Composite Beams with Ordinary Moment Frame 3

[EngineeringEric]

Working on a project with an 80ft clear span building that is supporting heavy upper floors with concrete deck and limited room for structures and windows all around. Great!

So looking at a composite girder lines spaced 15/20/25ft apart with infill beams. The girders that span 80ft will need to be a moment frame (R=3). Seismic may govern due to the weight vs projected area per line.

I can size the girders for gravity as pin-pin. I was then thinking of making them fix-fix for a lateral design. the connection would be sized for the maximum shear/moment of the envelope of reactions. To detail and evaluate the connection i am currently unsure.

Question:
1) would you do something else?
2) would you ever make the moment connection after/post composite action? so we don't load the column with moment due to deflection and loading of the girder. the only moment transfer is live and wind.
3) does this require a flexible/partial moment connection as opposed to a standard full moment so my girder never sees negative moment?
3a) pending the amount of moment from DL only, would a full moment after DL is engaged work?

Any references to AISC or industry references would be appreciated, especially if they answer the above questions to save you time.

 

[KootK]

1) I suspect that you'll find that most engineers disdain the Flexible Moment Connection business for the logical difficulties that it often presents. If you search that here, you'll find a number of threads on the merit -- or lack thereof - of FMC.

2) There's just no way in hell that I'd be willing to do FMC on something of this scale. I'd use fully restrained moment connections for all load sources coming in after a low estimate of DL when the connection gets locked up (your 3a) or do something altogether different. The last thing that you need is this heavy thing P-Delta-ing over because you estimated the connection flexibility wrong.

 

[EngineeringEric]

Thanks Kootk. I was not a fan of a flexible Connection... i can honestly say I've never detailed one partially due to this forum!

The locking the structure after-deadload moment is currently my favorite option but something i have not done before due to construction difficulties. Maybe there a good way to analyze a composite section that has negative moment over the outer 10% of its span? which would happen if the connection is welded prior to DL.

 

[KootK]

I'd be inclined to disregard composite action where the slab would find itself in tension. There's a load path for it with a mesh or rebar in the slab in that area but it gets convoluted.

If you go with the delayed connection locking, give some thought to another, minor concern: frame stability during construction. You'll be wanting to stack all that weight on a non-moment frame until the last minute. 80' is a fair ways to run temporary cable bracing etc.

Does this thing not have stair shafts?

 

[r13]

I don't know if the idea can fly or not, just give it a shot.

1. Design the girder for self and slab weight with un-shored condition, and pin-pin condition.
2. Design the connection for above loads.
3. Design the composite beam for live load with fixed-fixed end supports. Note that the N.A. should be above the upper flange of the girder to lock tensile force within the concrete slab, as the steel girder is simply supported, and can't develop tensile strength as truly fixed joint.

Obviously this method is not good for lateral load analysis, since the girder would be required to develop tensile strength at the ends. Need comments and suggestions here.

No good.

 

[EngineeringEric]

This thing is connected to an existing building but juts out to the side. the three sides are mostly windows and leaves the deck with an aspect ratio in the 1:1 zone (80x80). So an open-sided diaphragm is also an option but i don't like this huge mass with limited restraint, the induced torsion would probably be large enough that it will cause complications.

I would like to just disregard the limited tension and may.
Design would go as follows:
[ul]
[li]pin-pin for gravity. this provides the shear studs and member sizes[/li]
[li]Fix-Fix for lateral using the DL, LL, WL, SL, EL,. this provides columns and end reactions and drift[/li]
[li]Verify the beams moment is within the moment-envelope from the the pin-pin design[/li]
[li]Detail the connection to the column for the worst shear and worst moment.[/li]
[/ul]

The only concern i have about that path is that the connection is seeing moment from DL prior to composite action (Edit! if the connection is welded prior to concrete placement, the pre-composite state of the system, would this negative moment be an issue... i'm not sure) Maybe that isn't an issue?

 

[r13]

Maybe a pin connection can be easily modified to provide moment capacity after all dead load is in place.?

