A composite structural member and an arrangement of such composite structural members are constructed to provide a multi-functional under-floor airframe that reacts flight, handling and internal cargo loads, and is capable of absorbing energy when subjected to vertically applied compression loads, such as loads that may be applied in hard landing or crash situations. The composite structural member and arrangement of such composite structural members adds no parasitic weight to the aircraft, i.e., there is no need for additional energy-absorbing systems, structures or mechanism; and is lighter weight as compared to metal. The composite structural member and related parts of the arrangement of such composite structural members are designed to progressively collapse throughout the stroke (or displacement) that occurs when a dynamic compressive load is applied to the structural member and to work together with crashworthy seats to mitigate injuries to occupants.