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complete definition of live/dead load?

Ted_W's picture

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Allen, it means that if you have over 40 lbs of groceries you have to distribut them. Never set the bag right down on the floor. It'll go crashing through to the basement.

(post #172349, reply #6 of 23)

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That would be a dead load. Only thing is that an especially hefty unit would require beefing up the framing. Same as if yor replaced a shower stall with a cast iron tub.

(post #172349, reply #8 of 23)

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Alan,

I guess it means you can cover the floor with 40 lb., 1' cubes. Of course the closer to the bearing walls the stronger the joist is. Maybe all of the cubes could be stacked against the bearing walls. I guess its a question of how weight should be distributed. I've seen (and handled) cast iron radiators that I certainly wouldn't want in the middle of the room.

I think what we need here is a matheematical forula.

Anybody care to enter?

(post #172349, reply #16 of 23)

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Okay, I'm trying it now.

A bowling ball gently resting on my left foot... a little heavy but okay. Now I drop one on my right...

#*%&$@

Someone call a doctor, please.

(post #172349, reply #2 of 23)

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Allen, it means that if you have over 40 lbs of groceries you have to distribut them. Never set the bag right down on the floor. It'll go crashing through to the basement.

(post #172349, reply #22 of 23)

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On the surface this should be a no-brainer simple question, though i can't find the answer!

How is live/dead load defined? The unit is psf (i'm assuming pounds per square foot). We can't be talking point loads since most span tables assume 40psf. So it's a spread over some area. Can anyone explain?

I did a web search, checked a bunch of my books, no definition.

Thanks!

...alan

(post #172349, reply #1 of 23)

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Alan,


A simple way to view live and dead loads would be to say that the dead loads are that of the structure. Floor joints sheathing etc.. Live loads are always variable. Room furniture, floor coverings and of course the number of people and pets.



JosephFusco.Com

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"More than any time in history mankind faces a crossroads. One path leads to despair and utter hopelessness, the other to total extinction. Let us pray that we have the wisdom to choose correctly."

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(post #172349, reply #3 of 23)

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Good point ,Ted. We only load our grocery bags half -full so that we can set them on the floor.

That's why we wear our snow shoes in the house. Our feet aren't big enough to distribute the load.

(post #172349, reply #4 of 23)

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Would all the loads on a mortuary floor be dead load ?

Actually, Joe explained it pretty well. Live loads are temporary, and dead loads are permenant. Same thing applies to roof loading - Snow loads are live, the shingles are dead.

(post #172349, reply #5 of 23)

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Actually, I though snow loads were a third class of loads. BOCA discusses dead, live, and snow loads. If snow loads excede live loads, you use the snow load figure instead. Or so I thought.

The distribution thing works like this:

If you've got to design for a 40lb/sq ft load that means you take that 40 lb bag of groceries and place one bag on every square foot of floor (or roof).

Now here's one I've always wondered. If you're putting an airconditioning compressor on the roof, is it a live load or dead load?

(post #172349, reply #7 of 23)

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Sure, live load is live, dead is the structure. My span table sez 40lb live load. I routinely pack my groceries heavy, they must weigh more than 40 lbs. Not a bag has fallen through the floor yet.

so i'm still lost. What is that 40lbs actually measuring? An engineer with power tools is a dangerous thing... :)

...alan

(post #172349, reply #9 of 23)

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Span tables are for uniform loading. That means each sq foot is loaded the same. The reason the loads are so small is that they know you won't actually load every single square foot. You might put a heavier piano in one spot but you'll leave space in front of the TV, so it all evens out.

Usually the span is given as L/180 or L/360, or something similar.

What this means is at the given loading, the joist (or whatever is being rated) will deflect 1/180 of the span or 1/360 of the span at the center. (L/270 is also common). These tables are for a complete floor system, not just one joist.

To test this, you could spread sand evenly around the floor so that each square foot of floor had 40lbs of sand on it. So an L/360 span rating in a 10x10 room would need 4000lbs of sand (40 lbs for each sq ft.) evenly spread on the floor and the center of each joist should sink by 1/3" which is 1/360 of the span.

The span tables are made so that the maximum deflection is not exceded, has nothing to do with catastrophic failure. The joist will deflect to L/360 at only 40lbs/sq ft but failure will probably occur at several times that load.

