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Metal objects within a wavelength or two of the antenna could be reflecting your signal away from your audience, or otherwise creating tuning or matching problems. The structure shouldn’t present a problem in general, but nearby metal objects might inadvertantly form an undesirable antennal element. At 100 MHz, a wavelength is approximately 10 feet long (9.84 something, to be exact: 11811/F(MHz) = wavelength in inches).
The point is that anything metal that is 1/4 wavelength (2.46 feet) 1/2 wavelength (around 5 feet) 5/8 wavelength (6.15 feet) one wavelength (9.84 feet) or any combination of these, is going to interact with your antenna in what will likely be an undesirable way.
Others might have more exact information, but off hand I’d say you’re going to want to locate your antenna 35 to 50 feet away from any portion of the metal structure in order to be certain you’re free of parasitic influences. This might mean locating your transmitter and antenna high and in the clear, and then running audio and power out to the transmitter via a long cable.