Roof live load capacity 3

[glass99]

My client would like to build a roof deck on their NYC apartment, and I am looking at the live load carrying capacity of the existing roof of an old industrial building from the 1920's. The roof structure is 10" steel beams approximately at 6ft OC with an old style concrete deck.

I have run into this a few times now, but there is a lot of dead weight on these decks from the cinder fill (>120psf), and the roof has really no spare capacity whatsoever to the point of not even being ok for snow loads.

Has anyone else had similar experience?

 

[jayrod12]

Just build a new deck over the old one that transfers the load directly to the columns/walls below,

or, reinforce the existing roof to take the existing and new loads, so more steel joists and reinforced beams.

 

[glass99]

Jayrod12 - building a new floor which flies over the top of the existing deck is probably what we will do, but it is a significant amount of work in this instance because of the odd shape of the deck they want to build. There is also an existing roof deck which has been there for ~10 years, so everyone feels the roof is plenty strong as it is.

 

[jayrod12]

haha, of course they do. just like people feel they should be able to cantilever joists and still have a flush beam.

If the existing roof deck is that recent there should be available drawings no? why don't they contact the engineer that designed that one?

 

[SlideRuleEra]

glass99 - The 1917 NYC Building Code had the following minimum live load requirements:
"Flat, slope up to 20 degrees (1/3)": 40 PSF
"Steep, slope over 20 degrees (1/3)": 30 PSF
This is from page 265 of the 1923 Carnegie Pocket Companion. This book is available (free, of course) at this page of my website:

http://www.slideruleera.net/miscellaneous.html

It is reasonable to assume that these loads remained in force for all of the 1920's since they remain unchanged in the 1932 NYC Building Code, as quoted in the 1934 Carnegie Pocket Companion (have not finished scanning that one).

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[glass99]

Thanks SlideRuleEra, that's useful information. The certificate of occupancy usually has the live load rating of the occupied floors, which for these industrial buildings is a huge number like 150psf. You then get to the roof, and its barely capable of holding up its own weight. Though its own weight is really disproportionately high because they have a solid foot of cinderfill sitting there like a lead blanket. Folks have hard time getting this (including me sometimes).

 

[TomDOT]

Found some related articles for you

http://www.gostructural.com/magazin...n_existing____cinder_slab_buildings-7786.html

http://www.rci-online.org/interface/2009-04-koziol-giffin.pdf