New Load on Old Planks

[phamENG]

Looking for some history here. I'm adding a penthouse to the roof a building built in the 80's with precast/prestressed hollow core planks. Due to some "interesting" geometry, I have a brick line load (686plf) being introduced across the planks (effectively a point load on each) at about x=2.8ft.

Question 1: When did PCI switch to Strength Design (as opposed to working stress)? Working stress was replaced as the primary method in 1971, and was still available in ACI 318 until 2002 as an alternate design method. From the looks of it, PCI didn't switch until their 4th edition design guide was published in 1992 - there's a noticeable jump in allowable loads in the hollow core load tables at this point. I just want to confirm I'm using the right load combinations with the load tables from the correct era.

Question 2: Anyone see any significant problem with bearing 17' of brick on the planks? Assuming the load tables I'm using (3rd edition, 1985) are working stress, I'm at 0.33 utilization on available moment and .58 on shear for the weakest section listed.

Thanks for any help.

 

[Ingenuity]

My copy of the 2nd Edition of the PCI Manual states ultimate strength for flexural strength:

I don't have the 3rd Edition, but I would expect it to be similar limiting criteria, so therefore using ultimate strength for flexural capacity, with check on service stresses, and the lower value is the tabulated "safe superimposed service load".

How confident are you in knowing the as-built strand spacing and diameter?

Producers often varied the strand size and spacing to suit the specific project. In the past we have undertaken invasive probing and GPR scanning to determine the as-built condition. In extreme cases, extracted 4+ ft long strand samples and undertaken tensile testing to determine the mechanical properties.

With your proposed new additional loading close to the end of the plank, you will need to check that you have sufficient development of the strand in determining the capacity at the critical sections/s.

 

[jayrod12]

I'd be concerned about 17' of brick on an existing plank.

That's a fairly significant line load for something typically designed around uniform loading only. At 3 ft away from the support the concern would be shear capacity in my eyes over moment capacity. What about a steel stud or wood stud wall with brick veneer? It would reduce the new load significantly.

Was the original building designed for a penthouse to be added after?

 

[phamENG]

Strength design it is, then. Thanks, Ingenuity. We're not confident of anything but thickness yet. Contractor is aware that scanning will be in order - just trying to make the design reasonable enough that, regardless of what we find when work starts, it won't take too much to make it work. My thought with using the weakest generic plank from the PCI manual is that, hopefully, they wouldn't make anything significantly weaker than that.

jayrod, this is a veneer on a stud backup. We're converting a stair tower to an elevator. For clearance reasons, the machine room doesn't fit within the hoistway footprint. It was either bear the entire penthouse on the planks (machine room, car, counterweight, etc.), or put everything but the veneer on the existing CMU walls, cantilever the slab to get the clearance, and run the brick all the way down so it doesn't look like a lighthouse. That's why it's standing off 2.5 feet.

To determine available shear, I calculated the shear generated by the rated load which should be either shear, flexural, or service load stress. Seems like it should be a conservative limit to use, especially if shear isn't the governing limiting state. Does that sound reasonable?

Not ideal, but I have to make it work.

Thanks.

 

[Ingenuity]

phamODU:

I too share jayrod12 concerns wrt shear capacity.

What is your plank depth and span?

I assume the planks are topped.

Is it an end span condition, or is it an interior span where you could achieve negative moment continuity?

 

[phamENG]

6" planks with 2" composite topping per SEOR Record Drawings (no shop drawings of the planks survived). We have a 15' span, all simply supported.