A waypoint based approach to visibility in performance based fire safety design

TL;DR

The paper introduces a waypoint-based visibility map framework for performance-based fire safety design, solving the limitations of local visibility measures by integrating Jin's law along line-of-sight to exit signs. It defines a set of waypoints, computes an averaged extinction coefficient along rays, and constructs time-dependent and time-aggregated Boolean visibility maps that indicate safe egress regions. The approach accounts for viewing angle, visual obstructions, and sign type, and can produce ASET-like maps through post-processing, demonstrated via a design-fire scenario in a small office. Implemented as the open-source Python package FDSVismap, the method provides a practical, interpretable tool for post-processing CFD fire simulation data, though it highlights the need for careful parameter selection and potential computational optimization. The work suggests a path toward more credible, geometry-driven egress assessments and highlights areas for future enhancement, including multi-storey analysis and experimental validation of Jin's law under varied smoke conditions.

Abstract

In performance-based fire safety design, ensuring safe egress, e.g. by visibility of safety signs, is a crucial safety goal. Compliance with the building requirements is often demonstrated by simulations of smoke spread. Numerical models like the Fire Dynamics Simulator generally compute visibility as a local quantity using the light extinction coefficient, without the consideration of the actual light path to a safety sign. Here, visibility maps are introduced, providing an approach for post-processing fire simulation data. They indicate safe areas along egress routes, with respect to visibility. At each location, the available visibility is calculated using Jin's law, as an integrated value of the extinction coefficient along the line of sight to the closest exit sign. The required visibility results from the distance between those points. Additional parameters like view angle or visual obstructions are considered. The presented method allows for temporal visibility assessment, e.g. in an ASET-RSET analysis.

Figures (14)

• Figure 1: Flow chart for the computation of visibility maps $M_{i,j}$ and $\mathrm{ASET}_{i,j}$ maps based on FDS simulation data. The algorithm can be subdivided into geometric computations and visibility computations. Both processes build on the simulation's boundary conditions and results, and on the defined waypoint parameters
• Figure 2: Exemplary visualisation of the distance matrix $L_{i,j,k}$ (b) and view-angle matrix $A_{i,j,k}$ (a) for the waypoint $W_k$ as a function of the Cartesian coordinates $X_i~/~Y_j$ of the agent cells $i,j$
• Figure 3: The projection surface of the exit sign being observed changes according to the viewing angle $\theta$. $\theta$ can be described as a function of the orientation of the sign in the global $z$-plane, expressed by the rotation angle $\alpha_k$, and the viewer's position at the agent cell $i,j$.
• Figure 4: A ray-casting algorithm is first employed to detect agent cells $i,j$ that are concealed by obstruction cells. The line of sight between the waypoint $\mathrm{W}_k$ and the edge cells is rasterised by the Bresenham's line algorithm or an advanced algorithm involving anti-aliasing. In the next step, the algorithm is employed to identify all cells that are considered in computing the average extinction coefficient $\bar{\sigma}_{i,j,k}$ between the waypoint $\mathrm{W_k}$ and all unconcealed agent cells $i,j$. The domain is discretised in the same shape as the numerical grid of the fire simulation.
• Figure 5: In $U_{i,j,k}$ all agent cells $i,j$ are labelled as unconcealed (1, True) or concealed (0, False) when viewed from the waypoint $W_k$ depending on their location before or behind the first collision cell. When adjacent obstruction cells do not form an entirely closed barrier, a collision point may not occur with the rasterised line of sight. A more advanced ray-casting algorithm with a line thickness > 1 cell, e.g. using anti-aliasing, may then be applied to enforce an intersection.