In time, due to changing procedures or increased awareness, existing structures may be subject to inspection with regards to fire safety. One of such structures, the Heilige Urbanuskerk (Church of the Holy Urbanus), has been found to lack necessary precautions. Part of the strategy to approve the building, with its aberrant shape, for fire safety are CFD simulations. Just how accurate can a church be modelled? Without drawings?
A fire safety strategy has been developed in order to fulfil the requirements for safe use. One of the key issues is to allow for a safe evacuation of visitors in case of a fire. The capacity of the (emergency) exits was too limited. Despite this there was a feeling that there would be sufficient time for evacuation in the case of a fire due to the large volume of the church which may serve as a buffer for smoke. It was agreed with the fire safety consultant to investigate this suspicion by using CFD simulations. To perform the simulations, a 3D model of the church is required. Since the church was constructed in 1867, no drawings are readily available and the ones available are not digital or up to date. A model will need to be made from scratch. This raises the question: how does one obtain accurate measurements of such a complex structure? The answer: 3D scanning techniques.
A scanning device was set up by a scanning specialist inside the church at various locations and the interior was scanned. Later, these various scans were patched together to create a single 3D shell model. This model is constructed of a multitude of polygons and as such not directly usable in CFD software. It does however provide us with all the dimensions required and with the aid of the 3D scan a 3D volumetric model can be created. Additionally, on site measurements of the door openings were taken to model the exits later with a high level of accuracy. Even though in CFD simulations simplified models are used, this level of detail allows us to create a model that is more accurate and recognizable than would be achieved by manual measurements.
To determine if visitors can safely evacuate the building two variables need to be calculated, the Available Safe Egress Time (ASET) and Required Safe Egress Time (RSET). When ASET is larger than RSET, visitors can evacuate the building safely. The RSET was calculated according to Dutch building regulations based on response time, walking speed, distance to exits and the number of visitors. In this case the RSET was determined to be 320 s, even assuming one of the emergency exits is blocked by the fire, limiting the options for visitors to escape. The ASET is determined by running a simulation with a worst case design fire. The time it takes for conditions to become untenable was calculated at 360 s. This means that even with conservative assumptions there is sufficient time available to vacate the building.
The results have been evaluated by the appropriate authorities and have contributed to approval of the fire safety strategy for this monumental building as a venue for gatherings without making changes that would be detrimental to the building’s character. Modelling a building without useful drawings is a challenge. With the help of 3D scanning this is certainly possible. In the future we’re looking at ways to simplify the process of transforming a scan into a useful model.