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15 Apr 2016

Catastrophic roof structure failures.

The success of a safe roof structure lies in the finer detail, affirms Tinus de Jager as he reflects on the causes of roof structure failures, ways to combat such disasters, and the necessary interaction between roof erector and roof inspector – from an inspector’s perspective.

Many insightful and relevant articles have been published over the past few years about the roof truss industry
and the role of the Institute for Timber Construction SA (ITC-SA) with its affiliated members. For the most part, a lot of emphasis has been placed on the engineering systems, the  quality  control  of  the  engineered  product  (trusses),  and the rules that apply to the erection process of structures as determined by the relevant SANS codes and practices.

The importance of these aspects ought to be published more frequently, and be posted on every available social media outlet by the experts in the industry. Moreover, homeowners need to be educated and made aware of their responsibilities in appointing recognized professionals to oversee the construction processes of their newly erected structures.

IT’S IN THE DETAIL

The focus of this article is specifically aimed at the ‘finer detail’ as contained in available Roof Erecting Standards and Procedures Manuals.

This  industry  is  fortunate  to  have  many  highly  qualified, knowledgeable and experienced people as participating stakeholders. The timber engineering industry is probably as well equipped, structured and organized as any other industry. And that begs the question: Why are catastrophic roof structure failures, in this day and age, still prevalent?

It is quite disturbing, but these incidents do not often make news headlines, if at all. The lack of news coverage subsequently leaves  us  in  a  space  of  complacency,  creating  a  false  reality, because we are not being made aware of the extent of the problem and believe that all is fine in the industry. The sad and troubling reality is that all is not fine in the industry. And it is not because structures are inadequately engineered or manufactured, but because they are poorly erected and poorly inspected!

POORLY ERECTED AND POORLY INSPECTED

We have state-of-the-art tools available to design and manufacture very complex roof structures – which should also be  safe.  My  experience  in  conducting  roof  inspections  on timber roof structures over the past 16 years has, time and again,  confirmed  my  conclusion:  Most  failures  are  a  direct result  of  poor,  or  in  some  cases, noassessments of these structures prior to their occupation.

The ‘fate’ or ‘serviceability’ of a structure is determined largely by a thorough assessment of the structure by a competent person, prior to the application of loads to the structure.

Inspections are being done in various ways, and it is especially worrying  that  in  some  cases  they  are  done  ‘remotely.’  This practice is not only unethical, but dangerous and criminal.

The  ultimate  responsibility,  which  should  lie  with  the Competent  Person/Engineer  who  performs  the  final assessment,  is  sometimes,  somehow,  just  brushed  aside  to make  space  for  what  I  call  ‘excusitis.’  Consciousness  and mindfulness seem to be scarce commodities amongst some inspectors The logical next question is, how then, should assessments be conducted?

PROPER ASSESSMENTS

Inspectors are trained to have the required design documentation  at  hand  during  an  inspection.  This  is  vital  in checking  that  the  appropriate,  design-specific  requirements are being met.Together with the design output, the inspector should have extensive  knowledge  of  the  well  published  ‘Roof  Erecting Procedures’  as  contained  in  the  SANS10243  and  the SANS10400 (L) Codes of Practice, as well as the ITC-SA Roof Erecting Handbooks.In a recent publication of the SA Roofing Magazine, an expert explained,  amongst  other  very  important  topics,  the importance of bracing in a roof structure. The point was made that  the  importance  of  bracing  cannot  be  over-emphasized, but that this emphasis should go one step further…

BRACING, BRACING, BRACING

In light of recent catastrophic timber structure failures, I am prompted to expand a little on this very relevant and critical topic. It is in the implementation/application of the bracing detail where problems are created – and the only person that can practically identify these inadequacies is the roof inspector. My view is that: “Bracing is only as effective as its connection (nailing) to the primary roof structure.”

