Live load reduction factor 2

[longisland]

Hi,
I read the clause on the application of LLRF (ASCE 7-88) & need some clarification.
1) a foundation supporting multi-storey; some floors are storage space where LLRF is not applicable. What is the load capacity of the foundation? Is it the summation of loadings on each floor multiplied by the LLRF(on live loads) where applicable?
2) Supposed LLRF is not applied on beam members during analysis (beam supporting slab less than less than 400 square feet), what values are to be adpoted for the column supporting the beam assuming LLRF is applicable to the column? (Axial force & bending moment)

Thanks

 

[dougantholz]

I think I might be able to answer you question, but first a quick discussion about what live load reduction is. Design live load found in chapter 4 of ASCE 7 is a probabilistic load based on a worst case possible load. This means that in an office building, the worst case load is usually found under a bookshelf or filing cabinet. Not all of the area is loaded the same, therefore when you have members with large tributary areas, it is less likely that they will have a filing cabinet on each square foot, and therefore don't need to be designed for the full live load: hence live load reduction.
Now to answer your question. I am not familiar with the 88 version, but in later versions you are allowed to take a live load reduction (up to 40%) for members supporting one or more floors. So for a column that supports multiple occupancies, you would take the live load reduction for each floor, using the total area for that occupancy (even if it is spread out over multiple floors).
For your second question, when figuring the load for a column that has a trib area > 400 ft^2, the axial load can have a LLRF multiplied by it, and you eccentricity will be the same, so your moments will also have the LLRF applied.
Now as a side discussion, be careful to use load skipping on a column where you have live load applied from only one side to give a maximum moment. In this case, only use the trib area of the beam on the LLRF calculations.
HTH

 

[JAE]

I don't disagree with anything dougantholz says. Just another slant on it:

If you have a footing or column supporting a combination of storage (where LLRF is not applicable) and office loading (where it is), you can do the following -

1. Calculate the tributary area of ONLY the floors that have live loads that are reducable and add them together. This is your At. Do not include storage floor areas as these do not enter into the statistical valuation of the reduction.

2. With the At, calculate the reduction in live load. This is then applied to the floor live loads that ARE reduceable.

3. With the full storage live loads, and the reduced office/other live loads in your model or in your calcs, you then can analyze your structure, skipping live loads where appropriate (per dougantholz above) and obtaining your axial, shear, moment as needed.

4. Design your column/footing per these loads.

For various parts of your structure (per your question 2) you really have to analyze for each part, using the appropriate LLRF. For example, in a three story model we might have a computer model developed and create a series of separate load combinations for various amounts of At. So you might have a set of combinations (D, D+L, D+L+W) all with 0 reduction in live load; and then you'd have a separate set of combinations with perhaps a 20% reduction for your beams, and another set with 40% for columns.

Rather tedious, but that is how the code is written.

Conservatively, you don't have to take any reduction at all.