Live load required for two-way post-tensioned slab - office space

[structuralengr89]

I have the structural plans for 2 adjacent office buildings that are almost identical structurally.
They are both 8.5" two-way post tensioned slabs with the same mild reinforcing and column grids.
The plans state that both buildings were designed for 100PSF LL (reducible as allowed by code).

I checked the capacity of a column strip on the first building and for a trib area of 900 ft^2, I can reduce the LL 40% to 60 psf
Everything checks out and the strip is good for 100PSF Reducible LL.

Then I check an identical strip on the second building, the mild reinforcing is the same, spans are the same...everything is the same except there are about 50% less tendons in the strip on the second building.
I have to use a 50 psf LL reducible, which reduces 40% to 30psf to get the strip to work.

The original architectural plans show "open office space" in the area im checking...
I still can't figure out how on a set of plans that show 2 buildings on one sheet how there can be such a disparity in floor capacity. Possiblly the buildings were done by two different engineers...one designing for 100psf reducible and the other 50psf reducible???

Now the new owner wants to change the floor space...from an open office space with a bunch of cubicles...to office space with areas of light storage and corridors, etc.

We have typically designed office space LL for 100 psf for concrete buildings and 70-75 psf for steel framed buildings

Do any of you design office spaces for 50 psf? how do you handle the 80psf for corridors above the 1st floor..and light storage @ 125 psf?

Thank you!

 

[asixth]

I don't use live load reduction for individual floors, only for vertical elements and footings that support many levels.

50psf (2.5kPa) for office space is very restrictive, the loading code I use specifies a minimum 60psf (3kPa) for office space with many property managers requiring a minimum 80psf (4kPa) floor design so that the office space can rented as "premium office space".

 

[dcarr82775]

The Code says 50psf for a new office space (plus known equipment weights, etc), and there are a lot of office building designed for that 50psf. Depending on the version of the code there is also a 15 or 20 psf requirement for partitions. I agree with Asixth that I wouldn't reduce the load on a floor slab for that scenario, but you are technically allowed to do it (LL not the partition load).

Why would you design an office building for a different live load based on the material the building is constructed out of (concrete v steel)?

 

[connect2]

yes there are lots of offices designed for 50 psf and it is the ASCE 7-05 required minimum uniform load as well as the min for Canadian National Building Code, subject to some conditions etc. Interestingly, or perhaps not, in the US the minimum Partition Load is 15 psf and is considered a LIVE LOAD. In Canada the minimum is 20 psf and considered a DEAD LOAD. Issues of live load reduction then can't exist in Canada with the partition allowance but what was really problematic was that for many years the partition allownce because it's a DL contributed significantely to the Seismic Load. Finally in 2005 the NBC allowed for a reduction to 10 psf in the calculation of the effective seismic weight. In Canada alot of the OWSJ suppliers consider the 20 psf partition dead load to be movable ie live., when they are dealing with serviceability issues such as deflection and vibration. Makes sense. Who feels the partition allowance should be considered as a Dead or a Live load?

 

[msquared48]

I have used LLR for large beams and girders supporting large areas on a single floor with absolutely no qualms about it. For PS design, a column strip could be considered just such a girder.

I will have to disagree here about the application of the LLR here. It does get tricky though when combined with alternate loading scenarios. Tricky does not mean that it should not be used, just used carefully and judiciously.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com