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If a permanent room or platform is suspended from a larger structure (say, on a cable that is attached to a hook on a track in a ceiling) and designed to be in constant but not necessarily repetitive motion, does the load that the room/platform places on the larger structure count as a dead load or a live load?

On the one hand, it is permanent and won't ever be removed (like a dead load,) but on the other hand, it moves (like a live load.) I'm not talking about the loads that would be incurred by its motion, only by its weight.

Air
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Matt
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3 Answers3

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What is the difference between a Live or Dead Load?

A Live Load is typically a moving load. This can be vehicles on a bridge or occupants and vehicles in a building. Dead loads are (relatively permanent loads).

These are simple ways to differentiate the type of load, but why do we care?

The basic answer is, "Because the Code cares."

Why does the Code differentiate between a Live Load and a Dead Load?

There are two reasons why loads are sorted into groups:

  1. Different Load Combinations are described
  2. Different Load Factors are applied

The load combinations are standard groupings of loads that can occur simultaneously. The load factors take into account how accurately a load can be calculated.

In most Codes that use them, Live Loads have a higher load factor than Dead Loads. This is because it is easier to calculate the Dead Loads than it is to calculate the Live Loads. Especially with vehicles, you have little control over the weight of the actual vehicle.

Live Loads have more variability.

This is the only reason why there is a difference between what gets placed in the two groups. The live loads have a higher factor of safety.

What other factors apply?

Another thing to consider is that a moving load has the ability to create an additional Impact Load. If this is the case in this situation, then a separate Impact Load should be included.

What is the end result?

In the end, it may not matter which category gets assigned to the load. The main criteria will be how confident you are in the loading that will be imposed on the structure from the moving platform. If the load isn't likely to change, then you could consider it as a Dead (Permanent) Load. If it is likely to change, then be more conservative in calculating the load or apply a higher factor to it.

All codes will allow you to be more conservative if you choose to be.

hazzey
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Since you say a permanent room or platform, I'm assuming you're expecting it to be occupied and used for whatever function.

hazzey does an excellent job of describing the differences between dead and live loads, so I won't go into that here.

Because it is going to be occupied and used, I wouldn't lump the entire load into just dead or just live:

You can easily calculate the weight of the structure, you know the materials being used and you can find out their weights. Place this under dead load, and factor accordingly.

On the other hand, you have an idea of what will be going on in this room/platform, but you can't give as exact a number as above. Find out the appropriate loads for your use case. Place these under live loads.

Add the dead and live loads and boom, you have the weight you need. Since the room will have people on it, and people are unpredictable, placing the entire object under dead load seems irresponsible to me. However, separating the loads will turn out to be more economical than factoring the entire weight as a live load.

cKrug
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A moving load should always be classified as a live load even when its weight is known and remains the same. A dynamic body of mass has an extra degree of freedom. This positional uncertainty calls for a higher factor of safety. Your "permanent" room is not much different from a "permanent" mobile trailer in that both are capable of movement and that the gravitational load do not vary much.

A dead load, on the other hand, is permanent in both magnitude and position. I.e. it is a static load. A static load, being predictable, requires a lower factor of safety.

I would further argue that the classification of load depends on what you are designing. For the hook itself, you may even go for a higher safety factor by classifying it under crane load by reason that failure can result in catastrophic consequences to anyone below or inside the suspended room.

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