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[SEAOC] Response to Steel Frame Question

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The following question was posted to the list server on Oct. 7 1996.  

((Would someone know where I could find some information on the design of
Steel Moment Frames using the new requirements since the Northridge
Earthquake?

I have a copy of the "Steel Moment Frame Connections - Advisory No. 3
(SAC95-01)" and "Steel Moment Frame Structures - Interim Guidelines (FEMA
267)" but they basically give you the guidelines and theoretical background
and recent testing.  I am looking for some examples similar to those of the
Steel Manual.))

The following is a long answer - I hope you read all or at least most of it.-
 Unfortunately this is not a short answer subject, as evidenced by the SAC
Joint Venture?s expected 2-3 year Phase 2 program currently under way.

I share your frustration in trying to apply the criteria set forth in the
Advisories and Interim Guidelines.  The solution to the problem is
unfortunately rather complex.  Compounding the issue is the NEW CODE LANGUAGE
that was adopted by ICBO shortly following the Earthquake.  On September 19,
1995, The City of Los Angeles issued an Interdepartmental Correspondence
which outlines the cities requirements.  The following is taken from that
correspondence:

Subject:  WELDED STEEL MOMENT FRAME STRUCTURES: DESIGN OF MOMENT CONNECTIONS.

Studies conducted by the Department of Building and Safety/Structural
Engineers Associates of Southern California Task Force after the Northridge
earthquake revealed inherent structural problems in the beam-column
connections of welded steel moment frame structures.  It was concluded that
the prescriptive design for the beam-column connection specified in Sec.
91.2710(g)1B of the LABC was not adequate.

As a result, the International Conference of Building Officials (ICBO) passed
an emergency code change to the 1994 UBC (footnote reads -?The Department?s
Code Review Committee has also approved this code change for the upcoming
1996 Los Angeles Building Code.?) which reads as follows:

2211.7.1.1  Required Strength
The girder-to-column connections shall be adequate to develop the lesser of
the following:

1.  The strength of the girder in flexure.
2.  The moment corresponding to development of the panel zone shear strength
as determined by Formula (11-1).

2211.7.1.3-2  Connection Strength 
Connection configurations utilizing welds and high strength bolts shall
demonstrate, by approved cyclic test results or calculation, the ability to
sustain inelastic rotations and develop the strength criteria in Section
2211.7.1.1 considering the effects of steel overstrength and strain
hardening.

Effective immediately, prior to the issuance of any permit, the plans shall
be verified to comply with the above code change and the following
guidelines:

Special Moment Resisting Frames (SMRF):
<(BOLDFACE) Cyclic testing shall be required to justify the adequacy of the
connection strength.>  The testing program should replicate as closely as
practical the anticipated conditions in the field, including:

1.  Member sizes and material specifications,
2.  Welding process, details and construction condition,
3.  Cover plates, continuity plates, web tabs, bolts, and doubler plates,
4.  Joint configuration (e.g. beams on both sides).

While testing of all connection geometries and member combinations in any
given building is not practical, the engineer of record may submit an
interpolation of the current test results conducted by SAC for review.
-----------

With the above information, in conjunction with the Interim Guidelines and
Advisories, THE MAIN PROBLEM BECOMES DETERMINING AN APPROPRIATE DESIGN BASIS
AND INTERPOLATION PROCEDURE which can be implemented on a given project.  The
only short answer to the question is that there are currently no readily
available design examples similar to those in the Steel Manual.  There are a
number of design approaches discussed in the guidelines and other
publications such as AISC?s Modern Steel Construction magazine, however these
do not clearly address the rotational performance or interpolation
requirements.  There are proprietary connections which reportedly have
satisfied the cyclic testing and interpolation requirements.  As a design
engineer your task is to either develop a project specific connection, using
SAC or other ideas as a starting point, which can be proven, or interpolated
to, by cyclic test or calculation, or utilize one of the available
proprietary connections.

The use of a proprietary connection technology has been debated by some
engineers.  However, if you consider the wood industry as only one example,
there is extensive precedent for using proprietary ideas.  As engineers, we
can all design an I-section wood beam.  We would determine the required
section properties and then determine the requirements for attaching the
flanges to the web - VQ/I.  The simple attachment would be achieved by high
strength glue products.  The next step would be to communicate to the builder
how to slot the flanges to accept the glue and web piece, where to clamp the
pieces together and how long to maintain the pressure until the proper glue
strength has been achieved.  The last step would be to test or prove by
calculation that the I-section meets the design demand requirements.  Yes, we
could all do this or we could specify one of many wood fabricated products,
such as Trus Joist, that have already completed the design, testing and
interpolation and have satisfied the design requirements.  Agreed, this is a
simplified example, but it makes the point that the structural engineering
community has been using proprietary technology for years.

