<PAGE>
 
                                                                     Exhibit 4.1


                              EIGHTH AMENDMENT TO
                     AMENDED AND RESTATED CREDIT AGREEMENT


     THIS  EIGHTH AMENDMENT ("Eighth Amendment") made as of June 30, 1997 by and
among PITT-DES MOINES, INC., a Pennsylvania corporation, as borrower  (the
"Borrower"), PNC BANK, NATIONAL ASSOCIATION (formerly Pittsburgh National Bank),
WELLS FARGO BANK, N.A. and AMERICAN NATIONAL BANK, as lenders (individually
"PNC", "Wells" and "American" and a "Bank" and collectively the "Banks"), PNC
BANK, NATIONAL ASSOCIATION (formerly Pittsburgh National Bank) as agent for the
Banks (in such capacity the "Agent"), and PNC BANK, NATIONAL ASSOCIATION
(formerly Pittsburgh National Bank), as the issuer of Letters of Credit (in such
capacity the "Issuing Bank"), amends certain provisions of that certain Amended
and Restated Credit Agreement dated as of June 30, 1992 as previously amended by
the First Amendment to Amended and Restated Credit Agreement dated as of
November 10, 1992, the Second Amendment to Amended and Restated Credit Agreement
dated as of June 10, 1993, the Third Amendment to Amended and Restated Credit
Agreement dated as of December 16, 1993, the Fourth Amendment to Amended and
Restated Credit Agreement dated as of June 24, 1994, the Fifth Amendment to
Amended and Restated Credit Agreement dated as of December 8, 1994 and, the
Sixth Amendment to Amended and Restated Credit Agreement dated as of May 31,
1995 and the Seventh Amendment to Amended and Restated Credit Agreement dated as
of June 30, 1996 (said Amended and Restated Credit Agreement as so amended
herein referred to as the "Original Credit Agreement").


                                 WITNESSETH:

     WHEREAS, the Borrower, the Banks, the Issuing Bank and the Agent wish to
amend the Original Credit Agreement as provided herein.

     NOW, THEREFORE, in consideration of the mutual promises and the mutual
covenants made herein and in the Original Credit Agreement and other valuable
consideration and with the intent to be legally bound hereby, the parties hereto
agree as follows:


                                 ARTICLE I
                    AMENDMENTS TO ORIGINAL CREDIT AGREEMENT

     Section 1.01  Amendments to Subsection 9.1 of Original Credit
                   -----------------------------------------------
Agreement.  (i) The following definition set forth in Subsection 9.1 of the
---------
Original Credit Agreement is hereby amended and restated as follows:
<PAGE>
 
          "Governmental Person" means the government of the United States, the
     government of Canada or the government of any state or locality therein,
     any political subdivision or any governmental, quasi-governmental,
     judicial, public or statutory instrumentality, authority, body or entity,
     or other regulatory bureau, authority, body or entity of the United States
     or any state or locality therein, including the Federal Deposit Insurance
     Company, the Comptroller of the Currency or the Board of Governors of the
     Federal Reserve System, any central bank or any comparable authority.

          "Maturity Date" means December 31, 1999 or such later date as is
     ultimately determined in accordance with Subsection 1.1g hereof.

          "Significant Subsidiaries" means collectively General, Oregon and
     Strocal.

     (ii) The following new definition is inserted into Section 9.1 of the
Original Credit Agreement in alphabetical order:

          "Eighth Amendment" shall mean that Eighth Amendment to Amended and
     Restated Credit Agreement dated as of June 30, 1997 between the Borrower,
     the Banks, the Agent and the Issuing Bank.

          "General" means General Steel Corporation, a Washington corporation
     and a Subsidiary of the Borrower.

     Section 1.02  Amendments to Section 5.7(vi) of the Original Credit
                   ----------------------------------------------------
Agreement. Section 5.7 (vi) of the Original Credit Agreement is hereby amended
---------
to delete the reference to $7,500,000 and substitute in place thereof a
reference to $10,000,000.

