Steel Composite Beam Design 3

[JP20]

I’m looking into use three beams to make a composite beam to resist moment in the x-axis and also shear. One will be a 1.25” flat plate which will be sandwiched between two C-channels. I was thinking placing three bolts vertically in the web with them being 2’ o/c the length of the beam. Is this generally acceptable? What all design considerations will I need to make on the bolts? How do I ensure it will truly act as one?

 

[r13]

KootK,

Yes. Maybe I worried too much, but...

 

[JP20]

I am going to design the middle plate to have sufficient section modulus (S) to resist the applied moment on the beam, therefore not having to worry if the bending moment is transferred to the channels.

With that being said, I still need to analyze the bolts that connect the three members: would I just consider eccentricity in the plane of the faying surface (as discussed in 7-6 of AISC 13th edition)by looking at the elastic method, which gets resolved into a direct shear and a moment with e for eccentricity?

For example, say I have a max shear of 20k and a max moment of 90 ft-k derived from a shear-moment diagram. Would I analyze each bolt for the shear (20k) + the shear induced by the max moment? If I have bolts vertically in line @ 2' O/C... what's the best way to analyze/classify a "bolt group".. would taking the shear and moment at a point directly in between two vertically-aligned bolt groups be the worst case? - and if so do I share the load amongst one or two of the adjacent vertically-aligned groups? It's a tad confusing when you have "continuous" bolt groups.. any guidance on this would be very helpful..

Also, to Ganesh:

I read D.4 which led me to J3.5 which states my maximum longitudinally spacing shall not exceed 12". Is this restriction in regards to the three plates as behaving as one unit?

 

[Agent666]

Curious if a wide flange column section with continuous plate welded to bottom and lugs to the top is just not a simplier solution here?

 

[JoshPlumSE]

This is a bit off topic, but is anyone else bothered by the fact that this thread is titled "Steel Composite Beam Design"?

I understand that it's not a totally erroneous description. But, to me, the term "Steel Composite Beams" is almost referred to steel beams with a concrete slab over the top.

I know this is semantics, but a much better title for this thread would be "Built-up steel beam design".

 

[Ganesh Persaud]

JP20 the best way to analyze the bolt group, see AISC manual table 7-6 to 7-13.

These tables give the bolt group coefficient for typical. However at the beginning of chapter 7, there is a formula to calculate the bolt group coefficient, C’.

After computing this, see equation 10-4 to compute the max moment. I think there’s an equation there for shear.

 

[Ganesh Persaud]

After doing the checks, if the 2 ft spacing works and all checks with respect to the design forces, I don’t think the 12 inches would be necessary. However if you are not considering the weld, you may need to stiffen between the bolts placed at every 2 ft to Avoid any diagonal local shear.

 

[JP20]

Yes I realize the tables there but that still does not address how to appropriately apply the forces to the bolts..

 

[Ganesh Persaud]

You need to calculate the resultant bolt group force along the horizontal line.

Theta = sqroot (taninverse(Vu/Pu))

After this, calc the bolt clear distances which you would use, depending on the no of bolt along the line, or in no of bolts in a single column, check the bearing and tearout strengths and compare with the actual.

After which, calculate the net area of the section that’s carrying shear and get the uniform tension stress factor from AISC 14th fig c-j4-2.


There are eaquations utilizing These to calculate the shear resistance provided.

 

[Ganesh Persaud]

It’s similar to designing a shear plate subjected to axial and shear force.

 

[BAretired]

I agree with JoshPlum. When I read the title, I assumed we were going to be talking about composite beam design. Instead, we are talking about built-up-beam design. Semantics are important to clear understanding. Engineers should endeavour to use precise terminology.