A quick glance during the assessment of a completed structure might, more often than not, create a false sense of compliance, namely that all bracing members are visibly present in their designated positions, and therefore the structure appears to be  adequately  braced.  But  a  closer  inspection  (as  further illustrated in the photographs shown) might reveal that these members are either inadequately fixed or not fixed at all. And these  connections,  whether  they  are  screws  or  nails,  are critical in ensuring an effective bracing system.If one considers that structures are mostly designed optimally, for structural as well as economical reasons, there is absolutely nomargin for error or added negligence during the erecting phase. Every  misplaced  or  incorrectly  fixed  nail  or  screw  will compromise the strength of a joint or connection and can have far-reaching consequences.

NAILING IT

The correct ‘method of nailing’ is quite clear in SANS10243 p.16. For example: The general perception amongst roof erectors is that battens are  fitted  only  as  a  means  of  securing  roof  tiles,  but  their purpose goes much further than that.

  1. They are actually critical in ensuring the reduction of the effective  length  of  the  rafter  members,  against  buckling, especiallywhere multiple-ply girders are involved.
  2. They are vital in transferring the effect of the top cord bracing systems to the rest of the roof structure.

Numerous cases, as I have witnessed, have proven that this inadequacy  in  combination  with  other  non-compliances,  can create the perfect mixture for a catastrophic failure.

WHEN SHOULD INSPECTIONS BE CONDUCTED?

  • It is generally accepted that inspections on new structures should be done prior to the permanent loads (roof covering) being  applied. Unbraced or partially braced structures are doomed to fail once the loads are applied. There should be no compromise on this.
  • Any  modifications  or  alterations/additions  to  a  structure should  be  inspected  in  conjunction  with  the  relevant Rational Design of such additions or modifications.
  • Property owners must be made aware that roof structures must be checked regularly,according to a vital clause in the Construction Regulation 9(4) of 2003.

ROOF ERECTORS

Poorly trained and unqualified artisans, unfortunately, feature at the core of the problem. Their lack of understanding of the basic Roof Erecting Principles and Procedures has led to a multitude of bad practices in the industry for many years. The proliferation  of  these  bad  practices,  over  the  years,  has resulted  in  many  roof  structures  becoming  ‘monuments  of construction  inadequacies,’  in  direct  contradiction  to  basic design  assumptions.  We  are  today  witnessing  the  inevitable after-effects of such structures that do not perform over the long term. Proper and adequate training of artisans still remains one of the  main  challenges  facing  the  industry,  and  it  is  a  serious matter that needs to be addressed urgently and thoroughly. Until this problem is fully resolved, failures of roof structures will likely become more prevalent in the future.

ROOF INSPECTORS

Dedicated  and  passionate  roof  inspectors  have,  for  many years,  been  mentors  to  some  roof  erectors.  They  have provided assistance, guidance and unofficial training on site to many roof erectors – without any compensation. (And this will undoubtedly remain so until adequate, official training is fully in place.) This has subsequently resulted in the inspector being viewed as the ‘quality controller’ for these ‘contractors.’ One could perhaps hope for the introduction of compulsory written and practical tests, which would have to be successfully completed by currently accredited erectors before their annual ITC-SA membership can be renewed. Or, and this is my opinion, it warrants reconsidering the way in which roof erectors are evaluated for their competency to be accredited with the ITC-SA to begin with, as, in my experience, too many do not meet the requirements and expected standards.

SAFE ROOFS

The following six points form a very simple guideline that might help in establishing criteria for a ‘safe roof’:

  1. All trusses must be erected straight and plumb.
  2. Ensure that all bracing, as per the design requirements, has been installed.
  3. All  multiple-ply  girders  must  be  bolted  and  nailed,  or preferably  screwed  together,  in  accordance  with  the SANS10243 requirements and the ITC-SA standards.
  4. All cleats must be fully bolted and all truss hangers fully nailed and bolted where required.
  5. Torsional restraints must be installed at girders for all incoming  supported  trusses,  as  per  the  design specifications.
  6. Fly  rafter  restraints  must  be  installed  to  all  truncated trusses and truncated girders.

Note that the above points include a substantial amount of ‘finer  detail’  that  must  be  complied  with  in  order  to  make them  effective.  The  fine  print  on  these  details  must  be thoroughly studied and carefully applied.

For more information, the author can be reached via e-mail to: tinus@trussvision.co.za

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