EXHIBIT 3.121

                            CERTIFICATE OF FORMATION
                                       OF
                              TSI SPRINGFIELD, LLC

         UNDER SECTION 18-201 OF THE DEL. LIMITED LIABILITY COMPANY ACT

                  This Certificate of Formation of TSI Springfield, LLC, dated
as of August 3, 1998, is being duly executed and filed by Curt D. Buyum, as an
authorized person, to form a limited liability company under the Delaware
Limited Liability Company Act (6 Del. C. Section 18-101, et seq.)

                  FIRST. The name of the limited liability company formed hereby
is TSI SPRINGFIELD, LLC.

                  SECOND. The address of its registered office in the State of
Delaware is Corporation Trust Center, 1209 Orange Street, in the City of
Wilmington, County of New Castle. The name of its registered agent at such
address is The Corporation Trust Company.

                  IN WITNESS WHEREOF, the undersigned has executed this
Certificate of Formation as of the date first above written.

                                     /s/ Curt D. Buyum
                                     ------------------------------------
                                     Name: Curt D. Buyum
                                     Title: Authorized Person