A study for the Fire Service Research & Training Trust (FSR&TT) investigating and solving the problem of early loss of loadbearing capability of fixings hold M&E services to timber ceilings
“Preventing firefighter & evacuee entanglement, electrocution, and obstruction risks in timber buildings on fire study” aims to pre-emptively address a believed inevitable detrimental outcome of the necessary move to net-zero construction methods that may significantly raise the risk of firefighter and occupant harm from ‘entanglement’, ‘electrocution’ and ‘crush’ risks caused by detachment of ceiling mounted M&E structures in mass timber buildings. Such detachment of services also has the capability to render other fire protection systems (lighting, detection, smoke removal, alarm, suppression) inoperative, break down passive fire boundaries as large ventilation fall from their boundary interfaces, and even damage gas fuel lines.
Unresolved, it is believed that this is a problem waiting to happen and as such there is a duty to act upon this knowledge. In demonstrating and highlighting the problem, the project aims to educate those who can influence change to put in place the necessary training to reduce risk and inform on new standards required for the specification of anchoring equipment and methods in timber buildings to reduce the risk at point of source. Firefighting in buildings of combustible structure, particularly those of height, is associated with many new complex challenges for fire services. This study specifically addresses a potential terrible scenario of ‘rapid’ collapse of M&E systems in fire which might be impossible to risk assess on the spot.
In respect of FSR&TT objectives this project addresses:
a) Prevention of risks to attending firefighters, occupants and evacuees
b) Training of FRS personnel and standards makers by bringing this issue to front of centre
c) Ensuring firefighter effectiveness by highlighting a previously unknown risk, and in the longer term putting in place the standards and infrastructure that should lead to permanent change.
The study was prompted by a coincidental finding by Dale Kinnersley (FPA and RISCAuthority), whilst investigating how pipework attachment rules for fire sprinkler systems might have to be adapted in mass timber buildings.
OUTCOMES:
This study investigated the contribution that each discreet fixing parameter made to the loss of load bearing capability under fire conditions including, embedment depth, diameter, material, thread pitch, loading, fire challenge, and contact surface area. Counterintuitively, whilst larger fixing had greater native load bearing capacity, when subject to fire, their weakening was faster and more dramatic than smaller fixings.
To explain the disproportionate loss of strength in fixings a 2-model approach is presented by way of explanation defined by the ability of the fixing to conduct heat along its length to impact the wood where the thread grips.
‘Fat’ Fixing model
In this model, the ‘fatter’ fixing is able to conduct heat deep into the wood raising its temperature between the threads so there is a loss of retention strength at the grip points. Wood weakens at surprisingly low temperatures. Whilst we recognise easily the loss of strength associated with charring, lower temperatures that impact moisture content exert great influence even before the wood undergoes pyrolysis.
‘Skinny’ Fixing model
In this model, the geometry of the fixing is less capable of conducting heat deep within the wood and the increased length ensures that the thread necessary for full retention remains in unheated wood for the duration that it must remain effective.
Whilst the ambient load bearing capacity of ‘skinny’ fixings is less that for their large diameter counterparts there are certain advantages in their use, specifically not requiring pilot-holes to be drilled, that might greatly offset the additional effort required to install greater numbers of fixings.
This study has shown that the assurance of fixing performance under fire conditions demands:
- Knowledge of the ‘critical time’ for which M&E is required to stay in place to support safe evacuation of occupants, and intervention by the fire service.
- An understanding of the heat uptake rate of the timber ceiling so that the interface depth of the unheated wood is known at the ‘critical time’
- The selection of a fixing whose embedment of thread at depths greater than the unheated interface at the critical time assures full retention of the ambient load bearing capability of the fixing at the ‘critical time’
- The selection of the fitting performs as a ‘skinny’ fixing – minimising conduction along its length
- The fitting is made of hardened steel and will not melt or break in fire.
Whilst other solutions to the problem are certainly plausible, such as the angling of fixings into the timber, in conversation with suppliers and installers this simple set of requirements is practical, can be supported by existing products, and on balance requires little greater effort than the measures already taken but warranting significant performance enhancement under fire.
The adoption of these simple measures will significantly improve firefighter safety in the conduct of their duties in the already complex environment of mass timber buildings.
Dr James Glockling is a Principal Fire Protection Engineer within the Naval Engineering Team of BMT, consultant, and is the current Chair of BSI FSH/16 ‘Hazards to life from fire’. He has a degree in Chemical Engineering and PhD in nuclear engineering. Following post-doctorate study, he worked as, a lecturer in Chemical Engineering and Fire Safety Engineering, a forensic fire investigator, and ran research laboratories at the Loss Prevention Council (LPC), Building Research Establishment (BRE), and the Fire Protection Association (FPA) where he ran the UK insurance research scheme, RISCAuthority. Jim’s principal areas of expertise are in suppression and detection technologies and complex risk mitigation scenarios. Jim is also visiting Professor at the University of Central Lancashire and continues to work promoting resilience with the commercial built environment and maritime sectors.
The study on “Preventing firefighter & evacuee entanglement, electrocution, and crush risks in mass timber buildings on fire” was Prof. James Glockling, a Principal Fire Protection Engineer within the Naval Engineering Team of BMT, consultant, and currently the Chair of BSI FSH/16 ‘Hazards to life from fire’.
The project was funded by The Fire Service Research & Training Trust and supported by The RISCAuthority, with materials kindly donated from KLH and Midfix.