Live Load Reduction / Two Way Slabs 1

[mijowe]

I have a long span PT concrete office building that was being designed with a shallow beam and one way slab system. 45’ span beams and 28’ conventionally reinforced one way slabs. We are using an office load of 100 psf and due the large tributary area we used live load reduction to the beams on this 100 psf live load.

We have been asked to provide bay studies for a two way flat slab (with drop panels) to be compared and priced. Obviously to compare apples to apples I would need to use live load reduction for this two way slab as well. I have never considered reducing live loads to two way slabs, the bays typically are not large enough, however I suppose that it is possible. Does the code specifically restrict this? Does anybody regularly design two way slabs by reducing the live load?
Thanks for any input.

 

[Taro]

Which software programs have this "feature"? It would be good to know in case I ever come across it.

 

[rapt]

Taro,

Definitely ADAPT PT, presumably PTData and Posten as well as they cannot handle strip design and the default operation of the US FEM programs using banded/distributed tendon arrangements and setting a single design strip over the full width of the panel. The FEM programs allow you to define your own design strips to get around this but the default operation (used by many designers)does not do this.
Those FEM programs also ignore Mxy moments in design by default as well.

 

[slickdeals]

Guys,
On a side note, I have a question regarding LL reduction for columns.

I understand that the live load reduction factor for a particular column includes the influence area of all the members it supports. For example, if I had a column with a 24'x24' trib supporting 7 floors, then the influence area for the column on the first floor will have an influence area A = 7 * 24' x 24'. Using this I can compute the RLLF, with the limiting value being 0.4.

My question is thus:
Say, I am trying to compute the allowable load for a foundation including LLR. Assuming each floor has 100 kips load as live load, and the 7 story building will have a live load of 700kips at the base. Will the reduced live load for the foundation be 0.4 (assuming it controls) * 700kips or should it be a summation of the reduced live load for each floor calculated as RLLF for the floor * column live load at the floor.

I am confused as I thought it should be 0.4 * total live load , but the PCA simplified notes book does a summation of individually reduced live loads. (see the chapter on designing columns).

Am I misinterpreting something?

Thanks

 

[slickdeals]

See attached spreadsheet

 

[JAE]

The footing has its own tributary area (in your example it would be the same as the bottom column). The live load reduction is based on the total area and the total live load reduced based upon that total area.

 

[KBVT]

slickdeals - I have seen other engineers utilize incremental live load reduction factors on subsequent floors but this is not the correct application of the reduction factors. A live load reduction factor should be calculated for each member with the tributary area for that member used to determine the reduction. There are designs out there that assume a 100% loading of the top level for all column designs in the buiding!

 

[slickdeals]

@JAE,
For the foundation, assume that you end up with a RLLF factor of 0.4.

Would the foundation then be designed for 0.4 * SUM OF UNREDUCED LIVE LOADS = 0.4 * 7 floors * 100 kips?

What happens if you have a garage level below a retail level?

I will try to send a calc soon.

 

[slickdeals]

See calc.
Please help me understand this.

 

[slickdeals]

See Pag 5-20 of the Simplified Design book by PCA. i dont think that method is correct? Any thoughts?

 

[KBVT]

In theory, your first method is correct. But if you are designing to the IBC, your maximum reduction of the parking garage load would be 20% since you are supporting 2 or more floors. Your design load would be 0.40(93k) + 0.80(49.6k) = 76.88k

 

[slickdeals]

Correct, the only reason I did not add that factor was to keep the concept simple.

I am surprised why PCA would choose to do something like what they did in their publication.