Question on combining loads

[nmodie]

Hey Folks. I'm an architectural intern with a question that seems like it should be very simple and should have an answer readily available on the interwebs or in one of my structures books, but I cant seem to find it.

I am trying to combine uniformly distributed loads and point loads to design for deflection. Some people have recommended that a uniform load equivalent to the point load should just be added to the original uniform load, but that does not seem adequately accurate to me and would result in oversized structural members. I know that you can't simply add the deflections that you get from doing the standard equations for both of them together. Any help would be much appreciated.
Thanks,
Noah

 

[rb1957]

just normal straight-forward superposition works. you "can" replace the point loads with distributed loads, but this is typically unconservative (though not in a big way). It's clearly unconservative for central point loads, so it'll be accurate for a point load at some fraction of the semi-span (1/4 span?). it is an easy combination method, you need only one deflection curve (that always peaks at the mid-span, another easy calc).

Quando Omni Flunkus Moritati

 

[rb1957]

btw "straight forward superposition" means calcing the deflections for each load and adding them; you can't add the max deflection of each load (well, you can but you'll be conservative).

Quando Omni Flunkus Moritati

 

[desertfox]

Hi

page 16 of this link its gives an example.

http://www.freestudy.co.uk/statics/beams/beam tut3.pdf

 

[nmodie]

Thanks a lot folks!

 

[ztengguy]

btw "straight forward superposition" means calcing the deflections for each load and adding them; you can't add the max deflection of each load (well, you can but you'll be conservative).

Isnt this the same thing?

 

[EngineeringEric]

ZTengguy, it is only the same thing if the Max deflection is at the same location in every condition. if at L/2 and 2L/3 then the Sum would be greater than the actual deflection that we would calculate as SEs.

OR i think that is what he meant...?

 

[rapt]

All of the above answers do assume that you are dealing with a material whose stiffness is not load dependant! It does not work for concrete!

 

[JLNJ]

If most of my point loads are towards the middle third of the span I will back out the equivalent uniform load based on the bending moment and use that to the get rough deflection. It's from that point that I decide if a more exact analysis is required.

 

[rb1957]

"OR i think that is what he meant...?" ... that's what i meant

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