U.S. District Judge Joan Lenard imposed the sentence Wednesday on 24-year-old Burson Augustin. Four other men are scheduled to be sentenced over the next three days.
Prosecutors sought the maximum 30-year sentence for Augustin. Lenard ruled that sentencing terrorism enhancements should not apply to Augustin, who was not the leader.
Prosecutors said in court papers filed Monday that Lenard should reject pleas for leniency. Ringleader Narseal Batiste faces up to 70 years behind bars, with the other four men facing between 30 and 50 years.
The men were convicted in May of supporting al Qaeda, conspiring to wage war on the U.S. and other charges. There were two mistrials, and two other men were acquitted. All denied plotting terror attacks.
The 110-story Chicago skyscraper is now called the Willis Tower.