Lateral & longitudinal blocking loads during bridge bearing replacement 1

[AlanLord]

I have to replace the bearings for a one - 260' span , 50' wide , slab on 5 x steel I girder (10' high) bridge. The girders will be lifted simultaneously at the diaphragms at each abutment.

Question: What do I tell the contractor to do to block the bridge laterally (and longitudinally?) during jacking and bearing replacement? Do I specify that this blocking system must be designed for a percentage of the dead load to be resisted laterally? If so, how much?

 

[SlideRuleEra]

What do I tell the contractor...

As a former bridge contractor, I suggest you ask the contractor to submit a comprehensive plan to do the work. You could certainly ask that they address your specific concerns as part of that plan. If you don't like the answers... negotiate a suitable technical solution.

You did not mention the current status of the project:

If the contract has already been awarded and you specify details now, expect bills for extra cost.

If the contract has been bid but not awarded, you have a lot more leverage during negotiations.

The ideal approach is to ask for the plan submittal as part of the bidding process.

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[AlanLord]

Thanks Sliderule era (me too I used the slide rule!),

The contract has been awarded, the bridge is being built, but the bearings have to be changed before concreting the barriers and sidewalks etc, BECAUSE THE DESIGN ENGINEER DIDN'T DESIGN THE BEARINGS PROPERLY IN THE FIRST PLACE!. This is going on in Canada. Money is no object, we just want the contractor to do this job with just Safety in mind. We'll just pay him what he needs to get the job done. BUT, he doesn't have the knowhow to properly block the bridge, and is just proposing some frickin two by fours slapped on each side. I don't think this is kosher!!! I need to tell him to design Blocking Bracing able to withstand "X" amount of Kips laterally and longitudinally, but what - 5% of dead load? 10% ? I don't know.

 

[SlideRuleEra]

I would start with the 2% bracing rule that has a successful record, for many applications, for about a century. With that as a starting point, I would say 5% is reasonable. Working with an existing structure can offer a lot of surprises, that's why I consider going to 5% will give a margin for the unexpected. At least you have the advantage of current knowledge of the existing structure & materials that are still new, for that reason don't believe you need to go higher. Some projects we did were constructed up to 40 years earlier.

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[OSUCivlEng]

Sliderule,
Can you offer any references or anything that would back up the 2% rule? I have used 2% of the dead and live load when designing cross frames. My old boss introduced the principle to me, but I wish I had a text or something reputable to back it up.

 

[SlideRuleEra]

OSUCivlEng - The 2% rule was "created" by an informal experience based consensus of several of the leading engineers in the early 20th century (Thomson, Waddell, Ketchum, et. al.). Here is a good summary in "Guide to Stability Design Criteria for Metal Structures", by Ziemian that mentions Waddell:

https://books.google.com/books?id=D...EQQ6AEwBQ#v=onepage&q=2% bracing rule&f=false

In this 2009 Engineering Tips thread, Bridgebuster mentions Thomson's & Ketchum's contributions:

http://www.eng-tips.com/viewthread.cfm?qid=240591

Here are links to the books mentioned above:

Thomson 1908, "The Design of Typical Railway Bridges":

https://books.google.com/books?id=p...ign of Typical Steel Railway Bridges"&f=false

Waddell 1916, "Bridge Engineering, Volume 1":

https://books.google.com/books?id=n...QSCv6D4CQ&ved=0CCkQ6AEwAg#v=onepage&q&f=false

Waddell 1916, "Bridge Engineering, Volume 2":

https://books.google.com/books?id=b...QSCv6D4CQ&ved=0CCMQ6AEwAQ#v=onepage&q&f=false

Ketchum 1921, "The Design of Steel Mill Buildings:

https://books.google.com/books?id=V...ATFy5OIDA&ved=0CCMQ6AEwAQ#v=onepage&q&f=false

In each case, the 2% rule is not stated, but these texts seem to be the origin. Also, the value varied, you see mention of 2 1/2%, and other similar numbers.

Here are a couple of more modern sources:

"Structural Building Components Magazine", 2007:

http://www.sbcmag.info/Archive/2007/sep/0709_stability.pdf

AISC "Engineering FAQs", 2006:

https://www.aisc.org/DynamicTaxonomyFAQs.aspx?id=1646

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[AlanLord]

Thanks all!

 

[Lomarandil]

You also could reference the California Falsework Manual (section 3-1.06) as a basis for your minimum 2% load. That's pretty widely used in bridge construction.

I agree with SRE -- 2% is a good minimum value. 5% is not unheard of depending on risk, seismic loads, how confident you are in the contractor's means and methods, etc.

 

[AlanLord]

For your information Lomarandil, I'm not at all confident in the contractor! Sad, but that's reality. So 5% it is!