Period Difference between SAP and OPENSEES

[mkfotouhi]

Hi
I have made a 6 story steel frame building in SAP and OPENSEES but I get slight different period from them! What is the source of such difference?
SAP period:2.53 sec
OPENSEES period:2.35 sec
I have used nonlinearBeamColumn element with fiber section. I should mention that masses and sections and also constrains are exactly the same in both SAP and OPENSEES.

[silvia]

the fiber sections are the same, too?

[mkfotouhi]

No! The SAP model does not have fiber section. What is the influence of fiber section in period?!!

[silvia]

that's it.
read about it, it'd be good to know the model you are using.

[basir]

hi dr silvia
i have the same problem
i used dispBeamColumn element in my opensees model and exactly the same modeling in sap but i got these results also used fiver section in opensees
first mode period are as follows
sac 20 : opensees =3.12 sec , sap = 4.10 sec
sac 9 : opensees =2.10 sec , sap = 2.24 sec
sac 3 : opensees =0.95 sec , sap = 1.03 sec

[silvia]

they are very close, so check how axial stiffnesses are modeled.

[basir]

tanx for reply dr silvia
could you please tell me where can i find axial stiffness modeling information i saw some information in section aggregator manual script
buti dont know how should i implement it
i am using fiber section to define wsections could you send me an examle how i should consider the axial stiffness
and do you think the period wich is calculate using fiber section is correct it is very important for me to use correct period and mode shape
tanx in advance

[silvia]

the fiber section has axial and flexural stiffnesses calculated internally via an integration of the section.
you need to talk through this with someone in person, i'd find a colleague nearby.

[basir]

[quote="silvia"]the fiber section has axial and flexural stiffnesses calculated internally via an integration of the section.
you need to talk through this with someone in person, i'd find a colleague nearby.[/quote]

dear dr silvia
thank you very much
what do you think about following response
(sorry last time i sent you wrong data these are correct)

sac 3 period
drain-2dx = 1.03sec
opensees(elastic element) = 1.01sec
opensees(displacementelemnt) = 3.01sec

sac 9 period
drain-2dx = 2.24sec
opensees(elastic element) = 2.19sec
opensees(displacementelemnt) = 5.80sec

sac 20 period
drain-2dx = 3.84sec
opensees(elastic element) = 3.86sec
opensees(displacementelemnt) = 7.89sec

everything is the same in the opensees models except elements
now what do you think about period of displacementbase element
is it the correct period of structure?

[silvia]

the elastic sounds good.
i can't make a judgement about the inelastic because i have not seen it.
if you want to keep checking,
i'd do some intermediate steps:
1. inelastic element, elastic section
2. inelastic element, fiber section, elastic materials. -- here it is a matter of the discretization.

[basir]

hi dr
these are the answers for what you suggested

sac 3 periods
1-inelastic element, elastic section = 1.01sec
2-inelastic element displacment, fiber section, elastic materials =3.01sec
3-elastic element - = 1.01sec
4-inelastic displacement elemnt (elastic perfectly plastic material and hysteretic material) fiber section= 3.01sec

if you would like i will model it for sac9 sac 20 structures too

[silvia]

it seems fine, then.

[basir]

answers are as follows

sac 9 periods
1-inelastic element, elastic section = 2.14sec
2-inelastic element displacment, fiber section, elastic materials =5.80sec
3-elastic element - = 2.19sec
4-inelastic displacement elemnt (elastic perfectly plastic material and hysteretic material) fiber section= 5.80sec

sac 20 periods
1-inelastic element, elastic section = 3.86sec
2-inelastic element displacment, fiber section, elastic materials =7.89sec
3-elastic element - = 3.86sec
4-inelastic displacement elemnt (elastic perfectly plastic material and hysteretic material) fiber section= 7.89sec

[silvia]

i don't understand what all these data are.

[basir]

[quote="silvia"]i don't understand what all these data are.[/quote]

ok

i am modeling three structures
1-sac3
2-sac9
3-sac20
these structures are 2-dimentional and also they are moment resistant steel frame
i tried to model each one of them in 4 format because i didnt get a certain result for the period of these structures with fiber sections

the difrent modeling procedure which i used and the consequence first mode of vibrations period are as follows

for sac 3 structure :
1- i used ''element elasticBeamColumn'' to define elements and Tperiod=1.01 second
2- i used ''element dispBeamColumn'' which is associated with fiber section to define box section and Wsection and hysteretic material Tperiod=3.01second
3- i used ''element dispBeamColumn'' which is associated with elastic section Tperiod=1.01second
4-i used ''element dispBeamColumn'' which is associated with fiber section to define box section and Wsection and 'elastic' material Tperiod=3.01second

for sac 9 structure :
1- i used ''element elasticBeamColumn'' to define elements and Tperiod=2.19 second
2- i used ''element dispBeamColumn'' which is associated with fiber section to define box section and Wsection with hysteretic material Tperiod=5.80 second
3- i used ''element dispBeamColumn'' which is associated with elastic section Tperiod=2.14 second
4-i used ''element dispBeamColumn'' which is associated with fiber section to define box section and Wsection with 'elastic' material Tperiod=5.80 second

for sac 20 structure :
1- i used ''element elasticBeamColumn'' to define elements and Tperiod=3.86 second
2- i used ''element dispBeamColumn'' which is associated with fiber section to define box section and Wsection with hysteretic material Tperiod=7.89 second
3- i used ''element dispBeamColumn'' which is associated with elastic section Tperiod=3.86 second
4-i used ''element dispBeamColumn'' which is associated with fiber section to define box section and Wsection with 'elastic' material Tperiod=7.89 second

drain-2dx results are as follows
sac3 = 1.02
sac9 = 2.24
sac20 = 3.84