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Braced Frame Connection, Slip-Critical or Bearing Bolts?

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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 epages to be able to hear from other SEAINT members as well. To preserve his/her privacy I have dedacted 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
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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 2212.6.3.1 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 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 to direction of load. Oversize holes (with hole diameter being maximum of 3/16" greater than the bolt diameter). For standard holes, nominal bearing strength is limited to 2.4dtFu. There is agood commentary on this in the back of the Provisios that clarifies these and more.

However, all of the provisions and commentary in AISC sesimic 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 seimic design procedures and design your bracing conenction 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 occaisonally happens although not very fequesntly.

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 verbatum from "Behavior and Design of Steel and Composite Structures-Vol. I, Stel 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 (unfactored) 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 quate a lot of energy and cut out the path of transmission of the ground accelration 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 modrate 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 extablished 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 conenction 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)

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