<PAGE>




                                                                Exhibit 10.1a(5)

             FIFTH AMENDMENT TO CREDIT AGREEMENT AND LIMITED WAIVER

     This Fifth Amendment to Credit Agreement and Limited Waiver, dated as of
December 31, 2001 (this "Amendment and Waiver"), by and among STEEL DYNAMICS,
INC., an Indiana corporation (the "Borrower"), the lenders executing this
Amendment and Waiver on the signatures pages hereof and MELLON BANK, N. A., as
Agent for the Lenders under the Credit Agreement (Amended and Restated) referred
to below (the "Agent").

     Introductory Statements:

     A. Reference is made to the Credit Agreement (Amended and Restated), dated
as of June 30, 1994, as amended and restated as of June 30, 1997 (as amended to
date, the "Credit Agreement"), by and among the Borrower, the Lenders listed on
the signature pages hereof (collectively, the "Lenders") and the Agent.
Capitalized terms used herein and not otherwise defined herein shall have the
meanings ascribed to such terms in the Credit Agreement.

     B. The Borrower has advised the Lenders and the Agent that the Borrower is
in violation of Section 6.01(a) of the Credit Agreement in that the Borrower's
Leverage Ratio for the period of four consecutive fiscal quarters ending
December 31, 2001 exceeds 5.0 to 1 (such violation, the "Identified Violation").

     C. The Borrower has also advised the Lenders and the Agent that the
Borrower may, as of March 31, 2002, be in violation of Section 6.01(a) of the
Credit Agreement in that the Borrower's Leverage Ratio for the period of four
consecutive fiscal quarters ending March 31, 2002 may exceed 4.0 to 1 (such
violation, the "Identified March Violation").

     D. The Borrower has requested that, for the period from December 31, 2001
to, but not including, March 31, 2002 (the "Waiver Period"), the Required
Lenders waive (i) compliance with Section 6.01(a) of the Credit Agreement for
the period of four consecutive fiscal quarters ending December 31, 2001, (ii)
satisfaction of the conditions of lending under Section 4.02(d) of the Credit
Agreement to the extent, and only to the extent, that such conditions of lending
fail to be satisfied as a result of, and as a result solely of, the Identified
Violation and (iii) satisfaction of the conditions to conversion or renewal of
Interest Rate Options under Section 2.07(a) of the Credit Agreement to the
extent, and only to the extent, that such conditions to conversion or renewal
fail to be satisfied as a result of, and as a result solely of, the Identified
Violation.

     E. The Borrower has advised the Lenders that it is considering arranging
for the refinancing the Loans and the Specified Unsecured Bank Debt. The
Borrower has requested that if, and only if, on March 31, 2002, the Borrower
shall have theretofore received commitments from financial institutions to
provide financing on terms satisfactory to the Borrower in an aggregate amount
sufficient to pay the Loans and the Specified Unsecured Bank Debt in full and
the Borrower and such financial institutions shall be in process of finalizing
the documentation for such refinancing, and the Borrower shall have provided to
the Lenders a certificate to both such effects, then the Waiver Period shall be
extended to end on April 30, 2002 (or, if earlier, the date on which such
commitments terminate or on which the process of finalizing documentation for
such refinancing ceases) and the Identified March Violation, as well as the
Identified Violation, shall be covered by the waiver referred to in the
immediately preceding paragraph.

     F. The Borrower and the Required Lenders desire to make certain amendments
to the Credit Agreement.



<PAGE>


     Section 1. Waiver. The Required Lenders hereby waive, for the Waiver Period
only, (i) compliance with Section 6.01(a) of the Credit Agreement for the period
of four consecutive fiscal quarters ending December 31, 2001, (ii) satisfaction
of the conditions of lending under Section 4.02(d) of the Credit Agreement to
the extent, and only to the extent, that such conditions of lending fail to be
satisfied as a result of, and as a result solely of, the Identified Violation
and (iii) satisfaction of the conditions to conversion or renewal of Interest
Rate Options under Section 2.07(a) of the Credit Agreement to the extent, and
only to the extent, that such conditions to conversion or renewal fail to be
satisfied as a result of, and as a result solely of, the Identified Violation.

