50% live load reduction factor ?
hi again,
for that 30 story building, the negineer has used a 50% live load reduction. the national building code of canada has a formula to calculate this reduction factor and the reduction i calculate comes to 78%......
is there some empirical design formula that i may be missing here ??
thanks
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at the time of design the building wasn't fully leased.
50 years ago the allowance for simultaneous live load distribution was treated much simpler. i think the loading code was a small 30 page handout.
johnp.rz
i think ibc limits ll reduction to 20%
playing devils advocate, how are you determining the reduction? is it plainly stated that a 50% factor is used? is this factor plainly show across the entire building?
away from that, i have never know of a reduction that great.
if it is concrete, i hope the contractor is ready to shore the building to grade for most of its duration...
daniel toon
llrf is covered under cl.4.1.6.9 in nbcc in particular sentence 2 applies to areas other than assembly occupancies. if the tributary area is greater than 20m^2 (215 sft) the load is multiplied by 0.3+(9.8/b)^.5. for example if columns are spaced 20ftx20ft grid (ta=400sft or 37.1m^2),llfr=0.81. for a 30 storey bldg with same grid ta=30*37.1=1113m^2, therefore llrf=0.39.
other countries also use a 50% llrf:
australia; as/nzs1170.1:2002; llrf = 0.5 (min.) for supported area > 225 m2 (not applicable for certain load types). the formula given is virtually identical to "0.3+(9.8/b)^.5" as pasted by calculor, but with the 0.5 limit.
uk; bs6399
art 1:1996; reduction in live load = 50% (max.) for columns etc. supporting > 10 no. floors.
in the uk a 50% llrf can be used to calc the forces in columns only, when the number of floors is greater than 10.
this reduction is not to be applied to loading on beams.
another reduction factor of up to 25% can be used for live load on beams depending on the area of floor that the beam supports.
patswfc
by "columns etc" i meant the elements as listed in clause 6.2; "columns, piers, walls and their supports and foundations".
from a theoretical viewpoint it could be argued that transfer beams can be included if they support columns carrying a number of floors. common practice may indicate otherwise.
the above is based on the 1996 edition, please ignore if it has been amended since.