Live Load Reduction

[swivel63]

have you guys ever used live load reduction in a slab design? not a transfer slab carrying columns above or anythign like that.....but just a normal everday two-way slab that sits on columns above and below?

i've never done it before, but according to IBC chapter 1607.9.2, it doesn't appear that there's anything stating that live load reductions were only used for slabs or other horizontal members that were used in a transfer like fashion. for normal, everday slabs i've always used full live load and everywhere i've worked that's been the norm.

is this common practice? for concrete buildings, i don't think it would matter a whole lot considering that dead load is usually much bigger than live load anyway.

 

[271828]

Sure, lots of times. Just not for one-way slabs.

 

[msquared48]

Professionally, although it may be allowed in the code, I am not comfortable with using LLR on a slab, one or two way. No problem with beams and columns, as the logic is clear. I just cannot justify the logic of the reduction in my mind and still feel safe about it. I guess that's my problem.

Mike McCann
McCann Engineering

 

[pappyirl]

Why have specified live loads if they can be reduced?

Live load reduction is based on the probability that all slabs in a multi storey building are not at the maximum live loading at the same time. This applies only to columns, walls and transfer slabs and beams and foundations only in a building above a specified minimum height.

While there may be a party on Level 20, it is unlikely there are parties on levels 1 to 19 at the same time. The slab at level 20 still has to be designed locally to cater for the live load from the party at level 20, but there is a saving on the elements below due to the unlikely event of parties on all the other levels.

I regularly use the live load reduction on the elements specified above, but never on a single slab regardless of which way it spans.

 

[apsix]

"Why have specified live loads if they can be reduced?"

Because the probablity of the specified live load acting uniformly over a large area is small.
Logically the LL reduction is based on the area supported by the structural element in question, not the no. of floors in the building. However, local codes may specify otherwise.
Personally I don't reduce LL on slabs, although reduction on 2-way slabs clearly is allowed by some codes.

 

[msquared48]

Apsix:

You stated that...

"the LL reduction is based on the area supported by the structural element in question, not the no. of floors in the building"

The foregoing is true for any beam on a particular floor, but not for columns. Columns run floor to floor, support much more area in general in any particular structure, and consequently will see more reduceable live load area.

Mike McCann
McCann Engineering

 

[apsix]

Mike

What I was trying to say;
The LL reduction should logically be the same for a column supporting 1 floor with a supported area of 200 m2 as for a column supporting 10 floors, each with a supported area of 20 m2.

Upon reflection I suspect that it doesn't hold true for all loading cases. I was primarily responding to the previous post which stated LLR was applicable only to multi-storey buildings.

 

[msquared48]

As you stated, assuming the same live load rate, yes, I agree. Thanks.

Mike McCann
McCann Engineering