Live Load Reduction per IBC 2012

[design62713]

Per IBC 2012, section 1607.10, floor live loads can be reduced. I am designing a podium two way concrete flat slab, and want to see how much live load I can reduce. There are three wood floors above. Per equation 16-23, AT is tributary area in SF. How do I calculate AT for two way slab? Do I consider one bay (let's say 20'x20') area only or one bay area times 4 (three floors above? As we know, reduction factor depends on AT: higher AT, more we can reduce. Thanks in advance.

 

[n93s412]

You would only consider the tributary areas from the three stories above your 2-way slab if you have a column transferring those loaded areas to the middle of your slab.

 

[n93s412]

I guess more obviously you would consider the AT’s from the stories above if the tributary areas from the three stories above are being transferred to your two-way slab.

 

[asixth]

I don't think you should use live load reduction for the podium floor live load particularly if the bays are 20'x20'.

If the columns from the higher levels are offset and need to transfer at the podium level than you could reduce the live load component of the column load depending on how much area the column supports.

 

[JLNJ]

Are you trying to design the slab or the columns? If it's the slab, does the floor system support the floors above?

 

[design62713]

JLNJ,
I am trying to design slab, not columns. Yes, this slab supports three floors above and roof (wood framing). So, this is podium/transfer slab.

 

[JAE]

So your slab is carrying random wood framing (various layouts of load-bearing stud walls) across the lower concrete two-way slab?

If so, if the walls are fairly random - I'd use the area of one bay times the number of floors imparting live load to that lower slab.
That would include the live load directly on the concrete slab.
And per Table 4-2 - KLL = 1.0 for two-way slabs.

ASCE 7-05 and 7-10 state: [blue]Reductions are permissible for two-way slabs and for beams, but care should be taken in defining the appropriate influence area.
For multiple floors, areas for members supporting more than one floor are summed.[/blue]

 

[design62713]

Thanks guys for your help.