BA

 

[dhengr]

JP20:
Why not eliminate all of the bolts btwn. the channels and the center pl.? Instead, turn the channel toes in and weld them to the center pl. Now, you actually have closed sections which have some lateral stiffness and improve the pl. buckling picture as it acts as an axial member. The 1.25” center pl. is really overkill except at the pin holes where the shackle opening width at the pin is probably about 1.5” +/-, that’s a 6.5 or 8.5 ton shackle, or approx. your 20k sling force or shear force, on about a 20’ long beam/col. Alternatively, I agree with Agent666, and would pick a WF shape which worked for its combined loads, bending and axial and buckling and then weld attachment pls. to the top and bot. flgs. The top pad eyes would actually look much like the pin pl. (FIG. C3-3) in your earlier thread, and would be welded over and parallel to the WF web. They might be a thinner pl. with doubler plates (doughnuts) to match the shackle opening width, and with sloped sides to distribute the sling loads (tension and moment) to the top of the WF. The bot. pl. would be continuous and I would wonder if you really need that many bot. pin holes. Maybe 8” or 12” spacing gives plenty of adjustability. Right now, you are trying to make swiss cheese out of a piece of heavy pl. with your original sketch. Again, I caution that the pl. edges and pin holes need some special attention to assure no notches, slightly rounded/chamfered corners, etc. This helps assure that rough handling does not cause any sharp edge nicks or cracks. Also, the exact formulas (three formulas) from your earlier thread don’t really apply to this bot. pin pl.

 

[Agent666]

Actually I quite like dhengr's idea of turning the channels round and welding them to the plate. Far more elegant solution than what is originally proposed with welding (or bolting) web to the plate.

 

[JP20]

DHENGR:
Thx for the in-depth reply. Yes I decided to change it to how you described it, earlier today. We were going to bolt it so we could dismantle if need be.

My biggest question from your response is: Why do you say the three formulas do not apply to the bottom pinned connections?

I suppose that if I orientate it as such it will be continuously braced therefore eliminating the worry with dishing.

I will probably put a stiffener plate between the top holes since they will be approx. 6” from the top flanges of the channels.

BA & JoshPlum:
I worded it composite because composite means made of several different materials. One could argue that it is all steel but I argue that yes it’s all steel, but yet different shapes of steel with different properties and methods of producing, so therefore it’s considered composite in my opinion. Composite does not necessarily mean steel/concrete as an earlier reply suggests.

 

[BAretired]

If you say so, JP20, who am I to suggest otherwise?

BA

 

[r13]

On this engineering jargon, BA and Josh belong to the popular thinking camp. Me included.

Structural engineering. In structural engineering, composite construction exists when two different materials are bound together so strongly that they act together as a single unit from a structural point of view. When this occurs, it is called composite action. One common example involves steel beams supporting concrete floor slabs.

 

[JP20]

Retired13,

Lol. Just because Wikipedia said “concrete/steel”, it doesn’t mean composite cannot apply to how I used it.

 

[BAretired]

I don't think we should get too hung up over the definition of the word "composite". Two channels and a plate are certainly made up of different parts, so in that sense I would have to concede that JP20 used the term correctly. In Structural Engineering dialogue, however, a composite beam is normally understood to be a steel member connected to a concrete slab.

I move we call a truce.

BA

 

[KootK]

Before calling a truce, I would like to express my support for OP's position. Yeah, a great name for this thread might have been:

Fastening to Achieve Composite Behavior in a Beam Built Up From Vertical Plates.

Fat chance of that though. The original title of this is better than 2/3 of the threads here that have titles more like "My Aunt Like Peanut Butter".

"Composite" can mean a lot of things. Slab on deck, any manner of built up shape, and all kinds of things in the world of plastics etc. If a thread respondent has to put forth the minuscule effort of opening a thread to find out what it's all about, I think that's just fine. By the same token, if a potential respondent doesn't want to bother opening a thread because they feel that the title isn't sufficiently precise, I think that's just fine too.

 

[r13]

BA,

I disagree. Re-read the second explanation by UK, and the 4th (under noun) of the American language. For the former, we need the definition of "substance", we shall read as "a thing of distinct parts, that is any class of engineering material consisting of varies combinations of alloys..."

Somehow I think you would agree that it is an engineering jargon, rather than code defined terminology.

 

[BAretired]

I like Peanut Butter too.

retired13,
Forget about it. It isn't important enough to quarrel over it.

BA