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- To: seaint(--nospam--at)seaint.org
- Subject: Re: FEMA Bolted Connection
- From: Abolhassan Astaneh-Asl <astaneh(--nospam--at)ce.berkeley.edu>
- Date: Tue, 05 Jun 2007 10:27:38 -0700
Dear SEAINT friends: here is another question i had outside SEAINT pages and thought it is an interesting item to be posted here with my 2.5 cents response. Again the personal info is redacted. it would be interesting to hear from you on this.
========================= Here is my reply , the question is below. Dear xxxx: Thank you for the e-mail and kind words. Things are fine around here although I got pretty busy with the Mac Arthur Maze fire. Hope all is well with you and the family. As for your item on the bolted connections and the use of over-sized holes in the column splices, I don't know if your subscribe to the SEAINT posting page from SEAOSC? If not, I posted a response to a similar question there just this past weekend. i have entered the question and my reply below FYI. It would be great to hear from you what do you think? Am I off? By the way I was pretty interested and campaigned for getting this slotted hole and specially over sized hole allowance into the AISC seismic provisions for bolted connections. Of course the short slotted and over-sized holes should be in the gusset only and not in the member not to further reduce the net area of the member. I agree with you completely that OS holes help in erection and fabrication specially for large connections such as those large and complex multi plane gussets you sometimes have in your tall buildings. As for the column splices, everything that I know tells me that having slippage in the column splices will be beneficial. Having cyclic slippage of 3/8" or so that happens in over sized holes instead of the 3/16 or so of cyclic slippage expected in the standard hole splices will only help the situation. Of course one should check the additional drift , which simple calculation indicates to be in the order of 0.001. Specially important is the fact that unlike the bolted moment connections that the bolts are subjected to combination of gravity and seismic and can slip under gravity negative moment, the column splices cannot slip under gravity load since the column sections are in contact bearing at all times under gravity load and the bolts can only slip due to uplift of the column due to lateral loads. Professor Ray Clough of UC Berkeley, in 1970's did shaking table tests of steel structures with uplifting base of columns and showed that it is very beneficial if one lets the columns uplift. Later during 1980's , we did tests of uplifting column base plates in braced frames and showed that it is very beneficial to allow column bases to uplift a controlled amount. We also developed an innovative "uplifting base plates" to do just that. All of this is in my upcoming Steel TIPS report: " Seismic Behavior and Design of Steel Base Plates in Steel Braced Frames", By Abolhassan Astaneh-Asl, to be released in July 2007. The concept of uplifting base plate was used in seismic retrofit of the Golden Gate bridge and the Carquinez bridge in the Bay Area. The beneficial effect of column uplift was best manifested itself during the tests of steel shear walls we did a few years ago for GSA/MKA where the boundary columns and steel shear walls had bolted double lap plate splices at floor mid-heights. Because of the splice slippage, not only there was some damping but more importantly, the slippage of the bolted splice added to the ductility of the system and prevented large compressive strain built-up in the columns delaying column buckling. To summarize it, I think we should use bolted splices in columns more often and allow short slotted and even over sized holes in the splice plates (up to 1/4' gap in the hole). As for the drift, as I mentioned above, my simple calculation indicates that such an additional slippage might add 0.001 to the drift. Hope this helps and sorry for the length of this e-mail. As a famous person has said-don't remember the name now- I could not find the time to make it shorter. With your permission after redaction and removal of personal info, I will post this on the SEAINT. Best wishes. Hassan xxxx, yyyyy wrote:
Dear Hassan, I hope everything is well with you and family.Fema 350 allows for their pre-qualified Bolted Flange plate (BFP) connection to use Oversize hole for the plate while considering full bearing value of the bolt shear capacity. AISC Seismic provision also now recommends using OVS holes for Bracing connections based on the same Fema 350 prvisions. I like the concept very much since it facilitaes a site bolted connection for high demand connections while maintaining a manageable number of bolts. I would like to get your opinion on that. Also can this be extended to column splices?Thanks xxxxx * *
--------------------------------------------------------------------------7 Message:0007 , June 3, 2007 from SEAINT 7
-------------------------------------------------------------------------- From: Abolhassan Astaneh <astaneh(--nospam--at)ce.berkeley.edu> To: seaint(--nospam--at)seaint.org Subject: Braced Frame Connection, Slip-Critical or Bearing Bolts? Dear SEAINT Friends: I have received the e-mail below from a SEAINT list member privately and thought the question is very important and decided to reply to it on these pages to be able to hear from other SEAINT members as well. To preserve his/her privacy I have redacted the e-mail and my response. Here is the question followed by my 2.5 cents response. Please let me know what you think either on the list or my e-mail: astaneh(--nospam--at)ce.berkeley.edu =============== Dear Dr.Astaneh, I have been reading your posts on the SEAINT list and find them very informative and in great detail. I am a structural engineer working in xxxxx and a member of the list for more than xxx years. Although I use the posts almost daily, I have never put up a question in public domain. I have a question regarding steel bracing connections and would greatly appreciate your thoughts on it. STEEL BRACING CONNECTIONS We are designing a 1x story steel concentric braced frame structure in xxxx. The codes used are UBC-94 & AISC ASD-89. Seismic zone is 2A. The proposed connections are slip critical bolted connections using