flexural design - sap2000 question

[Fbnlvrs]

Hi,

I've got a question of an academic exercise.

I'm analysing a beam of a 2d Frame. The beam length is 6 meters (600 cm).

The section of the beam is a W16x57.
Lb=600 cm.

According to my calculations.

Lb>LP (464 cm)
And Lb > Lr (557 cm)

So for Flexural Design I should Use (F2-3).

The problem. Is when I use SAP2000 to design the beam. Sap uses MP (F2-1) ignoring the LTB.

I'll appreaciate your help

 

[rafteroftheyear]

Excerpt from A360-16
(a) When Lb ≤ Lp, the limit state of lateral-torsional buckling does not apply.

Did you check already Lp versus Lb?

 

[JStephen]

Is that next column over, to the right of your red box, the design check you're wondering about? Shows half the moment there?

 

[Celt83]

You have not defined or incorrectly defined the unbraced length. The LTB section of the report indicates a length of 1 for Lltb similarly the length for axial is also noted as 1. Review the manual on how to specify member unbraced lengths.

I'm making a thing:

http://www.thestructuraltoolbox.com

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

 

[Blackstar123]

Celt83,
The defined length of member is 600 which is also the unbraced length, so the factor for Lltb is suppose to be 1.

 

[Celt83]

Blackstar123:
Got it, not familiar with SAP so was just going off of the output. Odd they would label a factor “Lltb”

I'm making a thing:

http://www.thestructuraltoolbox.com

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

 

[Fbnlvrs]

Could it be, that due to the beam of the 2d frame has rigid connections in both ends. the software assumes that there is no LTB.

The maximum moment is in the connection and not in the center of the beam.


Update: The error in my hand calculation was I used Cb=1 and should Use Cb:2.3

 

[driftLimiter]

LTB can't be overcome by having a fixity at the end of the beam. Even if it is a torsional restraint. The unbraced length of the beam on the compression flange is what controls LTB. If that is what the software is assuming its sorely mistaken.

 

[Fbnlvrs]

My problem was de Cb factor I was using Cb=1 and is Cb=2.3

 

[JoshPlumSE]

My problem was de Cb factor I was using Cb=1 and is Cb=2.3

Yup, I think that's why they put the Phi*Mn (when Cb=1) in that one area. So, that you understand how they got from that value to the non-LTB one that they reported.