ASCE 7-10, Fluid load factor for basic combinations for strength and allowable stress design

Question

In ASCE 7-10, with regards to strength design,for fluid load, it mentions that where fluid loads are present, they shall be included with the same load factor as dead load D in combination 1 through 5 and 7. How about for load combination 6?

Load combination 6 is as follows:

0.9D + 1.0W,

which I expected to be

0.9D + 0.9F + 1.0W

However due to the statement in ASCE 7-10, I do not know what factor to assume for fluid load as fluid load will be present in my situation. The same applies to Allowable stress design load combination albeit with different combination.

Answer 1

The combination of loads shown in the code implies that loads shall be combined to test all possible structure behavior under different conditions.

Load combinations 1 to 5 incorporates the fluid load, while load 6 does not. It does not mean that fluid is not present in loading, instead directs the users to consider instances wherein fluid load will not be present e.g. during low tides, or low water table level.

Take note that sometimes having fewer loads considered leads to larger stresses. :)

One example of this case is in the analysis of slab in buildings. We used checkered load assignment (I'm not sure how they call it) wherein we assign floor loads alternately in checkered pattern to simulate behavior of slab when it is not loaded while the others are. This leads to different compression and tension stresses in beams and columns.