Live Load Reduction

[slickdeals]

Folks,
I want to gather some input from other engineers on live load reduction for horizontal elements on a floor (ie beams, slabs)

My previous firm had a internal policy that we would reduce floor live loads for post-tensioned and reinforced two-way slabs. Also, we would not reduce live loads on floor beams.

What are your typical standards? Is it uncommon to reduce floor live loads on a office floor designed for 4 KPa?

 

[DST148]

The Building Codes allow reduction in live loads subject to limitations for certain occupancies and magnitude of live loads. Most of the commercially available software have live load reduction capabilities.
We always reduce live loads for design of beams, columns, foundations of multistory buildings. However, we don't do it for low-rise office buildings.

 

[chris457]

When I worked on buildings, I generally would run live load reduction for column and foundation design, but design the horizontal elements for fully loaded floors.

If we were designing an office floor for any more than 2.4kPa the reasoning was to make sure it could handle filing loads later on (which always seemed to come up), so it wouldn't make a lot of sense to purposely up the live load only to use LL reduction to reduce it again.

 

[JAE]

chris457 identifies a proper view of engineering - that you use your common sense and engineering judgement in things like live load reductions. The code "allows" the reduction - doesn't demand it. So in cases where you suspect (fear?) that there might be possible overload (file cabinets vs. cubicles) then perhaps you use your judgement and apply what most contractors refer to as our "fudge" factor.

 

[msquared48]

I have never heard of reducing the live load for a slab of any sort, only joists, beams, columns and footings. And I never, but never, use LLR at the roof.

For small projects, I rarely consider it, only larger, multi-story ones.

Mike McCann
MMC Engineering

 

[ishvaaag]

We have the mandatory code CTE in Spain that addresses the issue as something one can do within what there specified. For example it does not prohibit the live load reduction for mats at mat foundations; and we have applied it some times, this meaning the loads going down through the column will be somewhat lower. It makes sense, (the code stipulates different owner per level to make the reduction and same kind of use), it is unlikely say 10 floors be fully loaded to the individually allowed capacity for the level, so the code itself allows for that.

CTE is directly visible from the web and so takes translation to english by google (even if a bit clumsy), if you want to look at the specifics.

 

[rapt]

slickdeals,

2 main problems with LL reduction on floor members,

1- as mentioned above, loading is often concentrated in an area such as light weight filing systems in offices.

2 - construction loading capacity for backpropping is much reduced so more floors need to be backpropped, leading to increased costs, so you have to weith up the reduced floor cost to the increased back-propping costs.

Because of this I would not do LL reduction for floor members, only columns/walls and transfer beams.

 

[dhengr]

Slick:

The whole concept of LL reduction was the forerunner of today’s probabilistic and statistical approach to loads in last three or four editions of most codes, particularly ASCE 7.

The thinking went something like this; what is the likelihood that this member is going to see our max. design LL, which is of course a somewhat arbitrary assumed value. In an office space for example, few floors a fully loaded to 50#/s.f., but most likely some areas will be loaded with storage and filing cabinets, so maybe 100#/s.f. is reasonable; and what is a reasonable concentrated load on 2 or 3s.f. area? Engineering judgement used to come into play here, and these three conditions could reasonably occur anyplace on a fl. system, so typical fl. framing, slabs, joists and the like usually got no reduction. But, it was also reasonable to think that it was unlikely that a major beam or girder which had several hundred s.f. of tributary area would all be fully loaded. Also the DL to LL ratio came into play. Generally, LL reductions on fl. members were fairly small, except those that had large tributary areas. Carrying this thinking a step further to columns or walls, after several stories and hundreds of s.f. of area supported, it is very unlikely that max. LL would exist everywhere; and the LL was reduced as the supported area grew, to some max. allowable % reduction. I think it was about 60% for columns and walls. Obviously, this would carry on to the foundation design.

With the latest thinking and machinations they have turned this relatively straight forward thought process into many research projects, much expensive publishing, many expensive software packages, and many pages of calcs. And, they think they can prove that you can save .05% in column and foundation concrete. That translates into 23.93" sq. conc. columns but your contractor doesn’t want to cut down all his 24" sq. column forms, so you should be able to back charge the code writing agencies for the lost savings.

 

[TXStructural]

Slick, that is why IBC/ASCE use trib area to determine how much reduction is allowed.

The engineer is obviously expected to exercise judgement, but the code does contemplate those issues.

 

[steellion]

Roofs: No
Floor Beams: Yes
Columns and Foundations: Yes, but limited to 50% reduction

 

[slickdeals]

The reason a 4 KPa load is used is because the Indian Code requires you to design an office without dedicated storage space for 4 KPa. If an office had dedicated storage space, it gets designed for 2.5 KPa

In IBC, you get a lesser load on columns/foundations, designing it for 80 psf (which includes an allowance for partitions), rather than designing it for 50 psf live and accounting for partition loads additionally.

I understand the whole probability concept, but you are designing live loads for a 1.6 overload factor. I don't wish to be too aggressive in my designs, but in India the Ultimate load combination is 1.5 D + 1.5 L. I know the "certain" loads are being upped by 1.5 instead of the 1.2 in US.

 

[JAE]

I disagree with the idea that roof live loads cannot be reduced.

This is not true.

In southern climates where there is little or no snow, roof live load reduction can and many times should be reduced. For large facilities, like dome stadiums, this is done all the time. Using the full 20 psf live load for a roof on a 10+ acre structure is overkill.

I agree that with snow loading (which isn't a live load anyway) you should not be reducing the load.

 

[Teguci]

I've almost always reduced live loads to the code allowed amount. If the floor tributary area was more than 400 SF than it gets reduced (not too many 20x20 two way slabs anymore though?).

I know we Americans are getting fatter, but we also take up more floor area and we don't jump as high. As for filing cabinets and library stacks, yes, we need to include that loading in our design.

 

[msquared48]

JAE:

I agree with you.

I have never done a structure without snow load, so I guess my perception of reality is tainted.

Mike McCann
MMC Engineering