huangyhg

uninhabitable attics
in asce 7-98 and ibc 2000 under residential, it gives loads of 10 psf and 20 psf for unihabitable attics without and with staorage, respectively. a wood truss supplier told me that this is a non-concurrent loading, in other words he does not have to add it to roof live load. i see no reference stating this in the codes.
am i missing something or is this a mis-interpretation on his part?
i believe this load would hbe added to the bottom chord (colatteral bottom chord live load). should this load be concurrent with the roof live load? yes, i believe it should.
this is definitely a misinterpretation by supplier (i guess that he has typical trusses designed for roof load, and he doesn't like to alter his/her calcs and design. the loads are concurrent.
the truss supplier spoke correctly. the 10 psf is for design of the bottom chord only. basically, it provides for a load for bending between panel points. there's also a prescription for a point load anywhere along the chord, so joe homeowner can walk on the truss chords to reach his box of ornaments.
otherwise, he must design the truss for a live load of 20psf basic roof live load, or a snow load - should that govern.
that's not to say you can't design for a greater load. if the owner knows he may wish to store more than christmas ornaments in the attic, you can specify a greater than minimum design bottom chord load and applied concurrent with the roof live load.
the truss manufacturers don't normally use a higher load than code prescription since their business is so competitive.
but being a structural engineer, i look for the opportunity to reinforce trusses to lift m-1 tanks to change their tracks in the garage. most people don't have such an appreciation of reinforced trusses.
i just had an email from our state plan review dept. (wisconsin) and he confirmed that the loads should be combined with the roof live load per the ibc.
i believe there may be other states interpreting this requirement like alohabob stated.
frankly, i cannot see anything in ibc that allows this interpretation unless the state has modified that particular requirement.
you got me going. i had to research this years ago.
i checked the ibc ....
uninhabitable attics without 10
storage ( my book has a misprint)
scuttles and accessible ceilings 200 lbs
load combinations 1605.3 formula 16-9 appear to make snow and roof live load exculsive of each other.
exclusion note of the ubc not here any more. i wonder if it will come back??
i checked old ubc's
'88 table 23-b ceiling live load 10 footnote 5 need not be considered with other live loads....
'94 table 16-b similar
formula 16-9 was exactly what i was looking for!
thanks, alohabob!
i respectfully disagree.
equation 16-9 from 1605.3 states (at least in my copy)-
d+l+(lr or s or r)
d is dead load
l is live load
lr is roof live load
s is snow
r is rain.
now, alohabob is correct that roof live load and snow are exclusive of each other. however, i would say that the attic load is a live load, not a roof live load. it is applied on the attic floor, which as far as i understand, is not a roof. therefore, i would say you should definately combine your attic load on the bottom chord with your roof live load on the top chord.
lppe you are correct.
the 10 psf for non-habitable attics without storage (or 20 psf for non-habitable attics with storage)is applied to the bottom chord as a live load.
the top chord of the truss is still designed for the roof live load (typically snow in the ne).
look at trusses as "pre-made" rafters and ceiling joists. wouldn't you design the ceiling joists for the 10psf (or 20psf) and the rafters for the roof live load. well now...the top chord is the rafter, the ceiling joist is the bottom chord.
you include both loads in the design.
now that we have that straight.....i really doesn't matter. if on your structural drawings you specify to include both loads, then the truss supplier should include both loads in the design. there is nothing in the code that says you can not exceed the minimumns.
alohabob, i disagree with you comment regarding truss suppliers not using higher loads than the code requires since the industry is very competetive. shouldn't we,as structural engineers who are ultimately responsible for the project, be telling the truss supplier what to design the trusses for. the design loads for should be clearly indicated on our drawings. every supplier would (should) be providing a similar bid based on the specified loading. if they don't look at the drawings for the loadings, and use the "minimumns" to bid the project and underbid the project that is their problem.
on many projects, the truss supplier submited copies of their shop drawings with different loads than i specified. i return with drawings a big rejected note across them and tell them to look at the drawings for the specified loads. they always resubmit with the corrected loadings.

if one requests that roof trusses be designed for both snow load or live load on the top chord simultaneous with attic load on the bottom chord, at least use the 0.75 multiplier on the live loads, as permitted by code. i still think a bottom chord will never see anything close to 10 psf, if it is an uninhabitable attic. the truss webs are too close together, and there is no flooring to allow for storage. and the occasional person walking in the attic won't be anywhere close to that load, either. i am okay with interpreting the code liberally, and saying attic load is a roof live load.
daveatkins
dave,
who said the attic load is a roof live load?
i know that i didn't say that. lppe didn't say that. i don't think aloha bob said it was a roof live load either.

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