How does RISA calculate a Distributed load?

[AuEng99]

I have mainly been in Land Development, but due to the economy I am trying to diversify and learn RISA 3D. I was trying to verify a result from RISA 3D. I just applied a load of 0.06 kips/ft to a beam 20 feet long with pinned connections. Per my 98 second edition LRFD Steel Design Manual, I had calculated that the moment(M) should be:

M=(wl^2)/8. So I would get M=(0.06 kips/ft*(20ft)^2)/8, which should equal -3 ft kip.

However, RISA calculates a value that is close, but not close enough for me. It calculates the Moment to equal -1.392 ft kip.

Can someone explain to me how it is calculating this value?

 

[JoshPlumSE]

There is almost certainly a modeling problem with your RISA model. I just cannot believe that the program could be that far off on such a simple structure. On more complex structures, there could be other explanations (shear deformation, P-Delta effect, et cetera) for why the program doesn't match hand calcs. But, for a simply supported beam it should be spot on.

I would recommend either sending your model into RISA tech support, or posting here on this forum.

 

[AuEng99]

Okay Josh, I am going to attach it.

 

[msquared48]

In your model, you are not releasing the ends of the beam so it is calculating the moment as the ends were fixed. Hence the lower positive moment. In essence you have modeled an inverted "U" frame with fixed supports.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[JoshPlumSE]

That's not a simply supported beam. Rather it is part of a portal frame. Probably closer to a fixed-fixed beam than a pinned-pinned beam.

In this sort of situation, the total difference between positive and negative moment should equal WL^2/8. In your case the w is actually 0.0701 k/ft (that includes the applied self weight of the beam). So, we're looking for the total moment to be 3.505 k-ft

RISA gives 2.126 k-ft at the column support and 1.378 at mid-span. = 3.504 k-ft total. That should be within a reasonable round-off error.

 

[AuEng99]

Thanks guys, you all have answered my question. The Force is now with me.