Section designer gives different values than imported section? 1

[dslyecix]

Hey everyone,

I am trying to design a stairway using channels for stringers. The loads are getting too high for the span, and so I thought to attempt a built up channel, using plates and angles. Interestingly, upon changing from a C18x58 channel to a built up section of approximately the same dimensions, my P_M ratios went from ~1.2 down to 0.3. I thought this seemed fishy because the section should be approximately the same.

To test, I imported a C18x58 section, and then created a new FSEC1 using the section designer, into which I put a single channel. I set this channel to be a C18x58. I then created two frame members, one using the imported C18x58 and the other using the section designer's imported C18x58.

Upon running the analysis under only dead load, I get different results! After comparing values (which all differ slightly, but not by any huge margins) I found that in the Steel Section Check, under the Moment Design heading, for the C18x58 Mr Capacity is listed as 101.29 kN*m whereas the section designed C18x58 has a Mr Capacity of 379.92 kN*m.

The same thing happens when comparing a C10x20, within and outside of the section designer. Under dead load only I find a section designer imported C10x20 to be at 0.061 while a straight added C10x20 is at 0.340.

This makes me extremely unwary of trusting any section that I have input through the section designer, as it seems to apply some sort of extreme "benefit" to the section that is not ascribed in the same way to anything you import the regular way.

Is there any reason that you can think of for why this would be happening?

 

[SteeliestFire55]

The difference in the calculated capacities has to do with how SAP2000 differentiates between section types.

A directly imported channel section will be classified as a 'Channel Section' and the critical length will be determined per a typical channel section for Flexural torsional buckling.

The section designer on the other hand does not determine the shape type and classifies all sections as 'General'. The general section flexural capacity is calculated as 0.6*Fy with no consideration for the compression flange unbraced length.

A quick check would be to increase the span length. You will notice that the directly imported C18x58 capacity will decrease whereas the Section Designer C18x58 capacity won't change.

Further information can be found in the SAP2000 Steel Frame Design Manual.

 

[stressed]

SteeliestF is correct. You have another option which should work fine, and that is to Define a new section using "Add New Property", then choose steel type and click channel type. There you can input whatever dimensions you wish to use and SAP should recognize it as a channel instead of a general section like in SD. You can use this Define/Add option for defining built-up sections for several common steel shapes (Wide flange, Tee, Channel, angles, etc.).

 

[dslyecix]

Thank you both! SteeliesFire55, your answer mirrors the response I received from the support ticket I opened as well. My beam length is quite long, which is of course why there was such a large discrepancy. Thanks for explaining what's going on.

Stressed, thanks for the input. I had forgotten about that option. It should work perfectly for what I need it for.