     Section 2. Extended Waiver. The effectiveness of the waiver granted by the
Required Lenders in the next succeeding sentence is subject to satisfaction of
the condition that on March 31, 2002, the Borrower shall have theretofore
received commitments from financial institutions to provide financing on terms
satisfactory to the Borrower in an aggregate amount sufficient to pay the Loans
and the Specified Unsecured Bank Debt in full and the Borrower and such
financial institutions shall be in process of finalizing the documentation for
such financing and the Borrower shall have provided to the Lenders a certificate
to both such effects. Subject to satisfaction of the condition set forth in the
immediately preceding sentence, the Required Lenders hereby waive, for the
Waiver Extension Period only, (i) compliance with Section 6.01(a) of the Credit
Agreement for the period of four consecutive fiscal quarters ending December 31,
2001 and for the period of four consecutive fiscal quarters ending March 31,
2002, (ii) satisfaction of the conditions of lending under Section 4.02(d) of
the Credit Agreement to the extent, and only to the extent, that such conditions
of lending fail to be satisfied as a result of, and as a result solely of, the
Identified Violation and/or the Identified March Violation and (iii)
satisfaction of the conditions to conversion or renewal of Interest Rate Options
under Section 2.07(a) of the Credit Agreement to the extent, and only to the
extent, that such conditions to conversion or renewal fail to be satisfied as a
result of, and as a result solely of, the Identified Violation and/or the
Identified March Violation. As used herein, the term Waiver Extension Period
means the period commencing on March 31, 2002 and ending on the earlier to occur
of (i) April 30, 2002 and (ii) the date on which the commitments referred to in
the first sentence of this Section 2 terminate or on which the process of
finalizing documentation for the financing referred to in the first sentence of
this Section 2 ceases.

     Section 3. Amendments.

     3.1. The Credit Agreement is hereby amended by adding thereto, as a new
Section 6.19 thereof, the following:

          6.19. Maintenance of Cash with Lenders. The Borrower shall not permit
     any of its cash to be held or maintained other than in one or more deposit
     accounts each of which is maintained by the Borrower with a Lender. The
     Borrower confirms that under the Security Agreement the Borrower is
     required to maintain perfection of the security interest of the Agent, for
     the benefit of the Lenders, in the Borrower's Cash Equivalent Investments.

     3.2. Section 6.03 of the Credit Agreement, entitled "Indebtedness" is
hereby amended by deleting the figure "$95,000,000" appearing at the end of
clause (i) thereof and inserting in lieu thereof the figure "$125,000,000".

     3.3. Section 6.04 of the Credit Agreement, entitled "Guaranties,
Indemnities, etc." is hereby amended by deleting the figure "$14,035,819"
appearing in clause (h) thereof and inserting in lieu thereof the figure
"$23,970,030" and by adding, immediately before the period at the end of such
clause (h), the phrase "and such guaranty shall be subject to reduction


                                      -2-
<PAGE>


as contemplated by paragraph 3 of the Agreement attached as Exhibit A to the
Fifth Amendment to this Agreement".

     3.4. Section 6.05 of the Credit Agreement, entitled "Loans, Advances and
Investments", is hereby amended by deleting the figure "$95,000,0000" appearing
in clause (g) thereof and inserting in lieu thereof the figure "$125,000,000".

     3.5. Section 6.10 of the Credit Agreement, entitled "Dispositions of
Properties", is hereby amended by inserting at the end thereof, after paragraph
(f) thereof, the following sentence:

          It is understood that the issuance by the Borrower of its treasury
          shares of common stock pursuant to the Agreement attached as Exhibit A
          to the Fifth Amendment to this Agreement shall be deemed not to
          violate this Section 6.10.

     3.6. Section 6.12 of the Credit Agreement, entitled "Dealings with
Affiliates", is hereby amended by inserting at the end thereof, after paragraph
(e) thereof, the following sentence:

          It is understood that the execution, delivery and performance by the
          Borrower of the Agreement attached as Exhibit A to the Fifth Amendment
          to this Agreement shall be deemed not to violate this Section 6.12.

     3.7. Section 3.15 of the Credit Agreement, entitled "Ownership and
Control", is hereby amended by adding thereto, immediately before the period at
the end of the third sentence thereof, the following clause: "and except for
performance by the Borrower of the Agreement attached as Exhibit A to the Fifth
Amendment to this Agreement".

     Section 4. Miscellaneous.

     4.1 This Amendment and Waiver shall become effective as of December 31,
2001 upon (i) the execution and delivery hereof by the Required Lenders and the
Borrower (ii) the execution and delivery by the Borrower and the lenders under
the credit agreement governing the Specified Unsecured Bank Debt of a waiver and
amendment substantially similar hereto and (iii) the payment by the Borrower to
the Agent, for the respective accounts of Lenders which shall have indicated
their consent to this Amendment and Waiver in writing to the Agent on or before
the date of execution and delivery hereof by the Required Lenders and the
Borrower of a waiver fee in the amount of 50 basis points times the aggregate
amount of the Commitments of each such Lender. By its acceptance hereof, the
Borrower agrees to pay such fee to each Lender which shall have indicated its
consent to this Waiver in writing to the Agent on or before January 29, 2002.