 

[KootK]

The only concern i have about that path is that the connection is seeing moment from DL prior to composite action.

I don't understand this bit. Can you elaborate? For the design of the moment connection, I think that I'd disregard composite slab action entirely in the determination of that and just pretend that the beam was non-composite. I feel that would be conservative and reliable. Perhaps excessively so, I don't know.

 

[EngineeringEric]

I think we are on the same page kootk, i just worded things poorly.

The concern i have is that i don't know if there are negative effects from dumping negative moment forces into the composite beam. or negative effects if they welded the beam-column connection prior to loading it. The deflection changes some... but i guess that is no different than we do for any non-composite beam. It is always welded prior to concrete and that imposes some static-deadload-moment in the connection.

I'm starting to think that a shored beam with fixed end moments welded prior to concrete place just works. I'll design the beam as pin-pin and then ill design the connection as fix-fix.

I may be too in my own head on this one!

Retired: that is one idea i've been kicking around. but temporary shoring and access are difficult, even with a W40!

 

[KootK]

The concern i have is that i don't know if there are negative effects from dumping negative moment forces into the composite beam.

There probably are some. Ideas:

1) I'd like to see your required, positive moment studs placed entirely between the inflection points. On an 80' beam, that shouldn't take much doing.

2) I'd like to see a lateral brace tie into your bottom flanges near the inflection points with a check on negative LTB in between.

I suspect that your choosing to design the beam as simple span for gravity loads, as I would, probably makes this stuff largely moot / pedantic.

...or negative effects if they welded the beam-column connection prior to loading it.

As I see it:

3) Any beam composite action stiffens the beam and, therefore, reduces the gravity moments on the columns.

4) The composite action will be prone to creep and be pretty much unpredictable in tension. So ignoring it all together, and thus overestimating the connection moments, appeals to me.

5) If you solve the temp bracing issue, you could plausibly shear connect your beams and columns, pour the slab with the connections boxed out, build the entire rest of the building, and then weld on some flange plates. You know, as a strategy to have to deal with less gravity moment.

 

[JoshPlumSE]

My thoughts on the concept of a composite beam with "Fixed" end moment connections. I'm talking fully fixed, not the partial / flexible moment connections.
1) Traditionally, when we design composite beams, we do them as pinned-pinned connections that don't have negative moments.
2) There is some commentary to chapter I.3 of AISC which talk about flexural strength for composite beams with negative moment demand. Though I have always taken this to be equal to the negative moment strength of the naked beam.... assuming no strengthening from the reinforcement in the slab.
3) For deflection design I have seen (older editions of AISC) that use a weighted I value between the naked beam and the composite beam, based on the % of length that's in negative vs positive bending. I don't have a problem with this. As long as you've got sufficient studs to transfer force in the positive moment regions. After all, I'm confident that your service level deflections due to dead and live load are going to be a good bit lower due to the composite behavior. I don't know if I can 100% quantify this, but the procedure shown in the older versions of the code commentary seemed reasonable to me.
4) For lateral design, I would design it like any other moment frame beam. Gravity dead load and live loads causing negative moments at the connections. The connections being required to take this + whatever seismic demand exists.

 

[JLNJ]

I completely agree with not trying to use composite members for moment frames.

How about full-depth trusses on alternating floors?

 

[EngineeringEric]

This is a single level, well second level is a large (huge) open seating area, not fixed seats even!. And we are limited to depth of structure. With a topping, 6" concrete, and 40" of steel we are are 50" and they already don't like i eliminated mechanical trunk lines. Now i have to work out ponding effects

I like the idea of leaving openings for the top flange connection. I was debating the ability for this due to the shear studs, but if i keep all studs required within the outer few feet i should be fine. This will eliminate about half of the 40% of the moment from gravity loads on the connection.

I always appreciate your guys input! you are amazing and need to get back to participating more on here. Thank you for all the ideas and bouncing this one off each other!

