This paper presents the results from an experimental investigation into the planar, multi-mode response of a metallic portal frame to a harmonic support excitation. Forcing frequency and amplitude sweeps near the frequencies of the eighth and tenth modes of the structure revealed multi-mode responses. In the frequency interval surrounding that of the eighth mode, two additional lower-frequency modes, the fourth and the sixth, were excited. The mechanism responsible for the multimode behaviour of the frame is a combination internal resonance of the additive type. In addition, for a brief interval, a non-resonant interaction involving the eighth mode and the first mode was observed. Forcing amplitude sweeps for forcing frequencies below the eighth linear natural frequency produced a saturation-type phenomenon and a chaotically modulated motion. For the frequency interval surrounding that of the tenth mode, the response contained contributions from seven modes; these modes were excited through the simultaneous satisfaction of several internal resonance conditions.
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