     4.2. The waiver granted by this Amendment and Waiver is limited to the
terms expressly provided herein. No implied waivers shall be inferred by the
execution of this Waiver and the exercise of any right or privilege under this
Waiver shall not affect any right, remedy, power or privilege of the Lenders or
the Agent provided under the Credit Agreement. By its acceptance hereof the
Borrower expressly ratifies and confirms the provisions of the Credit Agreement
and the Credit Agreement (as amended by Section 3 hereof) remains in full force
and effect among the parties thereto.

     4.3 This Amendment and Waiver may be executed in any number of counterparts
and by the different parties hereto on separate counterparts each of which, when
so executed, shall be deemed an original, but all such counterparts shall
constitute but one and the same instrument.


                                      -3-
<PAGE>


     4.4 By their execution and delivery hereof, the Required Lenders hereby
authorize and direct the Agent to execute and deliver this Amendment and Waiver.

     4.5 THIS AMENDMENT AND WAIVER SHALL BE GOVERNED BY, AND CONSTRUED AND
ENFORCED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK WITHOUT REGARD TO
CHOICE OF LAW PRINCIPLES.

     IN WITNESS WHEREOF, the parties have executed this consent as if the date
first above written.

                                            MELLON BANK, N.A., as Agent

                                            By /s/ Roger N. Stanier
                                            ------------------------------------
                                            Title: Vice President



                                            GE CAPITAL CFE, INC.



                                            By /s/ Michael Smith
                                            ------------------------------------
                                            Title: Duly Authorized Signitory




                                            KREDITANSTALT FUR WIEDERAUFBAU

                                            By /s/ Illegible
                                            ------------------------------------
                                            Title:

                                            By /s/ Illegible
                                            ------------------------------------
                                            Title:


                                            COMERICA BANK



                                            By /s/ James B. Haeffner
                                            ------------------------------------
                                            Title: First Vice President



                                      -4-
<PAGE>



                                            THE INDUSTRIAL BANK OF JAPAN,
                                              LIMITED

                                            By /s/ Walter R. Wolff
                                            ------------------------------------
                                            Title: Joint General Manager
                                                   and Group Head


                                            BANK ONE, INDIANA



                                            By
                                            ------------------------------------
                                            Title:



                                            WACHOVIA BANK, N. A.



                                            By /s/ Brad White
                                            ------------------------------------
                                            Title: Vice President



                                            BANK AUSTRIA CREDITANSTALT CORPORATE
                                                FINANCE, INC.



                                            By
                                            ------------------------------------
                                            Title:



                                            By
                                            ------------------------------------
                                            Title:



                                            NATIONAL CITY BANK OF INDIANA

                                            By /s/ Larry Mayers
                                            ------------------------------------
                                            Title: Senior Vice President




                                      -5-
<PAGE>




                                            LASALLE BANK NATIONAL ASSOCIATION

                                            By  /s/ James Thompson
                                            ------------------------------------
                                            Title:  Group Senior Vice President



                                            THE CHASE MANHATTAN BANK

                                            By  /s/ James H. Ramage
                                            ------------------------------------
                                            Title:  Managing Director



                                            HARRIS TRUST AND SAVINGS BANK

                                            By  /s/ Thad D. Rasche
                                            ------------------------------------
                                            Title:  Vice President



                                            WESTDEUTSCHE LANDESBANK
                                                GIROZENTRALE, NEW YORK BRANCH

                                            By  /s/ Donald F. Wolf
                                            ------------------------------------
                                            Title:  Director



                                            By  /s/ Elisabeth R. Wilds
                                            ------------------------------------
                                            Title:  Associate Director



                                            SUNTRUST BANK



                                            By
                                            ------------------------------------
                                            Title:




                                      -6-
<PAGE>




                                            NORTHERN TRUST COMPANY



                                            By   /s/ Mark E. Taylor
                                            ------------------------------------
                                            Title:   Vice President



                                            FIRST UNION NATIONAL BANK

                                            By   /s/ Eric Tan
                                            ------------------------------------
                                            Title:   Vice President



                                            THE HUNTINGTON NATIONAL BANK

                                            By   /s/ Angela Cecil
                                            ------------------------------------
                                            Title:   Vice President



                                            STEEL DYNAMICS, INC.



                                            By   /s/ Keith E. Busse
                                            ------------------------------------
                                            Title:   President


                                      -7-