Header/Coupling/Lintel Beam Alternatives 2

[Trenno]

Hi everyone,

I would like to investigate alternatives to traditionally reinforced coupling beams (longitudinal rebar and closed ligs). I'd like to get an idea of capacities achievable beyond the stock standard coupling beam. Note, seismic design is not governing.

Can anybody point me in the direction of design guides/references for the following:

- RC coupling beams with embedded steel sections (eg. I Beams with welded shear studs).

- RC coupling beams with embedded steel plate (eg. 50thk steel plate billets to take the shear).

- Not a huge fan of diagonally reinforced coupling beams, plus our span/depth ratios don't necessarily lend itself to that sort of behavior.

- Any other alternatives that I haven't mentioned?

Much appreciated.

 

[Deker]

I would start with AISC 341 sections H4 and H5 (free download). The commentary has a list of references you can dig into. Additional discussion and worked examples can be found in part 7 of the AISC Seismic Design Manual.

 

[Trenno]

bump. anyone else?

 

[KootK]

I figured Deker's response would point you to all the resourced you'd need and perhaps, most of the resources that are available. What do you feel you're missing at this point? Is this in reference to a particular project or more of a general survey kind of question?

Not a huge fan of diagonally reinforced coupling beams, plus our span/depth ratios don't necessarily lend itself to that sort of behavior

Two separate things (general fandom & aspect ratio) but, in general, you absolutely should be a huge fan of the diagonals. They're awesome in terms of performance and rationality. And the obvious constructabilty issues are easily resolved with appropriate sizing. But yeah, some aspect ratios are unworkable that way. I'm surprised that you'd concurrently have serious shear challenges in a beam too slender for the diagonals.

I think that part of the difficulty in providing a response to this thread lies in the fact that it's very difficult to separate this issue from seismic concerns these days. The overwhelming majority of the research is pointed in that direction. What kind of drift ratios are you concerned with? As you know, the rotational ductility demands of the coupling beams usually vastly exceed the translational ductility demands of the walls. So, even in a low seismic application, you may still be dealing with coupling beams that need to yield.

For slender coupling alternatives in aseismic projects, I might consider:

- not coupling.
- just fire protected, bare steel beams.
- allowing beams to yield and redistribute demand vertically.

I saw convincing presentation once on the use of what is basically a steel u-shaped bucket filled with studs and concrete. Simple and seemed to perform well. Off the top of my head, however, I don't know where the literature is. Maybe in Deker's stuff.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Trenno]

Sorry, yeah I was just seeing if there were any other design guides from around the world. Eurocodes etc.

I agree, even though a building may not necessarily by governed by seismic loads, it should be good practice to detail it with a seismic philosophy.

I'll get my hands on that AISC Seismic Design Manual and start to run some numbers. Thanks.

 

[hokie66]

I think Trenno is referring to coupling beams in concrete buildings with concrete shear walls. The most common need for these is to couple the two sides of the lift core. A major consideration, as he implied about the diagonal reinforcement, is constructability. That can be overcome, but in my opinion only by careful supervision. Lack of attention to detail in this area caused the Harmon Tower in Las Vegas to be demolished before completion of construction.

 

[MrHershey]

- RC coupling beams with embedded steel sections (eg. I Beams with welded shear studs).

This concept has become a lot better developed in the last couple years. UCLA's been doing a lot of research with support from the Pankow Foundation and MKA.

Presentation on the topic (PDF): Link

There's also a two-part article in the ASCE Journal of Structural Engineering on them: Part 1: Testing, Part 2: Modeling

Could have sworn I saw a rather lengthy, standard-looking document on this with modeling parameters and a bunch of other good stuff but now can't find it.

- RC coupling beams with embedded steel plate (eg. 50thk steel plate billets to take the shear).

This concept is less well developed (or at least documented), but a generally more palatable alternative in my eyes. It's similar to the embedded wide flange in concept but a bit more effective for shear strengthening in seismic regions because it doesn't also substantially increase your flexural strength like the wide flange version does. Also (again my opinion) easier to build as the flanges aren't getting in the way.

Work has been done at University of Hong Kong. Article: Link. Old ASCE article, don't know if it's the same: Link. There's also a PhD dissertation that has some great design examples, not sure if this sign up is free or not: Hong Kong University

Behavior is similar to the wide flange links, though anchorage is different. Wide flange links typically use a bearing couple to transmit forces/moments to the side walls while the plates have welded studs to do it.

For both of these concepts you have to pay attention to steel embedment in the adjacent walls and reinforce those walls enough so there's no "strong coupling beam-weak wall" behavior. Generally easier to justify for the plates in my opinion since they don't have as high moment strength as the wide flanges.

- Any other alternatives that I haven't mentioned?

Cary Kopcyznski has been exploring/utilizing fiber-reinforced link beams: Structural Journal Article, Concrete International Article

Haven't dug too far in but I think the main aim with the fibers is to reduce congestion and eliminate the diagonals in high seismic. I don't believe they're intended to allow you to break the cap on shear capacity like the steel-reinforced sections can.

 

[MrHershey]

Following up, was digging for something else and found the design document for the WF coupling beams I was referencing above but couldn't find before:
[URL unfurl="true"]https://www.scribd.com/document/290792571/UCLA-SRC-Coupling-Beam-Design-Document-Final-July-10-2014[/url]

Includes design guidelines for normal applications and modeling parameters if you're using them in performance-based applications.

Dissertation on the topic (which I haven't read yet, but assume it's similar to the document above), available for free from UCalifornia system website: [URL unfurl="true"]http://escholarship.org/uc/item/70k958f3[/url]

 

[Trenno]

They're great references MrHershey!

Interestingly the 2010 Seismic Provisions document has an error in the effective stiffness formula for SRC beams. They show the formula without the exponent of -1, resulting if an effective stiffness greater than the basic Ixx value of the steel section. This error is eluded to in the dissertation by John Motter.

I understand the embedment length dictates how much shear the wall/beam interface can develop. If the embedment length is constrained such that it can only develop say 50% of the shear strength of the steel section, it's logical that it could only develop that same fraction of the flexural capacity?

 

[MrHershey]

Not sure if that's a good assumption for steel. Not saying it isn't, just don't have a feel for it and would need to dig in. For concrete/rebar contributions I would imagine you can develop close to full capacity even if the structural steel elements are not anchored to develop 100%. You'd have anchorage failure/slip of the structural steel element but don't think that should affect the concrete/rebar portion's ability to develop its capacity.

For wide flange links designed using the UCLA/Motter method this calculation is pretty easy for moment. How much you're developed is based on the bearing couple you can develop within the wall (see below). Would seem pretty easy to back out your effective moment strength if you're not able to develop the full amount. Though important to remember that if you're not using full seismic loads (i.e. reducing by an R factor), that you may not want to consider being partially developed because the loads you're designing for are artificially low and you may not be able to provide the level of ductility that your R factor is assuming you're providing. I think I would only personally look at partial development during a performance-based process where I'm looking at full loads/deformations, or for a wind-controlled link in a low or moderate seismic region.

 

[Trenno]

Yep, so if Vn,embed is all based upon "a" which is half the clear span of the link beam, then the effective partial moment capacity is Vn,embed * a?

This is a bit more accurate than my previous statement, as you could have a really thick web providing a large shear strength of the section, but that won't help when you embed it a small distance.