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?
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?