<PAGE>   1
                                                                Exhibit 10.20(b)

                             MODIFICATION AGREEMENT


         BY THIS MODIFICATION AGREEMENT (the "Agreement"), made and entered into
as of the 10th day of March, 1999, WELLS FARGO BANK, NATIONAL ASSOCIATION, a
national banking association, whose address is 100 West Washington, Phoenix,
Arizona 85003 (hereinafter called "Lender"), and SCHUFF STEEL COMPANY, a
Delaware corporation, whose address is 420 South 19th Avenue, Phoenix, Arizona
85009 (hereinafter called "Borrower"), in consideration of the mutual covenants
herein contained and other good and valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, hereby confirm and agree as
follows:

SECTION 1. RECITALS.

     1.1 Borrower and Lender, as Lender, Arranger, Administrative Agent, Issuing
Bank and Swing Line Lender, entered into a Credit Agreement dated June 30, 1998
(the "Credit Agreement"), which provided for, among other things, (a) a
revolving line of credit (the "RLC") in the amount of $25,000,000.00, evidenced
by a Revolving Promissory Note dated June 30, 1998, executed by the Borrower
(the "RLC Note"), and (b) a revolving line of credit (the "Swing Line" and with
the RLC, the "Loans") in the amount of $5,000,000.00, evidenced by a Revolving
Promissory Line dated June 30, 1998, executed by the Company (the "Swing Line
Note" and with the RLC Note the "Notes"), all upon the terms and conditions
contained therein. All undefined capitalized terms used herein shall have the
meaning given them in the Credit Agreement. The Credit Agreement, the Notes and
all other agreements, documents and instruments relating to the Loans are
referred to as the Loan Documents.

     1.2 As of the date hereof, prior to the effect of the modifications
contained herein, the outstanding principal balance of the RLC is $0 and of the
Swing Line is $0.

     1.3 Borrower and Lender desire to modify the Loan Documents as set forth
herein.

SECTION 2. LOAN AGREEMENT.

     2.1 Section 6.11 of the Credit Agreement is hereby amended to read as
follows:

          6.11 Financial Covenants. Permit the following (collectively, the
     "Financial Covenants"):

               (a) Its Leverage Ratio at the end of any Fiscal Quarter to exceed
          3.75 to 1.0.

               (b) Its Interest Coverage Ratio at the end of any Fiscal Quarter
          for the prior twelve-month period to be less 
<PAGE>   2
          than 2.25 to 1.0.

               (c) Its Fixed Charge Coverage Ratio at the end of any Fiscal
          Quarter for the prior twelve-month period to be less than 1.25 to 1.0.

               (d) Its EBITDA at the end of any Fiscal Quarter to be less than
          $25,000,000.00 for the prior twelve-month period. For this purpose,
          EBITDA will be calculated on a pro-forma basis to include all entities
          acquired.

     2.2 Article 5 of the Credit Agreement is hereby amended by the addition of
the following Section:

          Section 5.12 Year 2000 Covenant. Borrower shall ensure that the
     following are Year 2000 Compliant in a timely manner, but in no event later
     than December 31, 1999: (a) Borrower itself; and (b) any other major
     commercial properties and entities in which Borrower holds a controlling
     interest. Borrower shall further make reasonable inquiries of and request
     reasonable validation that each of the following are similarly Year 2000
     Compliant: (x) all major tenants or other entities from which Borrower
     receives payments; and (y) all major contractors, suppliers, service
     providers and vendors of Borrower. As used in this paragraph, "major" shall
     mean properties or entities the failure of which to be Year 2000 Compliant
     would have a material adverse economic impact upon Borrower. The term "Year
     2000 Compliant" shall mean, in regard to any property or entity, that all
     software, hardware, equipment, goods or systems utilized by or material to
     the physical operations, business operations, or financial reporting of
     such property or entity (collectively, the "systems") will properly perform
     date sensitive functions before, during and after the year 2000. In
     furtherance of this covenant, Borrower shall, in addition to any other
     necessary actions, perform a comprehensive review and assessment of all
     systems of Borrower, and shall adopt a detailed plan, with itemized budget,
     for the testing, remediation, and monitoring of such systems. Borrower
     shall, within thirty business days of Lender's written request, provide to
     Lender such certifications or other evidence of Borrower's compliance with
     the terms of this paragraph as Lender may from time to time reasonably
     require.

