I'm building a linear voltage regulator using the LM 741 (part of school project) but everytime I connect a load than draws more than 1A the output voltage drops down. I'm not sure why this happens and what can I do to solve this ?
The TIP31C gain is not very high. In the ST datasheet, some binned parts are guaranteed not to have gain (beta) greater than 50. Other bins are guaranteed not to have gain greater than 24.
This means that your 741's output definitely has to produce at least 20 mA, and maybe as much as 42 mA to drive the TIP31 to deliver 1A.
This is somewhat mitigated by the fact that the TIP31's beta increases as it heats up, but still you're likely trying to get 10's of mA out of the 741.
The 741's short circuit output current is only 25 mA (based on the TI datasheet).
And the output voltage range spec implies an output equivalent resistance of about 150 ohms. (Because the typical output swing drops by 1 V when the ouput current is 6.5 mA)
So when sourcing 20 mA, the 741 output voltage is likely going to drop by about 3 V, giving a maximum output of about 11 V at this load.
Your maximum regulator output is then one Vbe drop lower, or about 10.3 V, lower than the 11 V you were trying to get.
So it looks like you were just trying to get a bit too much out of these parts.
(Also consider Peter Bennett's comment and double check that your 15 V supply is not sagging with 1 A load)
Agree with what The Photon has said, and Jack Creasey's comment on R2 should be connected to 15V.
But since you are stuck with the 741 and not enough base drive voltage, replace the TIP31 with something like below.