As the engineer of record for a steel moment frame project a number of
questions must be addressed.  Some key commentary and language from the LA
City correspondence and SAC/FEMA Guidelines as well as other correspondence
should be noted.

The prescriptive design beam-column connection has inherent structural
problems. (LA City Sept. 19,1995)

Tests conducted by Yang and Popov concluded that current code limitations on
panel zone strength and stiffness are inadequate for connections that rely on
significant panel zone participation to achieve rotational capacity.
(UCB/EERC - 95/08)

The causes for through-thickness failures of column flanges..., observed both
in buildings damaged by the Northridge Earthquake and in some test specimens,
are not well understood. The commentary concludes that "Given the many
complex factors which can effect the through-thickness strength of the column
flange, determination of a reliable basis upon which to set permissible
design stresses will require significant research (Section 7.5.1 FEMA
267-95/08)

There are a number of reasons that this connection [cover plated] can provide
unreliable performance.  These include continued reliance on poorly defined
through-thickness properties of the column flanges as well as somewhat
exacerbated demands on this property? (Hamburger, SAC Project Director,
SEAONC-96/04).

ETC. ETC. ETC. 

As a designer of a moment connection we must ask ourselves:

1)  Will the connection configuration selected rely on material properties
that are unknown or at best subject to sudden deterioration of strength?

2)  Does the design basis clearly predict the contributions provided by these
material properties and/or connection behavior such as the level of panel
zone deformation?

3)  Can calculations adequately predict the behavior of the material
properties, especially those which are questionable, in a manner which
justifies the use of these calculations as a method to prove the connection
can sustain inelastic rotations?

4)  Does the design of the connection adequately predict and address the
demand on the connection and the column?

5)  In light of questionable material properties, weldablility issues (i.e.,
construction realities), stress concentrations, etc. is interpolation for
this connection configuration reasonable?

6)  Do I, as the responsible engineer, understand the how the connection
design addresses the issues raised by the available information.

I believe the days of a prescriptive design example for moment connections
are gone.  The engineering community is left to design the steel moment
connection the same way we design a bolted brace frame connection or a
concrete moment frame connection.  We should look at this as an opportunity
to broaden our capabilities and understanding of how the demand is actually
met, rather than a burden to do something different.

I have attempted to write this in a manner which will evoke more questions
and get the engineering community to discuss the issues.  While the SAC Joint
Venture is undertaking and completing a GRAND task, it should be understood
that they do not now, and may not in the future supply all of the answers.
 There is a wealth of knowledge and information available.  SAC is attempting
to gather this information and provide a repository for distribution.  Los
Angels County has also completed a significant amount of work in their
attempt to keep capital investment projects on schedule.  They have issued a
Position Paper, dated August 14, 1996, on the Design and Construction of
Welded Moment Resisting Frame Systems.  In order to address the issues raised
by the position paper Los Angeles County formed a Technical Advisory Panel
(TAP) to review the implementation of SMRF systems.  The Position Paper and
TAP SMRF Bulletin are available from the county through the Construction
Quality and Contracting Division, by contacting Y. Henry Huang at
213-738-2832 or by fax at 213-386-4818.

Additional information can be obtained by contacting the proprietary
connection providers.  An article by Virginia Fairweather in the March 1996
edition of Civil Engineering provides a good overview of the available
connections.  Additional information regarding the use of the MNH-SMRF
Connection System can be obtained by contacting:

David Houghton
(phone) 310-542-2077
(fax) 310-542-5350
or
Rawn Nelson
(phone) 800-475-2077 or 
(fax) 714-540-0319

This system has been qualified for use on a number of Los Angeles County
Hospital projects and well as private hospital projects, in excess of 4.5
million square feet of floor space.  Past approvals include OSHPD, Los
Angeles County, and the City of Los Angeles.  Design software and a
construction specification are available for use by the Engineer-of-Record.
 In addition MNH-SMRF Systems provides assistance with plan check and design
review services including shop drawing review specific to the MNH-SMRF
Connection.  

Ken O?Dell
Myers, Nelson, Houghton, Inc.
email: mnhkodell(--nospam--at)aol.com

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