Hello,
My question is in regard to the RAMSJI.JST joist table in RAM Structural System. I have read the only explanation I could find regarding the discrepancy between the RAM joist design and using the SJI manual. Link to referenced wiki page here: http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/2054.ram-steel-joists-faq
However, after following this advice, I still am not getting the correct joist selection. Here are my exploration criteria and results:
Dead Load = 15 PSF; Roof Live Load = 20 PSF (Unreducible). I am using a 120'x120' building (60'x 60' grid) with joist spacings at 6'-0" OC.
Total Load / Roof Live Load = 210 / 120
Using the Default RAM criteria, which is RAMSJI.JST file with L/360 for Live Load Span-to-Deflection Ratio, and an L/240 for Net Total Load Span-to-Deflection Ratio.
The program selects a 36LH07. When I select the member and select "View Results", the report tells me:
Allowable loads: 236.5 / 157.7; Live Load Deflection = L/473; Total Load Deflection = L/270
I then change the Total Load Deflection Span-to-Deflection Ratio to 0 (as instructed by the RAM Steel Joist FAQ / wiki page referenced above).
The program selects a 32LH06. When I select the member and select "View Results", the report tells me:
Allowable loads: 239.4 / 123; Live Load Deflection = L/369; Total Load Deflection = L/211
However, the SJI manual lists the following:
36LH07 @ 60’ Span: Allowable loads: 274 / 160
32LH06 @ 60’ Span: Allowable loads: 242 / 125
In the RAMSJI.JST file and it lists the following:
36LH07 @ 59.67’ Span: Allowable loads: 274 / 160
36LH07 @ 60.67’ Span: Allowable loads: 266 / 153
Interpolating, 36LH07 @ 60’ Span: Allowable loads: 271.4 / 157.7
The interpolated values are very close to the SJI Manual values.
However, the Results Report shows the Allowable Live Load to be the same, but the Total Allowable Load is reduced by 34.9 PLF (or 12.86%) when compared to the values found in the RAMSJI.JST file.
In the RAMSJI.JST file and it lists the following:
32LH06 @ 59.67’ Span: Allowable loads: 242 / 125
32LH06 @ 60.67’ Span: Allowable loads: 234 / 119
Interpolating, 36LH07 @ 60’ Span: Allowable loads: 239.4 / 123.0
The interpolated values are very close to the SJI Manual values.
However, the Results Report shows the Allowable Live Load AND the Total Allowable Load to be the same.
There appears to be a reduction factor being applied to the Total Load in the program when L/240 Total Load is applied. Why would you reduce this load? Why would the Total Allowable Load of a 32LH06 (239.4) ever be more than a 36LH07 (236.5) when the span and joist spacing is the same?
Jeff