2D Frame Discrepancy 1

[XR250]

I ran this W8x21 moment frame in my 2D software (It is FrameMac on a Mac). I get a deflection of 0.4".
It is 10 ft. tall, 17'-6" wide and has a point load of 1,600 lbs ASD at the top right. My colleague ran the same frame on RISA 2D. He got the same base reactions but had a deflection of 0.5". We cannot figure out why they are different. I have been using this same frame software for literally 30 years and it has been vetted many times.
Does anyone feel like running this real quick and see what they get?

Thanks

 

[rowingengineer]

What analysis models are they using? Linear vs non-linear, includes shear deformation and nonlinear section changes?

 

[KootK]

Different treatments of P-Delta effects?

How come the FrameMac shows no beam rotation / deflection? The joints do not seem to be retaining their squareness under load.

 

[XR250]

Frame-Mac is linear with no P-delta. Here is a close-up of the beam....

 

[Ingenuity]

If your colleague re-runs the RISA model and changes the STIFFNESS adjustment to OFF (in GLOGAL MODEL SETTINGS, under CODES for AISC hot-rolled sections), then the lateral X displacement goes from 0.506" to 0.406" - same are your FRAME-MAC displacement.

 

[rowingengineer]

Sounds like old software.

 

[XR250]

For sure. It does not do any code checks. Just a sweet graphical interface to solve some matrix algebra.

 

[KootK]

If your colleague re-runs the RISA model and changes the STIFFNESS adjustment to OFF (in GLOGAL MODEL SETTINGS, under CODES for AISC hot-rolled sections), then the lateral X displacement goes from 0.506" to 0.406" - same are your FRAME-MAC displacement.

The confidence expressed in that statement suggests a man who's actually done the RISA modelling himself and, thus, knows the answer with some certainty. If so, "A" for effort.

 

[XR250]

Thanks to all. Looks like I should study the code a bit more

 

[rowingengineer]

When we use such simple software for anything more than a beam our code in our area oof the world makes us work out factors to adjust just about every result for p-d amplification. Which is why non-linear is just about standard in most office spaces.

 

[Ingenuity]

...knows the answer with some certainty.

'Twas my first experience with RISA some years ago such that I posted near the exact same question on E-T...and as a great brain trust that is E-T, I got the immediate answer.

 

[Ingenuity]

My old thread on the same issue: thread1419-424148

 

[JoshPlumSE]

Yes, depending on the design code used for Steel Analysis in RISA, there may or may not be a "stiffness adjustment factor" applied to the analysis per the "direct analysis method". This usually amplifies the deflection by 1/0.8 = 1.25, which is exactly what the amplification in this case was.