A-325 bolts. Assuming item 2 of section 218.104.22.168 of UBC controls, the connections need to be designed for 3 (Rw/8) times the force in the brace due to prescribed seismic forces. Since we are designing the connections at strength level using ASD, 1.7 * allowable stresses can be used. The strength of the bolts in bearing type connections (Type N) is about 30% more than the bolts in slip critical connections. I am assuming the connections will slip at strength level forces and the bolts will be in bearing condition. This will increase the drift of the structure but that is not critical. The question is: When designing the connections for strength level seismic forces, can we use the strength of the bolts in bearing condition rather than slip-critical condition? xxxx yyyyy Senior Engineer zzzzzzz Consulting Engineers P.C. (Address and Tel.) Email:xxx(--nospam--at)yyyy.com ======================= A. Astaneh: The current AISC Seismic Provisions (2005) has a section (7.2) on the use of bolted connections in Seismic Load Resisting System (SLRS) and a commentary in the back of the document on this subject. If I understood it correctly, it says that the bolts in the connections of diagonal members should be designed using bearing (and not slip critical strength) but the bolts should be tightened and the faying surface should have a Class A surface to deliver a coefficient of friction of 0.35. Also, it allows to use Short Slotted Holes perpendicular tothe direction of the load. Oversize holes (with hole diameter being maximum of 3/16" greater than the bolt diameter) are also allowed. For standard holes, nominal
bearing strength is limited to 2.4dtFu. There is a good commentary on this in the back of the Provisions that clarifies these and more. However, all of the provisions and commentary in AISC Seismic Provisions (2005 and earlier) focuses on seismic issues only. I wish, AISC had brought in some concerns and had some recommendation regarding wind behavior of seismically designed braced frames into the design of Seismic Lateral Load Resisting System. This is specially important in medium and low seismicity areas such as yours. But this is the way things are for now as long as separate code committees look into the wind and seismic and other loads acting on a single structures. What happens is that, at least in some cases, when seismic load is governing and you just follow the seismic design procedures and design your bracing connection for bearing capacity, when the service wind load is acting on your braced frame, the wind can cause slippage of the connection bolts in shear, creating not a safety problem but quite annoying noises to the otherwise satisfied occupants! Don't even mention what the owner would think of you as structural engineer! I have head that this actually occasionally happens although not very frequently. In any event, I use the following approach in design and teaching and recommend it in my consulting to others. For whatever it is worth, here it is: NOTES ON DESIGN OF BOLTED CONNECTIONS IN DIAGONAL BRACES (Adapted in content and not verbatim from "Behavior and Design of Steel and Composite Structures-Vol. I, Stael Structures", by and copyright 2007 Abolhassan Astaneh-Asl, Ph.D., P.E., all rights reserved. To be released Jan 1, '08) 1. Design the connection as bearing connections for the governing seismic or wind axial force in the brace. If seismic is governing, use additional provisions in Section 7.2 of the AISC Seismic Provisions (2005). If wind governs, use the AISC specification (2005). 2. Regardless of which load , wind or seismic, governs, use tightened bolts and at least a Class A faying surface with 0.35 coefficient of friction. 3. Check the design of connection for slippage under service (un-factored) wind load and ensure that the shear strength based on the slip-critical values is greater than 1.25x(design service wind load). This serviceability requirement ensures that the structure will not slip back and forth when there is high wind. 4. There is no need to check , or even avoid the slippage during moderate or strong earthquakes since such slippage during earthquakes that are relatively rare event (compared to service wind) can act as "poor person's friction device" and dissipate quite a lot of energy and cut out the path of transmission of the ground acceleration to the the masses of the upper floors (to some extent) resulting in reduced inertia forces. Both effects are beneficial. Besides, to me it looks like the last thing one should worry about during a moderate or major earthquake is the noise created by the slippage of otherwise perfectly safe bolts! You may have other things to worry about including falling objects such as that nice framed painting above your bed! 5. As for the drift, the research and shaking table tests of frames with bolted connections , for example, (Nader and Astaneh-Asl, 1990) have clearly established that during dynamic loading having slippage in the bolt holes dos not necessarily increase the drift! That is true for static loading and service wind, but, the drift due to seismic loads are created by dynamic inertia forces, which depend on stiffness, damping, character of the ground motion and mass. By changing the stiffness , which is reduced due to slippage, and damping which is increased during slippage, there is no reason to state that drift will increase if bolts slip. To be safe a rule of thumb can be that you can consider the drift might increase about 10-15% due to slippage. This is based on a number cases we have looked at. If the case is important of course you can always do time history-analysis using your ETABS or SAP. But, just taking the actual slippage in the bolt hole and converting it via geometry to establish drift will not be quite correct. It might be more reasonable to assume the connection will not slip. Do your typical elastic analysis, get the elastic drifts, convert them to inelastic drift using Cd, then increase it by 10-15% at the most. =================== Hope this helps and best wishes as always. "Hassan" Abolhassan Astaneh-Asl, Ph.D., P.E. Professor, UC-Berkeley (Contact info at www.ce.berkeley.edu/~astaneh) ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp* * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
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