100 or 100 = 25

huangyhg

100 or 100 + 25?
for years i have been using only 100 psf live load for public assembly areas on a roof, not adding snow (25 psf here).  professionally, i do not have a problem with this knowing that it is not very likely that people will be standing shoulder to shoulder on the deck in two feet of snow.
anyone know of an ibc or asce 7 code provision where the 25 psf snow load  should be added in addition to the 100 psf assembly load?  i can see just using the snow load and forgetting the assembly load for snow loads in excess of 100 psf too.  in the mountains here, 400 psf is not unheard of at 4000 feet.
just trying to verify that the codes are logical here
mike mccann
mccann engineering
check out our whitepaper library.
the ibc load combinations in section 1605 all use roof live load or snow, not both together.
nice to verify that the code is logical here.  thanks.  
mike mccann
mccann engineering
except that there is no 100 psf roof live load.
you apparently have 100 psf floor live load and 25 psf snow load.  floor live and roof snow are added together in the combinations.
i believe if the area is a public assembly area, as a public roof deck where residents or the public can congregate, then the 100 psf value must be used - see 2006 ubc table 1607.1, occupancy #28, hotels and multiple family dwellings, specifically "public rooms".  
what constitutes a room would be the only argueable issue here, but you have to consider the intent of the code.  i believe that a space does not have to be enclosed, either partially or completely, to be constituted a "room".  it gets down to the "public" access issue here.  hence i believe that section 1605 would apply here.
mike mccann
mccann engineering
jae,
ibc 1607.11.2 specifies the live loads to use for roofs and says to use 100 psf for assembly areas.  so the 100 psf is in fact a roof live load and does not need to be combined with snow.
taro-
i think that there is a subtle but important distinction.
that is in fact a live load that is applied to a roof, but it is not a roof live load.
this is taken from 2003 ibc commentary, 1602 definitions
live loads. those loads produced by the use and occupancy
of the building or other structure and do not include construction or environmental loads such as wind load, snow load, rain load, earthquake load, flood load or dead load.
   this definition identifies the scope of the type of loading included in section 1607. generally, live loads are not environmental loads or dead loads, but are transient in
nature and will vary in magnitude over the life of a structure.
live loads (roof). those loads produced (1) during
maintenance by workers, equipment and materials; and (2) during the life of the structure by movable objects such as planters and by people.
   this definition is needed for the proper application of the load combinations in this chapter. this definition clarifies that roof loads, such as snow loads, are not live
loads.
i would interpret this to mean is the 100 psf live loads are greater than the 25 roof live load, i would use 100 psf.
don phillips
structuraleit,
i disagree.  a live load that is applied to the roof is by definition a "roof live load" (as set forth in ibc section 1607.11).  the purpose of labeling it as a "roof live load" as opposed to any other kind of live load is so that it can be combined appropriately in section 1605.  that is, the effects of the live and snow loads are considered separately and the worst case is used for design.  the probability/reliability-based load combinations do not require the cumulative effects to be considered.  in other words (as mike originally said) the roof deck is not going to be packed with people standing shoulder-to-shoulder immediately after the 50-year design snow storm hits.
taro-
my apologies, i mis-read the posts.  i meant to agree with you and disagree with jae.
see the asce 7-05 commentary:
"occupancy related loads on roofs are live loads (l) normally associated with the design of floors rather than roof live loads (lr)"
although it makes sense to not include snow with such roof occupancy loads, i don't think it is allowed. i am not sure they want you to take a reduction as a floor load and include it in the roof load only combinations.