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- To: seaoc(--nospam--at)power.net
- Subject: [SEAOC] Response to Steel Frame Question
- From: MNHKODell(--nospam--at)aol.com
- Date: Wed, 23 Oct 1996 21:58:53 -0400
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: 2188.8.131.52 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). 2184.108.40.206-2 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 2220.127.116.11 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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