Can you reduce the floor live load when designing a continuous strip footing for a 2 story home?

[Arthur Farrow P.E.]

I have been looking for backup from the IBC, IRC and the ASCE that allows Live Load reductions
when designing a continuous strip footing.

For a two story house and roof snow load when computing loads by the linear foot a small reduction
would seem reasonable. All I could find was code reference to the member's tributary area.
However a one foot strip of footing that say has a tributary width of 10 to 20 feet would not
warrant any reduction.

But reason would suggest that two floors at 40 psf and roof snow loads sometimes being over 100 psf
should have some code rules that I could follow and have confidence that I'm within the law.

Sincerely,

Arthur Farrow P.E. WA, OR

 

[JAE]

If the footing has some manner of stiffness and strength along its length, such as a deeper grade beam where variations in the linear load can be spread out over some reasonable distance then you effective area would increase and you could then consider some level of LL reduction.

For a simple and relatively thin continuous footing I would not take the reduction.

A thin footing on a basement wall would certainly qualify.

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[hokie66]

I didn't think codes allowed reduction of snow load. I have never considered the possibility of reducing live loads in a house, especially on a strip footing. Very little if any benefit to be achieved.

 

[AaronMcD]

In ASCE 7-10, any one or two family dwelling structural members supporting more than one story can get a 30% reduction (0.7 x sum of floor live loads).

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[Ron]

I don't see the point. Unless you are trying to shave 2 inches of concrete off a 1000 tract homes it seems like you're trying to build a watch, not a house. No benefit. Look at your standard of care as well...could impact your liability more than it impacts the cost of the house.


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[Arthur Farrow P.E.]

Thanks everyone for replying

AaronMcD

Where is the excerpt from ASCE7-10 found in regards to reducing the floor live load.

I've seen that done when I worked for an engineer in the past but it was long before I
got my license and consequently did not ask what code he was using that allowed such reductions.

It makes sense to me because of the amount of live load that could accumulate from numerous floors
and the plain fact that the possibility of all of it being active at once is very remote.

Arthur

 

[spieng89]

Chapter 4 live loads you'll find the LL reduction provision. As some have stated, I don't see much benefit in trying to reduce live load for a house strip footing nor do I believe code allows for it. LL reduction applies to columns, beams. The difference in loading would create a minimal savings anyways in strip width and may not even matter given 18" minimum width is required in many residential codes.

 

[Arthur Farrow P.E.]

Thanks everyone for the replies.

After some research I've learned that I can reduce the live loads
on 2 story or more homes.

My reasons are:
The strip footings I design that support the roof trusses and
long span floor joists also have window openings.
AT the openings the roof load is dragged to the end of the header.
On multi story when openings are aligned that can create significant
point loads.

By designing the strip footing for that point load it eliminates the
need for many pad footings.

I found a couple publications that comment on live load reduction
as well as a publication that talks about actual live load vs
design minimums.

Arthur

 

[jayrod12]

So in that framing scheme you mention, are you planning of having localized wider portions of footing underneath the window framing? If not, then I don't really feel that a reduction should be allowed.

Generally speaking, I design walls, slabs and footings on a per metre or per foot design basis. Seeing as how in our code (NBCC) you can't reduce until you have at least 20m^2 of area, that means that your footing must support at least a 10m total trib width (5m per floor) and snow loads are non-reducible. 10m clear span residential floor framing would be extremely rare in my experience.

 

[KootK]

By designing the strip footing for that point load it eliminates the need for many pad footings.

I agree with this and it can be used to advantage to deal with heavy post loads without having to introduce a footing other than the conventional strip footing used elsewhere.

I found a couple publications that comment on live load reduction as well as a publication that talks about actual live load vs design minimums

Please share these if you're able.

In ASCE 7-10, any one or two family dwelling structural members supporting more than one story can get a 30% reduction (0.7 x sum of floor live loads).

Could you point me to a clause number for this?

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[XR250]

I'll do it if I need to make something work - most likely some beam in the crawlspace. Usually take 20% off the live load.
No house in history has ever had seen full floor and roof loads.

 

[Huevo]

Could you point me to a clause number for this?

4.7.2 Exception

 

[Arthur Farrow P.E.]

KEEP IN MIND THAT THERE ARE 4 TO 6 HUNDRED HOMES PER YEAR


ARTHUR

 

[Earth314159]

In our code, SL is not reducible but you can take the greater of 1.5SL+0.5LL and 0.5SL+1.5LL. This generally helps significantly for small buildings.

I will occasionally use a LL reduction if justifying an existing condition and depending on the ability of the foundation to redistribute the load. Break the live load contribution up into point loads. Use the largest span between point loads that the foundation can distribute as the width of the tributary area.

 

[KootK]

4.7.2 Exception

1) Thanks.

2) For a stem wall foundation building, I guess you'd have to be three stories before this would kick in for footing design without a basement (two suspended floors). That would rule out a lot of homes in some locations. Two story over basements could use it which are common in my area.

3) Where it did apply, I'd be down for using it. Sometimes this can be the difference between a 24" footing and an 18" footing which will make me friends in Res-world.



HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235