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Old 11-05-2008, 09:54 AM   #1
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Understanding live vs dead loading


I will be building a cabin on raised piers near the coast. I always like to understand what I'm being told by engineers, architects, etc., so can anyone point me to a good link that explains the concepts of joist and beam design as pertains to live loads and dead loads?

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Old 11-05-2008, 01:34 PM   #2
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Understanding live vs dead loading


If your building with 2x lumber try the southern pine site - southerpine.com will give you the live/ dead loads of nominal lumber sizes. the loads apply to what the structure will be supporting. Dead loads are weights of material, equipment or components that are relatively constant throughout the structure's life, while the live loads are loads not constant ie people, wind and snow. Here ins new york we like to design for 40psf live load and a 20 psf dead load. A snowier climate might raise the live load. There is also a factor of deflection, you'll often see things like L/360 L/240 which are your deflection factors. so for example a 10'-0" span = 120" so a L360 you divide 120 by 360 to get .33" of allowed deflection.
L360 is code for floors.

A google search of any of this will bring up sites with explanations and examples

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Old 11-05-2008, 02:04 PM   #3
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Understanding live vs dead loading


Dead loads are the loads of the parts and pieces of the building itself. That includes flooring, sheathing, walls, shingles, sheetrock, etc.

Live loads are everything else. Furniture, people, and anything else that can be moved in or out of a house.
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Old 11-06-2008, 05:47 AM   #4
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Understanding live vs dead loading


Last night I watched a show about a 6 story, 15 year old building in Singapore, which collapsed to nothing but a pile of rubble in less than 60 seconds. Experts were completely baffled by the cause, until they went back to the designer's original calculations. This is what they found (from Wikipedia):

"It was found that the weight of these additions was inconsequential; however this line of investigation led to the discovery that the original structural engineer had made a serious error in calculating the building's dead load, the weight of the building itself. The structural engineer had calculated the building's live load, the weight of the building's potential inhabitants, furniture, fixtures and fittings. However the building's dead load was completely omitted from the calculation. This meant that the building as constructed could not support its own weight. Collapse was only a matter of time and after three different supporting columns had failed in the days before the disaster, the other columns, which took on the added weight no longer supported by the failed columns, could not support the building."

Ouch.
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Old 11-06-2008, 06:30 AM   #5
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Understanding live vs dead loading


the same as water pressure measures taken at hydrants: static & dynamic,,, excellent tip on the site - THANKS ! ! !
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Old 11-06-2008, 10:33 AM   #6
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Understanding live vs dead loading


There are things that can adversely affect live load like water beds. They can put more weight on a structure that it was designed for. I remember floors falling in when water beds were first introduced. They just make a vinyl envelope and people filled them up with water. No big deal.

Until the powers that be realized that there was over 3 tons of water on a floor that was only meant to handle 2500 pounds. Over time the fibers on the bottom of the joists would pull apart and would lose it's weight bearing abilities. Wood joists have two things that make them strong. Compression which affects the top part of the joist and tinsil strength which affects the bottom part. Wood is very good at both of these.

I had one of these waterbeds. I remember having to remove it becasue the ceiling on the apartment below was losing plaster as the floor slowly sank over a period of 6 months.

Then manufacturers quickly made some changes to the design and made them thinner so they held less water and thereby decreased the weight. Some places had a requirement for renting, NO WATERBEDS.

Also when they talk about live versus dead weight, dead means over the life of the structure and live is more transient but still considered relatively long term as things are shifted around. Several good examples are having a party. Most of the time you exceed the live load weight since there are people who weigh 250 pounds per sq ft and another standing a few feet away who weighs the same. So now you have 1/4" ton in just a few feet which is way beyond the 40psf the floor is rated for. But since these people will not stay there it is a transient load and the structure can handle it.

I have seen cars sitting the the living room of a home after the driver lost control. Then there is the weight of the Paramedics, firefighters and cops and you have way beyond the 40psf. Again, this is just a transient load. If that was left on there the building would start to sag.

