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Re: Positive connection

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Daryl,

It's good you reconsidered and replied!

I hope I understand how the continuous bridge would thermally lift off its 
central support, but cannot come up with the force that would push it laterally 
 off the abutments.

Agreed - there should be some connection against the vertical force, and 
thermal loading - especially, on a continuous-beam bridge - should be 
considered.

In my case, a simple-span steel bridge was laterally restrained by the 
C-shaped (three sided) reinforced concrete walls/keys on both abutments. 
The superstructure and the keys should be properly designed for the applicable 
 forces, including impact. The thermal etc. gaps (1"-2") allow some very 
limited lateral movement of the superstructure that is "floating" on the 
low-friction pads.  The gaps are duly covered with sliding plates.  The 
anchors - placed in the oversized holes with heavy plate washers - resist 
uplift (although there is none), but do not restrict lateral movement.

Thanks,

Steve

______________________________
V. Steve Gordin PhD
Licensed Structural Engineer
SGE Consulting Structural Engineers
2081 Business Center Drive #105
Irvine CA 92612
949-552-5244
sgordin(--nospam--at)sgeconsulting.com

On Apr 21, 2014, at 9:47 AM, Daryl <h.d.richardson(--nospam--at)shaw.ca> wrote:

> Steve,
> 
> I was not going to reply to this line but your most recent posting (below) 
> caused me to reconsider. 
> In the late 1970s there was a bridge under construction for use in Calgary's 
> Light Rail transit (LRT) system.  This bridge slipped off its foundations 
> and landed up-side-down in the Elbow River.  As I understand it, the bridge 
> was a two-span steel box girder with a concrete top deck held latterly by 
> pins, not anchor bolts; and thermal effects (snow on the deck combined 
> with a low angle of the sun heating the lower part of the box girder) 
> caused it to lift above the pins allowing it to slide off the supporting 
> piers and into the river.  Had the LRT rails been installed or had there 
> been anchor bolts with nuts the accident probably wouldn't have happened. 
> Sometimes it's useful to have a positive connection! Regards to all, Daryl 
> Richardson ______________________________________________________________ 
> On 21/04/2014 9:49 AM, Steve Gordin wrote: 
>> Good morning,
>> The question arose during a plancheck for one of my designs, and was 
>> related to the issue of whether a concrete shear key (or similar component) 
>> can be classified as a positive connection for a pedestrian bridge - even 
>> though it is not a steel attachment and allows some movement. I got the 
>> reply from AISC Solution Center (below) this morning. Apparently, they 
>> did quite a research… At the end of the day, the term appears to directly 
>> correlate to the Merriam-Webster's definition of "positive:" formally 
>> laid down or imposed; fully assured; independent of changing circumstances; 
>> relating to or constituting a motion or device that is definite, unyielding, 
>> constant, or certain in its action; not fictitious; directed or moving 
>> toward a source of stimulation. Evidently, some smart engineer once coined 
>> the term - but forgot to define it... They way I see it now, the definition 
>> for the positive connection should be something like: "A structural 
>> connection against all applicable forces that is fully assured without 
>> the consideration of friction." Have a good week, Steve ______________________________ 
>> V. Steve Gordin PhD Licensed Structural Engineer SGE Consulting Structural 
>> Engineers 2081 Business Center Drive #105
>> Irvine CA 92612
>> 949-552-5244
>> sgordin(--nospam--at)sgeconsulting.com
>> Begin forwarded message:
>>> From: "Solutions Center" <Solutions(--nospam--at)aisc.org>
>>> Subject: RE: Positive connection
>>> Date: April 21, 2014 5:38:02 AM PDT
>>> To: <sgordin(--nospam--at)sgeconsulting.com>
>>> Steve,
>>> It does not appear that the AISC Specification, or the IBC define 
>>> “positive connection”.  In fact, the only place that I found that 
>>> provided a definition is in the FEMA E-74 document “Reducing the Risks 
>>> of Nonstructural Earthquake Damage, Section 6.6.1 Positive Connections. 
>>> There it states: “The objective of nonstructural anchorage or restraint 
>>> details is to provide what engineers refer to as a positive connection 
>>> between the item and a hard attachment point, such as a structural wall, 
>>> braced partition, concrete floor, or built-in countertop. Positive 
>>> connections generally consist of some combination of screws, bolts, 
>>> cables, chains, straps, steel angles, and other steel hardware that 
>>> transfer seismic loads to structural framing. Positive connections do 
>>> not rely solely on the frictional resistance produced by the effects of 
>>> gravity. Frictional resistance between the base of an object and the 
>>> floor or mechanical friction connections such as C-clamps or thumbscrew 
>>> clamps are not considered positive connections. The most common nonstructural 
>>> connection details for wall attachments, floor or ceiling attachments, 
>>> countertop attachments, and attachments between adjacent items are 
>>> discussed below. GENERAL INTEREST SIDEBAR Earthquake Forces Keep in mind 
>>> that although heavy objects are hard to move by hand, their weight (mass) 
>>> interacts with the shaking (accelerations) of an earthquake to produce 
>>> large inertial forces. Those forces mostly act sideways to make the 
>>> object slide or tip, and there are also vertical motions in earthquakes 
>>> that temporarily "lighten" an object and reduce frictional resistance.” 
>>> You can find this online at <http://cirrus.mail-list.com 
>>> /seaint-seaosc/09421005.html> Also, the term positive connection is 
>>> sometimes used in relation to the OSHA regulations related to rigging 
>>> and lifting, though I was unable to find a definition supplied by OSHA 
>>> or even the use of this exact term in the regulations – though I may 
>>> have missed it. It can be inferred from the context in which the term 
>>> is used that a positive connection is one which cannot accidentally 
>>> become disengaged (i.e. slide off). For instance OSHA seems to deem a 
>>> shackle through a lifting lug to be a positive connection, but a sling 
>>> around a trunnion is not a positive connection. I suspect the term is 
>>> typically not used in conjunction with the regulations related to steel 
>>> erection since bolts provide a positive connection and regulations 
>>> relative to their use in steel erection are pretty explicit. This 
>>> definition of positive connection is also used informally in other 
>>> applications. The term positive connection can also refer to electrical 
>>> continuity and is also used this way in OSHA documents.” I hope this 
>>> helps.  Please let me know if you have any additional questions. Sincerely, 
>>> Carlo Lini, P.E., LEED AP BD+C Staff Engineer American Institute of Steel 
>>> Construction 866.ASK.AISC lsm/eng How did we do? Your opinion matters 
>>> to us: www.aisc.org/TellAISC for your chance to win a free Steel 
>>> Construction Manual! This document has been prepared in accordance with 
>>> information made available to the American Institute of Steel Construction 
>>> at the time of its preparation. While it is believed to be accurate, it 
>>> has not been prepared for conventional use as an engineering or construction 
>>> document and should not be used or relied upon for any specific application 
>>> without competent professional examination and verification of its 
>>> accuracy, suitability, and applicability by a licensed engineer, architect 
>>> or other professional. AISC disclaims any liability arising from information 
>>> provided by others or from the unauthorized use of the information 
>>> contained in this document.

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