Live Load Reduction for Foundations or Footings 1

[kgengr]

Here's an easy one for ya'll:
What KLL do you use for foundation design? I was looking for a code reference or commentary mentioning footings, but to no avail. Strictly by code -- I'd guess it falls to "All other members not identified" KLL = 1. Thanks in advance for your input.

 

[strucguy]

If it's a column footing, I would go by the KLL you use for that specific column. If the strip footing that supports a bearing wall, I do not consider any live load reduction.

Good Luck!

 

[slickdeals]

Why don't you consider any live load reduction for a bearing wall based on influence area?

 

[kgengr]

slickdeals - Do you matching the KLL of the member you are supporting for your footings typically?

 

[msquared48]

Slick:

Looking at the numbers, for live load only, for a one foot strip, say 25 foot bay, it would take up to the seventh floor to overcome the 400 square foot break point in ASCE7-10, section 4.7.2, no reduction could be taken until you reached the seventeenth floor... (400 / 25 = 16).

Hence, the tendency to ignore LLR for strip footings.

Mike McCann
MMC Engineering

 

[slickdeals]

But wouldn't a bearing wall be able to distribute loads over the length of the footing. Let's for instance forget that we are designing it on a per foot basis and think of a 40 ft long bearing wall every 25 ft. Seems like we should be able to account for having probabilistic live load reduction.

 

[jayrod12]

I tend to agree with slickdeals but the work we do is generally wood framed on concrete walls and so for the area calculation we use a footing length that is equal to the concrete wall height above, i.e. 10 foot tall foundation wall supporting 25 foot width per floor then we use 250sq ft per supported floor. We are fairly confident that the concrete wall will spread any loads over at least that length(in fact if you were to use the 45 degree "rule" you could in theory assume a 10 foot high wall will spread its load over 20ft).

 

[msquared48]

I can see that logic too if you treat the bearing wall as a deep "beam", but spreading out the load is really not part of the solution here. The code is not really clear on that though. Any reduction in live load allowed is due solely to the relative probability of not being fully loaded to the design live load, based on the total area supported - the more area, the more chance of not being fully loaded.

Mike McCann
MMC Engineering

 

[jayrod12]

Mike,

I see what you are saying. I still feel that LLR should be able to be used for a strip footing as the wall will redistribute the loaded area to a neighbouring unloaded area and as such looking at only a 1 ft width of footing seems way too conservative.

 

[JLNJ]

For walls, if I have some two-way action (such as in a CMU wall) I will use the area of the entire panel for things such as wind load reduction. A 12" strip is just to penal.

The footing just needs to carry the same load as the wall or column which sits on it.