recorder global-local force

[Elina]

I'm recording forces at the ends of a column. Firstly, I use "localForce" and then "globalForce".
Both cases give me the same values for Fx,Fy,Fz. I mean that I have: Fx=FY, Fy=FX and Fz=-FZ. But when I look at the moments (Mx,My,Mz) I see that the values are different. Why is this??

I'm also confused with the sign of the forces. For example, for a column of a building under time history analysis, I have Fy (Shear along y-local axis) with opposite sign and same value at the two ends of the column. Shouldn't these be of the same sign???

Thank you!!

[fmk]

put the following in your script and post the answer, replacing 12 with the element you are interested in.

puts [eleResponse 12 localForces]
puts [eleResponse 12 globalForces]


no .. the element must be in equilibrium!!!

[Elina]

Can I choose the time step for the results from the command you proposed or it only has to do with where you place it in your script???

By writing "puts [eleResponse 2110 localForces]
puts [eleResponse 2110 globalForces]" after the dynamic analysis, I get:

390454.57 1362.49 2073.84 126.33 762.50 -2173.29 -375886.72 -1362.49 -2073.84 -126.33 6081.18 6669.52

1362.49 390454.57 2073.84 2006.81 22.64 20878.53 -1362.49 -375886.72 -2073.84
6495.95 -22.64 -25736.35

Forces are the same but moments are not..
OpenSees >

[fmk]

does the element have offsets associated with the coordinate transformation? if yes, that is the problem (local forces are for end of element and not at the nodes)

if not, can you post the nodes, materials, section and element command for this element so i can see what is going on.

[Elina]

# Units: N,m

node 1 0.00 3.00 0.00 ;
node 2 0.00 7.10 0.00 ;

set fc [expr -18000000.]; # CONCRETE Compressive Strength for B225 (+Tension, -Compression)
set Ec [expr 11000000000.*pow([expr -$fc/1000000],0.3)]; # Concrete Elastic Modulus
set nu 0.2;
set Gc [expr $Ec/2./[expr 1+$nu]]; # Torsional stiffness Modulus
# unconfined concrete
set fc1U $fc; # UNCONFINED concrete, maximum stress
set eps1U -0.0020; # strain at maximum strength of unconfined concrete
set eps2U -0.00350; # strain at ultimate strength of unconfined concrete

set Fy [expr 400000000.]; # STEEL yield stress
set E0 [expr 200000000000.]; # modulus of steel
set Fu [expr 690000000.]; # ultimate stress in tension
set Esh [expr 5600000000.]; # tangent at initial strain hardening
set esh 0.013; # strain corresponding to initial strain hardening
set eult 0.13; # strain at peak stress

uniaxialMaterial Concrete04 100 $fc1U $eps1U $eps2U $Ec
uniaxialMaterial Concrete04 138 -18521181 -0.002117 -0.004658 26405432582
uniaxialMaterial ReinforcingSteel 153 $Fy $Fu $E0 $Esh $esh $eult;

############################################################################################################################################################################################
proc BuildRCrectSection {id HSec BSec coverH coverB coreID coverID steelID numBarsTop barAreaTop numBarsBot barAreaBot numBarsIntTot barAreaInt nfCoreY nfCoreZ nfCoverY nfCoverZ} {
set coverY [expr $HSec/2.0];
set coverZ [expr $BSec/2.0];
set coreY [expr $coverY-$coverH];
set coreZ [expr $coverZ-$coverB];
set numBarsInt [expr $numBarsIntTot/2]; # number of intermediate bars per side

# Define the fiber section
section fiberSec $id {
# Define the core patch
patch quad $coreID $nfCoreY $nfCoreZ -$coreY -$coreZ $coreY -$coreZ $coreY $coreZ -$coreY $coreZ

# Define the four cover patches
patch quad $coverID 2 $nfCoverY -$coverY $coverZ -$coreY $coreZ $coreY $coreZ $coverY $coverZ
patch quad $coverID 2 $nfCoverY -$coreY -$coreZ -$coverY -$coverZ $coverY -$coverZ $coreY -$coreZ
patch quad $coverID $nfCoverZ 2 -$coverY $coverZ -$coverY -$coverZ -$coreY -$coreZ -$coreY $coreZ
patch quad $coverID $nfCoverZ 2 $coreY $coreZ $coreY -$coreZ $coverY -$coverZ $coverY $coverZ

# define reinforcing layers
layer straight $steelID $numBarsInt $barAreaInt -$coreY $coreZ $coreY $coreZ;
layer straight $steelID $numBarsInt $barAreaInt -$coreY -$coreZ $coreY -$coreZ;
layer straight $steelID $numBarsTop $barAreaTop $coreY $coreZ $coreY -$coreZ;
layer straight $steelID $numBarsBot $barAreaBot -$coreY $coreZ -$coreY -$coreZ;
}; # end of fibersection definition
}; # end of procedure
############################################################################################################################################################################################

set cover 0.030; # rectangular-RC-Column cover
set IDconcCover 100;
set IDSteel 153;
set barAreaF18 254.00e-6; # longitudinal-reinforcement bar area

BuildRCrectSection 34 0.60 0.30 $cover $cover 138 $IDconcCover $IDSteel 3 $barAreaF18 3 $barAreaF18 2 $barAreaF18 15 10 15 10

set GJCol30x60 [expr 0.1*$Gc*0.00675];
uniaxialMaterial Elastic 154 $GJCol30x60

section Aggregator 1034 154 T -section 34

geomTransf Linear 1 0 0 -1 -jntOffset 0.050 0.600 0.000 0.050 -0.200 0.000
element dispBeamColumn 2110 1 2 8 1034 1 ;




Is there anything else you need about the model??

Thank you!

[fmk]

the reason is the joint offsets, they are not taken into account in the computation of local forces, the local forces thus are for the ends of the element and not at the nodes.

[Elina]

Thank you very much..
Everything is clear now!