Shape looks like the common orb spiders I have in my neighborhood. If they are building orb webs then it is most likely on of those guys. The shape of the back end kind of gives it away. I haven't had them in my yard much lately which is both good and bad. We used to get some HUGE ones - like over an inch in diameter. If you stepped on them they would push back.
Spider Identification Chart - Venomous or Dangerous?