RISA 3D - Reported Mf Changes between SPF member and Steel member 1

[puszka]

Hi everyone,

I am designing some beams for a porch and I decided to throw a couple whose loading was more complex in a 2D plane in RISA 3D. I noticed that the required member sizes were smaller than I had anticipated, and when I looked at the detail report it showed the Mf to be about 75% of what traditional beam formulae would give for the same condition.

When I switched the member to steel while keeping everything else equal, the Mf jumped back up to what the beam formulae give (wf*l^2/8).

Can anyone help me understand why this is happening?

 

[Celt83]

without screenshots showing spans, loading, support conditions/end releases, Shear, and Moment plots not much anyone is going to be able to comment on.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[puszka]

 

[puszka]

Here is an example. Everything is the same between the two. 5m span, 1kN/m load. Support conditions are pinned (one with Mx locked for stability so it will analyze). I'm new to RISA and I'm still learning the settings, which I imagine this is a part of.

 

[puszka]

 

[jayrod12]

Are the runs for the same load combo? Because the run on the left says envelope, the one on the right says deflection.

I see in your second post that run is both the same load combo. But the difference in moments is closer, ~11.5% difference. Do you have self weight turned on in the basic load cases section? If so, that could be a source for some of the difference.

 

[puszka]

@jayrod - yes I just caught that -- the new table image shows them both on Deflection 1

 

[puszka]

@jayrod

You were very correct -- it was a mixture of the weight differences which I forgot were turned on as well as a mistake in my hand arithmetic :S

thanks!!

 

[puszka]

For anyone else who might read this -- after triple checking my arithmetic wasn't wrong. RISA was only fully analyzing serviceability combinations in wood members and hence the Mf was lower than expected since it wasn't part of LSD. By selecting the specific combination I was able to see the expected Mf and Vf, though the 'Envelope' was only showing the worst case of the unfactored load combinations.

Thought it might be useful to know that there can be a different operation of the software depending on what material you are using!

 

[jayrod12]

Make sure that under your design tab in the load combinaton tab you have Wood checked for all of them. That should avoid potential issues like this as well.