This is very likely an araneid (or Orb-weaver spider) in the family Araneidae.
Without further evidence, it's hard to narrow down to a species.
Neoscona crucifera (Hentz orbweaver, spotted orbweaver or barn spider)
Source: Will Cook 2001
- Relatively variable in color (and sometimes pattern), but is usually rusty-red or golden orange color.
- This site suggests females are reddish-brown to brown and males are lighter brown with the sides of the carapace nearly black.
- Dorsal abdomen is brown and hairy; abdomen underside = black, with two white spots
- The epithet crucifera refers to the cross-like pattern on their back
- Legs have alternating light and dark brown bands.
- Size: females = 9.5-19 mm long; males = somewhat smaller
- Web: very large (up to 0.61 m diameter) and often on buildings and other man-made structures
- Range: Eastern/Central USA + Mexico
According to Will Cook,
Several other species of Neoscona are confusingly similar...Other species usually have a bolder pattern.
We can rule out Neoscona domiciliorum because Illinois is west of its range and the colroation is different. [Source].