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Re: Positive connection

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Steve,

There was a slight cross slope, perhaps because it was on a curve or transitioning into a curve; and it was on teflon slide bearings. Some of the details have become a little vague, perhaps because I was not involved in the project and it was 35+ years ago. (35+ years ago! Where has all that time gone!!!)

Regards,

Daryl
_____________________________________________________________

On 21/04/2014 11:19 AM, Steve Gordin wrote:
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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