Abstract

This thesis presents an experimental investigation into the optimization of fire protection to mass timber construction, with particular focus on the performance of sprinkler systems with consideration to the fixing of pipe networks and other services to cross laminated timber (CLT) ceilings, in order to ensure fire safety.

The built environment accounts for 25% of the UK’s annual carbon footprint which is largely attributed to the significant dependence upon concrete and steel. The government’s roadmap, “Timber in Construction Roadmap 2025”, sets out an ambition for timber to play a significant role in reducing the use of concrete and steel in construction.

The first phase of the study focused on optimizing the wetting potential of sprinkler systems in rooms with exposed timber surfaces as might be found in CLT buildings. Testing was
undertaken to assess the capability of an assortment of currently available sprinkler head types, operating pressures, and mounting orientations. Experimental test facilities were
created to measure water distribution to the floor and walls, and ceiling wetting patterns were measured and recorded photographically for subsequent analysis. The results demonstrate that effective wetting of exposed ceilings and walls can be achieved with currently available equipment without compromising water delivery to room contents. These findings suggest that crucial modifications to sprinkler system design, particularly in head selection and placement, can significantly enhance fire suppression performance in timber buildings. The data generated support the need for updates to UK sprinkler standards to accommodate the unique challenges posed by exposed combustible timber surfaces, due to their combustibility.

Further improvements might be possible if new sprinkler head designs were developed specifically for this application. The second phase examined the structural integrity of screw fixings used to suspend mechanical and electrical (M&E) services from CLT ceilings, including sprinkler systems, at ambient (pre-fire), under fire and post-fire conditions. Pull-out tests were conducted to evaluate load capacity in non-fire conditions, and these measurements were used as baseline fire test results. Using a purpose-built test rig, fire tests were conducted to evaluate pull-out strength, both during and after fire exposure. Variables included screw embedment depth, shank diameter, and CLT adhesive type. Fire test results indicated that load capacity degradation could be both sudden and catastrophic, primarily driven by surface charring along the screw thread driven by thermal conduction of heat from the flame zone deep into the timber through the shank of the fixing. Both mechanisms greatly reduced the timber’s ability to hold fixings under load demonstrating ‘strength-under-fire’ to have a different relationship to the fixing embedment depth than for the non-fire situation. These effects were found to intensify with increased screw diameters which facilitated conduction of heat.

Whilst the presence of a sprinkler system will inherently protect fixings from experiencing extreme heating, the ability of other M&E to remain in place must be considered as another layer of protection. It is common practice to run heavy ducting, electrical cable trays, and service pipework from the ceiling of mass timber buildings to create an attractive industrial aesthetic, or because it is the only option. Early and rapid detachment of these services could at best impede evacuee egress and firefighter access, and at worst lead to life-loss and injury through entanglement, crush, and electrocution. Detachment may additionally cause the breakdown of building compartmentation allowing fire spread between connected compartments. This situation could be considered analogous to the Shirley Towers fire where firefighter lives were lost through entanglement with fallen services – in that instance the detachment was due to weak fixings (plastic) in a strong substrate (concrete), here the fixings are strong (steel), but the substrate (timber) weakens significantly in fire. The data will underpin the development of new standards for fixing M&E services into timber, due to the loss of strength during and after fire.

Together, these findings provide critical insights into the fire resilience of mass timber buildings and offer practical recommendations for improving fire protection strategies through
sprinkler system optimization and revised guidance on M&E screw fixing performance.