Keywords: Truss-Z, modular skeletal system, organic design, discrete structural optimization, retrofitting, pedestrian ramp.

The Truss-Z (TZ) is a concept for a modular skeletal system for creating free-form ramp links and networks among any number of terminals in space. The main structure of the system is built of only two types of modules which can be connected in four different ways. This paper presents a case study of retrofitting a single branch of the TZ ramp structure in a given three-dimensional environment. The spatial configuration of modules is automatically generated, and the only input required are the coordinates of the initial unit, the target zone defined by a cuboid and the spatial configuration of the environment where the TZ structure is to be installed.

This is a multi-objective optimization problem with two criteria: quantitative and qualitative - the number of modules to be the smallest (economical optimization) and the condition that none of the modules collides with any other objects (satisfaction of the geometrical constraints). The case study is done for the Musashikosugi railway station in Tokyo, where the pedestrian traffic flows through a large stairs with 39 steps at only one intermediate landing and total elevation of over 6 metres. Such a communication path requires tremendous effort on pedestrians, especially the elderly and persons with mobility problems. There are also an escalator and elevator, however in case of a fire or a major earthquake, they cannot be used, and in such a case evacuation may become very difficult. Because of these concerns, retrofitting of a ramp, that is a step-less system would greatly improve the mobility, comfort and safety of the users. This case represents a large class of pedestrian access problems which can be solved with the TZ system.

Since the number of modules to construct such a linkage is relatively large, the number of all possible configurations, that is the search space, is enormous. Moreover, due to the geometrical constraints it is also possible that in certain situations there is no allowable solution.

Because the system is modular, the optimization of the shape of such a link is discrete. Therefore the search for the optimal solution has a combinatorial nature. At first a preliminary configuration created manually is presented, followed by implementation of backtracking. Next, an evolutionary algorithm (EA) is implemented. The encoding of a single three-dimensional branch of TZ, the selection method, the fitness function, the operations of mutation and crossover are briefly described. In particular, the calibration of fitness function parameters, which is usually the most challenging part in a meta-heuristic approach, is explained and briefly discussed.

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