A system and method of manufacture providing reinforced structurally functional load-bearing members, including but not limited to using thermoplastic materials, such as High Density Polyethylene (HDPE), reinforced such as with an aluminum alloy or carbon fiber core element. Among its possible uses, the present invention has application for provision of structural support members, such as an illustrative I-joist product having a vertical center member preferably comprising HDPE, and top and bottom flanges having structurally meaningful reinforcement. The center member and flanges preferably comprising HDPE provides a relatively hard, durable, substantially weather-resistant structure.