     2.3 Schedule 3.1 of the Credit Agreement is hereby amended to read as
attached hereto.

     2.4 Exhibit B of the Credit Agreement is hereby amended to read as attached
hereto.

                                      -2-
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SECTION 3. OTHER MODIFICATIONS, RATIFICATIONS AND AGREEMENTS.

     3.1 All references to the Credit Agreement in the Loan Documents are hereby
amended to refer to the Credit Agreement as hereby amended.

     3.2 Borrower acknowledges that the indebtedness evidenced by the Notes is
just and owing, that the balance thereof is correctly shown in the records of
Lender as of the date hereof, and Borrower agrees to pay the indebtedness
evidenced by the Notes according to the terms thereof, as herein modified.

     3.3 Borrower hereby reaffirms to Lender each of the representations,
warranties, covenants and agreements of Borrower set forth in the Notes and the
Credit Agreement, with the same force and effect as if each were separately
stated herein and made as of the date hereof.

     3.4 Borrower hereby ratifies, reaffirms, acknowledges, and agrees that the
Notes and the Credit Agreement, represent valid, enforceable and collectible
obligations of Borrower, and that there are no existing claims, defenses,
personal or otherwise, or rights of setoff whatsoever with respect to any of
these documents or instruments. In addition, Borrower hereby expressly waives,
releases and absolutely and forever discharges Lender and its present and former
shareholders, directors, officers, employees and agents, and their separate and
respective heirs, personal representatives, successors and assigns, from any and
all liabilities, claims, demands, damages, action and causes of action, whether
known or unknown and whether contingent or matured, that Borrower may now have,
or has had prior to the date hereof, or that may hereafter arise with respect to
acts, omissions or events occurring prior to the date hereof and, without
limiting the generality of the foregoing, from any and all liabilities, claims,
demands, damages, actions and causes of action, known or unknown, contingent or
matured, arising out of, or in any way connected with, the Loans. Borrower
further acknowledges and represents that no event has occurred and no condition
exists that, after notice or lapse of time, or both, would constitute a default
under this Agreement, the Notes or the Credit Agreement.

     3.5 All terms, conditions and provisions of the Notes and the Credit
Agreement are continued in full force and effect and shall remain unaffected and
unchanged except as specifically amended hereby. The Notes and the Credit
Agreement, as amended hereby, are hereby ratified and reaffirmed by Borrower,
and Borrower specifically acknowledges the validity and enforceability thereof.

SECTION 4. GENERAL.

     4.1 This Agreement in no way acts as a release or relinquishment of those
rights securing payment of the Loans. Such rights are hereby ratified,
confirmed, renewed and extended by Borrower in all respects.

                                      -3-
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     4.2 The modifications contained herein shall not be binding upon Lender
until Lender shall have received all of the following:

          (a) An original of this Agreement fully executed by the Borrower.

          (b) An original Guaranty, an original Security Agreement and original
     UCC-1 Financing Statements executed by each Subsidiary of Borrower.

          (c) Such resolutions or authorizations and such other documents as
     Lender may require relating to the existence and good standing of the
     Borrower and each Subsidiary and the authority of any person executing this
     Agreement or other documents on behalf of the Borrower and each Subsidiary.

     4.3 Borrower shall execute and deliver such additional documents and do
such other acts as Lender may reasonably require to fully implement the intent
of this Agreement.

     4.4 Borrower shall pay all costs and expenses, including, but not limited
to, reasonable attorneys' fees incurred by Lender in connection herewith,
whether or not all of the conditions described in Paragraph 4.2 above are
satisfied. Lender, at its option, but without any obligation to do so, may
advance funds to pay any such costs and expenses that are the obligation of the
Borrower, and all such funds advanced shall bear interest at the highest rate
provided in the RLC Note and shall be due and payable upon demand.

