Performing a solar structural analysis - lightweight concrete

[OHEcoEng]

Hi, everyone. I am, once again, looking at an existing building's capacity for a ballasted PV system, which wil add approximately 4 psf dead load.

I have been through ASCE 7-05 calculations using several variables for the existing roof, but keep coming to the conclusion that the roof structure can't support it's own weight PRIOR to placement of a ballasted PV system.

This building utilizes a series of OWSJs supported by wide-flange beams and columns.

The roof itself consists of 20 ga. decking, lightweight concrete, and EPDM.

The roofer has lost his records (built in 1997) and I can't get a good idea of concrete deck thickness. The plans I have do not specify.

The joist I am analyzing is a 30K10, 6' OC, 43' span.

Here are my assumptions (dead load):

1. Sprinklers/lights: 5 psf
2. Ductwork: 4 psf
3. Suspended ceiling: 2 psf
4. Metal decking: 2.5 psf
5. EPDM roofing: 1 psf
6. Joist self weight: 2.5 psf


The lightweight concrete deck is specified as 3500 psi/115 pcf. I have been assuming that it is 2" thick at drains, increasing in thickness at 1/4" per ft. to the halfway point to the next drain, or approx. 22 ft (drains are 44' apart) This would give a concrete thickness of 7.5" at the thickest. This thick portion runs on/parallel to an entire joist in some locations. With the slope considered over the tributary area of the joist, this averages 7.125" of lightweight concrete concrete supported by one joist.

At this thickness, I am looking at 68.3 psf dead load from the concrete alone. Total dead load in this case (using numbers above) is 85.3 psf. The joist capacity (ASD) is 65.6 psf (43' span, 6' oc spacing).

This exceeds the allowable without snow load, drifting, wind, and live loads considered.

What the hell am I doing wrong?? I have read over and over that MINIMUM lt. wt. concrete roof deck thickness is typically 2", and minimum slope is .25"/ft. These are the assumptions I made. Does anyone know anything different?

The roofer did say that there was no polystyrene utilized under the concrete.

Any help in sorting this out would be greatly appreciated!!!

 

[hawkaz]

Are you sure this is lightweight structural concrete and not lightweight insulating concrete? There is a big difference in densities.

 

[paddingtongreen]

@hawkaz, OP tells you it is specified at 3500psi/115pcf.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[OHEcoEng]

hawkaz:

this material is actually referred to as "insulating concrete fill" in the plans. The roof cross section (no dimensions given) also shows a "block" with cross hatching within the concrete. It would appear to me that this material might be EPS or other rigid insulation embedded within the "fill".

The specs for the strength and density came right off of the plans under the masonry section.

Thanks!!

 

[OHEcoEng]

Oh, yeah...riminder: the roofer said there was no EPS or other insulation embedded within the "fill"...but something has to be wrong given the loads calculated.

 

[paddingtongreen]

If they ordered lightweight concrete from the plant, the roofer wouldn't know, vermiculite granules look like ordinary aggregate when they are covered in the cement mix.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[OHEcoEng]

I am really at a loss here. I've uploaded the concrete specs. and a section from the roof in question. What do you all think? Given the issues I'm having, I am leaning toward the assumption that there actually is EPS under 2" - 3" of insulating concrete fill.

Any opinions? Is there actually a chance that there is 7" of concrete here?

 

[MikeHalloran]

Measure the actual roof slopes, and/or get out the core drill...

Mike Halloran
Pembroke Pines, FL, USA

 

[OHEcoEng]

Well, Mike. That's where I was leaning. I guess I'll have a core sample in a day or two...bah...

 

[Ron]

What you are dealing with is lightweight insulating concrete, not lightweight structural concrete. Lightweight structural concrete might have been specified elsewhere on the project but that's not what is on your roof deck.

That material may or may not have polystyrene incorporated in it, as the roofer noted. Since he remembers that it did not have polystyrene in it, its a good indication it is LWIC not structural concrete.

It typically sits on a vented deck, but is also poured onto non-vented decking. The decking is usually from 20 to 26 gauge material. Rarely is it thicker than 20 gauge.

The material consists of a small amount of portland cement and vermiculite or perlite aggregate. It is/was commonly marketed under the brand name "Zonolite", which in 1997 was still a W.R. Grace product. It was later bought by Siplast. It would typically have a compressive strength of around 125 to 150 psi and a dry density of 35 to 40 pcf.

Since it was built in 1997, it could also be cellular concrete fill. That material commonly had no polystyrene fill in it and was placed in a similar manner to the Zonolite. Elastizell is one of the producers of this material. It has similar unit weights, maybe slightly higher up to about 50 pcf, and higher compressive strengths, up to around 350 psi.

 

[MikeHalloran]

On second thought, don't go drilling the stuff, at least not dry:

http://www.aolnews.com/killer-in-th...n-cancer-causing-zonolite-insulation/19731468

http://www.allaboutmalignantmesothelioma.com/asbestos-zonolite.htm

http://www.asbestos.com/companies/w-r-grace.php

http://www.asbestos.com/products/cement/asbestos-finishing-cement.php

Mike Halloran
Pembroke Pines, FL, USA

 

[OHIOMatt]

I would doubt that they placed the concrete to make the roof slope. Is it possible that the roof steel is sloped or that they use a fill such as insulation to create the slope?

I think that you are likely on the right track with eps under the concrete.