You will go through immigration i.e. passport control on your way to your flight in Frankfurt.
Actually, you will not go through Immigration. Immigration is the process of entering a country (or zone); in this case entering Schengen. At immigration they verify that you are qualified to enter Schengen, and they enter into their computers that you are entering with a 90/180 day Visa. It is at emigration, they essentially check you out of the Schengen zone.
It all depends on how your ticket(s) are written. If you are coming into and leaving Frankfurt on a single ticket using the same concourse (probably the same airline), A/Z, B, or E/F, and you have carry-on luggage or it is checked through to your US destination, I believe you will just go from the in-Schengen gates to the outside-Schengen gates by going through emigration - usually a quick, simple process.
If your luggage is not checked through, you will have to leave security to pick up your luggage, recheck it, then go back through security and emigration to get to your gate. That will take some time.
If your luggage is checked through, but you have to change concourses, stay on the airside, go through emigration in that concourse and take the SkyLine shuttle to the new concourse, staying on airside. As I understand it, based on an email from FRA administration, you will probably have to go through security to get into airside in the new concourse, but it will probably be a shorter process, and you will end up outside of emigration.
If you take the "Landside" SkyLine, you will have to leave security to get to it, change concourses on the airside SkyLine, then go back through security to get to the new gate.
So, if you are fortunate enough to be flying from Venice to Houston all on Lufthansa/United, your luggage is checked through all the way, your flight form Venice is arriving at an A gate, and your flight is departing from a Z gate, 1:15 is probably way more time than you need, assuming your flight from Venice arrives at the scheduled time. At the root of the A concourse is the emigration desk; check out and take the escalators upstairs to the Z level and find your gate.