What are the considerations for live load ratings in structural design?

[soronemus]

So I'm looking at some technical drawings here (mechanical engineering co-op) and the live load rating is 100 pounds per square foot (PSF) for this particular drawing. My question is this: why is it so low? Just me standing with my feet together exerts roughly 230 pounds per square foot (i'm 230lbs, my feet take up ~1 square foot). The structure this drawing depicts is regularly walked on by workers and I must be misunderstanding the concept if just standing on it exceedes its designed strength.

I'm just now learning about live load though google searches, seems to be more civil's field, so go easy on me lol.

 

[SteamKing]

The live loading of 100 pounds per square foot means that the entire walking surface is assumed to be loaded this amount.

For example, if the walking surface were 3 feet wide and 10 feet long, the total live load on the walkway would be:

3 ft *10 ft *100 lbs/sq.ft. = 3000 pounds

So, for this walking surface, even at 230 lbs for yourself, you would need a lot of company to get 3000 pounds of load.

 

[soronemus]

You the man SteamKing.
So it is definately treated differently for structures than it is for solid parts like I usually deal with as a mechanical engineer.

When they determine that the live load rating is 100 PSF what assumptions are made? For example is that the 3000 pounds of force can be held safely at -any point- on the structure where people would be walking? Is the surface people are usually walking on such as metal grating assumed to distribute any point load evenly across adjacent i-beams?

What kind of factor of safety is used for drawings made up by PE structural engineers in manufacturing plant cases?

Do they do all of the shear/bending moment diagrams and stress calculations, fatigue strength calculations, and other number crunching for everything they approve, or do they use a lot of 'rules of thumb'?

 

[SteamKing]

You the man SteamKing.
So it is definately treated differently for structures than it is for solid parts like I usually deal with as a mechanical engineer.

I really can't say, since I don't have a lot of experience in designing machine parts, except for one course in Machine Design I took at college. However, the basic principles of structural analysis are the same for designing a widget, an airliner, or a supertanker: the size and type of the loading may vary somewhat across this spectrum, but the procedure for calculating the actual stress values is the same, and a lot of the same analysis tools are used as well.

When they determine that the live load rating is 100 PSF what assumptions are made? For example is that the 3000 pounds of force can be held safely at -any point- on the structure where people would be walking? Is the surface people are usually walking on such as metal grating assumed to distribute any point load evenly across adjacent i-beams?

The 3000 pounds of total force I used in the previous example was just to illustrate the total load which the walkway is designed to support. The load would still be assumed distributed for design purposes at 100 pounds/sq. ft. The structural details of a walkway designed to support 100 pounds/sq.ft. versus one which can support a concentrated load of 3000 lbs. would be different, however.

The response of a certain grating material, for example steel or a composite material, to a point load would require a separate analysis.

What kind of factor of safety is used for drawings made up by PE structural engineers in manufacturing plant cases?

It depends on what kind of structure/mechanism is being designed or analyzed. For example, in the US, most steel structures are designed to a code of some sort, like the AISC (American Institute of Steel Construction), which gives safety factors for loads in bending, shear, and the like.

http://en.wikipedia.org/wiki/American_Institute_of_Steel_Construction

Other codes, for different structures like road bridges, come from similar organizations like AASHTO:

http://en.wikipedia.org/wiki/American_Association_of_State_Highway_and_Transportation_Officials

and there is a lot of consultation among these different associations which goes into developing design and construction standards for various structures.

Do they do all of the shear/bending moment diagrams and stress calculations, fatigue strength calculations, and other number crunching for everything they approve, or do they use a lot of 'rules of thumb'?

Hopefully, a structural engineer with a PE would use engineering principles in designing or checking a structure, even one which is used as a walkway. There are a few rules of thumb one can use to quickly eyeball whether or not a structure is properly designed, but the competent engineer always draws his conclusions from an engineering analysis of the structure/mechanism.