 

[Blackstar123]

I can't seem to understand what everyone is trying to convey regarding fully fixed composite beams. My excuse is that it’s almost the end of the week and my brain is shutting down.
I’ve recently designed a composite beam floor system of almost similar bay width (i.e. 20 m) for un-shored construction. The design is at very preliminary stage and is sent to cost estimation department only.
I’ve provided a composite beam at every 5m along the length of building and laterally braced them at every 2.5 m along the width of the building. I have braced the bottom flange at the first two lateral beams from the ends to prevent lateral buckling. Connections will be designed for full plastic capacity. However, end moments coming from actual joint stiffness were more than the plastic capacity of the section, so I just released the extra moment from ends. I’ve not yet decided on a particular type of connection, but most probably I’ll provide an end plate type of connection. Columns are concrete filled tube sections. Tubes section will be built with 20mm th. Plate and have a cross-section of 900x900 mm. Composite steel beams are also built up with plates. I've attached a part snapshot of my model.

Design loads are as follows
Before composite behavior: Dead load of wet concrete and a construction live load of 100 psf
After composite action is achieved: Dead load of normal weight concrete, live load of 300 psf and seismic load (Zone 2B as per UBC).
Connections, for both phases, are considered fixed.

My above technique seems so simple in comparison to what it’s being discussed above and my tired brain cannot seem to pick up if it’s missing something regarding to design or detailing. My comprehension is down to its lowest.


Euphoria is when you learn something new.

 

[KootK]

The critical bit of information that you haven't shared is: what's your lateral system? Eric's is the beam and column frames acting as moment frames. That system may not work for you given that your beams are plastifying under gravity loads. Given your very large columns, your lateral system could conceivably be cantilevered columns. Or shear walls. Or braced frames. Can you share that detail?

 

[JLNJ]

Your P Delta stiffness will need to be reduced another 80% in addition to the 80% frame softening.

If your connections are maxed out under gravity loads, you might need to consider the girders fixed/pinnned under lateral.

 

[r13]

EngineeringEric,

For single span with fix-fix design, I think you didn't fully utilize the benefit of composite action in the positive moment area. If there is still room allow for additional deflection, why not release some end moment, so the entire beam is more uniformly utilized?

 

[Blackstar123]

Well for lateral loads, I've also designed it as a moment frame.
My understanding is that, sure plastic hinges will be formed at beam ends but joint will still be able to resist a moment equals to plastic moment.
I haven't checked P delta yet, but I've provided a reasonable margin in design capacity ratio. (Without PD effects, DCR = 0.7 in columns)
If I remember correctly, moment due to gravity loads were more than twice of EQ moments, But this can be due to the reduced joint stiffness.I'll confirm this fact tomorrow.
No shear walls or braced bays.
Columns are also supporting steel columns of peb structure at every 20m. I've only considered the reactions of those columns in my model.

 

[Blackstar123]

KootK,
Thank you so much. You've saved me from a bucket load of embarrassment.

The critical bit of information that you haven't shared is: what's your lateral system? Eric's is the beam and column frames acting as moment frames. 

So what I found out is that, even without any end release, beam joints are not really stiff enough to act like a frame for a lateral load. I can't berate myself enough for missing such a vital detail.

Truth is, I can't really afford to design the lateral system as a cantilever because of the shamefully lowest value of R which is 2.2

I've increased my beam depth from 750 to 1000 mm, but still the end moments of column at beam-column interface is only 40% of the end moment at fixed support.
(M=800 kN-m at top, M=2000 kN-m at support)

I don't know is this enough to justify using an R = 5.5 for IMRF?
I'll really appreciate all kind of inputs.

P.S. Should I make a separate post for this problem? I know I'm stealing the limelight here from EngineeringEric questions.

 

[EngineeringEric]

I don't mind you expanding this thread, but it may help to make a new one and link them... but also I don't know (Edit: feel free to expand the knowledge in this thread!!!)

Are your columns extending to upper levels as well?
Can you increase their stiffness or make them W-sections?
I'm assuming you also don't have adjacent structures to utilize