When I was a firefighter I did a lot of training. One of the issues I brought up was water load. We can put 1000 gpm of water into a building and if it doesn't move out quickly we can actually collapse it. On a training burn to learn we had done all of our entries and now the building was burnt to the point of being too dangerous to do any more entries. I went up to the second floor and boarded up one bedroom door. Then we stuck the snorkel into the window and let fly. In a matter of 8 minutes the whole house collapsed as we had pumped in 32 tons of water.

My traines's got the message.

If you look at previous posts you will see a lot of older homes that have sagging issues. Many if not all homes that were made in the early 1900's had structural defects with undersized joists. Most were 2x6's spanning 12 feet. We know today that this is not acceptable and use 2x8's or 2x10's.
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Old 11-06-2008, 11:17 AM   #7
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Yeah I see that all the time when we are asked to design a dormer or extension to an existing house. The client often doesn't realize that ceiling joists (attic floor joists) are not design as floor joists. In most homes we can't just rip off your roof and add a second floor. The floor structure whether the size of the joists or there span was not design for this.

Now even in our new buildings we design the attic joists as floor joists because most people end up using it as an extra bedroom, storage, or have there HVAC units in their. Contractors often don't like the added expense but its worth it when you don't have saggy and spongy floors.
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Old 11-06-2008, 11:47 AM   #8
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Understanding live vs dead loading


Quote:
Originally Posted by Rkeytek View Post
Yeah I see that all the time when we are asked to design a dormer or extension to an existing house. The client often doesn't realize that ceiling joists (attic floor joists) are not design as floor joists. In most homes we can't just rip off your roof and add a second floor. The floor structure whether the size of the joists or there span was not design for this.

Now even in our new buildings we design the attic joists as floor joists because most people end up using it as an extra bedroom, storage, or have there HVAC units in their. Contractors often don't like the added expense but its worth it when you don't have saggy and spongy floors.
And don't get me started on trusses...grrrrrrrr.

I did a storage space in an attic for my neighbor. Of course he had trusses. I had to haul up 2x8's and beef up the flooring as it was not meant to handle the weight we were going to put on it. Then I had to remove the vertical supports and make a knee wall to support the undersized 2x4 rafters.

And of course there was the wiring to move.....

I hate wasted space. He now has over 300 sq ft (1200 cu ft) of space to store seasonal items up there. His wife loves it.
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Old 11-07-2008, 10:12 AM   #9
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Understanding live vs dead loading


Thanks for the link.
I guess where I'm getting lost is not the concept of dead vs live, but how they come up with the values.
For example, the charts say use use a certain weight for dead load that represents the building components. If I'm in the process of determining those components, how do I know what weight to use? Obviously a 2x8 will be 2/3's the weight of a 2x12. Are the charts just assuming a worst case?
The same goes for the roof loading. I see charts for asphalt singles, but I'm planning on using metal roofing. Are there charts that let you figure this, or do I have to get manufacturers weight?
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Old 11-07-2008, 10:49 AM   #10
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Most lumber charts will include the weight of the lumber in there calculations and for wood joists its not much dead load.

You can get the manufacturers spec for weight per square foot to know what the weight for the roof will be, than look into what the average annual snow load is, and what the code rates the req'd wind load for your area.

OR ... do what most people do (even architects) and use the "standard" that has been proven before and is just copied to every job after that.

The same rafters we used in a Spanish title roof on one project will most likely be used in the next Spanish tile roof.

Metal roofing is the way to go, the stuff is strong, relatively light , especially compared to those roofs that have 5 layers of asphalt stacked on them, and it LASTS.
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Old 11-07-2008, 12:06 PM   #11
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Understanding live vs dead loading


Minimum design live loads are prescribed by code, and are available in the code book. Sometimes exceptionally high live loads (waterbeds, fish tanks, etc) are accounted for in the structure's design.

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