Connector for Metal Deck Slab Noncomposite Action 5

[Eren Jaeger]

Hi!

I have a metal deck slab over main steel beams (supported by columns) and secondary steel beams (supported by main beams). My questions are:

1. Is it required for all the steel beams, main and secondary, to be fully or partially composite with the metal deck slab? I read from AISC that for special moment frames, composite action for steel beams that are part of this moment frame are only allowed to resist gravity loads. If that's the case, then I would prefer the secondary beams to be the one that only acts compositely and for the main beams (which are part of my special moment frame) to be noncomposite. Is that okay?

2. If it is allowed to choose some to be noncompostite, should there be a shear connector also for noncomposite action between steel beam and metal deck or are metal decks just directly placed above steel without any shear connectors to achieve noncomposite action?

3.1. If there really is no shear connectors required for noncomposite action, is it okay during construction? I think we still need to somehow hold the metal deck in place above the main steel beam to avoid movement during concrete pouring?

3.2. If there really is no shear connectors required for noncomposite action, then the metal deck will no longer provide lateral support for the compressive top flange?

Thank you.

 

[KootK]

I'm hoping that you'll hear from Deker, Sandman21, or one of our other west coast guys but, until somebody shows, I'll have to do.

This is a must have, free reference for this kind of stuff if you don't have it already. Link. The clips below are taken from that reference and, I believe, answer at least part of your question.

As for the rest:

1) You absolutely do want some kind of nominal shear connection between the deck and the beam for just the reasons that you mentioned as well as some others:

a) erection stabilty.
b) beam bracing.
c) diaphragm bracing against shear buckling.
d) sometimes diaphragm in plane shear transfer.
e) uplift if it's a roof.
f) problably some other stuff I've not considered...

2) At special moment frame beams, the salient issues are these:

a) In the past, premature fractures in the plastic hinge zones have resulted as a result of stud welding embrittling (sp?) things. To address this, the plastic hinge zones are now "protected zones" where the studs are not allowed. So these local areas must either go without deck fastening or, as described in the clip below, the fastening must be limited to less detrimental methods such as powder actuated fasteners and spot welds. I would have to think that the latter option is the better in most situations.

b) For the purposes of capacity design methodology, there has been some concern that having composite flexural capacity by accident will cause designers to underestimate the capacity of the beams to be used in the over-strength design of other aspects of the system. As described in the second clip below, we deal with this by placing restrictions on the kinds of connections that are allowed when a structural concrete slab is present.

 

[r13]

This paper will be a good starter for your questions Link. More information can be found from SDI's publications. However I wonder your design approach that leaves the main beams without the benefit of composite action, but allow the secondary supports to be composite. It seems illogical, and complicating the design to me. Please elaborate your situation, if you don't mind.

 

[Deker]

I think KootK covered it. I personally see no benefit in omitting shear studs on the frame beams if you're going to be providing them everywhere else, even if the frame beams are designed as non-composite. Consider that shear studs can replace puddle welds to secure the deck to the beams, so labor is pretty much a wash. As KootK mentioned, they provide a more robust diaphragm shear transfer mechanism than puddle welds. Additionally, there are a few prequalified connections that allow you to omit lateral bracing adjacent to the plastic hinge when shear studs are provided on the frame beams (see below). Detailing requirements are specific to the type of connection you are using, so you'll need to read the relevant chapter in AISC 358 to know which ones apply to your project.

 

[sandman21]

I am assuming that you are using a conventional SMF system. As composite action is taken into account when designing for C-SMF/OMF, composite moment frames see AISC section G. Also note that the power-actuated fasteners up to .18" and for deck support only. We have them provide arc welds where studs are not permitted as even letting them use fasteners in the area can cause them to think they can put them anywhere. We also still provide bracing at the protected zone even when we use studs.

 

[Eren Jaeger]

Thank you guys. So I guess it is really much advantageous to use the composite action of deck for both main and secondary beams.

I have this now in mind; Use the benefit of composite action for both the main and secondary beams while being mindful that studs are not allowed in the plastic hinge zones or protected zones and use powder actuated fasteners and puddle welds instead. I should also be aware of the allowed moment connections that can be used when a concrete slab is present and should make it such that the slab will not inhibit the performance of the connection which can be achieved by using compressible materials around the connection.

However I wonder your design approach that leaves the main beams without the benefit of composite action, but allow the secondary supports to be composite. It seems illogical, and complicating the design to me. Please elaborate your situation, if you don't mind.

I thought of this idea because I read somewhere that one way slabs are preferred for steel structures and it gave me the impression that using composite action for both the main and secondary beam will induce two-way action for the slab even if the (Longer span / Shorter span) ≥ 2 is satisfied. Will the slab still be one-way regardless if both main and secondary beam are composite as long as the ratio 2 is satisfied?