     4.5 Notwithstanding anything to the contrary contained herein or in any
other instrument executed by Borrower or Lender, or in any other action or
conduct undertaken by Borrower or Lender on or before the date hereof, the
agreements, covenants and provisions contained herein shall constitute the only
evidence of Lender's consent to modify the terms and provisions of the Credit
Agreement. Accordingly, no express or implied consent to any further
modifications involving any of the matters set forth in this Agreement or
otherwise shall be inferred or implied by Lender's execution of this Agreement.
Further, Lender's execution of this Agreement shall not constitute a waiver
(either express or implied) of the requirement that any further modification of
the Loans or of the Notes or the Credit Agreement, shall require the express
written approval of Lender; no such approval (either express or implied) has
been given as of the date hereof.

     4.6 Time is hereby declared to be of the essence hereof of the Loans, of
the Notes and of the Credit Agreement, and Lender requires, and Borrower agrees
to, strict performance of each and every covenant, condition, provision and
agreement hereof, of the Notes and the Credit Agreement.

                                      -4-
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     4.7 This Agreement shall be binding upon, and shall inure to the benefit
of, the parties hereto and their heirs, personal representatives, successors and
assigns.

     4.8 This Agreement is made for the sole protection and benefit of the
parties hereto, and no other person or entity shall have any right of action
hereon.

     4.9 This Agreement shall be governed by and construed according to the laws
of the State of Arizona.

     IN WITNESS WHEREOF, these presents are executed as of the date indicated
above.

                                    SCHUFF STEEL COMPANY, a Delaware corporation



                                    By:   /s/ Kenneth F. Zylstra
                                    Name: Kenneth F. Zylstra
                                    Its:  Vice President and Chief 
                                          Financial Officer

                                    COMPANY


                                    WELLS FARGO BANK, NATIONAL
                                    ASSOCIATION, a national banking association



                                    By:   /s/ Tim Dillingham
                                    Name: Tim Dillingham
                                    Its:  Vice President

                                    LENDER

                                      -5-
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                                  SCHEDULE 3.1

                                  PRICING GRID

<TABLE>
<CAPTION>
                   Eurodollar Rate
Leverage Ratio         Spread          Base Rate Spread     Facility Fee Rate
--------------     ---------------     ----------------     -----------------
<S>                <C>                 <C>                  <C>  
3.50. or higher         3.00%               1.000%               .500%
3.25. or higher         2.75%                .750%               .500%
3.00. or higher         2.50%                .500%               .375%
2.50. or higher         2.25%                .250%               .375%
below 2.50.             2.00%                   0%               .250%
</TABLE>
<PAGE>   7
                                   EXHIBIT "B"

                             COMPLIANCE CERTIFICATE
                         FOR FISCAL QUARTER/YEAR ENDING
                             ________________, 19__


Wells Fargo Bank, National Association
100 West Washington
Phoenix, Arizona  85003

Attn: Timothy J. Dillingham                           Date: ____________________
      #4101-251

Dear Ladies and Gentlemen:

     This Compliance Certificate refers to the Credit Agreement dated as of June
30, 1998 (as it may hereafter be amended, modified, extended or restated from
time to time, the "Credit Agreement"), among SCHUFF STEEL COMPANY, a Delaware
corporation ("Borrower"), the Lenders named therein, and WELLS FARGO BANK,
NATIONAL ASSOCIATION, as Administrative Agent for the Lenders and as Arranger,
Issuing Bank and Swing Line Lender. Capitalized terms used and not otherwise
defined herein shall have the meanings assigned to such terms in the Credit
Agreement.

     Pursuant to Section 7.1 of the Credit Agreement, the undersigned certifies
that:

     1. Enclosed are the required financial statements for the [fiscal quarter]
[fiscal year] ending _________________ ("Reporting Period") for Borrower as
required under Section 7.1 of the Credit Agreement.

     2. To the best of the undersigned's knowledge, no "Event of Default" or
"Default" has occurred [or if so, specifying the nature and extent thereof and
any corrective actions taken or to be taken].

     3. As of the last day of the Reporting Period, the computations below were
true and correct:

I.   SECTION 6.11(a) LEVERAGE RATIO

     Numerator:                              Net Funded Debt _________________A

          Divided by