These are the fare rules for a U class ticket I have, emphasis is mine where I think it applies:
Code:
COMBINABILITY
DOUBLE OPEN JAWS NOT PERMITTED.
ADD-ON CONSTRUCTION IS ADDRESSED IN MISCELLANEOUS
PROVISIONS - CATEGORY 23.
END-ON-END
END-ON-END COMBINATIONS PERMITTED. VALIDATE ALL FARE
COMPONENTS. SIDE TRIPS PERMITTED WITH NO RESTRICTIONS.
PROVIDED -
COMBINATIONS ARE FOR CARRIER DL/AF/KL/AZ
END-ON-END
FOR TRAVEL ON/BEFORE 22JUN 12
END-ON-END COMBINATIONS PERMITTED WITH US-CANADA
TRANSBORDER/DOMESTIC FARES. VALIDATE ALL FARE
COMPONENTS. SIDE TRIPS PERMITTED WITH NO RESTRICTIONS.
PROVIDED -
COMBINATIONS ARE FOR CARRIER DL/CO/UA/AA/AS/WS/AC/HA/
US/9K/WP/JV/ZK
END-ON-END
FOR TRAVEL ON/AFTER 04SEP 12
END-ON-END COMBINATIONS PERMITTED WITH US-CANADA
TRANSBORDER/DOMESTIC FARES. VALIDATE ALL FARE
COMPONENTS. SIDE TRIPS PERMITTED WITH NO RESTRICTIONS.
PROVIDED -
COMBINATIONS ARE FOR CARRIER DL/CO/UA/AA/AS/WS/AC/HA/
US/9K/WP/JV/ZK
OPEN JAWS/2-COMPONENT CIRCLE TRIPS/MULTI-COMPONENT CIRCLE
TRIPS
FARES MAY BE COMBINED ON A HALF ROUND TRIP BASIS WITH DL
FARES IN ANY RULE AND TARIFF.
-TO FORM SINGLE OPEN JAWS.
A MAXIMUM OF 2 FARE COMPONENTS PERMITTED.
MILEAGE OF THE OPEN SEGMENT MUST BE EQUAL/LESS THAN
MILEAGE OF THE SHORTEST FLOWN FARE COMPONENT.
-TO FORM 2-COMPONENT CIRCLE TRIPS
-TO FORM MULTI-COMPONENT CIRCLE TRIPS
A MAXIMUM OF 2 FARE BREAK POINTS PERMITTED.
NOTE - THE CITY PAIRS BELOW ARE CONSIDERED THE SAME POINT-
BWI-WAS FLL-MIA LAX-ONT LAX-SNA NYC-EWR OAK-SFO
-TO FORM MULTI-COMPONENT CIRCLE TRIPS
A MAXIMUM OF 2 FARE BREAK POINTS PERMITTED.
NOTE - THE CITY PAIRS BELOW ARE CONSIDERED THE SAME POINT-
BWI-WAS FLL-MIA LAX-ONT LAX-SNA NYC-EWR OAK-SFO
OAK-SJC SFO-SJC LAX-BUR
ROUTING 501 *** TRAVEL MUST BE NONSTOP OR DIRECT ***
I have no idea if that's what a "side trip" is but I don't see any problem with what you're doing, you're not abandoning a trip, it's a trip within a trip.