13] NZ Building Act 2004, s116b, http://legislation.govt.nz/act/public/2004/0072/144.0/DLM306886.html



Melanie Tobeck