Live Load Reduction on Discontinuous Column

[sgs114]

Hello,

I have a 6 story office building that has a series of interior columns supporting a concrete slab. Floors 3-6 are identical, so the columns stack. At the 2nd floor the column grid shifts, so the columns land on reinforced concrete beams. The building will also be on piles and a grade beam at the foundation. My questions is in regards to the loading to design the beam, first level columns, and piles for.

First, looking at the 2nd floor beams with a point load from the interior columns, do I apply a point load to the beam based on the reduced live load from the column (K(ll) = 4)? Or do I take my un-reduced live load, and apply the beam K(ll) factor (2) and design the beam based on that? The 2nd approach is more conservative, but then I am designing my beam for a load my column is not designed to support?

Similar questions for the first floor columns (though the first floor columns would have the same K(ll) factor as the columns above, so that is not as big of an issue) and piles. Basically, once you reduce your live load to an element, do you simply carry that reduced load all the way to the foundation design? Or do you adjust the load based on the element it is supporting at different locations (beams, columns, piles) with different K(ll) factors? Thanks for any help.

SGS

 

[JAE]

Basically, once you reduce your live load to an element, do you simply carry that reduced load all the way to the foundation design? Or do you adjust the load based on the element it is supporting at different locations (beams, columns, piles) with different K(ll) factors?

The latter is correct.
Each element has its own tributary/effective area which generated the load to it. That area is used to determine the LL reduction.



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[DST148]

@sgs114>>> The reduction factor at a given story would depend on the cumulative influence area at that level.
See thread176-405000.
It has a solved example from ASCE at the end of the thread.
Here are my thoughts on the total live load on the column on the 1st floor from that solved example:
I have not included live load on the roof since 1.0L > (0.75L + 0.75Lr) for 1st floor column.

(315 sft) x (8 floors) @ 20 psf = 50.4 Kips ------ Reducible Live Load
(315 sft) x (8 floors) @ 15 psf = 37.8 Kips -------Non-Reducible Partition Load
(315 sft) x (1 floor) @125 psf = 39.4 Kips -------Non-Reducible Storage Load
---------------------------------
Total Live Load = 127.6 Kips
---------------------------------
The solved example has 143.3 Kips on the 1st floor column.
Let us know your thoughts on my comutations above.

 

[rapt]

The way loading codes are currently written, yes JAE is correct.

We are having an interesting discussion on this at the moment in regards to Robustness rules for the Australian codes.

Robustness requires that the failure of any individual member will not lead to progressive collapse or collapse disproportionate to the initial member collapse. This is very difficult with transfer beams.

The only solutions with a transfer beam is to either provide another load path through the structure, or over design the beam as a "structurally significant member" so that it is more likely to survive an event that could cause its failure under normal circumstances and avoid the resulting progressive collapse.

Part of the over-design option is to increase the load factors on transfer beams and the columns supporting them, and another part of it is to design them for the full un-reduced live loading.

The discussions are on-going.

With PT transfer beams, I have always over designed them anyway, as an increase in strength will only cost 10-20% of the reinforcing cost in the beam which is petty cash compared to the overall building cost! And it allows me to sleep better at night!