Why Use PR Connections? 1

[phamENG]

This doesn't apply to any specific technical issue - just a general curiosity. I'm familiar with partially restrained connections, can usually identify them pretty easily, and can model them well enough if I need to. So my question is this - why? The AISC spec commentary states "The potential benefits of using PR connections of various types of framing systems are discussed in the literature." Anyone have copies of this "literature?" I've searched and can't find anything that discusses the reasons for using them. I did find an article from AISC (around 2005/2006) that suggested there's only a single firm in the country that actually uses them frequently in their designs.

I don't want to confuse this with "flexible moment connections." I know that's a conservative alternative to PR connections that's analytically much easier. I'm curious about who uses them in design and what practical benefits it brings either to the engineer, the structure, or the project as a whole.

Thanks.

 

[r13]

I believe it is meant to fully utilize the flexural strength of a structural member, an idea similar to moment redistribution in continuous structures.

 

[phamENG]

How does it use more strength than a fully restrained connection? In a fully restrained connection, you can design it to handle the full moment capacity of the member being connected. I've known a few engineers who would specify the full capacity on all moment connections (whether this was out of a desire to be extremely conservative or just plain lazy I couldn't tell). This seems to me to be more of a servicability/performance thing.

One possibility has come to mind - a large, single story steel frame with braced frames on the perimeter and a flexible roof diaphragm. I could see some benefit of using a line of PR connections at the building center line. It would allow you to reduce diaphragm deflections while minimizing the necessary increase in column size and/or foundations. Maybe? Not sure if I'm on the right track here...

 

[r13]

A fixed end beam, -M = WL²/12, +M = WL²/24. Is this beam fully utilized at the positive moment region? If deflection is not a big concern, can we make ±M = 3WL²/48?

 

[phamENG]

Ahhh. I think I see what you mean now. Bring the peaks of the BMD closer to the average, and you have a more economical section. I like it.

I feel like this, like so many neat analytical and detailing things, only provide real benefit at scale - where the cost of the effort to design the connection (and possibly the connection itself) is eclipsed by the savings in steel. Do you have a feel for that sweet spot? If we compare a 25' bay building with 16" deep beams, 100psf LL and 20psf DL, simple span would be a W16x57, FR W16x36, and partially restrained using your moment would be W16x31. From FR to PR, we're only saving 5plf, or 125#/beam. Beams from the mill are running at $33.75 cwt right now, so we'd save $42/beam. Added fabrication/erection costs will eat into that some since it will be more than just a simple shear connection. Will $42 be enough to offset the cost of two more complicated connections? In a high rise that 125#/beam could add up and save on foundations, but that'll take a lot to accumulate, too. Then there's the added analysis and detailing costs. I could see a single, simple PRC easily adding $800 in cost to a job - you'll need a lot of them to offset all these added costs.

 

[dhengr]

PhamENG:
We were doing this back in the late 60’s and early 70’s, the latest new idea to save steel, and felt that we could get by with slightly lighter beams, as apposed to designing them as simple beams, and/or slightly lighter connections than fully fixed connections. It was more work than normal design, particularly as relates to connection design and overall serviceability. Look back at ASCE and AISC Structural Journals starting in about 63-65 for a number of papers and articles on the subject. We were starting to use computers, but mostly with inhouse written programs, and remote time-sharing. We were just automating our hand calcs. and methods for batch processing, and to follow the latest codes. With the software you have today, you should be able to tackle these type problems with more ease than we could. The trick will be to tabulate a whole big bunch of end connections, for many beam sizes, with the right strengths, stiffnesses and end rotations so they can be matched with each simple beam, span length, load and moment.

I think we called these partially fixed connections, and we paid particular attention to the strength, stiffness and rotation of each connection. We were trying to match the +M and -M to pick the lightest beams. Retired13 has it right in his 17DEC19, 17:59 post. Of course, it didn’t always work because you fell btwn. two beam sizes, or didn’t want to muck up the works by introducing a new beam size to the mill order or fl. plan.

 

[r13]

Also note, this concept is quite useful while qualifying an old structure to carry some additional loads, if moment redistribution is possible.

 

[phamENG]

Thank you both very much. That makes a lot more sense now.