     Section 1.03  Amendments to Schedules to the Original Credit Agreement.
                   --------------------------------------------------------
Schedule 3.1b and Schedule 3.9 to the original Credit Agreement are deleted and
Schedule 3.1b and Schedule 3.9 attached to this Eighth Amendment are substituted
in place thereof.

     Section 1.04  No Other Amendments.  The amendments to the Original Credit
                   -------------------  
Agreement set forth in Sections 1.01 through 1.03 hereof do not either
implicitly or explicitly alter, waive or amend, except as expressly provided in
this Eighth Amendment, the provisions of the Original Credit Agreement.  The
amendments set forth in Sections 1.01 through 1.03 hereof do not waive, now or
in the future, compliance with any other covenant, term or condition to be
performed or complied with nor do they impair any rights or remedies of the
Banks, the Issuing Bank or the Agent under the Original Credit Agreement with
respect to any such violation.

                                   ARTICLE II
                    BORROWER'S SUPPLEMENTAL REPRESENTATIONS

     As an inducement to the Banks, the Issuing Bank and the Agent to enter
into this Eighth Amendment hereunder, the Borrower represents and warrants that:
<PAGE>
 
     Section 2.01  Incorporation by Reference.  Borrower hereby incorporates
                   -------------------------- 
herein by reference and repeats herein for the benefit of the Banks, the Issuing
Bank and the Agent the representations and warranties made by it in Sections 3.1
through 3.20, both inclusive, of the Original Credit Agreement and for purposes
hereof such representations and warranties, shall be deemed to extend to and
cover this Eighth Amendment.


                                  ARTICLE III
                                 MISCELLANEOUS

     Section 3.01  Ratification of Terms.  This Eighth Amendment shall be
                   ---------------------
construed in connection with and as part of the Original Credit Agreement; and
the Original Credit Agreement is hereby amended and modified to include this
Eighth Amendment. Except as expressly amended by prior Amendments to the Credit
Agreement and this Eighth Amendment, the Original Credit Agreement and each and
every representation, warranty, covenant, term and condition contained therein
is specifically ratified and confirmed.

     Section 3.02  Counterparts.  This Eighth Amendment may be executed in any
                   ------------ 
number of counterparts and by the different parties hereto on separate
counterparts, each of which when so executed and delivered shall be an original,
but all of which together shall constitute one and the same instrument. Delivery
of an executed counterpart of a signature page to this Eighth Amendment by
telecopier shall be effective as of delivery of a manually executed counterpart
of this Eighth Amendment.

     Section 3.03  Capitalized Terms.  Except for proper nouns and as otherwise
                   -----------------
defined herein, capitalized terms used herein shall have the meanings ascribed
to them in the Original Credit Agreement, as amended hereby.

     Section 3.04  Conditions Precedent.  This Eighth Amendment shall become
                   --------------------
effective (the "Amendment Effective Date") on the date on which Borrower shall
provide to the Banks, the Issuing Bank and the Agent the following:

          (A) A duly executed counterpart original of this Eighth Amendment;

          (B)  A certificate of the chief financial officer of the Borrower
certifying that, as of the date of this Eighth Amendment, no Event of Default
shall have occurred and be continuing and no event, condition, act or omission
has occurred and is continuing which, with the passage of time, the giving of
notice or both, would constitute a Event of Default, or would result from the
execution of this Eighth Amendment;


          (C)  A certified copy of the corporate action of the Borrower
authorizing the execution and delivery of the performance under this Eighth
Amendment;

          (D)  A good standing certificate for the Borrower issued by the
Secretary of State of the state of its incorporation and of the jurisdiction of
its principal place of business;
<PAGE>
 
          (E)  A lien certificate for the Borrower issued by the Department of
Revenue of the state of its incorporation and of the jurisdiction of its
principal place of business;


          (F)  A certificate of the secretary or assistant secretary of the
Borrower certifying the names of the persons authorized to sign this Eighth
Amendment and all other documents and certificates delivered hereunder together
with the true signatures of such persons;

          (G)  Such other instruments, documents and opinions of counsel as
the Agent shall reasonably require, all of which shall be satisfactory in form
and content to the Agent and its special counsel, Tucker Arensberg, P.C.

