In answer to this question, consider the following. The building’s structural steel and concrete reinforcing in all likelihood constitute a dense metallic cage, particularly throughout any given reinforced concrete floor, assuming that either deliberate or inadvertent electrical continuity has been established between these structural components. Concrete reinforcing steel is typically so dense that it provides what can be considered, for most practical purposes, a quasi-equipotential surface on each floor. Touch and step voltages inside the building are therefore not expected to represent a safety concern, provided that electrical equipment grounds are bonded to the building steel near the equipment. Nevertheless, you can model such a building with MALZ or HIFREQ and compute touch and step voltages.
A sketch of one proposed model is attached. In this model, we essentially specify the earth surface as being the surface of the floor of interest to you (e.g., the third floor, if you are studying a GIS on that floor of the building). We then specify what is assumed here to be a concrete floor as your uppermost soil layer (wet concrete has a resistivity on the order of 50 to 100 ohm-m; dry concrete is more like 2,000 to 10,000 ohm-m). Beneath that, model an air layer, representing the air gap between the floor you are modeling and the basement or first floor that is in contact with the earth in reality. Specify a relatively high resistivity for this air layer (e.g., 100,000 to 1,000,000 ohm-m). Note, however, that run time will increase as a function of the resistivity ratios between different layers, so the idea is to find a large value (it must be large compared with the resistivity of the concrete floor above and the earth below, say on the order of 100 times greater) that will yield reasonably accurate results without taking forever to run. In order to model the building, you simply need to model a skeleton of the building steel and rebar, except in the area where touch and step voltages are of concern, where you might want to increase the density of the steel conductors modeled. Although the upper floor concrete layer is infinite in lateral extent in the model, this should have little impact on touch and step voltages computed inside the building, since the floor is relatively thin and bounded with metallic elements, which carry current to the native soil below. Specify observation points for the uppermost floor of interest at z=0 (“Earth Surface in Computer Model” level) and compute touch and step voltages. Observation points can also be specified for the lowest level floor that is in contact with the earth. You can use either MALZ or HIFREQ for the simulation (in the case of MALZ, simply ignore conductors above the “Earth Surface in Computer Model” level).
You might want to run an initial simulation with a minimal model (very few conductors), in order to see to what degree the resistivities you choose for the air and for the top floor concrete affect your results, before running a complete model with many conductors.
To compute touch and step voltages outside the building, switch to a model that represents only the below-grade components of the building, with the true soil structure (no air layer or aboveground concrete floor). Granted, this model will fill the basement (if there is one) with earth, but this should not significantly affect touch and step voltages outside the building.