Composite Beam and Deck?

[SteelPE]

I have a question that was asked by a contractor that made me a little sick to my stomach.

The contractor is involved with a job where the steel erector installed the wrong size shear studs. The slab construction was a 4-1/2” slab on 2” composite deck with a required shear stud length of 3-1/2” (the minimum for 2" deck according to the AISC). The erector decided to weld shear studs that were less than 3-1/2”. The mistake was never picked up until after the slab was poured.

Everyone is now learning the AISC requirements of extending the stud 1-1/2” above the height of the deck very quickly. However, they are trying to figure out how to fix the problem. I suggested one of 4 possibilities all of which are undesirable.

1) Rip out the slab and place new shears studs and re-pour slab.
2) Drill holes in the slab (evenly spaced) weld on new proper sized studs and set in non-shrink grout.
3) Install new beam in-between existing to support the floor and place new column under the girders (this is above a basement).
4) Reinforce the existing beams to support the floor w/o composite action.

Is there any other options I would be missing?

 

[Teguci]

Check the design to see if composite action is required.

 

[SteelPE]

Teguci,

That was the first thing we did. The loading information on the plans in nebulous. It appears as if the beams are 20% overstressed w/o composite action. Girders are another issue as we may be able to justify LLR (something the EOR did not include in his design).

 

[Teguci]

skyhooks or strong magnets (smile)!

I think your list is complete.

 

[WillisV]

5) If this is a very large project you could consider having a testing lab perform a reasonable number of push-out tests with the site conditions (short studs) to determine a site-specific reduction factor for the short stud strength and see if the beams are acceptable when using this lower shear stud strength.

 

[SteelPE]

WillisV

Interesting, I would assume you would have to place the studs in the "weak" position as you would be unable to confirm whether or not the studs were placed otherwise.

What would you apply for a FOS on the results of the shear test (would one even be required)?

 

[WillisV]

SteelPE, rather than worry too much about trying to derive the actual value (not enough data to derive a phi factor), I would simply look at the percent reduction based on comparing to control tests of studs that meet the AISC specification. So if you did 10 tests, 5 with short studs in weak position and 5 with AISC standard studs in weak position, and the average short stud strength of that set pf pushout tests was 20% lower than the average of the standard pushout test set, I would apply a 20% reduction to the typical AISC stud strength equations and check my beams that way.

 

[hawkaz]

How about load testing the slab. You can give them the loading and required defelction criteria- if it behaves poorly, then they need to do something else.

I have load tested slabs with drums of water or sheets of drywall

 

[rowingengineer]

I would think pinning the beam to the slab from below without having to core from above should be on the list.

http://www.nceng.com.au/

"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected” Petroski 1992

 

[SteelPE]

Willis V,

That's a decent idea.

hawkaz,

That idea was also discussed but we were not sure who we could get to perform the test. Also, failure of this test would result in collapse as some of the beams are transfer girders.

rowingengineer

How would you pin the beam to the slab from below?

The contractor brought this up during our conversation. They were interested in drilling holes from the underside and installing Hilti epoxy anchors. I didn't think this would be a good idea as installation would be very difficult and that these anchors would not be as "stiff" as a properly cast in place stud.

Do you have a different idea for pinning the slab from the underside?

 

[rowingengineer]

SteelPE,
epoxy anchors upside down is very hard to complete but can be done, I would be thinking screw anchors or similar.

http://www.nceng.com.au/

"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected” Petroski 1992

 

[Forensic74]

I would investigate the load test option further. Are you anticipating brittle failure?

I'm not so sure hacking up the slab with epoxy anchors will improve anything.

How much bottom flange reinforcing do you need? Can it be done with one bar and minimum welding?

Other option would be to hire on a consultant from a firm in a larger city like NYC. You sound like you may have limited means. They do all sorts of strengthening of composite beams all the time.

 

[SteelPE]

The EOR is from a relatively medium sized consulting firm and they have walked away telling the contractor to propose solutions for their review. They are helpful but want to part in planning something that may end up costing the erector some large $ to fix. All possible solutions will end up running through them (at least for right now).

Strengthening beams from the underside is not all that difficult and should not require a large consulting firm. I can tell you that they increased the bending strength of the existing beams by 50% and increased the moment of inertia of the existing beams by 91%. So I am skeptical about being able to only add a few bars and achieve this amount of strength increase. This is why we proposed option #3 as it would be less field labor intensive, and we wouldn't have to even consider clear height conflicts that may arise.

Also, as noted in the original post, the loads are a little vague as the EOR varied the LL on the floor throughout the structure. So defining the loading on the members is going to be a chore.

As it stands today, the erector is shopping the design around trying to find an engineer that will bite on accepting the initial design with no testing or strengthening required. They obviously don't want to spend any $ fixing their mistake. I don't want to be the engineer who says yes to that proposition.

 

[spats]

I did a seach on "retrofit shear studs", and this is one of the results I found:

http://www.utexas.edu/research/ctr/pdf_reports/0_4124_1.pdf

It happens to be for bridge girders, but the same principles apply.

Also, have you considered asking AISC about the 1 1/2" above deck rule? You say they are less than 1 1/2"... how much? If it's 1" maybe they can be salvaged. I would think that the 1 1/2" rule would be less important for primary girders (deck parallel) than secondary beams (deck perpendicular). The AISC 360-05 Commentary says that design rules for composite construction were established to keep composite construction with formed steel deck within the available research test data. It makes me wonder if there is new data, since the original composite test data seems to date back about 35 years.

 

[SteelPE]

spats,

That was interesting. I only "skimmed" through it, but it appears that they recommend using an epoxy anchor or a double nutted threaded rod that gets attached through holes in the top flange of the girders.