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Deflection differences using AISC WF section vs manually input props
2

Deflection differences using AISC WF section vs manually input props

Deflection differences using AISC WF section vs manually input props

(OP)
I have a simple span beam (wind column actually), spanning 60', uniform load, and input the W18x55 section from AISC database within RISA-3D. Single load case, the most basic of simple beams.

From RISA-3D: Max moment at 232 k.ft. Max deflection 7.3" at midspan.

Hand calc: Max moment at 232 k.ft. Max deflection 5.8" at midspan.

So I investigate RISA further, to explain the 125% deflection differences. I copy the file and in the new file manually define my Izz section props within RISA (890 in4 that equals the AISC value) and I get 5.8" deflection. Correct answer.

I am a somewhat new RISA-user, so I assume I am doing something wrong when selecting the AISC database within my model - does any one have any ideas on what I may be doing wrong?

I can attach the two RISA files, if needed.



RE: Deflection differences using AISC WF section vs manually input props

RISA includes shear deformations if you have that checked in your global dialogue box. Just click on the Global Parameters and if you uncheck it the deflections should match the hand calculations.

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RE: Deflection differences using AISC WF section vs manually input props

(OP)
JAE:

Thanks for the reply.

I did try that before I post this tread: 7.325" including SHEAR deformation and 7.287" without. Both a long way off 5.8".

It is something to do with Izz using the W18x55 even though RISA reports the Izz as 890 in4 - just cannot figure out what it is! Grrr!

RE: Deflection differences using AISC WF section vs manually input props

.00624*L^2*M/I (L in ft, M in k-ft, I in4)gives 5.86 inches which is SS defl for uniform loading. Shear deformation, albeit small, would increase this value a tad. Don't know why there are other deflections unless the supports are translating.

Dik

RE: Deflection differences using AISC WF section vs manually input props

Post a beam-loading diagram - might help me or others look at it with you.

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RE: Deflection differences using AISC WF section vs manually input props

Two sources for difference that I can think of:
1) Go to the Codes tab of the (Global) Model Settings. If you're using AISC 13th or 14th edition, then there is a flexural adjustment to stiffness (per Direct Analysis Method). This is approximately 20% and is intended to decrease the flexural buckling strength and column frames. In order to better approximate the inelastic buckling that steel frames actually experience. Though this is not intended to be used for service level deflection calculations.
2) The shear deformation setting which is on the solution tab of the (Global) model settings.

RE: Deflection differences using AISC WF section vs manually input props

(OP)
Shotzie and JoshPlum:

Thank you. Riddle solved. It was the DAM stiffness adjustment factor.

JAE and dik:

The loading was uniform:




Turning 'OFF' the stiffness adjustment the displacement is now 5.8".

Thank you, all.

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