Complex sentence: The tallest building under construction is the Shanghai Tower; it will have a height of when completed, and consequently will be the tallest building in China and second tallest building in the world after the Burj Khalifa.
Split into simpler sentences: The tallest building under construction is the Shanghai Tower, which will have a height of and become the tallest building in China when completed. It is also slated to be the second tallest building in the world after the Burj Khalifa.