Slab Overstress - The Real World Affect? 1

[mark_1155]

Was wondering your thoughts on what these numbers with the attached layout would physically do to the structural slab? It's residential loading on that level with a bearing wall above.

Slab is 5" with composite deck, 1 1/2", 20 ga, headed studs exist on beams at about 12" o.c.

Loads from bearing wall (non-factored loads):

800 plf dead
960 plf live (full live load reduction taken)
1600 plf live (no reduction) - edited... all plf. Double wood bearing wall above.

There is no steel support underneath the bearing wall from above and it is 1'-9" from the main girder. What will happen most likely?

 

[MIStructE_IRE]

No one here can answer that. Have you undertaken a structural analysis?

 

[mark_1155]

MIStructE...
I'm fairly certain some people can chime in. The only spec. I forget was 6x6 W2.1 mesh called out. Yes... the calculation of bending was done for various loadings (just dead, dead and a little live, etc.) and all bypass any composite deck span table... by a lot.

 

[Ron]

What will happen most likely?

With a 1600 psf live load, I would bet on collapse.


A Great Place For Engineers to Help Engineers

Follow me there.....

 

[MIStructE_IRE]

Hi, So if you know that you’re exceeding the allowable load “by a lot” then what is the question? Sorry, I don’t mean to sound harsh, but I’m not entirely sure what you’re looking for.

 

[rapt]

I'm with you MIEStructE IRE,

If the stresses are above allowable then it fails. There is no concept of "well I don't think it will actually collapse until more than 50% above allowable so I will accept it at 50% over stressed".

Allowable it the maximum. Anything above fails.

 

[XR250]

Perhaps you can design a a beam at the top or bottom of the wall and bring the load down to discrete points 6'-10" O.C.?
Otherwise, seems like a bad idea. I imagine, in reality, the slab will distribute the load to adjacent deck flutes. Not something i would count on, however.

 

[mark_1155]

XR250,

I assume some distribution as well and 2 way action. But I think what's going to happen is 1/4" cracks at all beams and then a plethora of spider cracking in that area. Over time I think creep will set in and there will be very unhappy condo owners.

The EOR says it's fine, I say it's not. And now the owner can see first hand what happens as well.

 

[Tomfh]

What’s the EOR’s rationale for saying it’s ok?

 

[mark_1155]

His exact words... It's close enough.

 

[Tomfh]

How overstressed in your view? What percent?

 

[mark_1155]

How much? I'm going on recollection but allowable M was around 50k-in (LRFD). Dead load just as one way slab was about 70k-in (likely low on dead load also, I used 20 but each floor has gyp 1 1/2). Put in the live load between 3 and 4 times over allowable (actually with LRFD and 1.6 factor, make that 4 to 5 times). That didn't even put the load from the floor in question which has it's own uniform loading and obviously the concrete.

I have a hard time visualizing the bending stresses going to the main girder, certainly shear forces. I am assuming all cracked concrete at beams. I'd put it about 3.5 times over allowable in bending from the load it will likely see during it's life. So if allowable is 50 k-in, what it actually sees is around 175 k-in.

Haven't even considered the web crippling of the deck itself at main girder. I guess that all depends on where the flutes hit.

 

[MIStructE_IRE]

Is this an existing occupied building? If so, I think you now have a duty of care, if not a moral obligation, to do something about it. Even report to your local authority so your conscience is clear if and when this does collapse.

 

[wannabeSE]

How are you calculating the capacity of the deck? Are you assuming a 1' strip width or have you assumed a different width or performed more rigorous analysis? Does the wall above have any strength/stiffness to span between beams? Have you asked the EOR why they believe it is fine?

Unless there is reinforcing (more than temperature and shrinkage)specified over the beams, composite decks are typically designed as simple span.

 

[kipfoot]

I'll suggest that the 'real world' effect depends on the bearing wall stiffness. If that's a masonry wall, then it will arch from beam to beam and very little load will be imparted to the slab. Then it's a question of the shear capacity of the W21 beams.

 

[mark_1155]

They are wood walls, sheetrock. It's going to deliver the load direct to that slab.

I didn't run any calcs at first, just mentioned was a bad idea to leave out the steel.

Let's say we take that load, spread it over 3', include first floor loading, make an assumption the inner 1.5' sees 70% of the load, reduce live load from 40 to 24, ignore any tile areas, ignore the weight of unit separation walls, go light on general dead load... I'm still around 100 k-in LRFD. Allowable is, I believe, around 45k-in.

There is steel under all other bearing walls, it was missed during design. It's not wanting to admit to the owner they messed up the design. It, in no way, works on paper. I think it will excessively crack and cause serviceability issues both during construction and down the road. I do not think the slab will shear into failure.

 

[Tomfh]

Your saying it’s 3x overloaded, and EOR is saying it’s “close enough”. I don’t know any engineer who would say 3x overload is acceptable, so it sounds like you’re both assessing it differently?

 

[mark_1155]

How do you quote when replying? I see the option on my phone but it never posts on my phone. So only can post on a computer.

He's saying 1'-9" offset is "close enough". Bearing wall runs parallel to main beam, 1'-9 offset center to center.

 

[jayrod12]

Does the wall have wood sheathing on it? If so, I wouldn't be overly surprised if nothing bad ever happens. The wall in that case would act as a deep plywood beam and deliver the load almost directly to the W21 beams.

That being said, I agree it's a terrible idea to rely on a mechanism like that.

 

[mark_1155]

Wall above is gyp shear wall. It will likely be first to show signs of cracking.

70%, I don't know if too conservative. Most engineers would likely take the depth of slab from top of flutes (3 1/2), and spread the load at a 45 degree from edge of wall to edge of wall and check slab. If no good, put steel.

I'll ask GC to take some pictures in a couple months in that area, will post them if I get them.