If you want the formulas, here's an excellent site some engineer set up for those of us who forget the formulas.
http://ourworld.compuserve.com/homepages/mjvanvoorhis/T307.htm

(post #172349, reply #10 of 23)

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Alan, it doesn't mean that if you cover your floor with 41lbs./sq. ft. that your floor will collapse. Deflection is a part of the equation. More deflection is allowed on roofs than floors. A floor works as a unit, but is often engineered as parts, as a unit most floors are stronger than they are actually designed( this may not be true with all engineered lumber systems.) I have seen more than one floor made with 2x6 joists spaning 10' . They are still standing after 70 years, but they sag and bounce. That sag and bounce is the deflection. If you can look at a table for any engineered lumber product,(which are readily available at any lumber yard) you will see different tables for different applications that relate to the amount the material will deflect under a given load, L/180, L/240, L/360.

(post #172349, reply #11 of 23)

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A dead load is a static load (even temporarily stationary). A live load is also known as a dynamic load, it implies an inertia vector that's higher than what would be imparted by the same mass at rest (e.g. a child skipping, for part of the skipping cycle they are exerting no load, and for another part of the cycle they could be exerting a force that greatly exceeds their weight).

There's a simple experiment they do in first year engineering to demonstrate the difference. First, put a one pound weight gently onto your bare toes: that's a static weight; next, drop the same weight onto your toes from chest height: that's a dynamic load. Which felt heavier ?

(post #172349, reply #12 of 23)

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Phill:

I don't think the building code considers live loads as dynamic loads. Live loads are the stuff that gets moved around, TV's, piano's, and people. Dead loads are the stuff that doesn't move like the bath tub and heating ducts. So many temporarily stationary loads like furnature are considered live loads in the building code and not dead loads as you claim.

Because the code talks about such low numbers for live loads, I'm sure they are not talking about dynamic loads. As difficult as the code is, it wasn't written for engineers. Maybe by engineers, but not for them. No builder or architect is gonna figure out the duration of impact when the skipping child hits the floor, his velocity and mass, and calculate the impact force.

I've never heard dynamic loads called "live loads" but if they are, it's interesting trivia but not applicable to construction.

Sorry, Phill: I really don't want to tangle with you again and I mean no disrespect, but bad information is dangerous in construction.

(post #172349, reply #13 of 23)

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You guys are quick!

Thanks for the answers.

I'm doing a remodel with a large glass block shower plus the associated floor/wall tile. I want to make sure that the current floor system is capable of handling the new load.

...alan

(post #172349, reply #14 of 23)

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Are the students that drop the weight on their toes from chest height allowed to continue their engineering studies?

Rich Beckman

(post #172349, reply #15 of 23)

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Rich,

Yes

KK

(post #172349, reply #17 of 23)

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Rich, believe it or not, in my first semester class there were 3 people who volunteered to do this experiment - the prof actually had one holding the weight ready to drop it before he stopped him and told our classmate that maybe engineering wasn't the best profession to choose.

(post #172349, reply #18 of 23)

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Ryan is correct in his description of live/dead loads. I know of no building code that considers inertia loads in design.

Deflection calcs have nothing to do with "bounce" in a floor. Is means that if the floor joist (or whatever) is loaded to the maximum design load, it can't deflect more than L/360. (or whatever ratio you use)I'll grant you that a floor with lots of deflection will probably also have a lot of "bounce", but they aren't directly related.

The university of Virginia did some research on floor vibration that's really interesting. I wish I knew where more of it was on the 'net. If I find it, maybe I'll get a thread started on it.

(post #172349, reply #19 of 23)

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Be very careful using I/360 as a deflection criteria. Most home that that my firm does use I/480 or higher. The reason is vibration. If you use I/360 for a long span (15' or greater) the floor will bounce when someone walks on it ant he owner will not be happy.

(post #172349, reply #20 of 23)

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RD...So true about long spans...Had an Arch/homeowner/GC construct a floor that spanned 20' using 2x12 dougfir joists 16"OC...What a great trampoline...To his credit he also engineered a fix.

near the stream,

aj

(post #172349, reply #21 of 23)

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I can beat that one AJ. Last winter I went to start a frame and the homeowner/big time contractor had stocked the job with 2x8's, one floor span was 17'-6".

I walked and told him to send all the stock back. In the meantime he got togeather with the building inspector (who is also the police chief) and they determined that 2x10's would be enough, spaced 12"oc with 2 rows of bridging.

Idiots, they are all idiots...

(post #172349, reply #23 of 23)

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alan... just caught this...

everything i've read in this thread rings true..

but...
you're describing a ((point load))....

b handling the new load. >>

that is NOT a uniformly distributed load.. you have to make spoecific provisions for point load to transfer to the structure...

other examples might be .. commercial freezers in a residential kitchen..

or large soaking tubs in the master bath.. anything out of the ordinary should be looked at as if it might be a point load..

b but hey, whadda i no ?