CarlosEstay
Structural
Hi,
I am doing some checks to the auto hinge function in ETABS v18 and v19 with unsatisfactory results that I would like to get reviewed/commented by someone else.
My definitions in ETABS:
[ul]
[li]Simply supported RC frame: two columns (L=3m) and 1 beam (L=8m)[/li]
[li]RC rectangular beam 300x600mm with As top and bottom, both ends, 1000 mm2[/li]
[li]RC column 400x400mm (no rebar specified)[/li]
[li]No rigid offsets[/li]
[li]Rebar steel A615: fy=413MPa (nominal) fye=517 MPa (expected)[/li]
[li]Concrete: f'c=40MPa (expected)[/li]
[li]Auto hinge located at 0m and as per ASCE41-17 Table 10-7 concrete beams - flexure item i[/li]
[/ul]
The hinge input parameters are:
[ul]
[li]DoF: M3[/li]
[li]V2 = 50kN (user defined)[/li]
[li]Transverse reinforcing conforming[/li]
[li]Reinforcing ration: from current design[/li]
[li]Drop Loads After Point E[/li]
[/ul]
Then I checked the generated hinge property in the 'Define' menu with the following results:
[pre]Point, Moment/SF, Rotation/SF
B 1.0 0
C 1.286642 0.025
D 0.2 0.02525
E 0.2 0.050[/pre]
The ETABS calculated 'Moment SF' = 260.2291 kNm which is the same as my calculation of section strength using the expected material properties. Also, in this case, the parameter 'a' is equal to 0.025.
The issues are:
1.- The 'Moment SF' value, which is the section strength, is assigned to point 'B' (not 'C' as per ASCE41, FEMA356, ATC40)
2.- The slope between point 'B' and 'C' is calculated automatically by ETABS as k2 = 0.1*6*E*Ieff/L (where L=clear length of beam and E*Ieff is the effective stiffness of the beam using the reduction factors from ASCE41-17)
3.- The moment at point 'C' is artificially increased from 'B' to 'C' and its value is 'B'+k2*'a', hence the 1.322902 factor applied to Moment SF at point 'C'.
My understanding from looking at ASCE41, FEMA 356 and ATC 40 is that point 'C' is the maximum strength (typically calculated using ACI318 and using expected material properties) and it includes strain hardening and over-strength (included in the 1.25 factor applied to the steel rebar properties).
They also talks about the slope between 'B' and 'C' is something between 0 and 10% of the elastic effective stiffness, but that is considering 'B' as the 'effective yielding point', not the ultimate strength.
It seems that ETABS is not considering all of the above and the 'strength' values in the plastic hinge are totally overestimated (unconservative). Does anyone have had to deal with this in the past? Is it an ETABS issue with the auto hinge function for RC beams/columns not working as it should be, or I have misinterpreted the documentation?
Any comments/discussion would be much appreciated. Thanks in advance.
Regards,
Carlos
Note1: I have also applied the ASCE41-13 auto hinge with same results except for the point 'C' moment factor is 1.1 instead of 1.286642
Note2: attached is my ETABS v19.1.0 model for reference.
I am doing some checks to the auto hinge function in ETABS v18 and v19 with unsatisfactory results that I would like to get reviewed/commented by someone else.
My definitions in ETABS:
[ul]
[li]Simply supported RC frame: two columns (L=3m) and 1 beam (L=8m)[/li]
[li]RC rectangular beam 300x600mm with As top and bottom, both ends, 1000 mm2[/li]
[li]RC column 400x400mm (no rebar specified)[/li]
[li]No rigid offsets[/li]
[li]Rebar steel A615: fy=413MPa (nominal) fye=517 MPa (expected)[/li]
[li]Concrete: f'c=40MPa (expected)[/li]
[li]Auto hinge located at 0m and as per ASCE41-17 Table 10-7 concrete beams - flexure item i[/li]
[/ul]
The hinge input parameters are:
[ul]
[li]DoF: M3[/li]
[li]V2 = 50kN (user defined)[/li]
[li]Transverse reinforcing conforming[/li]
[li]Reinforcing ration: from current design[/li]
[li]Drop Loads After Point E[/li]
[/ul]
Then I checked the generated hinge property in the 'Define' menu with the following results:
[pre]Point, Moment/SF, Rotation/SF
B 1.0 0
C 1.286642 0.025
D 0.2 0.02525
E 0.2 0.050[/pre]
The ETABS calculated 'Moment SF' = 260.2291 kNm which is the same as my calculation of section strength using the expected material properties. Also, in this case, the parameter 'a' is equal to 0.025.
The issues are:
1.- The 'Moment SF' value, which is the section strength, is assigned to point 'B' (not 'C' as per ASCE41, FEMA356, ATC40)
2.- The slope between point 'B' and 'C' is calculated automatically by ETABS as k2 = 0.1*6*E*Ieff/L (where L=clear length of beam and E*Ieff is the effective stiffness of the beam using the reduction factors from ASCE41-17)
3.- The moment at point 'C' is artificially increased from 'B' to 'C' and its value is 'B'+k2*'a', hence the 1.322902 factor applied to Moment SF at point 'C'.
My understanding from looking at ASCE41, FEMA 356 and ATC 40 is that point 'C' is the maximum strength (typically calculated using ACI318 and using expected material properties) and it includes strain hardening and over-strength (included in the 1.25 factor applied to the steel rebar properties).
They also talks about the slope between 'B' and 'C' is something between 0 and 10% of the elastic effective stiffness, but that is considering 'B' as the 'effective yielding point', not the ultimate strength.
It seems that ETABS is not considering all of the above and the 'strength' values in the plastic hinge are totally overestimated (unconservative). Does anyone have had to deal with this in the past? Is it an ETABS issue with the auto hinge function for RC beams/columns not working as it should be, or I have misinterpreted the documentation?
Any comments/discussion would be much appreciated. Thanks in advance.
Regards,
Carlos
Note1: I have also applied the ASCE41-13 auto hinge with same results except for the point 'C' moment factor is 1.1 instead of 1.286642
Note2: attached is my ETABS v19.1.0 model for reference.