     Section 3.05  Original Credit Agreement.  From and after the Amendment
                   -------------------------
Effective Date, all references in the Original Credit Agreement to the Original
Credit Agreement shall be deemed to be references to the Original Credit
Agreement as amended hereby.

     Section 3.06  Entire Agreement.  This Eighth Amendment contains the
                   ----------------
entire agreement between the parties relating to the subject matter hereof;
there are merged herein all prior representations, promises and conditions,
whether oral or written, in connection with the subject matter hereof, and any
representation, promise or condition not incorporated herein shall not be
binding upon the parties.

     Section 3.07  Severability.  Whenever possible each provision of this
                   ------------
Eighth Amendment shall be interpreted in such manner as to be effective and
valid under applicable law but if any provision of this Eighth Amendment or any
part of such provision shall be prohibited by or invalid under applicable law,
such provision of part thereof shall be ineffective to the extent of such
prohibition or invalidity without invalidating the remainder of such provision
or the remaining provisions of this Eighth Amendment.

     Section 3.08  Governing Law.  THIS EIGHTH AMENDMENT AND THE RIGHTS AND
                   -------------
OBLIGATIONS HEREUNDER SHALL BE CONSTRUED IN ACCORDANCE WITH AND GOVERNED BY THE
LAWS OF THE COMMONWEALTH OF PENNSYLVANIA WITHOUT REGARD TO THE PROVISIONS
THEREOF REGARDING CONFLICTS OF LAW.

     Section 3.09  Headings.  The headings of this Eighth Amendment are for
                   --------
purposes of reference only and shall not limit or otherwise affect the meaning
thereof.

     Section 3.10  References.  All notices, communications, agreements,
                   ----------
certificates, documents or other instruments executed and delivered after the
execution and delivery of this Eighth Amendment may refer to the Original Credit
Agreement without making specific reference to this Eighth Amendment, but
nevertheless all such references shall include this Eighth Amendment unless the
context requires otherwise.

     Section 3.11  Taxes.  The Borrower hereby agrees (i) to pay any and all
                   -----
stamp and other taxes and fees payable or determined to be payable in connection
with the execution, delivery, filing and recording of this Eighth Amendment and
such other documents, and (ii) to save each of the Banks, the Issuing Bank and
the Agent harmless from and against any and all
<PAGE>
 
liabilities with respect to or resulting from any delay in paying or omission to
pay such taxes and fees.

     Section 3.12  Costs and Expenses.  The Borrower hereby agrees to pay
                   ------------------
all costs and expenses of the Banks, the Issuing Bank and the Agent (including,
without limitation, the reasonable fees and the disbursements of the Agent's
special counsel, Tucker Arensberg, P.C.) in connection with the preparation,
execution and delivery of this Eighth Amendment and the related documents.

                 [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]
<PAGE>
 
     IN WITNESS WHEREOF, the parties hereto, with the intent to be legally bound
hereby have caused this Eighth Amendment to be duly executed by their proper and
duly authorized officers as of the day and year first above written.


<TABLE>
<CAPTION>

ATTEST:                  (SEAL)                PITT-DES MOINES, INC.

 
<S>      <C>                                   <C>      
By       /s/   Thomas R. Lloyd                 By       /s/   Richard A. Byers
               ---------------                          ---------------------------
Name:          Thomas R. Lloyd                 Name:          Richard A. Byers
Title:         General Counsel & Secretary     Title:         Vice President Finance and 
                                                              Administration
 
                                               PNC BANK, NATIONAL ASSOCIATION (formerly
                                               Pittsburgh National Bank) as a Bank, as the
                                               Issuing Bank and as the Agent


                                               By       /s/   Lawrence W. Jacobs
                                                       ----------------------------
                                               Name:          Lawrence W. Jacobs
                                               Title:         Vice President


                                               WELLS FARGO BANK, N.A.


                                               By       /s/ Judy Pallios
                                                        ---------------------------
                                               Name:        Judy Pallios
                                               Title:       Vice President


                                               AMERICAN NATIONAL BANK


                                               By       /s/ James R. Popp
                                                        ---------------------------
                                               Name:        James R. Popp
                                               Title:       Vice President

</TABLE>