Also, I read from AISC 341 chapter E for SMF that "Structural steel beams in SMF are permitted to be composite with a reinforced concrete slab to resist gravity loads." The way I understand this is that composite action of deck and SMF beams is only useful in resisting gravity load combinations but not those with seismic and wind so I have to create 2 separate checks: 1) composite for gravity LC, 2) noncomposite for seismic and wind LC. So I though it is much simpler if I would just assumed all main beams to be noncomposite and check it against gravity, seismic and wind LC and also benefit from composite action for secondary beams. What are your thoughts about this? I will be happy to hear from you retired13

 

[Eren Jaeger]

thanks sandman21. Yes, I was referring to the conventional SMF system. Thank you for leading me into this. So it is C-SMF where we can use the composite action of steel beams and reinforced concrete slab to resist seismic forces but I just want to clarify (sorry, I hadn't mentioned it before) if this is applicable if my column is just pure steel? What I mean is that for columns that are not composite? From what I read, this is for composite or reinforced concrete columns only?

 

[Eren Jaeger]

when using arc spot welds on plastic hinge regions, are computations pretty much similar to when using shear studs but using the capacity solved similar to plug welds?

 

[r13]

Eren,

Please read the linked articles, which represent my response to you. Link, Link, Link

 

[sandman21]

..concrete slab is present and should make it such that the slab will not inhibit the performance of the connection which can be achieved by using compressible materials around the connection.

The compressible material requirement is only true if the pre-qualified connection requires the the materiel.

I thought of this idea because I read somewhere that one way slabs are preferred for steel structures and it gave me the impression that using composite action for both the main and secondary beam will induce two-way action for the slab even if the (Longer span / Shorter span) ≥ 2 is satisfied. Will the slab still be one-way regardless if both main and secondary beam are composite as long as the ratio 2 is satisfied?

Also, I read from AISC 341 chapter E for SMF that "Structural steel beams in SMF are permitted to be composite with a reinforced concrete slab to resist gravity loads." The way I understand this is that composite action of deck and SMF beams is only useful in resisting gravity load combinations but not those with seismic and wind so I have to create 2 separate checks: 1) composite for gravity LC, 2) noncomposite for seismic and wind LC. So I though it is much simpler if I would just assumed all main beams to be noncomposite and check it against gravity, seismic and wind LC and also benefit from composite action for secondary beams. What are your thoughts about this? I will be happy to hear from you retired13

The span ratio has nothing to do with the slab. I would suggest reading the commentary of AISC 358 as it goes over the reasons for the ratio. The conc. over metal deck slab will be spanning one-way, the direction of the deck regardless of the beam being SMF. Also the SMF beams are fixed at each end and the composite action would put the conc. in tension. You will find that the typical size of the moment frame beams when accounting for end fixity will be more than enough and you could ignore composite action in the middle portion of the beam.

 

[r13]

Prequalified connections per AISC 358 with notes -

 

[r13]

if the (Longer span / Shorter span) ≥ 2 is satisfied

One way action will be more dominant once the ratio is approaching 2 and getting larger (if beams in both directions are composite), why make the secondary beams composite for such short span then?

 

[sandman21]

That is an extremely old 358 current has 10 connections. AISC 358-16 -S1

 

[r13]

Yeah, quite a change. It looks like they got ride of the limitations on the SMF, but kept the exception for bracing at the reduced section.

 

[r13]

The linked paper answers your question on ASCE 241 assertion on SMF that, "Structural steel beams in SMF are permitted to be composite with a reinforced concrete slab to resist gravity loads.", without addressing lateral load cases. Link

Preface

1.1.4.1

Hope this helps.

 

[HTURKAK]

I screened the previous responds ..I am sure all of them are providing valuable information and documents....

I just want to remind the RC beam design for SMRF; Under reverse load during a large EQ, hinge development affects both the top and bottom of beams and leads to cracking and spalling of cover on the top and bottom of the beam . In order to improve ductility, confinement of the concrete core is necessary. For this reason, closely spaced transverse steel needed.
The same reasoning is valid for composite action of beams for steel SMRF. During a large EQ event, the concrete topping around hinging zones of the beams will crack and composite action will deteriorate rapidly in subsequent cycles. That is, one can not assign a hysteresis loop for composite action.

I understand that the standard is allowing composite action for steel beams that are part of this moment frame are only to resist gravity loads to justify the SWAY MECHANISM and STRONG COLUMN -WEAK BEAM requirements.

However, totally ignoring the contribution of composite action for the link shear strength should not be reasonable ...The overstrength produced by the composite slab effect should be considered to estimate the maximum forces that the shear link imposes to the other structural elements.

 

[Eren Jaeger]

Thank you guys for all your responses. It is getting clearer to me now but I really still have to dig into all the references you shared me. Thanks. All of them will surely be helpful for me to have a better understanding on this subject.