Complex sentence: It was the first heavy bridge designed and built by China's own technical strength, and therefore considered as one of the greatest accomplishments and a proof of the spirit and ability of the Chinese people in Mao Zedong's era.
Split into simpler sentences: It was the first heavy bridge designed and built by China's own technical strength. Therefore, it is considered as one of the greatest accomplishments and a proof of the spirit and ability of the Chinese people in Mao Zedong's era.