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[SEAOC] More on Steel Frame Connections - Response to DWish[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaoc(--nospam--at)seaoc.org
- Subject: [SEAOC] More on Steel Frame Connections - Response to DWish
- From: MNHKODell(--nospam--at)aol.com
- Date: Mon, 25 Nov 1996 13:03:23 -0500
Dennis Sorry it has taken so long for me to get back to you on this. Your note a month or so ago (10-23-96) brings up even more good questions that are not truly being addressed by the current research programs. I write this back to you with unfortunately more questions and fewer answers. Your project specific question raises yet another issue that appears to be just outside of the current scope of the SAC investigations. That is, What do we do for Ordinary Moment Frame Connections when the expected loading should not require plastic behavior?? In my reading and discussions on the subject I have found two main opinions: 1) Increase the design force to insure elastic performance. This is actually the approach that the City of Los Angeles outlined in the Interdepartmental Correspondence that I quoted from last time. I did not include the OMRF section, sorry. It reads: Ordinary Moment Resisting Frames (OMRF): Cyclic testing is not required. However, all components of the structure and their connections shall have adequate strength to resist the design earthquake elastically. The connections for OMRF structures shall be designed using a lateral force reduction coefficient (Rw) of unity while the beams and columns shall be designed using the standard lateral force reduction coefficients (Rw) specified by the code. OMRF?s with a Flexible Diaphragm: Supervisors may approve a less stringent connection design on a case by case basis. -------------- and 2) The ORMF connection still has to be capable of developing a measure of ductiliy. The current recognized measure of ductility is the ability to develop the capacity of the beam. Within the realm of my discussions with a number of different researchers this second opinion seems to stem from the recognition that a number of the pre-Northridge joints failed at stress levels well below the elastic limit of the material. This fact appears to be getting ?lost in the shuffle?. Although most of the published information focuses on large scale projects with concrete floor diaphragms, we need to remember that a number of small, 2-3 story buildings with plywood diaphragms had the same type of damaged connections as the bigger buildings. I had the misfortune of having to crawl through the open web joists in the second floor level of one such building that had approximately 30% of it ORDINARY MOMENT FRAME connections damaged. The damage was very consistent with that discussed for large buildings including, HAZ tearing, column flange divot pullout, and beam flange cracking. One significant item was the percentage of TOP FLANGE damage. Also consistent with larger buildings, no significant evidence of plastic yielding of beam or column flanges was observed. There were however, a number of connections that had evidence of panel zone yielding, (by evidence, I refer to the visible flaking of the primer.) I?m not sure that your statement suggesting SAC Guidelines recommends the avoidance of bolted connections is entirely accurate. My thoughts: It is my understanding that SAC/FEMA is recommending that any connection proposed for a project either be tested or based on a connection which has a design procedure founded on the results of a test program. Obviously there needs to be considerable caution when extrapolating test results. A number of bolted moment connections have been tested through various organizations including SAC/FEMA, NIST, UC Berkely and IFC/Kaiser; the last one being a proprietary connection. The test results for bolted connections have varied. Most are able to achieve significant rotational performance, however they do not as readily achieve the strength requirement for the given beam. (It should be noted that, in many opinions this still satisfies the needs.) One of the restrictions to using bolted connections is the limitation to smaller beam sizes or significant bolting requirements. Although, I believe the IFC/Kaiser connection has been able to achieve the flexural strength of a W36x150; I?m not sure of the actual details. A common failure mode is the deterioration of strength with sometimes sudden failure at the bolt holes due to the reduction of net area and potential for stress risers at the bolt holes. A typical hysteric loop has signs of pinching, due to slip of the bolts. The question becomes; does the slip create the potential for increased drift of the overall frame system? I personally feel that the slippage is not as great an issue as the potential for failure at the reduced net section. I do agree with you whole heartedly that the cost of a testing program is prohibitive for a small job. MNH-SMRF has spent close to 1.5 million unsubsidized dollars in testing, developing, and qualifying their connection. Even a scaled back testing program for a building that uses only one beam-to-column size range and therefore needs only to test one configuration can expect to spend over $50,000. I?m willing to bet this exceeds your design fee for a small job by a factor of 3-4. The final item in your note appears to describe a connection which eliminates the backer bar concerns. From your description it sounds like you are discussing the repair condition and not the concepts for the new construction. Regardless, the main focus at this point is to get the hinge to form away from the face of column. Your frustration is noted in your closing paragraphs. As a suggestion you recommend resolving the design issues as soon as possible. As I see it, the main issue holding back most currently available connection designs is a validated design value for the through thickness strength of the column flange material. To my knowledge, with the exception of the MNH-SMRF and IFC/Kaiser connections, ALL other proposed connections are adaptations of the pre-Northridge connection which rely on this through thickness property. Interestingly enough, rumor has it that the 8.3 million dollar budget for SAC Phase II allocates less than $100,000 to the through thickness issues while it allocates $400,000 to review the socio-economic impact issues. Seems to me that providing a good design basis and connections which will not fracture suddenly, will reduce the socio-economic impact without spending so much to study it. One of the most important things we, as the design community, need to do is make sure that SAC addresses all of our questions. At the SEAOC Convention in Hawaii, SAC presented the scope of work completed and proposed. Unfortunately they did not allow questions to be raised from the floor. These questions include the big, as well as, the little jobs. Some of the time it appears that the committees is charge become so focused on the specifics that the broader picture gets lost. I suggest that we need to express our concerns to the SAC oversight committee and make sure reasonable and rational answers are provided. In response to your frustration that it seems we are faced with paying SEAOC, SAC or AISC fees for seminars, we have to remember that most of the fees go to renting the space where we can all meet in order to discuss the issues. I am aware of the upcoming SEAOC Short Design Seminars in December to present the miscellaneous proprietary connections (see yesterdays mail). AISC is also going to have a series presenting the current options and design requirements, look for this in the near future. Ken O?Dell email: mnhkodell(--nospam--at)aol.com ...
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