reinforced pan-poured raised deck - beam sag

[markls8]

I've just had a Stampcrete raised deck installed on my house using "I" beams and a corrugated-type galvanized steel pan. One area spans 24'-9", and the supporting beam (W6X20) has deflected down about 1" (maybe more, haven't been up there to measure it yet) under the weight of the concrete, which is 7" thick, fibrated, with 6X6 welded wire mesh. Control joints have been cut. It is my understanding that rebar is what is used to give concrete structural strength in an application such as this, and fibres and 6X6 mesh are for crack control. I believe that once the concrete cracks it will no longer function as a structural unit in concert with the steel and the strength of the entire deck will be solely dependant on the supporting beams and pan, because of the absence of rebar.
The W6X20 beam is located about 5 feet proximally to a parallel W8X21 which bands around the outside edge of the deck. This larger beam is deflected somewhat less.
I guess my question is, am I over-reacting by being concerned about this deflection, (this is our "forever" house) and what kind of deflection tolerance maximum is considered safe? (expressed as a % of length?) It "feels" fairly solid when I jump on it.
ps. the upper deck concrete was finished flat and with a proper slope for runoff and has not bowed any farther since it was finished a couple of weeks ago. ie. it hasn't "concaved" in : )
Thanks in advance for your input.
Regards - Brian

 

[Taro]

A 6" beam spanning 24'-9" seems too small. Depending upon the spacing of the beams, I would expect to see a 12" deep beam based on typical rules of thumb. Who sized the beams?

 

[markls8]

My architect sized the beam. The 8" beam was originally a 6" beam with a support column in the middle of it. When that column was removed it was changed to 8". The architect did not suggest that the beam which has deflected (which is parallel to the larger and about 5' away from it) be replaced with a larger one, but the builder and I agreed that it should be made larger as well, but the structure ended up being built with the 6" beam, and I didn't notice it until after the pour. The architect specified 5" of reinforced concrete, but there ended up being 7" to satisfy a step-down distance from a doorway, and I guess 6X6 mesh counts as reinforcing (can anybody comment on this ?).
The builder is away on vacation for a few days and I'm going to meet with the architect on Wednesday, but I was hoping to have my burning curiosity somewhat satisfied by then.
I measured the arc of the beam by snapping a chalk line today and it measures 1 - 11/16 " deflection.
My proposed solution will be to place a second 6" beam which will have its ends jacked up to give about 3/16" of deflection and welded there. Hopefully a this amount will exert enough preload force for adequate safe supplementary support of the deck but not exert enough force to distort and crack the Stampcrete deck. They should be able to confirm this at the time.
Thanks Taro for your reply. Regards - Brian.

 

[Rjeffery]

Could have the beam been designed with a camber and installed incorrectly, ie upside down?

 

[JedClampett]

I, too suspected that the beam was too shallow for the span. I did a quick check on Enercalc. I assumed that the beam supports 2'-6" of the concrete filled deck based on your description. The dead load is then 220 lb./ft. Using this and the beam self weight, I got 16.18 ksi stress, which is acceptable for a laterally suppoted member. Deflection is 1.62 inches, which is almost exactly what you measured. For your span, the deflection to length ratio is 1/175 which is kind of high. You'd like to keep it to about half that.
As far as your solution, the best solution is to use a deeper member. A W8 x 18 lowers deflection to 1 inch. Or, if there's headroom, you could jack up the existing member and weld a WT (WT4 x 9?)with its flange down to the bottom flange. When the jacks are removed, the deflection will be lessened. Note that this only works because the shears are low.

 

[DTGT2002]

As the concrete is in place an hardened, I would ask for proof that the current framing is safe, from the contractor who overloaded the framing members, and from the architect. The deflection to beyond what might be considered acceptable for the span, as far as commercial construction is concerned.

I think all is well, and attempts to jack up the slab will likely lead to cracks in the slab.

The wire mesh is indeed reasonable reinforcing for a composite steel pan deck, with a proper size wire.

What are the supports for the deck? If you were deeply unsatisfied with the response of the contractor and architect, could a centered colummn be placed under the long span?

Do you have the plans? Once any questions of life safety have been taken care of, the final issue will be who's pocketbook feel the pain of any erros that occured.

Good luck,


Daniel

 

[markls8]

Rjeffrey - good thought, but the beam was uncambered at installation.
JedClampett - L/360 is the deflection I've worked with for wood I's. From what you said, it looks like this is what one shoots for in steel as well. As Daniel points out, I cannot risk cracking the concrete, so my goal now is to just prevent farther live-load deflection, and to ensure the long-term safety of the structure. The underside of the deck will in the future be an enclosed soffit,(it's kinda ugly now, but I plan on living with it by painting it until such time as we can enclose it) so I would like to not place anything lower than the lowest point of this sagging beam. The sagging beam will also increase the cost of installing the soffit because strapping is now required to ensure a flat ceiling.
DTGT2002 - IIRC the wire mesh was about 1/8" dia. I kept a sample of it and will measure it. It is good to know that indeed, the wire mesh can be structural, and not just for crack control as some have maintained to me in the past. The centre support column (they're all HSS 4" dia.) was removed to make room for a swimming pool which is partly underneath the deck, so we cannot place a support anywhere near it now. I certainly will persue proof of safety from both parties as you have suggested.
I expect that since both the architect and the builder are of excellent repute that I will end up satisfied one way or another. But my need for some knowledgeable background input from this forum before meeting with them has been well-met. Thankyou. - Brian.

 

[markls8]

Epilogue...
- the 6X6 wire mesh wire is 5mm in dia.
- a steel member running perpendicular to the beam in question was for some as of yet undetermined reason left out - the likely culprit of this whole problem
- we will be seeking the input of a Structural Engineer, and the Architect has suggested that the beam can be strengthened with four lengths of welded-in "reinforcing steel" bar running along its length inside the "I", enough to strengthen it against farther deflection in the future.