Girder-Slab System -- Non-composite Moment Capacity 1

[edled]

Hello all,

Currently working on my first Girder-Slab project. For those of you unfamiliar, the general concept is that you have hollow core precast planks bearing on "D-Beams", steel beams formed from the bottom half of a castellated beam + a top flange plate, with a grouting/reinforcing layout that creates a composite floor system in the final condition. The D-Beam design tool spreadsheet provided by Girder-Slab is very helpful for sizing the steel beams, but one thing I noticed was that the non-composite design check simply uses the plastic moment as the capacity of the D-Beam. Surely LTB also needs to be checked for these beams in the construction, non-composite phase? You end up with some long unbraced lengths on these assuming you can't count the planks as bracing the beams. I've reached out to Girder-Slab's engineering department, but have yet to hear back. Curious what everyone's thoughts and experiences are.

 

[KootK]

These closed shapes are wickedly stiff torsionally and, thus, not prone to LTB.

 

[KootK]

Separate from LTB, a legitimate concern is just the ordinary design of the beam for torsion. Particularly any support connections that might need to resist torsion. In my experience, this is often handled by way of sequencing. Some combination of:

1) Stager the erection side to side to ameliorate torsion.

2) Fasten planks to the beam such that the planks restrain beam roll.

3) Shore the planks, or the beam, until the roll restraining connections can be made.

This handling of direct torsional stress is the bit that might be fruitful to discuss with the supplier.

 

[structSU10]

Imay be mixing it up with delta beams but I thought these were temporarily shored until the reinforcing was grouted in.

 

[edled]

@KootK I think you may be talking about Delta beam re: closed shapes. It's confusing because Girder-Slab refers to their beams as "D-Beams" for whatever reason. These are open sections.

Definitely hear you with regards to sequencing and limiting torsion, already had a few discussions about it. I'm more talking about a standard LTB condition when both sides are loaded with planks. Even if the planks were spot welded to the beam, they're resting on the bottom flange so I'm not sure what restraint they'd offer the top flange.

When I first mentioned it to the Girder-Slab engineer a few weeks ago he said something about the top flange plates being robust enough that it's not a concern, but when you actually run the numbers per section F4 of the AISC spec, you do get a reduced capacity because of LTB.

Edit: @structSU10 regarding shoring, we're aiming for no shoring, as I believe is the norm for a Girder-Slab building. The perimeter (loaded on one side) beams will be wide flanges loaded concentrically on the top flange. I believe Deltabeam often has the perimeter shored due to torsion concerns.

 

[KootK]

@KootK I think you may be talking about Delta beam re: closed shapes. It's confusing because Girder-Slab refers to their beams as "D-Beams" for whatever reason. These are open sections.

That's exactly what I was talking about. My bad.

I'm more talking about a standard LTB condition when both sides are loaded with planks.

LTB may still not be an issue in a similar way to how it's not with closed sections. If Iyy > Ixx, there the possibility of LTB is somewhere between remote and non-existent.

Even if the planks were spot welded to the beam, they're resting on the bottom flange so I'm not sure what restraint they'd offer the top flange.

Note that top flange buckling is not the same thing as LTB, even if they have similarities. LTB is the entire cross section rotating about a point in space horizontally coincident with the beam shear center. The bottom flange has to rotate and sway too.

Even if the planks were spot welded to the beam, they're resting on the bottom flange so I'm not sure what restraint they'd offer the top flange.

Plenty. The weld and bearing against the plank soffit will form a moment resisting couple that will rotationally stabilize the bottom flange. The the web will cantilever up from the bottom flange and stabilize the top flange. But, yeah, a sequencing solution is probably the way to go with this.

 

[edled]

Thanks for the input KootK. I'll try to circle back with the Girder-Slab engineer and discuss my concerns, and see if he has any formal explanation/confirmation of the non-composite capacity.

-Ed

 

[KootK]

Do report back and let us know what they say. You've got me curious now.

I was thinking about this in the shower this morning and thought of something additional. Once you get the planks in place and welded along the entire length of the beam, I do feel that LTB is precluded. And likely local top flange buckling as well. However, if erection starts from one end that will always leave a length of the beam between the last installed plank and the beam end that could potentially LTB if Ix > Iy.

I've done a lot of work in precast engineering with similar systems. As someone who's an EOR most of the time, it was a process of adaptation where I had to learn to accept that the erectors and suppliers of these systems usually know their craft quite well and, via experience and thoughtful erection techniques, are able to make a go of a lot stuff that doesn't always pass a conservatives paper check and often makes me cringe when I've got my EOR hat on.

 

[jayrod12]

Everytime someone has proposed the girder slab system, they're usually just finished a job with delta beams from Peikko and they thought that project went so smoothly that we must do it again, only cheaper.

In comes girder slab claiming substantial cost savings because your local steel supplier can fabricate the pieces. But what they don't like to volunteer early on is that the entire design and detailing is the onus of the EOR. Which is drastically different from the way Peikko operates.

Where I am, delegated design is super common, and although as EOR we are still liable for any of the designs in the project, we aren't required to do detailed design of their systems at all. To a layman, girder-slab and delta beam are the same, so both the contractors proposing it, and the owners hearing about cost savings, give us a hard time when we reject it as an option. Or give us a hard time when we tell them that the original proposal we wrote was based on Delta beams from Peikko, and if they'd like to use girder slab, then we'll need to charge more fees.

Not a single project of ours in the office (quite a few hollow-core projects per year) has ever ended up going with the system. The contractors always end up just going with Peikko's delta beams.

 

[KootK]

But what they don't like to volunteer early on is that the entire design and detailing is the onus of the EOR.

Ew... I did not no that. Now I understand the degree of OP's concern.